Patent application title: ZWITTERIONIC POLYSACCHARIDES FOR PROMOTION OF IMMUNE SYSTEM MATURATION AND HEALTH
Dennis L. Kasper (Brookline, MA, US)
Dennis L. Kasper (Brookline, MA, US)
Sarkis M. Mazmanian (Porter Ranch, VA, US)
THE BRIGHAM AND WOMEN'S HOSPITAL, INC.
IPC8 Class: AC08B3700FI
Class name: Carbohydrates or derivatives nitrogen containing sulfur containing
Publication date: 2012-12-06
Patent application number: 20120309955
Certain zwitterionic polysaccharides, including those naturally expressed
by commensal B. fragilis in the gut, interact with cells of the immune
system and affect the TH1/TH2 balance so as to promote health.
Nutritional formulas and nutritional supplements containing isolated
preparations of such zwitterionic polysaccharides, and methods for
preparing the nutritional formulas and supplements, are provided. Also
provided is a method of promoting immune system maturation in an infant
involving enteral administration of a nutritional formula or nutritional
supplement of the invention.
1. A nutritional formula or nutritional supplement composition,
comprising an isolated zwitterionic polysaccharide consisting essentially
of repeating units, wherein each repeating unit comprises two to ten
monosaccharides, and free amino moiety and a negatively charged moiety
selected from the group consisting of carboxylate, phosphate,
phosphonate, sulfate, and sulfonate.
2. The composition of claim 1, wherein the zwitterionic polysaccharide is a naturally occurring bacterial capsular polysaccharide.
3. The composition of claim 1, wherein the zwitterionic polysaccharide is a B. fragilis capsular polysaccharide A (PSA).
4. The composition of claim 3, wherein the PSA is PSA1.
5. The composition of claim 3, wherein the PSA is PSA2.
6. The composition of claim 1, wherein the zwitterionic polysaccharide is a B. fragilis capsular polysaccharide B (PSB).
7. The composition of claim 1, wherein the zwitterionic polysaccharide is selected from the group consisting of Shigella sonnei Phase I lipopolysaccharide 0-antigen, Streptococcus pneumoniae type 1 capsular polysaccharide, and Streptococcus pneumoniae group antigen C substance.
8. The composition of claim 1, wherein the nutritional formula or nutritional supplement is a nutritional formula.
9. The composition of claim 1, wherein the nutritional formula or nutritional supplement is a nutritional supplement.
 This application is a divisional application of Ser. No. 12/223,563, filed Apr. 3, 2009 which is a national stage filing under 35 U.S.C. §371 of international application PCT/US2007/003160, filed Feb. 5, 2007, which claims the benefit under 35 U.S.C. §119(e) from U.S. provisional application Ser. No. 60/765,800, filed Feb. 6, 2006, the entire contents of each of which are herein incorporated by reference.
BACKGROUND OF THE INVENTION
 Immediately after a sterile birth, mammals are initiated into an organized and life-long process of colonization by foreign organisms. Shaped by eons of evolution, some host-bacterial associations have developed into prosperous relationships creating diverse environments. No better example exists in biology than the astounding numbers of bacteria harbored by the lower gastrointestinal tract of mammals (Hooper et al., 1998). By young adulthood, humans and other mammals are host to ˜1012 viable bacteria per gram of colonic content, consisting of 500-1000 microbial species and outnumbering host cells by 100-fold (Hooper and Gordon, 2001). The magnitude of this interaction between commensal bacteria and mammals must predictably exert fundamental influences on the physiology of both. The most impressive feature of this relationship may be that the host not only tolerates but has evolved to require colonization by commensal microorganisms for its own development and health.
 Autochthonous (indigenous) bacteria in the mammalian gut have long been appreciated for potential benefits to the host: provision of essential nutrients, metabolism of indigestible compounds, defense against colonization by opportunistic pathogens, and contributions to the development of the intestinal architecture (Hooper et al., 2000; Hooper et al., 2002). For some years workers have sought to understand how and why the immunocompetent gut environment allows the presence of multitudinous foreign organisms. Researchers have proposed that certain commensal bacteria have evolved to aid in the host's health; several organisms are being studied for probiotic (beneficial) potential (Guarner and Malagelada, 2003; Rastall, 2004). The "hygiene hypothesis" suggests that the appropriate bacterial constitution of the human microflora is a factor in protection from allergy and asthma (Umetsu et al, 2002; Von Hertzen and Haahtela, 2004). Investigations have shown that the interactions of commensal bacteria with Toll-like receptors are critical for intestinal homeostasis (Rakoff-Nahoum et al., 2004). The intimate relationships between commensal microorganisms and the host immune system are increasingly evident (Macpherson and Harris, 2004; Noverr and Huffnagle, 2004).
 The mammalian immune system is a dynamic and remarkable organ. In recognizing, responding, and adapting to countless foreign and self molecules, the immune system is central to processes of health and disease. CD4.sup.+ T cells, a major component of the immune system, are required for vital aspects of proper immune function, from reactions to infectious agents to control of autoimmune reactions and cancers (Janeway et al., 2001). Effector CD4.sup.+ T cells are of two general subtypes: T helper 1 (TH1) and T helper 2 (TH2), each carrying out distinct and opposing activities. The proper balance between TH1 and TH2 immunologic responses is critical to overall human and animal health (Neurath et al., 2002; Sheikh and Strachan, 2004). A role for commensal bacteria in establishing this equilibrium has been postulated (Bowman and Holt, 2001; Rook and Brunet, 2002).
 Bacteroides fragilis (B. fragilis) is a ubiquitous and important gram-negative anaerobe that colonizes the mammalian lower gastrointestinal tract. Bacteroides spp. are among the earliest-colonizing and most numerically prominent constituents of the gut microflora (Kononen et al., 1992). Although capsular polysaccharides are common in many bacterial species, B. fragilis elaborates an unprecedented eight distinct surface polysaccharides (Krinos et al., 2001). Several of these polysaccharides have a characteristic zwitterionic structure, with both positive and negative charges in each repeating unit (Tzianabos et al., 1993).
 Zwitterionic polysaccharides (ZPSs) are unique T cell-dependent antigens that specifically mediate the proliferation of CD4.sup.+ T cells in vitro (Brubaker et al., 1999; Tzianabos and Kasper, 2002). Adoptive transfer experiments have shown that responses to polysaccharide A (PSA), the most immunodominant ZPS of B. fragilis, are conferred by CD4.sup.+ T cells, not by B cells or other T cells (Tzianabos et al., 1999). PSA is internalized and processed within endosomes of antigen-presenting cells (APCs) (Cobb et al., 2004). Subsequent presentation of processed polysaccharide by major histocompatibility complex class II (MHC II) molecules activates CD4.sup.+ T cells and represents a previously undescribed pathway of antigen presentation. Thus ZPSs appear to have evolved novel biological activities shaped by co-evolution with the host immune system.
SUMMARY OF THE INVENTION
 The invention is based in part on the discovery by the applicants that enteral administration of a zwitterionic polysaccharide, such as the bacterial capsular polysaccharide PSA isolated from B. fragilis, can influence overall immune homeostasis through the zwitterionic polysaccharide's ability to mediate establishment of a proper TH1/TH2 balance for the host, a fundamental aspect of healthy immunologic function. More particularly, according to the invention ZPSs can be included in nutritional formulas and nutritional supplements and administered to a subject to help promote and maintain a desirable balance between TH1 and TH2 throughout the immune system. Establishment of a proper TH1/TH2 balance using the compositions and methods of the invention promotes a state of health characterized by diminished risk of having or developing a number of immune-mediated conditions associated with a default TH2 phenotype, e.g., atopy (allergy predisposition), allergic asthma, and autoimmunity. Compositions and methods of the invention are thus useful whenever it is desirable to provide a TH1 stimulus to the immune system of a mammalian subject. In particular, the compositions and methods of the invention are useful in stimulating a proper TH1/TH2 balance in human infants and other human subjects that have an immature immune system or predisposition to develop a default TH2 phenotype.
 In one aspect the invention is a nutritional formula or nutritional supplement composition, comprising an isolated zwitterionic polysaccharide consisting essentially of repeating units, wherein each repeating unit comprises two to ten monosaccharides and a positively charged free amino moiety and a negatively charged moiety selected from the group consisting of carboxylate, phosphate, phosphonate, sulfate, and sulfonate.
 In one aspect the invention is a method for preparing a nutritional formula or nutritional supplement composition. The method according to this aspect of the invention includes the step of combining an isolated zwitterionic polysaccharide, said polysaccharide consisting essentially of repeating units, wherein each repeating unit comprises two to ten monosaccharides and a positively charged free amino moiety and a negatively charged moiety selected from the group consisting of carboxylate, phosphate, phosphonate, sulfate, and sulfonate, with a nutritional formula or nutritional supplement that is otherwise free of the isolated zwitterionic polysaccharide.
 In one aspect the invention is a method of promoting immune system maturation in an infant. The method according to this aspect of the invention includes the step of enterally administering to the infant an effective amount of a nutritional formula or nutritional supplement composition, said composition comprising an isolated zwitterionic polysaccharide consisting essentially of repeating units, wherein each repeating unit comprises two to ten monosaccharides and a positively charged free amino moiety and a negatively charged moiety selected from the group consisting of carboxylate, phosphate, phosphonate, sulfate, and sulfonate.
 In one embodiment the zwitterionic polysaccharide is a naturally occurring bacterial capsular polysaccharide.
 In one embodiment the zwitterionic polysaccharide is a B. fragilis capsular polysaccharide A (PSA). In one embodiment the PSA is PSA1.
 In one embodiment the PSA is PSA2.
 In one embodiment the zwitterionic polysaccharide is a B. fragilis capsular polysaccharide B (PSB).
 In one embodiment the zwitterionic polysaccharide is selected from the group consisting of Shigella sonnei Phase I lipopolysaccharide O-antigen, Streptococcus pneumoniae type 1 capsular polysaccharide, and Streptococcus pneumoniae group antigen C substance.
 In one embodiment the nutritional formula or nutritional supplement is a nutritional formula.
 In one embodiment the nutritional formula or nutritional supplement is a nutritional supplement.
 In one embodiment the enterally administering is orally administering.
 In one embodiment the infant is 0-6 months old.
 In one embodiment the immune system maturation is an increase in a T helper 1 marker to a T helper 2 marker.
 In one embodiment the T helper 1 marker is a cytokine selected from interferon gamma (IFN-γ) and interleukin 2 (IL-2).
 In one embodiment the T helper 2 marker is a cytokine selected from interleukin 4 (IL-4) and interleukin 5 (IL-5).
BRIEF DESCRIPTION OF THE DRAWINGS
 FIG. 1. Cellular and Physical Immune Maturation in Germ-Free Mice Requires PSA Production During Intestinal B. fragilis Colonization.
 (A) Flow cytometry (FC) analysis of anti-CD4-stained splenic lymphocytes from conventionally colonized (CNV) and germ-free (GF) mice reveals depletion of CD4.sup.+ T cells in the absence of colonizing microflora in the gastrointestinal (GI) tract. Results are representative of 4 experiments with pools of 3-5 mice.
 (B) Quantitation of fecal colony forming units (CFU) during mono-colonization of GF mice with wild-type B. fragilis or an isogenic mutant deficient in PSA production (B. fragilis ΔPSA) reveals equivalent counts of viable bacteria from fecal pellets. Inset: Immunoblot of bacterial extracts with anti-PSA after SDS-PAGE separation and electro-transfer to PVDF membranes shows the lack of PSA expression by B. fragilis ΔPSA.
 (C) FC of anti-CD4-stained splenic lymphocytes from GF mice shows that intestinal mono-colonization of mice with B. fragilis results in complementation of CD4.sup.+ T cells. In the presence of every other antigen produced during colonization with B. fragilis ΔPSA, the absence of PSA results in no increase in CD4.sup.+ T cell counts. Results are representative of >10 experiments with single or pools of 3-5 mice.
 (D) FC of splenic lymphocytes from CNV and GF mice colonized with wild-type or B. fragilis ΔPSA reveals no significant differences in CD19.sup.+ B cells (upper panels) or CD8.sup.+ T cells (lower panels) in the CD4.sup.- fraction. Results are representative of 4 experiments with pools of 3-5 mice.
 (E) Hematoxylin and eosin (H&E) stained sections of spleens from CNV mice (right), GF mice mono-colonized with wild-type B. fragilis (left) and with B. fragilis ΔPSA (center). White pulp containing lymphocytes appears as darker-staining follicular structures (arrows). The lack of large, well-defined follicles in mice colonized with B. fragilis ΔPSA is a measure of T cell depletion and reflects developmental defects in organogenesis. All images were taken at the same magnification.
 FIG. 2. Purified PSA Treatment Is Sufficient for Expansion of CD4.sup.+ T Cells.
 (A) FC of anti-CD4-stained splenic lymphocytes from GF mice treated intraperitoneally with PSA or phosphate buffered saline (PBS) reveals PSA-dependent restoration of CD4.sup.+ T cells. Results are representative of 2 experiments with pools of 4 mice.
 (B) FC shows that oral treatment of conventional C57BL/6 and BALB/c mice with purified PSA (PSA) results in an increase in CD4.sup.+ T cell proportion among splenic lymphocytes over controls (PBS). Results are representative of 4 experiments with pools of 3-5 mice.
 (C) FC shows that oral treatment of conventional mice with purified PSA does not affect proportions of CD8.sup.+ T cells or CD19.sup.+ B cells among the CD4.sup.- splenic lymphocyte population.
 FIG. 3. PSA Is Specifically Recognized by Dendritic Cells (DCs) in the GI Tract and in Vitro, with Consequent Cell Activation.
 (A) Oral treatment of mice with Alexa-594-labeled PSA results in antigen uptake by CD11c.sup.+ DCs from mesenteric lymph nodes (MLNs). FC of CD11c.sup.+ gated cells (boxed left panel, middle) analyzed for the presence of PSA (horizontal axis of right panels) shows co-localization of PSA with DCs. FC of isolated MLNs reveals no PSA associated with CD4.sup.+ T cells from the same lymph nodes (boxed left panel, bottom). Isotype control is shown in the top left panel. Shaded histograms represent unstained control, and thick black lines denote Alexa-594 signal from PSA on the horizontal axis. No PSA is detectable in splenic tissues (data not shown).
 (B) Confocal microscopy of anti-CD11c-labeled (originally green) and 24-hour PSA-treated (originally red) bone marrow-derived dendritic cells (BMDCs) in culture illustrate antigen in endosomes and surface display (arrowheads). Central image is the XY-plane of a medial Z-section, upper and side panels are assembled Z-stacks.
 (C) FC of BMDCs cultured for 24 hours with PSA shows activation by up-regulation of MHC II (horizontal axis) among CD11c.sup.+ cells (vertical axis).
 (D) FC of CD11c.sup.+ BMDCs cultured for 24 hours with PSA shows activation through up-regulation of the co-stimulatory molecule CD86 (B7.2).
 FIG. 4. PSA Induces CD4.sup.+ T Cell Proliferation and TH1 Cytokine Production in Vitro.
 (A) CD4.sup.+ T-cell proliferation by [3H]thymidine incorporation increases in response to irradiated BMDCs (1×106) incubated with PSA. Treatment of co-cultures with NAc-PSA results in no increase in cell proliferation. Lipopolysaccharide (LPS) treatment or DCs or T cells alone do not support CD4.sup.+ T-cell proliferation. Results are representative of 3 experiments.
 (B) PSA treatment stimulates IFN-γ in DC-T cell co-cultures, as measured by enzyme-linked immunosorbent assay (ELISA) of culture supernatants after 48 hours of treatment. NAc-PSA treatment has no effect. Treatment with anti-CD3, LPS, and staphylococcal enterotoxin A (SEA), all known stimulators of TH1 cytokine expression, results in IFN-γ production. DCs or T cells alone treated with PSA do not support cytokine expression.
 (C) PSA treatment does not stimulate expression of IL-4 in DC-T cell co-cultures. Treatment with anti-CD3 and SEA, known stimulators of TH2 cytokine expression, results in IL-4 production.
 FIG. 5. PSA Signals Through the IL-12/Stat4 Pathway to Mediate TH1 Cytokine Production, Which Requires Presentation by MHC II.
 (A) PSA stimulates expression of IL-12, the TH1 determining signal, in DC-T cell co-cultures (1×106 of each cell type). NAc-PSA treatment has no effect. Anti-CD3 and SEA serve as controls for IL-12 expression by BMDCs. Results are representative of 2 experiments.
 (B) IL-12 is required for PSA-mediated IFN-γ production. IFN-γ expression is abolished in DC-T cell co-cultures treated with PSA (100 μg/mL) in the presence neutralizing antibody to IL-12. Neutralization of IL-12 does not inhibit anti-CD3-mediated IFN-γ expression, which is IL-12 independent.
 (C) PSA signals through Stat4 to induce IFN-γ secretion by T cells. IFN-γ expression from DC-T cell co-cultures treated with PSA (100 μg/mL) is reduced when CD4.sup.+ T cells are purified from spleens of stat4 knockout rather than wild-type mice (FIG. 4B).
 (D) MHC class II expressed on DCs is necessary for PSA-mediated IFN-γ expression by CD4.sup.+ T cells. IFN-γ production by DC-T cell co-cultures treated with PSA (100 μg/mL) is reduced when BMDCs are from MHC II knockout mice. NAc-PSA (100 μg/mL) treatment of wild-type DCs (MHC II.sup.+/+) serves as the control for PSA-specific IFN-γ expression.
 FIG. 6. Colonization of GF Mice with PSA-Producing B. fragilis Corrects TH1/TH2 Imbalances Associated with Cytokine-Mediated Pathologies.
 (A) IL-4 production from splenic CD4.sup.+ T cells stimulated in vitro with anti-CD3/anti-CD28 (2 μg/mL each), reveals that PSA is required to correct the TH2 skew in GF mice. Compared with CNV mice, GF mice overproduce IL-4 (first and second bars). Intestinal colonization with B. fragilis (third bar) reduces the expression of IL-4 from levels in GF mice. B. fragilis ΔPSA colonization fails to correct the TH2 skew (fourth bar). Results are representative of 2 experiments from pools of 4 mice.
 (B) IFN-γ expression by splenic CD4.sup.+ T during colonization indicates increased TH1 cytokine production in CNV than in GF mice (first and second bars). PSA production by intestinal B. fragilis is required for the increase in IFN-γ expression and immune homeostasis (third bar); homeostasis is not seen in the absence of PSA (fourth bar).
 (C) Intracellular cytokine staining and FC of in vitro stimulated (500 ng/mL PMA, 5 μg/mL ionomycin) cultures of splenic CD4.sup.+ T cells for 4 hours in the presence of brefeldin A shows that IFN-γ (horizontal axis) is produced specifically by CD4.sup.+ T cells (vertical axis) during bacterial colonization. PSA production by B. fragilis is required for the specific increase in IFN-γ expression to levels similar to those for CNV mice. GF and B. fragilis ΔPSA-colonized mice express low levels of TH1 cytokines. Results are representative of 3 experiments with pools of 3-5 mice.
 (D) Thymic histology of germ-free mice (H&E) colonized with wild-type or B. fragilis ΔPSA for over 1 year reveal follicles (arrows) within the inner medullary compartment in the absence of PSA. None of 5 B. fragilis-colonized compared to 3 of 5 B. fragilis ΔPSA-colonized mice displayed follicles.
 (E) FC of thymic tissues recovered from groups of differentially colonized GF mice (10 per group) reveals the anomalous presence of CD19.sup.+ B cells in B. fragilis ΔPSA-colonized mice, a condition likely resulting from increased TH2 cytokine production in the absence of PSA.
DETAILED DESCRIPTION OF THE INVENTION
 It has now been found that maturation of the mammalian immune system is profoundly influenced by the specific direction of an immunomodulatory molecule expressed by the symbiotic bacteria, B. fragilis. This organism, a ubiquitous constituent of the mammalian lower gastrointestinal microflora, elaborates a zwitterionic polysaccharide (i.e., PSA) that directs the development of CD4+ T cells; the eventual result is the correction of immunologic defects found in the absence of bacterial colonization. Impaired systemic CD4+ T cell maturation, aberrant TH1/TH2 lineage differentiation, and defective lymphoid organ development can all be corrected by PSA or by PSA production during B. fragilis colonization. Dendritic cells in gut-associated lymphoid tissues (GALT) apparently sample B. fragilis and/or PSA from the intestine and after activation migrate to lymphoid organs and signal TH1 lineage differentiation through the IL-12/STAT4 pathway. As TH1 cytokine production opposes the TH2 default phenotype, this process contributes to protection from disease by creating appropriate cytokine balances in the immune system.
 It has been hypothesized that reduced exposure to immunomodulatory molecules of beneficial commensal bacteria that provide protection from unrelated immune diseases results from improved sanitation and antibiotic use. This "hygiene hypothesis" posits that deviations in gut microflora composition may be the environmental factor underlying development of atopy and asthma in genetically predisposed individuals.
 It has now been discovered according to the invention that feeding of PSA or other zwitterionic polysaccharides having the charge characteristics of PSA to newborn infants would provide the appropriate TH1 stimulus to their immune systems that would stimulate the infant to develop with a favorable TH1/TH2 balance. As described in greater detail below, the feeding of the polysaccharide can be accomplished through enteral administration of infant formulas, nutritional supplements, pills, or liquid suspensions, wherein these agents contain the isolated zwitterionic polysaccharide. In practice this can be done daily at a dose sufficient to maintain TH1 stimulation over the course of the first six months to one year of life, allowing full development of a competent and healthy immune system.
 Also as described in greater detail below, the polysaccharides can be PSA, PSB, or other zwitterionic polysaccharides characterized by repeating units, each repeating unit bearing a charge motif. In one embodiment, such as in PSA, the charge motif includes one positive charge and one negative charge on each repeating unit. In other embodiments, the charge motif includes at least one positive charge and at least one negative charge on each repeating unit. The positive charge is in the form of a free amino group and the negative charge is in the form of a carboxylate group, a phosphate group, or other similarly negatively charged residues.
 The molecular size of the molecule to be fed could be as small as one or two saccharide units. Recent studies by the inventors have shown that a disaccharide comprised of only one non-acetylated amino sugar and one uronic acid is sufficient to stimulate T-cell proliferation. This is important because it suggests that the natural hydrolysis of these molecules which might take place in the acidic environment of the stomach would still allow carbohydrate molecules to be absorbed that remain active on the immune system.
 A nutritional formula as used herein refers to a nutritional formulation suitable for enteral administration. Nutritional formulas are well known in the art and are sometimes referred to as enteral formulas. Such preparations typically include a water-based formulation containing a source of nutritional carbohydrate, amino acids and proteins, fat, vitamins, minerals, and optionally other components such as nucleic acids. In one embodiment a nutritional formula refers to an infant formula suitable for administration to human infants. The nutritional formula can be formulated and presented either as a liquid or as a dry powder for reconstitution. A nutritional formula of the invention further specifically includes at least one zwitterionic polysaccharide useful for promoting immune system maturation, as disclosed herein.
 A nutritional supplement as used herein refers to a composition suitable for enteral administration either as a supplement or as an additive to any one or more components of a diet. A nutritional supplement generally can include at least one source of any one or combination of nutritional carbohydrates, amino acids and proteins, fats, vitamins, minerals, and optionally other components such as nucleic acids. In certain embodiments the nutritional supplement can be formulated and presented as a liquid, as a dry powder, as a capsule, or as a pill. Other formulations are also contemplated, including bars, sprinkles, cereals, gels, pastes, and the like. A nutritional supplement of the invention further specifically includes at least one zwitterionic polysaccharide useful for promoting immune system maturation, as disclosed herein.
 A zwitterionic polysaccharide as used herein in one embodiment refers to a naturally occurring polysaccharide having certain structural features including the presence of repeating units, each with at least one positively charged moiety and at least one negatively charged moiety. A zwitterionic polysaccharide as used herein in one embodiment refers to polysaccharides that have been modified to include the structural features including the presence of repeating units, each with at least one positively charged moiety and at least one negatively charged moiety. The zwitterionic polysaccharides useful according to the invention generally have a plurality of repeating units, wherein each repeating unit comprises two to ten monosaccharides and a positively charged free amino moiety and a negatively charged moiety selected from the group consisting of carboxylate, phosphate, phosphonate, sulfate, and sulfonate. Molecular weights of the zwitterionic polysaccharides useful in the invention typically have molecular weights between 500 Da and 2,000,000 Da, although smaller and larger polysaccharides can also be used. For example, the polysaccharide can be as small as one or two saccharide units. Thus a disaccharide including only one non-acetylated amino sugar and one uronic acid is sufficient to stimulate T-cell proliferation.
 Polysaccharides useful according to the present invention include those naturally occurring polysaccharides that include the requisite charged groups. These polysaccharides may be derived from bacterial sources. Bacteria used as starting materials to obtain capsular polysaccharides can be obtained commercially from a number of sources. For example, the B. fragilis, NCTC 9343 and ATCC 23745 may be obtained from the National Collection of Type Cultures (London, England) and the American Type Culture Collection (Manassas, Md.). Polysaccharide A and polysaccharide B can be purified from the above bacteria following the protocol of Pantosti et al. (1991) Infect Immun 59:2075-2082.
 In addition to the naturally occurring polysaccharides, polysaccharide repeating units that consist of at least one N-acetyl sugar and at least one uronic acid (sugar with a negatively charged carboxyl group) can be modified to produce the immune response of the present invention. A polysaccharide repeating unit containing at least one N-acetyl sugar and at least one uronic acid can be de-N-acetylated to create a free amino group and thus will yield a polysaccharide with the correct charge motif. Molecules which may be de-N-acetylated include Salmonella typhi capsular polysaccharide (VI antigen), Escherichia coli K5 capsular polysaccharide, Staphylococcus aureus type 5 capsular polysaccharide, Group B Streptococcus type III capsular polysaccharide, and Rhizobium meliloti exopolysaccharide II. These polysaccharides and their modification have been described in U.S. Pat. No. 5,679,654, the entire contents of which is incorporated herein by reference.
 De-N-acetylation can be accomplished by conventional chemistry techniques well known to those of ordinary skill in the art. One suitable method involves the use of alkali with or without sodium borohydride. Twenty mg of polysaccharide is dissolved in 2M NaOH (3 ml) and sodium borohydride is added (50 mg). The solution is heated to 100° C. for 5 h. Following neutralization with acid, the solution is dialyzed against distilled water in the cold and freeze-dried. DiFabio, J. L, Michon, F., Brisson, J. R., Jennings, H. J., Weasels, M. R. Benedi, V. J., Kasper, D. L. (1989) Structure of the capsular polysaccharide antigen of type IV groups B Streptococcus. Can J Chem 67:877-882.
 Bacterial polysaccharides which possess imine groups (C═NH) in addition to free carboxyl groups may be modified and used to produce the immune response of the present invention. Many of the Pseudomonis aeruginosa O-specific side chains possess imine groups. Imine groups can be reduced with sodium borohydride (NaBH4) to create free amino groups (NH3.sup.+). An example of a compound which may be reduced with sodium borohydride to create free amino groups is Pseudomonas aeruginosa Fisher 7.
 A bacterial capsular polysaccharide as used herein refers to a polysaccharide which occurs in nature as a component of the capsule of a bacterial cell. For use in the present invention, a bacterial capsular polysaccharide is isolated from the bacterial cell in which it occurs in nature. As used herein, isolated means removed from other components with which a polysaccharide may be found in nature.
 A naturally occurring bacterial capsular polysaccharide as used herein refers to a bacterial capsular polysaccharide that is not modified from how it occurs in nature except for being isolated.
 B. fragilis polysaccharide A (PSA) as used herein refers to B. fragilis capsular polysaccharide A as disclosed, for example, in U.S. Pat. No. 5,679,654. This polysaccharide has a tetrasaccharide repeating unit containing one cationic free amine and one anionic carboxylate in each repeating unit. Tzianabos A O et al. (1992) J Biol Chem 267:18230-5; U.S. Pat. Nos. 5,679,654 and 5,700,787. PSA is also known as PSA1.
 PSA2 as used herein refers to B. fragilis capsular polysaccharide A as disclosed, for example, in Wang Y et al. (2000) Proc Natl Acad Sci USA 97:13478-83, and Kalka-Moll W M et al. (2001) Infect Immun 69:2339-44. Bacteroides fragilis PSA2 has a pentasaccharide repeating unit containing mannoheptose, N-acetylmannosamine, 3-acetamido-3,6-dideoxyglucose, 2-amino-4-acetamido-2,4,6-trideoxygalactose, fucose, and 3-hydroxybutanoic acid. PSA2 is zwitterionic and carries one cationic free amine and one anionic carboxylate in each repeating unit.
 B. fragilis polysaccharide B (PSB) as used herein refers to B. fragilis capsular polysaccharide B as disclosed, for example, in U.S. Pat. No. 5,679,654. This polysaccharide has a repeating unit that includes six monosaccharides.
 Shigella sonnei Phase I lipopolysaccharide O-antigen, Streptococcus pneumoniae type 1 capsular polysaccharide, and Streptococcus pneumoniae group antigen C substance are additional zwitterionic polysaccharides disclosed for example in U.S. Pat. No. 5,679,654 and useful in the instant invention.
 Immune system maturation as used herein refers to development of an immune system characterized by a TH1/TH2 balance that is associated with a healthy adult phenotype. A healthy adult phenotype is to be contrasted with a default TH2 phenotype that is characteristic of atopy, allergic asthma, or certain types of autoimmune disease. Immune system maturation can be assessed by measuring, either at a single point in time or serially over a relevant span of time, relative contributions of at least one TH1 marker and at least one TH2 marker.
 An infant as used herein refers to a human less than 12 months old. In one embodiment an infant is 0-9 months old. In one embodiment an infant is 0-6 months old. In one embodiment an infant is 0-3 months old.
 As used herein, enterally has its usual meaning and refers to any route involving administration to at least one component of a gastrointestinal tract. Enteral administration can be accomplished by any suitable method, including oral feeding (e.g., sucking, chewing, swallowing); nasogastric delivery; orogastric delivery; gastric delivery; duodenal, jejunal, or other small intestinal delivery; enema and other forms of delivery to the large intestine. In one embodiment enterally refers to orally.
 A T helper 1 marker (TH1 marker) as used herein refers to an objectively measurable manifestation of a TH1 immune phenotype. TH1 markers include, without limitation, certain cytokines including interferon gamma (IFN-γ) and interleukin 2 (IL-2), as well as certain immunoglobulin isotypes, e.g., IgG1 in humans and IgG2a in mice. Methods for measuring TH1 cytokines and immunoglobulin isotypes are well known in the art and can include, without limitation, appropriate cytokine-specific or isotype-specific enzyme-lined immunosorbent assay (ELISA), bioassay, quantitative reverse transcriptase-polymerase chain reaction, and the like.
 A T helper 2 marker (TH2 marker) as used herein refers to an objectively measurable manifestation of a TH2 immune phenotype. TH2 markers include, without limitation, certain cytokines including interleukin 4 (IL-4) and interleukin 5 (IL-5), as well as certain immunoglobulin isotypes, e.g., IgE in humans and in mice. Methods for measuring TH2 cytokines and immunoglobulin isotypes are well known in the art and can include, without limitation, appropriate cytokine-specific or isotype-specific ELISA, bioassay, quantitative reverse transcriptase-polymerase chain reaction, and the like.
 An increase in a T helper 1 marker to a T helper 2 marker as used herein refers to skewing a T helper subtype predominance toward a TH1 phenotype and/or away from a TH2-dominant phenotype, e.g., away from a TH2 default phenotype. Any increase in the ratio of TH1 to TH2 results in an increase in a T helper 1 marker to a T helper 2 marker. In one embodiment such increase can be measured at a single point in time, with reference made to some standard or control. Alternatively or in addition, in one embodiment such increase can be measured serially over a relevant span of time, comparing results from one time point to results from another time point, for a single subject. Methods for measuring TH1 markers and TH2 markers are as described above.
 The present invention is further illustrated by the following Examples, which in no way should be construed as further limiting. The entire contents of all of the references (including literature references, issued patents, published patent applications, and co-pending patent applications) cited throughout this application are hereby expressly incorporated by reference.
Mice and Bacterial Strains
 Conventional specific pathogen-free (SPF) mice of strains C57BL/6NTac, BALB/cAnNTac, B6.SJL-Ptprca/BoAiTac-H2-Ab1.sup.tm1GLM N13 (MHCII.sup.-/-, and B6.SJL-Ptprca/BoAiTac (MHCII.sup.-/- control) were purchased from Taconic Farms (Germantown, N.Y.). C.129S2-Stat4.sup.tm1Gru (Stat4.sup.-/-) mice were purchased from Jackson Laboratory (Bar Harbor, Me.). These mice were housed in conventional cages at Harvard Medical School (HMS). Germ-free Swiss-Webster mice were from Taconic Farms and housed in sterile isolators (Class Biologically Clean, Madison, Wis.). Mice were screened weekly for bacterial, viral, and fungal contamination. All procedures with mice were performed according to guidelines of the HMS Office for Research Subject Protection.
 B. fragilis strain NCTC 9343 and its isogenic PSA deletion mutant have been described (Coyne et al., 2001). For mouse colonization, ˜1×108 CFU of bacteria grown in BHI medium were spread on food and bedding. Mice were colonized for at least 60 days before examination. Results are for mice up to 8 months post-colonization (except thymic pathology, which requires aging over 1 year).
Lymphocyte Isolation from Tissues
 Lymphocytes were isolated from tissues (Tzianabos et al, 2000). In brief, spleens, thymuses, and MLNs were disrupted into single-cell preparations, and enriched for lymphocytes over a Histopaque 1083 gradient (Sigma, St. Louis). Cells were washed with PBS and used directly or fixed with 0.5% PFA for 1 hour at 4° C.
Flow Cytometry, Fluorescence-Activated Cell Sorting and Intracellular Cytokine Staining
 Directly fluorochrome-conjugated monoclonal antibodies were used (BD Pharmingen, San Diego). For surface staining, lymphocyte preparations were washed twice in FC buffer (PBS with 2% FBS) and resuspended in 100 μL. 1×106 cells were incubated with antibodies at 2 μg/mL for 30 min at 4° C. For intracellular cytokine staining, cells were resuspended in 100 μL of Cytofix/Cytoperm buffer for 30 min at 4° C., washed with Perm/Wash buffer, and incubated with fluorochrome-conjugated anti-cytokine antibodies for 30 min at 4° C. Cells were washed and analyzed on a model FC500 Cytometer (Beckman Coulter, Fullerton, Calif.), and all data were analyzed with RXP Analysis Software (Beckman Coulter). FACS was performed on a BD FACSAria, and cell purity was always >99%.
PSA Purification and Animal Treatment
 PSA from B. fragilis NCTC 9343 was prepared as previously described (Tzianabos et al, 1992). For some studies, PSA was treated with acetic anhydride to neutralize the positively charged amino group (Tzianabos et al., 1994). Mice received 50 μg of PSA in 1.5% sodium bicarbonate/PBS intraperitoneally or intragastrically 3 times a week for 2 weeks. Rag2.sup.-/- mice received 30 μg of PSA (PBS controls) in alternating intragastric and subcutaneous treatments 3 times a week.
In Vitro Cytokine Stimulation and Proliferation Assays
 For analysis of splenic cytokines, lymphocytes were isolated as above. CD4.sup.+ T cells were purified with the MACS CD4 Sorting Kit (Miltenyi Biotec, Auburn, Calif.). Cell purity was >97% CD4.sup.+. The remaining lymphocytes were used as APCs after gamma-irradiation. 3×105 cells of each type were mixed in a 48-well plate, and anti-CD3/anti-CD28 (2 μg/mL) was added. Supernatants recovered after stimulation for 48 and 72 hours were analyzed by ELISA.
 For co-cultures, CD4.sup.+ T cells were purified from splenic lymphocytes with a CD4.sup.+ T Cell Subset Kit (R&D Systems). Cell purity was >95%. BMDCs from femurs of mice were purified. Cells were cultured for 8 days in C-RPMI-10 with GM-CSF (20 ng/mL; Biosource, Camarillo, Calif.). Cells were >90% CD11c.sup.+. 1×106 purified CD4.sup.+ T cells were mixed with 1×106 purified CD11c.sup.+ BMDCs and incubated at 37° C./5% CO2.
 ELISA plates were from precoated kits (Biosource). T-cell proliferation assays were done with 1×105 cells of each type (APCs irradiated) after incubation for 96 hours. For the last 8 hours before harvest, wells were pulsed with thymidine (1 μCi/well). Cells were washed, harvested and counted by liquid scintillation (Wallac; now PerkinElmer, Boston). Data were expressed as mean cpm±SD for triplicate wells.
Histological Tissue Analysis
 Paraffin-embedded mouse tissues were stained with H&E. Sections were evaluated in blinded fashion by a single pathologist (R. T. Bronson, HMS).
Mono-Colonization of Germ-Free Animals with B. fragilis Results in CD4.sup.+ T Cell Expansion
 The effects of bacterial colonization on immune maturation in animals were investigated, exploring the role of the microbial flora in systemic T cell development. Germ-free mice, animals born and raised in sterile isolators devoid of microbes, were used. Initially, spleens were harvested from both conventionally colonized and germ-free mice and were analyzed for total CD4.sup.+ T cells by flow cytometry (FC). All groups of mice had similar splenic total lymphocyte counts (average: 1×108). Consistent with seminal observations of a positive immunologic role of autochthonous bacteria (Dobber et al., 1992), the lymphocyte population purified from spleens of conventional SPF (specific pathogen-free) mice with a diverse gut microflora contained a greater proportion of CD4.sup.+ T cells than that of germ-free mice (FIG. 1A). Previous studies have documented the beneficial role of commensal bacteria in intestinal development (Hooper, 2004). The observed alteration in CD4.sup.+ T cell proportions of splenic lymphocytes highlights the profound effects of bacterial colonization of the gut on the systemic immune response.
 SPF mice harbor a diverse and complex microbial flora. To stringently investigate the influence of specific bacterial constituents of the gut flora on the host immune system, germ-free mice were colonized with a single bacterial species. This approach allows "real-time" measurements of responses to bacterial colonization in animals with a naive immune system. The model microorganism B. fragilis was chosen because of its prominence in the normal microbial gut flora and its production of known immunomodulatory molecules (Kononen et al., 1992; Tzianabos and Kasper, 2002). In the absence of competing bacterial species, germ-free mice mono-associated with B. fragilis strain NCTC 9343 were readily colonized to high levels (>1010 CFU/g of feces; FIG. 1B). Flow cytometry of splenic lymphocytes from these mice showed a nearly complete restoration of CD4.sup.+ T cells to conventional proportions (FIG. 1A and FIG. 1C). Thus B. fragilis mono-colonization was sufficient to correct CD4.sup.+ T cell deficiency in spleens of germ-free mice. No other bacterial species alone has been shown to correct lymphoid defects in germ-free animals (Cash and Hooper, 2005).
Immunomodulatory Effects of B. fragilis Require Production of PSA
 At least 2 of the 8 capsular polysaccharides of B. fragilis are ZPSs, a unique class of bacterial molecules with immunomodulatory properties (Tzianabos and Kasper, 2002). We wondered whether PSA, the most immunodominant and highly conserved ZPS, plays a role in splenic T cell expansion during B. fragilis commensalism. PSA-deficient B. fragilis ΔPSA were used to mono-colonize germ-free mice (Coyne et al., 2001). The level of intestinal colonization by the mutant was indistinguishable from that by the isogenic parent strain, as assessed by fecal CFU counts (FIG. 1B). Examination of splenic lymphocyte populations from mice colonized with B. fragilis lacking PSA but expressing all other antigens produced by this organism revealed an inability to correct CD4.sup.+ T-cell deficiencies in germ-free mice (FIG. 1C). In pooled experiments (n=4), the average proportions of CD4.sup.+ T cells were: Conventional: 17.82%±2.1; B. fragilis: 18.05%±1.9; B. fragilis ΔPSA: 10.95%±2.3; and Germ-free: 11.15%±1.5. The effects were specific to CD4.sup.+ T cells, as the proportions of CD8.sup.+ T cells and CD19.sup.+ B cells from splenic lymphocytes (FIG. 1D) were indistinguishable between conventional and either mono-colonized or germ-free mice, as previously observed (Pereira et al, 1986). Together, these results show that B. fragilis colonizing the gut of germ-free mice requires PSA production to correct host systemic CD4.sup.+ T cell deficiencies during commensalism.
PSA Production by B. fragilis Directs Lymphoid Organogenesis
 Commensal bacteria have long been appreciated for their positive impact on development of gut-associated lymphoid tissues (GALT, including Peyer's patches) and intraepithelial lymphocytes (IELs) and production of mucosal IgA (Hooper, 2004). The experiments in this example examined whether cellular immune maturation after bacterial colonization was also manifested in the morphological and ultrastructural development of peripheral lymphoid tissues. Germ-free animals have recently been reported to display defects in splenic structural development (Macpherson and Harris, 2004). Histological sections of spleens from germ-free mice colonized with wild-type B. fragilis or B. fragilis ΔPSA were examined. Spleens from mice mono-colonized with B. fragilis appeared normal, with well-formed lymphocyte zones appearing as defined follicles (white pulp) similar to those in conventional mice with a complete gut flora (FIG. 1E). The interspersed red pulp was densely packed with red blood cells and neutrophils. Spleens from germ-free mice colonized with B. fragilis ΔPSA showed gross anatomical depletion of the lymphocyte zones similar to that in uncolonized germ-free mice (Macpherson and Harris, 2004). Follicles were smaller, less defined, and more fragmented than in germ-free mice colonized with wild-type B. fragilis or conventional mice (FIG. 1E). The overall size and shape of spleens from all groups were comparable. This finding suggests a role for PSA specifically in lymphocyte development. Thus changes in CD4.sup.+ T cell expansion that are mediated by PSA produced by intestinal bacteria are consistent with the correction of physical and developmental defects in secondary lymphoid tissues. This observation reflects the importance of the beneficial relationship between commensal microorganisms and host physiology.
Purified PSA is Sufficient to Expand T Cells in Germ-Free and Conventional Animals
 The inability of a PSA-deficient mutant to cause T-cell expansion showed that the activity of this immunomodulatory molecule is required during B. fragilis colonization. Using chromatography to purify PSA extracted from the surface of B. fragilis, it was investigated whether PSA alone is sufficient to counter the CD4.sup.+ T cell defects in germ-free mice. Purity was assessed by various methods, including H1-NMR, spectroscopy, and gel electrophoresis. The preparation was devoid of contamination by protein, nucleic acid, and endotoxin (LPS). Uncolonized germ-free mice were treated intraperitoneally with purified PSA and then assessed for splenic T cell expansion. Purified PSA was found to restore CD4.sup.+ T cell proportions among splenic lymphocytes in germ-free mice to conventional levels (FIG. 2A).
 To assess whether the route of administration to mice of various backgrounds affects PSA's immunomodulatory properties, both conventionally colonized C57BL/6 and BALB/c mice received purified PSA intragastrically. This treatment recapitulates intestinal exposure of animals to PSA during colonization. Oral treatment led to a specific increase of splenic CD4.sup.+ T cells in conventional mice (FIG. 2B), showing that PSA's effect is not exclusive to germ-free animals. CD8.sup.+ T cell and CD19.sup.+ B cell ratios were unaffected by PSA (FIG. 2C) as formerly shown (FIG. 1D). These studies demonstrated that specific recognition of purified PSA by host immune components in the intestines resulted in splenic CD4.sup.+ T cell expansion.
 In further experiments germ-free mice are administered purified PSA intragastrically. Effects of oral treatment of germ-free mice is assessed by measurement of splenic CD4.sup.+ T cells, CD8.sup.+ T cell and CD19.sup.+ B cell ratios, and TH1 (e.g., IFN-γ) and TH2 (e.g., IL-4) cytokine production.
PSA is Specifically Recognized by Dendritic Cells, with Consequent Cell Activation
 All CD4.sup.+ T cell reactions require instruction to T cells by APCs (Kidd, 2003; Kapsenberg, 2003). PSA is recognized by APCs and then presented to T cells in vitro (Kalka-Moll et al., 2002). The cellular mechanism of PSA recognition in the intestine and subsequent signaling to expand splenic T cells is unknown. To characterize the APC responsible for PSA effects in animals, mice were fed fluorescently labeled PSA by gavage and then cells were recovered from mesenteric lymph nodes (MLNs). As shown by flow cytometry, PSA specifically associated with CD11c.sup.+ dendritic cells (DCs; FIG. 3A), and not with CD4.sup.+ T cells or CD19.sup.+ B cells (FIG. 3A and data not shown) in the MLNs. The notion of in vivo DC recognition of PSA appears appropriate, as DCs are the only APCs known to sample luminal contents from the intestine and migrate to lymph nodes to initiate T cell reactions (Rescigno et al., 2001; Mowat, 2003). Consistent with this hypothesis, no PSA was recovered from spleens of orally treated mice despite splenic T cell expansion. In addition, confocal microscopy revealed that PSA was internalized by primary cultured bone marrow-derived DCs (BMDCs) and was subsequently displayed on the cell surface (FIG. 3B). Thus it appears that DCs sample intestinal PSA by antigen uptake and migrate only as far as the local lymph nodes. These results agree with those of Macpherson and Uhr (2004), who observed that commensal bacteria are internalized by intestinal DCs that migrate to MLNs only.
 We next assessed whether the association of PSA with BMDCs leads to cell activation and maturation, as only mature DCs can activate T cells. PSA-mediated maturation of DCs was measured by up-regulation of MHC II among CD11c.sup.+ cells from 22% to 43% after PSA treatment (FIG. 3c), a process required for efficient antigen display to the receptor on CD4.sup.+ T cells (Banchereau and Steinman, 1998; Thery and Amigorena, 2001). Treatment also increased expression of the costimulatory molecules CD80 (B7.1) and CD86 (B7.2) in a dose-dependent manner (data not shown and FIG. 3D, respectively), a result further showing that PSA interacts with and induces maturation of DCs. These findings are consistent with the observation that DCs, not B cells, mediate intestinal antigen presentation to T cells after conventionalization of germ-free animals (Yamanaka et al., 2003).
PSA Induces T-Cell Proliferation when Presented by DCs In Vitro
 To further investigate cellular and molecular events underlying immune responses to PSA, an in vitro co-culture system using primary cells to assess the ability of purified PSA to induce T-cell proliferation and cytokine expression was developed. As shown above, PSA treatment led to CD4.sup.+ T cell expansion in mice. Incubation of PSA with CD11c.sup.+ BMDCs and naive splenic CD4.sup.+ T cells led to a dose-dependent increase in T-cell proliferation (FIG. 4A). Neutralization of the positive charge by chemical modification of PSA (N-acetylated [NAc] PSA) resulted in no incorporation of [3H]thymidine--a marker for cell replication. This extends to DCs our observation that the zwitterionic structure of PSA is critical for biological activity (Kalka-Moll et al., 2002; Tzianabos and Kasper, 2002). Purified E. coli LPS did not induce T-cell proliferation in mice (FIG. 4A; LPS). PSA-mediated CD4.sup.+ T-cell proliferation required both DCs and T cells; either cell type alone resulted in no incorporation of radiolabel (FIG. 4A). Together, these results show that DCs can direct T-cell proliferation in response to purified PSA in vitro.
PSA Induces T Cell Cytokine Production in Dendritic Cell Co-Cultures
 The two subtypes of effector CD4.sup.+ T cells, TH1 and TH2, are defined by expression of the cytokines interferon γ (IFN-γ) and interleukin 4 (IL-4), respectively (Janeway et al., 2001). As shown above, PSA induces CD4.sup.+ T cell expansion in B. fragilis-colonized mice and in vitro. To further characterize the effects of PSA-mediated T cell activation, cytokine profiles using purified cellular components were assessed. Co-culture of DCs and CD4.sup.+ T cells in the presence of PSA yielded dose-dependent up-expression of the TH1 cytokine IFN-γ (FIG. 4B). The level of IFN-γ production associated with PSA was comparable to that associated with several known potent IFNiγ inducers (anti-CD3, LPS, and staphylococcal enterotoxin A [SEA]) and required both DCs and T cells (FIG. 4B). Specificity was evidenced by the lack of TH1 cytokine production after NAc-PSA treatment (FIG. 4B).
 TH1 cytokine production suppresses TH2 responses; conversely, TH2 cytokine expression inhibits TH1 responses. Normal immune responses require a controlled balance of these opposing signals. Examination of IL-4 expression in response to PSA treatment revealed no cytokine production by purified CD4.sup.+ T cells (FIG. 4C; 100 μg/ml PSA). Anti-CD3 and the superantigen SEA are potent stimulators of both classes of cytokine (FIG. 4C). As TH2 cytokine production is a "default pathway" in many systems (Kidd, 2003; Amsen et al. 2004) and TH1 cytokine production is antagonistic to TH2 expression, the specific stimulation of IFN-γ by PSA in vitro may provide a mechanism for establishing commensal-mediated homeostasis of the host immune system by balancing TH1/TH2 responses.
TH1 Cytokine Production in Response to PSA Treatment Requires Signaling Through the IL-12/Stat4 Pathway and MHC II Expression
 A unique immunologic molecule, PSA is the only carbohydrate studied to date that is internalized into APCs and displayed by MHC II to T cells (Cobb et al., 2004)--a process previously believed to be reserved for protein antigens. We further characterized the molecular pathway for PSA-induced TH1 cytokine production. Many of the molecular signaling events involving T-helper cytokine expression are well characterized. The major pathway of IFN-γ up-regulation and TH1 cell differentiation involves DC secretion of IL-12, which binds to the IL-12 receptor on T cells and signals to activate the TH1-specific transcription factor Stat4 (Trinchieri, 2003). PSA stimulation of DC-T cell co-cultures elicited a dose-dependent increase in IL-12 production (FIG. 5A). DCs alone stimulated with PSA secreted IL-12 (FIG. 5A, DC 100 μg/ml PSA), although at levels lower than those found for CD4.sup.+ T cells. This is the first bacterial polysaccharide shown to signal IL-12 secretion by APCs, as previously shown for classical protein antigens (Macatonia et al., 1995). NAc-PSA, lacking the essential positive-charge motif, did not stimulate cytokine production. To determine whether IL-12 is required for TH1 cell differentiation, IFN-γ expression we measured after PSA treatment in the presence of increasing concentrations of neutralizing antibody to IL-12 (Heufler et al, 1996). IL-12 neutralization abolished PSA-mediated IFN-γ secretion by CD4.sup.+ T cells in vitro (FIG. 5B). Ablation of IL-12 signaling does not affect anti-CD3-mediated IFN-γ expression, as this signal is APC-independent, acting directly on T cells. To determine whether IL-12 signaling that results in TH1 lineage differentiation involves the Stat4 transcription factor, DCs from wild-type mice were incubated with CD4.sup.+ T cells from stat4 knockout mice and IFN-γ expression in response to PSA treatment was measured. The absence of Stat4 greatly reduced TH1 cytokine production (FIG. 5C); thus PSA specifically induced DCs to signal T cell differentiation through Stat4 transcriptional regulation.
 To investigate whether antigen presentation of PSA was required for cytokine signaling, DCs were purified from MHC II-deficient mice and DC-T cell co-cultures were treated with PSA or NAc-PSA. The level of IFN-γ expression was significantly higher in wild-type (MHC II.sup.+/+) mice treated with PSA than in MHC II knockout mice (MHC II.sup.-/-), which expressed amounts similar to a NAc-PSA control (FIG. 5D). Together, these results demonstrate that TH1 cytokine responses to PSA require MHC II expression by APCs and involve signaling through the IL-12/Stat4 pathway to induce T cell activation and proper cytokine expression.
PSA is Required for Appropriate CD4.sup.+ T-Helper Cytokine Production During Colonization
 A proper TH1/TH2 balance is critical for human and animal health; over- or underproduction of either response is associated with immunologic disorders. The effects of PSA on TH1/TH2 cytokine responses in colonized animals were investigated, again using germ-free mice. CD4.sup.+ T cells from mouse spleens were purified and tested by ELISA for cytokine production. FIG. 6A shows overproduction of the TH2 cytokine IL-4 in spleens of germ-free mice compared with levels in conventional mice. This result is consistent with previous reports of the appreciably TH2-skewed profile of mice devoid of bacterial contamination and reflects the human neonatal (precolonization) cytokine profile (Kirjavainen and Gibson, 1999; Prescott et al., 1998; Adkins, 2000; Kidd, 2003). This "default" TH2-bias in the absence of bacterial colonization again highlights the profound contributions of the microflora to immune development and provides a model to test the effects of symbiotic bacteria on the establishment of appropriate host cytokine production.
 Mice colonized with wild-type B. fragilis alone displayed a level of IL-4 production similar to that in conventional mice with a complex microflora (FIG. 6A); this similarity shows the organism's sufficiency to correct systemic immune defects. Moreover, mice colonized with B. fragilis ΔPSA produced TH2 cytokines at elevated levels, similar to those in germ-free mice (FIG. 6A). Thus the expression of a single bacterial antigen allows B. fragilis to correct the IL-4 cytokine imbalance found in uncolonized animals.
 Examination of IFN-γ production by purified splenic CD4.sup.+ T cells revealed that germ-free mice, which are TH2-skewed, were deficient in production of this prototypical TH1 marker when compared to conventional mice (FIG. 6B). Colonization with wild-type B. fragilis alone was sufficient to correct the defect in IFN-γ expression in germ-free mice, with levels nearly as high as those in conventional mice (FIG. 6B). Lack of PSA production by the B. fragilis mutant during colonization of germ-free mice resulted in low-level production of TH1 cytokines (FIG. 6B). These results were corroborated by intracellular cytokine staining of splenic lymphocytes from each group, which confirmed that IFN-γ production was attributable to CD4.sup.+ T cells (FIG. 6c). The production of IL-2, another TH1 cytokine, by CD4.sup.+ T cells in gnotobiotic mice also required PSA production (data not shown). Together, these results demonstrate that intestinal colonization of germ-free mice by B. fragilis alone was sufficient to establish a proper systemic TH1/TH2 balance within the host--a fundamental aspect of the mammalian immune response.
Absence of PSA Production by B. fragilis During Colonization is Associated with TH2-Mediated Pathologies of the Thymus
 Throughout these studies, specimens obtained at necropsy were subjected to histological examination. A rare pathology of the thymus exclusively in mice colonized with B. fragilis ΔPSA was noticed. Thymic tissues from germ-free mice colonized with wild-type B. fragilis appeared normal, with a darker-staining outer corona and a uniform and homogeneous inner medullary compartment. Surprisingly, at >1 year of age, the majority of mice colonized with B. fragilis ΔPSA displayed the outgrowth of B cell-like follicles in the thymic medulla (FIG. 6D). Flow cytometry showed that these tissues contained CD19.sup.+ B cells (FIG. 6E) not found in the normal thymus. This rare condition appeared to be similar to human thymic hyperplasia, in which B cells are found in follicles of the medulla (Kasper et al., 2005). The latter condition is associated with numerous autoimmune disorders, most notably myasthenia gravis, a B cell-mediated pathology (Malhotra et al., 1992; Infante and Kraig, 1999). These disorders, as well as B-cell outgrowths, are mediated by overproduction of TH2 cytokines by CD4.sup.+ T cells (Zhang et al., 1997; Janeway et al., 2001). It is compelling to speculate that the inability to restore proper TH1/TH2 balance in germ-free mice through appropriate commensal colonization results in an aberrant TH2 response, which may lead to immune-mediated pathologies.
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 The foregoing written specification is considered to be sufficient to enable one skilled in the art to practice the invention. The present invention is not to be limited in scope by examples provided, since the examples are intended as a single illustration of one aspect of the invention and other functionally equivalent embodiments are within the scope of the invention. Various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and fall within the scope of the appended claims. The advantages and objects of the invention are not necessarily encompassed by each embodiment of the invention.
Patent applications by Dennis L. Kasper, Brookline, MA US
Patent applications by THE BRIGHAM AND WOMEN'S HOSPITAL, INC.
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