A soluble suppressor T cell factor protects against experimental intraabdominal abscesses

Finding a needle in a haystack: Bacteroides fragilis polysaccharide A as the archetypical symbiosis factor

Starting from birth, all animals develop a symbiotic relationship with their resident microorganisms that benefits both the microbe and the host. Recent advances in technology have substantially improved our ability to direct research toward the identification of important microbial species that affect host physiology. The identification of specific commensal molecules from these microbes and their mechanisms of action is still in its early stages. Polysaccharide A (PSA) of Bacteroides fragilis is the archetypical example of a commensal molecule that can modulate the host immune system in health and disease. This zwitterionic polysaccharide has a critical impact on the development of the mammalian immune system and also on the stimulation of interleukin 10-producing CD4⁺ T cells; consequently, PSA confers benefits to the host with regard to experimental autoimmune, inflammatory, and infectious diseases. In this review, we summarize the current understanding of the immunomodulatory effects of B. fragilis PSA and discuss these effects as a novel immunological paradigm. In particular, we discuss recent advances in our understanding of the unique functional mechanisms of this molecule and its therapeutic potential, and we review the recent literature in the field of microbiome research aimed at discovering new commensal products and their immunomodulatory potential.

The yin yang of bacterial polysaccharides: lessons learned from B. fragilis PSA

Over the past several years, there have been remarkable advances in our understanding of how commensal organisms shape host immunity. Although the full cast of immunogenic bacteria and their immunomodulatory molecules remains to be elucidated, lessons learned from the interactions between bacterial zwitterionic polysaccharides (ZPSs) and the host immune system represent an integral step toward better understanding how the intestinal microbiota effect immunologic changes. Somewhat paradoxically, ZPSs, which are found in numerous commensal organisms, are able to elicit both proinflammatory and immunoregulatory responses; both these outcomes involve fine-tuning the balance between T-helper 17 cells and interleukin-10-producing regulatory T cells. In this review, we discuss the immunomodulatory effects of the archetypal ZPS, Bacteroides fragilis PSA. In addition, we highlight some of the opportunities and challenges in applying these lessons in clinical settings.

Bacteroides: the good, the bad, and the nitty-gritty

SUMMARY

Bacteroides species are significant clinical pathogens and are found in most anaerobic infections, with an associated mortality of more than 19%. The bacteria maintain a complex and generally beneficial relationship with the host when retained in the gut, but when they escape this environment they can cause significant pathology, including bacteremia and abscess formation in multiple body sites. Genomic and proteomic analyses have vastly added to our understanding of the manner in which Bacteroides species adapt to, and thrive in, the human gut. A few examples are (i) complex systems to sense and adapt to nutrient availability, (ii) multiple pump systems to expel toxic substances, and (iii) the ability to influence the host immune system so that it controls other (competing) pathogens. B. fragilis, which accounts for only 0.5% of the human colonic flora, is the most commonly isolated anaerobic pathogen due, in part, to its potent virulence factors. Species of the genus Bacteroides have the most antibiotic resistance mechanisms and the highest resistance rates of all anaerobic pathogens. Clinically, Bacteroides species have exhibited increasing resistance to many antibiotics, including cefoxitin, clindamycin, metronidazole, carbapenems, and fluoroquinolones (e.g., gatifloxacin, levofloxacin, and moxifloxacin).

The love-hate relationship between bacterial polysaccharides and the host immune system

This article explores the fascinating relationship between the mammalian immune system and the bacteria that are present in the mammalian gut. Every human is an ecosystem that hosts 10(13)-10(14) bacteria. We review the evidence that immunomodulatory molecules produced by commensal bacteria in the gut have a beneficial influence on the development of certain immune responses, through eliciting the clonal expansion of CD4(+) T-cell populations. This process seems to contribute to the overall health of the host by offering protection against various diseases and might provide supporting evidence at a molecular level for the 'hygiene hypothesis' of allergic immune disorders.

Animal models simulating anaerobic infections

Animal models simulating human disease have played an important role in our understanding of the pathogenesis and treatment of infections caused by obligately anaerobic bacteria. These models helped document the primary source of such infections as the host's own normal microflora. In addition, the polymicrobial nature of anaerobic infections was documented by using animal models for intraabdominal sepsis. Subsequent studies using animal models have led to an understanding of the nature of the host immune response to abscess causing agents and have been instrumental in defining the molecular basis for the virulence and protection provided by the polysaccharide capsule of Bacteroides fragilis. Animal models have also been important components for studies of toxigenic clostridial diseases, such as antibiotic associated colitis and ulcerative colitis. A discussion of some of these models is provided in this review.

Effect of molecular size on the ability of zwitterionic polysaccharides to stimulate cellular immunity

The large-molecular-sized zwitterionic capsular polysaccharide of the anaerobe Bacteroides fragilis NCTC 9343, designated polysaccharide (PS) A, stimulates T cell proliferation in vitro and induces T cell-dependent protection against abscess formation in vivo. In the present study, we utilized a modification of a recently developed ozonolytic method for depolymerizing polysaccharides to examine the influence of the molecular size of PS A on cell-mediated immunity. Ozonolysis successfully depolymerized PS A into structurally intact fragments. PS A with average molecular sizes of 129.0 (native), 77.8, 46.9, and 17.1 kDa stimulated CD4+-cell proliferation in vitro to the same degree, whereas the 5.0-kDa fragment was much less stimulatory than the control 129.0-kDa PS A. Rats treated with 129.0-kDa, 46.9-kDa, and 17.1-kDa PS A molecules, but not those treated with the 5.0-kDa molecule, were protected against intraabdominal abscesses induced by challenge with viable B. fragilis. These results demonstrate that a zwitterionic polysaccharide as small as 22 repeating units (88 monosaccharides) elicits a T cell-dependent immune response. These findings clearly distinguish zwitterionic T cell-dependent polysaccharides from T cell-independent polysaccharides and give evidence of the existence of a novel mechanism for a polysaccharide-induced immune response.

IL-2 Mediates Protection Against Abscess Formation in an Experimental Model of Sepsis

Little is known regarding the mechanism by which T cells control intraabdominal abscess formation. Treating animals with polysaccharide A (PS A) from Bacteroides fragilis shortly before or after challenge protects against abscess formation subsequent to challenge with different abscess-inducing bacteria. Although bacterial polysaccharides are considered to be T cell-independent Ags, T cells from PS A-treated animals mediate this protective activity. In the present study, we demonstrate that CD4+ T cells transfer PS A-mediated protection against abscess formation, and that a soluble mediator produced by these cells confers this activity. Cytokine mRNA analysis showed that T cells from PS A-treated animals produced transcript for IL-2, IFN-γ, and IL-10, but not for IL-4. The addition of IL-2-specific Ab to T cell lysates taken from PS A-treated animals abrogated the ability to transfer protection, whereas the addition of Abs specific for IFN-γ and IL-10 did not affect protection. Finally, administration of rIL-2 to animals at the time of bacterial challenge prevented abscess formation in a dose-dependent manner. These data demonstrate that PS A-mediated protection against abscess formation is dependent upon a CD4+ T cell-dependent response, and that IL-2 is essential to this immune mechanism.

Polysaccharide-mediated protection against abscess formation in experimental intra-abdominal sepsis

Abscess formation is a major complication of intra-abdominal sepsis that causes significant morbidity and mortality. In such cases, Bacteroides fragilis is the predominant anaerobic isolate. In a rat model of intra-abdominal sepsis, the capsular polysaccharide complex (CPC) from B. fragilis promotes abscess formation and when administered sub-cutaneously, protects against this host response by a T cell-dependent immune mechanism. In the present study, the polysaccharide A (PS A) component of CPC protected animals against challenge with live heterologous bacterial species (mixtures of anaerobes and facultative organisms) that are most commonly isolated from intra-abdominal abscesses in humans. Protection against heterologous bacterial challenge was transferred by T cells. Administration of PS A shortly before or even after challenge with B. fragilis protected against this host response. In experiments designed to simulate fecal contamination of the human peritoneal cavity, PS A protected animals against abscess formation induced by a rat cecal contents inoculum. The surprisingly broad protective activity of PS A indicates that this molecule is likely suppressing a nonspecific host tissue reaction that forms in response to a variety of abscess-inducing organisms and that it might be useful in preventing abscess formation associated with intra-abdominal sepsis in the clinical setting.

CD4+ T cells mediate preexposure-induced increases in murine intraabdominal abscess formation

We have previously shown that an increased number of Escherichia coli/Bacteroides fragilis intraabdominal abscesses are produced in mice after preexposure to small numbers of live E. coli or B. fragilis. Splenic lymphocyte subset changes and the importance of different elements of the immune response in this system were studied. Preexposure to bacteria induced a significant increase in the percentage of splenic T cells without altering the CD4/CD8 ratio. The passive transfer of either 10(7) mixed splenic lymphocytes, 5 x 10(6) mixed T cells, or 2.5 x 10(6) CD4+ T cells from preexposed animals to naive siblings 24 hr prior to abscess induction resulted in increased abscess formation. Transfer of serum, B cells, < 10(7) lymphocytes, CD8+ T cells, or any cell type from naive animals did not change abscess number. The bacterial composition of abscesses changed only in animals receiving either serum or B cells from donors preexposed to B. fragilis, where an increased number of B. fragilis per abscess was found. The CD4+ T cell response can be altered by transient infections and is critical to subsequent abscess formation, and a concurrent humoral response may play a role in determining an abscess' ultimate bacterial composition.

In vivo effects of rufloxacin and ciprofloxacin on T-cell subsets and tumor necrosis factor production in mice infected with Bacteroides fragilis

We investigated the in vivo effects of rufloxacin and ciprofloxacin on T-cell subsets and tumor necrosis factor production in mice infected with Bacteroides fragilis. These quinolones did not alter the helper/suppressor ratio but did modulate the kinetics of tumor necrosis factor production in infected animals. This result correlated with the observed therapeutic efficacies of the quinolones.

Effect of arginine on immune function in rats with obstructive jaundice

In this study we used an experimental model produced by bile duct ligation (BDL) in rats to demonstrate relation between immune function and susceptibility to infection associated with obstructive jaundice and immunoregulatory effect of arginine in BDL rats. Our experimental results showed that thymus weight, content of lymphocytes in thymus and responsiveness to Con A in vitro were dramatically reduced in BDL rats as compared to rats with sham operation and that the impairment of lymphocyte function was significantly related to the susceptibility to infection. Arginine, as a T cell stimulator, markedly improved immune function and decreased susceptibility to infection of BDL rats.

Altered lymphocyte subsets and natural killer cells of patients with obstructive jaundice in perioperative period

Sepsis is a common and occasionally lethal complication of obstructive jaundice. The reasons for the increased susceptibility to infection are not fully understood. This study was conducted to examine lymphocyte subsets and natural killer cells of patients with obstructive jaundice in perioperative period. In these patients, when compared with normal controls, there was a significant reduction in the percentage of Leu 4 (pan T lymphocytes), Leu 3a (T helper cells) and Leu 7 (natural killer cells) before operation, and the immune suppression induced by surgical operation was more marked and persistent.

Arginine supplementation and its effect on established peritonitis in guinea pigs

To evaluate the efficacy of supplemental arginine with nutritional support in the presence of sepsis, eighty-eight gastrostomized female Hartley guinea pigs were implanted with osmotic pumps effusing an Eschericia coli/Staphylococcus aureus mixture. Animals were randomized and infused for two weeks with isocaloric and isovolumetric diets containing 0%, 2%, 4%, or 6% supplemental arginine as arginine hydrochloride. Survival was 12/22 (54%) in 0%, 9/22 (41%) in 2% and 4%, and 2/22 (9%) in 6%. Analysis by chi-square test of independence was significant (p = 0.0141) with 6% survival lower than the others. Median survival was 11 days in 0%, 8 days in 2% and 6%, and 9 days in 4%. Median survival was longer in 0% than in 2% or 6% (Kruskal-Wallis ANOVA: p = 0.02). Nitrogen balance was significantly lower in 6% compared to 0% on days 2 through 10, and lower than 2% and 4% on days 6 and 9. Nitrogen balance was higher in 0% than in 2% on days 4, 6, 10, and 13. Serum albumin and C3 were lower in all experimental groups than normal controls (ANOVA: p = 0.01). Comparison of liver, spleen, adrenals, gastrocnemius, and carcass weights, cell-mediated immunity as determined by contact sensitivity to DNFB, and transferrin showed no significant differences. There was a positive dose-response effect seen amongst the experimental groups for the amino acids arginine, ornithine, and citrulline in relation to the amount of supplemental arginine. This study suggests that dietary arginine supplementation does not enhance survival in a guinea pig model of established peritonitis.

Intraperitoneal host cellular responses and in vivo killing of Bacteroides fragilis in a bacterial containment chamber

A bacterial containment chamber was used to evaluate the peritoneal cellular response to Bacteroides fragilis during intraperitoneal challenge. This containment system was also used to determine the fate of bacteria within the peritoneal cavities of animals immunized, either actively or through adoptive transfer of cells or cell lysates, with the capsular polysaccharide of B. fragilis. This system demonstrated that the dominant cell types in the peritoneal cavities within 48 h of implantation of the containment chambers containing B. fragilis were neutrophils and macrophages. However, the early cellular response in immunized animals included an increase in the lymphocyte population within 4 h of challenge which was not detected in naive animals. In immunized animals, a later dramatic increase in the lymphocyte population at approximately 4 to 6 days following implantation of the containment chambers occurred. This increase in the lymphocyte population in immunized animals coincided with a decline in the viable bacterial counts within the chambers from 10(8) to 10(9) CFU/ml to less than 10(2) CFU/ml. A similar decline was not seen in naive animals challenged in the same manner. Killing of B. fragilis within containment chambers occurred when spleen cells, T cells, or lysates of T cells from actively immunized animals were passively transferred to naive recipient animals. It was shown that the factor responsible for bacterial killing was not antibody mediated, since bacteria contained within dialysis sacs with an exclusion of 50 kilodaltons were still killed in this model. Moreover, removal of T cells from adoptively transferred cell populations before transfer abrogated the decline in viable bacterial populations. The postulated mechanisms by which this bacterial killing occurred are discussed.

Induction of macrophage procoagulant activity by Bacteroides fragilis

Fibrin deposition in the peritoneal cavity during acute peritonitis appears to predispose the host to abscess formation by providing an environment for bacterial proliferation protected from host defenses. The purpose of the present study was to determine whether the potent abscess-inducing anaerobe Bacteroides fragilis could promote fibrin deposition by inducing mononuclear cells to express procoagulant activity (PCA). B. fragilis stimulated PCA in a dose-dependent fashion, achieving a maximum at 10(7) CFU/ml. Heat-killed B. fragilis induced comparable levels of PCA, while a nonspecific phagocytic stimulus, latex beads, was not stimulatory. B. fragilis was capable of inducing PCA even when phagocytosis was blocked by preexposure of cells to latex beads. The results suggested that phagocytosis was neither necessary nor sufficient for the generation of PCA. Cell separation studies showed that PCA was solely produced by macrophages and that lymphocytes did not augment its production. These studies suggest one potential mechanism by which B. fragilis might initiate abscess formation.

Use of lymphokines in treatment of experimental intra-abdominal abscess caused by Bacteroides fragilis

The role of cell-free soluble factors (lymphokines) derived from mitogen-activated splenic cells of mice previously immunized against Bacteroides fragilis was evaluated in the treatment of B. fragilis intra-abdominal abscess (IAA). Neither clindamycin nor lymphokines alone were effective against an established B. fragilis IAA, but the combination of clindamycin and lymphokines decreased the abscess size and bacterial counts in the majority of animals. This suggests that the synergy of lymphokines with clindamycin effects cure of IAA caused by B. fragilis and that lymphokines might have an application as adjuncts to conventional antimicrobial therapy in this setting.

Stimulation of T cell immunity by arginine enhances survival in peritonitis

T cell-mediated immunity may play a role in host responses to infection. Arginine is a known thymic and T cell stimulator which enhances host allogenic, mitogenic, and anti-tumor responses. We, therefore, examined the effect of arginine on the survival of rats with severe and lethal peritonitis induced by cecal ligation and double-needle puncture (CLP). In Experiment 1, arginine HCl (100 mg) was given bid by gavage starting immediately after CLP. In Experiment 2, the same dose of arginine was given by gavage bid for 3 days pre-CLP and continued thereafter. In Experiment 3, arginine was administered iv post-CLP (100 mg tid). Arginine had no effect on overall survival in Experiment 1. In Experiments 2 and 3, arginine therapy significantly increased survival at all times. A separate experiment was carried out to determine the reason for the differential response to arginine administered via gavage or iv post-CLP (Experiments 1 and 3). Nonseptic rats showed a 400% increase in plasma arginine 30 min after gavage with 100 mg arginine (P less than 0.001). No rise in plasma arginine was noted when arginine was administered by gavage post-CLP. The impaired intestinal absorption or markedly increased utilization of arginine in this septic model may explain why no improved survival was seen in Experiment 1. The mechanism for the improved survival with arginine therapy seen in Experiments 2 and 3 may be related to its known thymic and T cell immunostimulatory effects.

The biology of peritonitis and implications for treatment

Peritonitis continues to be one of the major infectious problems confronting surgeons. Despite the many advances in antimicrobial agents and supportive care, mortality from diffuse suppurative peritonitis remains unacceptably high. The authors review the anatomy and defense capabilities of the peritoneal cavity and then focus on the polymicrobial nature of peritonitis and how, through microbial interactions with host defenses, there is either resolution or persistent infection. They also consider possible infection-potentiating agents in the peritoneal cavity and experimental modes of therapy.

Effect of immunization on experimental Bacteroides gingivalis infection in a murine model

BALB/c mice were immunized with an invasive (A7A1-28) or noninvasive (381) Bacteroides gingivalis strain, Bacteroides intermedius, or Ringer solution. All immunized mice were subsequently challenged with the invasive B. gingivalis strain and examined for septicemia or secondary spread of the infection or both. Mice immunized with the invasive B. gingivalis strain localized the infection to the challenge site. Mice immunized with the noninvasive B. gingivalis strain, B. intermedius, or Ringer solution developed spreading infections. These data suggest that immunization with an invasive B. gingivalis strain can alter the course of subsequent infections.

The role of Bacteroides encapsulation in the lethal synergy between Escherichia coli and Bacteroides species studied in a rat fibrin clot peritonitis model

Encapsulation in Bacteroides species has been thoroughly studied in vivo as a virulence factor in abscess formation. Its pathogenic role in lethal infections caused by a mixture of pathogens has been less well investigated. Our previous studies using the rat fibrin clot peritonitis model have demonstrated lethal synergy between Bacteroides fragilis and Escherichia coli. In order to determine the synergistic role of the encapsulation of the Bacteroides component in this model, inoculations of E. coli plus one of seven Bacteroides strains of differing degrees of encapsulation were assessed for their effect on mortality. Both unencapsulated Bacteroides strains tested (B. distasonis 1244, B. vulgatus 4300) produced an early lethal synergistic effect with E. coli while the heavily encapsulated strain, B. thetaiotaomicron 1603 did not do so. The four other Bacteroides strains tested were encapsulated and their synergy with E. coli was demonstrated. Control Gram-positive strains, Streptococcus faecalis and Staphylococcus aureus, did not alter mortality when mixed with E. coli in this model. These studies support the concept that virulence factors other than encapsulation are important in the outcome of polymicrobial infections in which Bacteroides species play a part.

T-lymphocyte involvement in abscess formation in nonimmune mice

Athymic mice formed significantly smaller abscesses than euthymic mice in response to the intraperitoneal inoculation of an abscess-inducing mixture of Escherichia coli, Bacteroides fragilis, and autoclaved colonic contents, an abscess-potentiating agent. Adoptive transfer of nonimmune, Thy-1-positive spleen cells to athymic mice restored their ability to make abscesses of sizes similar to those in controls, indicating that T lymphocytes contribute to abscess formation in normal mice.