Immunological unresponsiveness in primed B lymphocytes

The need for central and peripheral tolerance in the B cell repertoire

The immune system normally avoids producing antibodies that react with autologous ("self") antigens by censoring self-reactive T and B cells. Unlike the T cell repertoire, antibody diversity is generated within the B cell repertoire in two phases; the first occurs by gene rearrangement in primary lymphoid organs, and the second phase involves antigen-driven hypermutation in peripheral lymphoid organs. The possibility that distinct cellular mechanisms may impose self tolerance at these two different phases of B cell diversification may explain recent findings in transgenic mouse models, in which self-reactive B cells appear to be silenced both by functional inactivation and by physical elimination.

Induction of self-tolerance in mature peripheral B lymphocytes

In transgenic mice, mature peripheral B lymphocytes in lymphoid follicles, like immature B cells, are rendered tolerant by encounter with self-antigen, provided receptor occupancy by self-antigen exceeds a critical threshold. The tolerant state of the B cell is closely correlated with down-regulation of membrane IgM but not IgD antigen-receptors. Identical changes in antigen-receptor expression occur in a subset of follicular B cells in nontransgenic mice, suggesting that clonally silenced self-reactive cells are common in the peripheral B-cell repertoire.

Adjuvant activity of Escherichia coli heat-labile enterotoxin and effect on the induction of oral tolerance in mice to unrelated protein antigens

The ability of Escherichia coli heat-labile enterotoxin (LT) to influence the induction and maintenance of tolerance was examined in animals primed orally with a soluble protein antigen, ovalbumin (OVA), or in animals primed orally with two unrelated protein antigens administered simultaneously, OVA and bovine serum albumin (BSA). LT is immunologically and structurally related to the cholera enterotoxin (CT), which has been shown to be capable of abrogating oral tolerance to protein antigens when delivered simultaneously with the antigens. In this study, simultaneous administration of LT with OVA was shown to prevent the induction of tolerance to OVA and to increase the serum anti-OVA IgG response 30- to 90-fold over OVA-primed and PBS-primed animals, respectively. This effect was determined to be a function of the enzymatically active A subunit of the toxin since the B (binding) subunit alone was unable to influence tolerance induction. Animals fed LT with OVA after the initial OVA prime developed a significantly lower serum IgG and mucosal IgA anti-OVA response than those fed LT with OVA in the initial immunization, indicating that prior exposure to the antigen reduces the effectiveness of LT to influence tolerance and its ability to act as an adjuvant. LT was not able to abrogate tolerance once it had been established. Serum IgG and mucosal IgA responses in animals receiving LT on only a single occasion, that being upon first exposure to antigen, were equivalent to responses after three OVA/LT primes, indicating that commitment to responsiveness occurs early and upon first exposure to antigen.(ABSTRACT TRUNCATED AT 250 WORDS)

T cell tolerance studied at the level of antigenic determinants. I. Latent reactivity to lysozyme peptides that lack suppressogenic epitopes can be revealed in lysozyme-tolerant mice

Whether T cell tolerance represents direct inactivation of antigen-specific T cells via recognition of antigen plus major histocompatibility complex, or via T suppressor (Ts) cells, or a combination of these mechanisms, remains to be clarified. This problem was investigated using a novel approach based on the finding in several systems that T helper/proliferative (Th/Tp) cell-inducing antigenic determinants are dissociable from Ts cell-inducing determinants. Thus, peptide probes containing known sites that stimulate T proliferative activity, as well as peptides from distinct sites assumed to bear Ts-inducing determinants, were used in studying hen (chicken) eggwhite lysozyme (HEL)-tolerant mice. The clear prediction from clonal deletion model is that Th/Tp response potential to short peptides in the tolerant mouse would not exist, while regulatory suppression models predict the coexistence of antigen-reactive cells and antigen-specific regulatory cells that prevent their expression. Adult mice, treated with 2 mg HEL in saline, were tolerant to HEL in complete Freund's adjuvant (CFA). Latent T cell proliferative responses could be revealed to determinants within two HEL peptide probes, which lacked the amino-terminal region of the molecule. This responsiveness suggested two conclusions: first, Ts cells directed against the amino terminus of lysozyme exist in the tolerant genetic responder B10.A; second, these Ts regulate the activity of functional antigen-reactive T cells directed against epitopes elsewhere on the molecule, but only in the presence of the complete molecule, HEL. Examination of neonatally induced tolerance did not reveal any latent responsiveness, supporting the hypothesis that clonal deletion or anergy is the relevant mechanism in this situation. Possible reservations in these explanations of the two tolerant states, plus analysis of the more complex "split tolerance" resulting from 20 mg HEL in saline treatment in adults, are discussed. The approach of dissociation of proliferation-inducing determinants from suppression-inducing determinants clarifies our understanding of the tolerant state and holds promise for more definitive exploration of mechanisms of T cell tolerance.

Primed lymphoid cell tolerance. II. In vivo tolerization of highly tolerogen-sensitive hapten-primed, potentially IgG-producing B cells

The relative ease of tolerizing IgM-bearing versus IgG-bearing B cells was investigated. Previous work had shown that IgG-bearing trinitrophenyl (TNP)-specific B cells from mice primed and boosted with TNP-keyhole limpet hemocyanin (TNP-KLH) are highly susceptible to tolerization in vitro by TNP presented on an unrelated carrier. TNP-OVA was used as tolerogen, as it may represent a more general class of tolerogens than those which are nonmetabolizable or immunoglobulin containing. This study showed that highly primed B cells are tolerizable in vivo using TNP-OVA, with the IgG response to TNP-KLH easier to tolerize than the IgM response. To determine if the ease of tolerization of the IgG response in vivo was due to intrinsic differences in B-cell precursors of the IgM and IgG responses, tolerance was performed in vitro with B cells of defined surface isotypes. A T-independent antigen, TNP-endotoxin, was employed to minimize T-cell effects. At least 10 times as much TNP-OVA was required to tolerize B cells bearing the IgM surface isotype than those with the IgG surface isotype. Thus, the ease of inhibition of the IgG response as compared to the IgM response in vivo by preexposure to TNP-OVA may be at least partially explained by inherent differences in IgM and IgG B-cell precursors.

Long-term suppressor cell lines. I. Demonstration of suppressive function

Suppressor T cell (Ts) lines have been generated and maintained in culture for over one year. Cells from these lines suppress proliferation by primed lymph node cells and antibody production by primed spleen cells. Ts were originally obtained from either spleens of mice rendered tolerant to human gamma globulin (HGG) or short-term cultures of normal spleen cells. Ts from these two sources have previously been shown to exhibit antigen specificity in that only HGG-specific immune responses were suppressed. Prolonged culture of Ts, however, leads to 2 events: an enrichment for suppressive activity in the cultures; and a polyclonal activation of Ts which is demonstrated by the acquired ability of Ts cell lines to regulate responses to antigens other than HGG. Ts cell lines still contain HGG-specific Ts which can be isolated by their capacity to bind to HGG-coated plates. Only HGG-specific immune responses are suppressed by the HGG-binding Ts.