Enhancement and suppression of the Peyer''s patch immune response by systemic priming

Mice were immunized by a single injection of sheep red blood cells (SRBC) in Freund's complete adjuvant (FCA), and their Peyer's patch (PP) immune response was studied by in vitro culture, or transfer of PP cells into irradiated recipients. After both intra-peritoneal (i.p.) or sub-cutaneous (s.c.) immunization, PP cells showed greatly enhanced in vitro IgM plaque forming cell (PFC) responses to SRBC. The in vitro enhanced response was antigen-specific and was apparent up to 11 weeks post-immunization. It was also shown that PP from primed mice contained nylon wool non-adherent cells (N/A) which could enhance normal PP PFC responses in vitro. Peyer's patch cells from i.p. immunized mice transferred into irradiated recipients generated greatly enhanced splenic IgG and IgA anti-SRBC responses when compared to the response generated by normal PP cells. In contrast, PP cells taken from mice immunized s.c. showed suppressed IgG and IgA responses in irradiated recipients. Furthermore, whereas N/A cells from the PP of i.p. immunized mice greatly enhanced the response of normal PP cells in the irradiated recipients, N/A cells from the PP of s.c. immunized mice noticeably suppressed this response. These data show that systemic immunization can markedly alter PP immune function, and that these effects are probably T cell mediated.     

Mucosal homeostasis: role of interleukins, isotype-specific factors and contrasuppression in the IgA response

Oral ingestion of antigen elicits immune responses at mucosal sites where humoral immunity is largely due to antibodies of the IgA isotype. This is often accompanied by suppression of systemic responses to the same antigen, a state termed oral tolerance. This IgA response is regulated by interactions between T cell subsets found at IgA inductive tissues, i.e., the gut-associated lymphoreticular tissue (GALT) or Peyer's patches (PP). PP T helper (Th) cells support IgA responses, and interleukins 5 (IL-5) and IL-6 can augment secretion of this isotype. Subsets of Th cells may also express Fc receptors for IgA (Fc alpha R) and secrete Fc alpha R as an IgA-binding factor (IBF alpha). Membrane-derived Fc alpha R is a glycoprotein of 38,000 M.W. and this molecule induces selective increases in IgA secreting cells (as determined by the ELISPOT assay) in PP B cell cultures. Fc alpha R+ T cell lines have been shown to secrete IBF alpha as well as IL-5 both of which promote IgA synthesis. Recombinant IL-5 (rIL-5) and rIL-6 induce IgA synthesis mainly by PP B cell blasts, and principally act on surface IgA-positive (sIgA+) B cells for these responses. Another form of mucosal regulation is provided by T contrasuppressor (Tcs) cells, which abrogate oral tolerance when adoptively transferred to mice and restore systemic responsiveness to the antigen sheep erythrocyte (SRBC). Tcs cells from mice systemically primed with SRBC support IgM and IgG subclass responses, while Tcs cells from orally primed mice support IgM, IgG subclass and IgA anti-SRBC responses. These Tcs cells are CD3+, CD4-, 8- and are antigen-specific. These regulatory cells may use the gamma-delta (gamma-delta) form of T cell receptor for antigen recognition.     

Evaluation of antisera used for detecting enterotoxigenic Escherichia coli in Sao Paulo.

The usefulness of antisera in detecting enterotoxigenic Escherichia coli (ETEC) strains in Sao Paulo was evaluated. Polyvalent antisera detected 49% of ETEC isolates and were more effective in identifying E. coli that produced heat-labile and heat-stable enterotoxins and in strains that produced only heat-stable enterotoxin. ETEC strains not detected by the antisera belonged to different serogroups not isolated in Sao Paulo before; 34% of these strains had undetermined O antigens, and most of them produced only heat-labile toxin. A variation of serogroups over time was especially observed among strains that produced heat-stable toxin. The importance of H-antigen determinations in the effectiveness of ETEC diagnosis by serological methods became evident, as non-ETEC strains were also detected by polyvalent antisera, but their serotypes were different from those of ETEC strains. Although antisera can be used to identify O:H types of ETEC strains with accuracy, serotyping cannot be recommended for routine diagnosis. However, such a procedure may be useful for studying outbreaks of ETEC diarrhea if the involved serotypes are already known.     

IgA antibody response to intragastric immunization in high and low immune responder lines of mice.

Two lines of mice selected for genes controlling the character "quantitative agglutinin production" to heterologous erythrocytes were immunized orally by the intragastric administration of sheep red blood cells (SRBC). The quantity of circulating IgA anti-SRBC and the numbers of splenic plaque-forming cells of IgA and IgM classes were related to the genetic status of the animal lines.     

Oral administration of SSG, a beta-glucan obtained from Sclerotinia sclerotiorum, affects the function of Peyer's patch cells.

The effect of orally administered SSG, a beta-1,3-glucan obtained from the culture filtrate of a fungus, Sclerotinia sclerotiorum IFO 9395, on the function of Peyer's patch (PP) cells was investigated in comparison with that on spleen cells in mice. Oral administration of SSG enhanced the proliferative response of PP cells to a T-cell mitogen, concanavalin A (Con A), and a B-cell mitogen, lipopolysaccharide (LPS), although the response of spleen cells was not affected. Peyer's patch cells taken from mice which had received oral administration of SSG two days before, showed enhanced plaque-forming cell (PFC) response to sheep red blood cells (SRBC) after antigen (SRBC) stimulation for 5 days in vitro. These results suggest that oral administration of SSG can modulate the mucosal immune response.     

Mice Refractory to Lipopolysaccharide Manifest High Immunoglobulin A Responses to Orally Administered Antigen

Lipid A-nonresponding C3H/HeJ mice manifested high immune responses to orally administered (either by feeding or by intragastric immunization) heterologous erythrocytes when compared with syngeneic lipid A-responding C3H/HeN mice. Prolonged consumption of horse erythrocytes resulted in a significant secretory immune response in both C3H mouse strains as evidenced by high salivary agglutinin titers. Although salivary agglutinin titers were only slightly greater in C3H/HeJ mice than those of C3H/HeN mice, serum agglutinin titers and immunoglobulin A (IgA) levels were consistently higher (two- to fourfold) in C3H/HeJ mice. The appearance of anti-horse erythrocyte plaque-forming cell responses in spleens of immunized animals was followed by an increase in salivary anti-horse erythrocyte agglutinin activity. Peak levels of both responses were attained after approximately 3 weeks of immunization. Differences in immune responsiveness between C3H mouse strains were also evident at the cellular level since splenic IgA anti-horse erythrocyte plaque-forming cell responses in fed C3H/HeJ mice were twofold higher than those in similarly treated C3H/HeN mice. This higher response pattern was also observed when C3H/HeJ mice manifested threefold higher splenic IgM and IgA plaque-forming cell responses to intragastrically administered sheep erythrocytes. Thus, higher responsiveness was observed in the C3H/HeJ mice given heterologous erythrocytes by the oral route. Furthermore, levels of serum IgA in 10- to 12-month-old nonimmunized C3H/HeJ mice were higher than those of C3H/HeN mice. These findings suggest that lack of host responsiveness to lipopolysaccharide affects the manifestation of subsequent immune responses to orally administered antigens. The possible mechanisms and implications of this high responsiveness are discussed.     

Specific IgM Enhances and IgG Inhibits the Induction of Immunological Memory in Mice

The effect of priming mice with IgM anti-SRBC (sheep erythrocytes) together with SRBC or IgG anti-SRBC together with SRBC on the development and expression of memory cells was studied. Mice primed with specific IgM and SRBC showed a much more efficient secondary plaque-forming cell and serum antibody response after challenge with SRBC in an adoptive transfer system than did controls primed with SRBC only. The expression of this enhanced memory of IgM-primed spleen cells was counteracted by the high levels of internal IgG anti-SRBC (also the result of priming with IgM) when the mice, instead of being tested in adoptive transfers, were challenged directly. The antigen-specific feedback suppression of the primary antibody response by specific IgG antibodies was also seen to inhibit partially the development of memory cells. The suppressive effect on priming could be demonstrated both in adoptive transfer systems and after direct boost of the same mice that received the primary immunization. Both the IgM enhancement and the IgG suppression of memory cell development were antigen-specific, since no effect on the antibody response to a non-cross-reacting antigen, horse erythrocytes, was seen. The effect of these up- or down-regulations of immunological memory could be demonstrated after secondary injections as long as 90-280 days after priming.     

Enhancement of in Vitro Immune Responses of Murine Peyer's Patch Cultures by Concanavalin A, Muramyl Dipeptide and Lipopolysaccharide

Immunofluorcscent and esterase staining of Peyer's patch (PP) cell preparations from six mouse strains showed that each strain possessed approximately eqtial numbers of T and B cells but less than 1% esterase-positive cells (macrophages; Mψ). The addition of concanavalin A (Con A) to either lipopolysaccharide (LPS)-responsive C3H/HeN or LPS-non-responsive C3H/Hej PP cultures immunized with sheep erythrocytes (SRBC) resulted in good anti-SRBC plaque-forming cell responses, whereas the addition of Con A to PP cultures from SRBC orally primed C3H/HeJ mice resulted in significantly higher in vitro responses to trinitrophenyl-conjugated SRBC than similarly treated cultures from C3H/HeN mice. On the other hand, muramyl dipeptide (MDP) promoted higher responses in PP cultures derived from either normal or carrier-primed LPS-rcsponsive (C3H/HeN) mice than were observed with C3H/HeJ PP cultures. A similar pattern of responsiveness was seen when LPS prepared by a phenol-water extraction procedure (LPS(Ph)) was added to PP cultures. When LPS prepared by a bntanol-water extraction method was used, PP cultures from both C3H/HeN and C3H/HeJ mice were enhanced; however, the response of C3H/HeN PP cultures was significantly greater than that seen with C3H/HeJ PP cultures. The enhanced responsiveness of C3H/HeN PP cultures to MDP was probably due to an effect on the Mψ, since addition of MDP to either C3H/HeN or C3H/HeJ PP cultures containing C3H/HeN splenic adherent cells resulted in significantly enhanced immune responses. C3H/HeJ splenic Mψ did not promote adjuvant responses. LPS(Ph) augmentation of immune responses required that Mψ (spleen) and PP cultures be derived from LPS-re-sponsive C3H/HeN mice. These results demonstrated that Con A, MDP, and LPS promoted immune responses of PP cultures to the T-dependent antigen, SRBC. Evidence is presented that Con A enhanced T-helper-cell activity, whereas MDP and LPS required the presence of LPS-responsive Mψ for augmentation of immune responses.     

Immunosuppression caused by antigen feeding II. Suppressor T cells mask Peyer's patch B cell priming to orally administered antigen

Peyer's patch (PP) cells transferred into sublethally irradiated recipients generated substantial IgM, IgG and IgA anti-sheep red blood cell (SRBC) plaque-forming cell (PFC) responses in the recipient spleen. If donor mice were given SRBC orally for 4–5 weeks prior to transfer, the adoptively transferred PP IgG and IgA responses were considerably suppressed, although the IgM responses were often unaffected. Co-injection of PP cells from antigen-fed mice with PP cells from normal mice resulted in marked suppression of the normal PP IgG and IgA response. However, treatment of PP cells from antigen-fed mice with anti-Thy-1.2 plus complement prior to cotransfer completely abrogated suppression of the IgG PFC response and partially abrogated the suppressed IgA response. B cells from the PP of antigen-fed mice, when transferred into SRBC-primed irradiated recipients (to provide T cell help) generated 2–3 times more IgG and IgA PFC than comparable numbers of B cells from the PP of normal mice. Thus antigen feeding generates suppressor T cells in PP which can mask the expression of B cell priming to orally administered antigen.     

Conditions for Induction of Specific and Polyclonal Antibody Production by Cowan 1 Bacteria and by Pokeweed Mitogen

Cowan 1 bacteria and pokeweed mitogen (PWM) were used to induce the formation of direct plaque-forming cells (PFC) against sheep erythrocytes (SRBC) by human peripheral blood lymphocytes in vitro. It was necessary to absorb the serum supplement with SRBC before culture to obtain anti-SRBC PFC. Alternatively, sheep serum could be added to the cultures. The PFC response was specific, and the response was equally high in cultures with a mixture of absorbed and non-absorbed serum as in cultures with absorbed serum only. Cowan 1 and PWM could also induce synthesis and secretion of both IgM and IgG polyclonal antibodies. Absorption with SRBC or addition of sheep serum had no effect on this synthesis. Thus it seems likely that the induction of anti-SRBC PFC by Cowan 1 or PWM needs the presence of SRBC antigen and is the result of a synergism between mitogen and antigen. Consequently, the anti-SRBC PFC response obtained after stimulation with Cowan 1 or PWM in SRBC-absorbed serum does not reflect a true polyclonal antibody response.     

Characterization of antibody responses of local lymph nodes to antigen given under the oral submucosa.

We studied the function of submandibular lymph nodes (MLN) in the oral mucosa immune system as compared with that of inguinal lymph nodes (ILN) in the cutaneous one. Primary IgM, IgG and IgA antibody responses in MLN to sheep red blood cells (SRBC) as a model antigen given submucosally occurred more extensively than those in ILN to the antigen injected subcutaneously. Particularly, definite IgA synthesis was seen in MLN but not in ILN. This IgA synthesis was shown to be originated locally in oral submucosal lymphoid tissue or MLN but not in gut-associated lymphoid tissue (GALT). This suggested that the oral mucosal tissue including MLN acts like Peyer's patches in GALT for IgA synthesis. When mice were administered with SRBC and bacterial lipopolysaccharide (LPS) submucosally, the adjuvant effect of LPS was only observed on the capacity of MLN cells for secondary antibody response in vitro. This contrasted to the marked augmentation by LPS of both the primary antibody response in ILN and capacity for in vitro secondary antibody response of ILN cells of mice given SRBC and LPS subcutaneously. The radioactivities detected in the local lymph nodes and other tissues of mice given 51Cr labeled SRBC submucosally or subcutaneously were comparable with each other. MLN, however, contained more Ig+/B220+ B cells and less Thy1+/Ly-1+ T cells than ILN did, and the L3T4/Ly-2 ratio of T cell subpopulations in MLN was lower than that in ILN. Partially corresponding to this observation, the B cell-dependent area was developed more extensively in MLN than in ILN. This difference in cellular composition and organization might in part explain the reason why MLN and ILN display distinct modes of response and sensitivity to the action of LPS.     

The effect of maternal antigenic stimulation upon the active immune responsiveness of their offspring.

Pregnant mice were stimulated by sheep erythrocytes (SRBC) and the active immune responses of their offspring were investigated. The offspring whose mothers were stimulated with SRBC did not develop either IgM or IgG plaque-forming cell (PFC) target cells. From the dose response of pregnant mice for inducing suppression, enough doses (10(8)-10(10) cells of SRBC) for inducing primary anti-SRBC PFC could establish suppression in the young. Both intravenous and intraperitoneal administration of SRBC induced almost complete suppression of the specific PFC response (98.6-95.2%), but only partial suppression (57.8%) was induced by subcutaneous injection. For suppression to take place, female mice had to be injected with SRBC from 2 days before fertilization to day 16 of gestation. Suppression of the PFC response was not obtained when SRBC were given 3 days before fertilization or just 24 hr before delivery. This suppressive effect on the PFC response persisted until the 15th week after birth. In these newborn mice, detectable amounts of specific anti-SRBC antibodies were found. After exchanging mothers and newborns, the stimulated newborns fostered by normal mothers were still unresponsive following antigenic stimulation, even though a specific antibody from the mother was not detected. However, normal newborns nursed by stimulated mothers could respond to SRBC injection, no matter how the specific antibodies were transferred. Possible mechanisms of immunosuppression of the PFC response by antibody transmitted by the mother to her offspring are discussed.     

Major histocompatibility complex restriction of maternally induced suppression in young adult mice.

This study focused on the mode in which maternal T cells induce suppression of plaque-forming cell (PFC) response in offspring. The maternal T cells of C57BL/6J pregnant mice, which had been intraperitoneally injected with 2 x 10(8) of sheep red blood cells (SRBC) on day 12 of gestation, were transferred, 5 days after immunization, into (C3H/HeJ x C57BL/6J)F1 normal pregnant mice on day 12 of gestation. The (C3H/HeJ x C57BL/6J)F1 x C3H/HeJ offspring of (C3H/HeJ x C57BL/6J)F1 recipient pregnant mice were reared to more than 6 weeks of age, and their anti-SRBC PFC responses were examined. Suppression of anti-SRBC PFC response was observed in H-2bxk but not H-2k offspring. Thus, maternal T cells of SRBC-immunized pregnant mice induce suppression of anti-SRBC PFC in offspring with restriction to major histocompatibility complex (MHC) haplotype utilized in maternal T-cell responses during pregnancy. Maternal CD4+ T cells are responsible for the MHC-restricted induction of PFC suppression in offspring. Furthermore we demonstrated, in this report, using adoptive transfer of maternal T cells from SRBC-immunized pregnant mice and in vitro secondary PFC assay in the offspring, that maternal T-cell-mediated suppression results from the development of CD4+ suppressor T cells in offspring. Moreover, the activation of suppressor T cells in offspring depends on the recognition of SRBC antigens presented in association with the same MHC haplotype as that utilized in the maternal T-cell response during pregnancy. Thus, the maternal T cells of SRBC-immunized pregnant mice generate a repertoire of suppressor T cells in their offspring.     

Protein malnutrition reduces the IgA immune response to oral antigen by altering B-cell and suppressor T-cell functions.

Mice fed a protein-deficient diet containing 2% ovalbumin for 6 weeks (PD) show a reduced spleen IgA plaque-forming cell (PFC) response after oral immunization with sheep red blood cells (SRBC), compared to the response of mice fed a control diet containing 20% ovalbumin (C). This reduced IgA PFC response was partially restored by reconstituting the PD mice with an i.v. injection of 5 X 10(5) Peyer's patch (PP) cells from C mice prior to immunization. Reconstitution with B-cell enriched PP, but not helper T-cell (Th) enriched PP, from C mice resulted in an equally elevated IgA response, suggesting that malnutrition induced alterations in the PP B cells yet allowed partially functional Th cells. Reconstitution of PD mice with 1 X 10(7) PP cells from C mice yielded a very low IgA PFC response. This suppression was relieved by removal of Lyt-2+ T cells from the control PP cells prior to transfer. Transfer of spleen, but not PP, cells from oral-immunized PD donor mice to C mice prior to oral immunization resulted in a suppressed IgA PFC response of the recipient mice that was not seen when immune C cells were transferred. Removal of Lyt-2+ cells from the PD spleen cells prior to transfer abolished this suppression. These results suggest that protein deprivation is accompanied by the stimulation of Lyt-2+ suppressor T cells that suppress the IgA response after oral immunization.     

IMMUNOPATHOLOGICAL STUDY OF GLOMERULAR IgA DEPOSITION IN ddY MICE

Serial renal biopsies were performed to observe in detail the age-related glomerular IgA deposition in ddY mice. Moreover, lymphocytes from spleens and Peyer's patches (PP) were immunologically examined in the mice at 10 months of age. The incidence of IgA deposition in the glomeruli was 0% at 3 months, 33% at 7 months, 60% at 10 months and 100% at 13 months, but IgA deposits were less intense than IgG and IgM deposits. Serum levels of IgA and polyclonal IgA responses of PP cells were significantly higher in the mice with glomerular IgA deposits. However, neither the Lyt-l+/Lyt-2+ cell ratio nor the polyclonal IgA responses of spleen cells were related to IgA deposition. Therefore, gut-associated lymphoreticular tissue (GALT) probably plays an important role in the pathogenesis of the high IgA response and glomerular IgA deposition observed in this strain. These results suggest that ddY mice would be valuable as a model of spontaneous glomerular IgA deposition, as well as being useful for studying the possible relationship between the mucosal immune system and glomerular IgA deposition. ACTA PATHOL JPN 38 : 1 -10, 1988.     

Mucosal Homeostasis: Role of Interleukins,Isotype-specific factors and Contrasuppression in the IgA response

Oral ingestion of antigen elicits immune responses at mucosal sites where humoral immunity is largely due to antibodies of the IgA isotype. This is often accompanied by suppression of systemic responses to the same antigen, a state termed oral tolerance. This IgA response is regulated by interactions between T cell subsets found at IgA inductive tissues, i.e., the gut-associated lymphoreticular tissue (GALT) or Peyer's patches (PP). PP T helper (Th) cells support IgA responses, and interleukins 5 (IL-5) and IL-6 can augment secretion of this isotype. Subsets of Th cells may also express Fc receptors for IgA (FcαR) and secrete FcαR as an IgA-binding factor (IBFα). Membrane-derived FcαR is a glycoprotein of 38,000 M.W. and this molecule induces selective increases in IgA secreting cells (as determined by the ELISPOT assay) in PP B cell cultures. FcαR⁺ T cell lines have been shown to secrete IBFα as well as IL-5 both of which promote IgA synthesis. Recombinant IL-5 (rIL-5) and rIL-6 induce IgA synthesis mainly by PP B cell blasts, and principally act on surface IgA-positive (sIgA⁺) B cells for these responses. Another form of mucosal regulation is provided by T contrasuppressor (Tcs) cells, which abrogate oral tolerance when adoptively transferred to mice and restore systemic responsiveness to the antigen sheep erythrocyte (SRBC). Tcs cells from mice systemically primed with SRBC support IgM and IgG subclass responses, while Tcs cells from orally primed mice support IgM, IgG subclass and IgA anti-SRBC responses. These Tcs cells are CD3⁺, CD4^?, 8^? and are antigen-specific. These regulatory cells may use the gamma-delta (γ-δ) form of T cell receptor for antigen recognition.     

Antibody-coated Sheep Erythrocytes Suppress the Ability of Polyclonal B-Cell Activators to Induce Plaque-forming Cells against Sheep Erythrocytes

The primary immune response to untreated sheep erythrocytes (SRBC) in vitro was suppressed by the addition of antibody-coated SRBC. A mixture of SRBC and antibody-coated SRBC also suppressed the induction of anti-SRBC plaque-forming cells by the polyclonal B cell activators lipopolysaccharide, purified protein derivative of tuberculin, and native dextran. Injection of a mixture of SRBC and antibody-coated SRBC into mice led to an increased response to SRBC. It seems plausible from the in vitro findings that the Fc part of antibodies complexed to an antigen can exert a negative signal on antigen-specific B cells that cannot be overcome by positive signals delivered by polyclonal B cell activators.     

Enhancement of Immunoglobulin A Production in Peyer's Patches by Oral Administration of a Traditional Chinese Medicine,Xiao-Chai-Hu-Tang (Shosaiko-To)

Abstract

The Peyer's patches contain a large number of precursor cells committed to the production of immunoglobulin A (IgA) and play an important role in IgA response in the mucosal immune system. We investigated the induction of IgA producing cells in Peyer's patches by plaque forming cell assay after oral administration of a traditional Chinese herbal medicine, Xiao-chai-hu-tang (Japanese name: Shosaiko-to). The number of total IgA producing cells in the Peyer's patches detected by the protein-A plaque assay was increased about two-fold by Shosaiko-to administration and the numbers of both anti-SRBC and anti-HRBC IgA producing cells were also increased by such a treatment. On the other hand, when SRBC alone were administered orally, only the number of anti-SRBC IgA producing cells was increased. Further, we examined T-cell subpopulations in the gut-associated lymphoid tissues after oral administration of Shosaiko-to by flowcytometry. Marked alternations in T cell subpopulations were not detected in the Peyer's patches, though TcR TcR γδ⁺T-cells in the intraepithelial lymphocytes and Thy 1.2^?TcRαβ⁺T cells in the mesenteric lymph nodes were slight increased. These results showed that orally administered Shosaiko-to acts as a polyclonal B-cell activator which induces IgA production in the mucosal immune system.     

Recent progress toward an enterotoxigenic Escherichia coli vaccine

Enterotoxigenic Escherichia coli(ETEC) is the most common cause of bacterial diarrhea in children in Africa, Asia and Latin America and in travelers to these regions. Despite this, no effective vaccine for ETEC is available. ETEC causes disease by colonizing the small intestine with colonization factors, most of which are fimbriae, and production of heat-labile and/or heat-stable enterotoxins. Antibodies against heat-labile enterotoxin and the colonization factors have been shown to be protective, and local immunity in the gut seems to be of prime importance for protection. Hence, several inactivated and live candidate ETEC vaccines consisting of toxin antigens, alone or together with colonization factors, have been evaluated in clinical trials. In this review, the authors describe ETEC vaccine development in progress and the rationale for constructing different types of vaccines. They also discuss possibilities of enhancing immune responses to candidate ETEC vaccines, particularly in children.     

Immunosuppression caused by antigen feeding. I. Evidence for the activation of a feedback suppressor pathway in the spleens of antigen-fed mice

Sheep red blood cells (SRBC) administered by the oral route to normal mice elicited no detectable splenic anti-SRBC plaque-forming cell (PFC) response until 8 weeks of antigen feeding. At this time a splenic IgA anti-SRBC PFC response was detected. On the other hand, spleen cells taken from mice given oral SRBC for 1-5 weeks showed striking changes in their in vitro anti-SRBC responsiveness as compared to spleen cells from normal mice. This was evidenced by enhanced early (days 3-4) in vitro responses, followed by suppressed late (day 5-6) in vitro responses. Both early enhancement and late suppression were T cell-mediated. Early enhancement appeared to be mediated by helper T cells of the Lyt-1+2.3- phenotype. Late suppression was also mediated by Lyt-1+2.3- cells, but Lyt-2-bearing cells had to be present in culture for suppression to occur. Lyt-2-bearing cells could be replaced with normal T cells. Furthermore, elimination of cells bearing I-J-encoded determinants from the T cell population isolated from the spleens of antigen-fed mice also partially relieved suppression. Thus, antigen feeding appears to activate a feedback suppressor pathway in which Lyt-1+2.3-, I-J subregion determinant-bearing T cells can suppress immune responses by causing normal T cells to become suppressor effectors. No evidence was found to show that antigen feeding induced Lyt-1-2.3+ suppressor cells in the spleen, nor were any serum suppressor factors detected.