Class II MHC/peptide complexes are released from APC and are acquired by T cell responders during specific antigen recognition.

T cell expression of class II MHC/peptide complexes may be important for maintenance of peripheral self-tolerance, but mechanisms underlying the genesis of class II MHC glycoproteins on T cells are not well resolved. T cell APC (T-APC) used herein were transformed IL-2-dependent clones that constitutively synthesized class II MHC glycoproteins. When pulsed with myelin basic protein (MBP) and injected into Lewis rats, these T-APC reduced the severity of experimental autoimmune encephalomyelitis, whereas unpulsed T-APC were without activity. Normal MBP-reactive clones cultured without APC did not express class II MHC even when activated with mitogens and exposed to IFN-gamma. However, during a 4-h culture with T-APC or macrophage APC, recognition of MBP or mitogenic activation of responder T cells elicited high levels of I-A and I-E expression on responders. Acquisition of class II MHC glycoproteins by responders was resistant to the protein synthesis inhibitor cycloheximide, coincided with transfer of a PKH26 lipophilic dye from APC to responders, and resulted in the expression of syngeneic and allogeneic MHC glycoproteins on responders. Unlike rested I-A- T cell clones, rat thymic and splenic T cells expressed readily detectable levels of class II MHC glycoproteins. When preactivated with mitogens, naive T cells acquired APC-derived MHC class II molecules and other membrane-associated proteins when cultured with xenogeneic APC in the absence of Ag. In conclusion, this study provides evidence that APC donate membrane-bound peptide/MHC complexes to Ag-specific T cell responders by a mechanism associated with the induction of tolerance.

An autologous self-antigen differentially regulates expression of I-A glycoproteins and B7 costimulatory molecules on CD4- CD8- T helper cells.

During inflammation, T helper cells transiently express class II major histocompatibility complex (MHC) glycoproteins and present antigens to other T cells. To assess involvement of self-antigens in the generation of T cell antigen-presenting cell (T-APC) activity, rat (R) myelin basic protein (MBP) was used to stimulate a rat CD4-CD8- T cell clone. RMBP induced T cell surface expression of class II MHC glycoproteins and T-APC activity, although RMBP did not elicit interleukin (IL-2) production or proliferation. When added to culture with the strong agonist guinea pig (GP) MBP, RMBP acted as a partial antagonist and inhibited responses of IL-2 production, proliferation, and T cell expression of B7.1. RMBP did not, however, efficiently antagonize GPMBP-induced I-A expression on T cells. These findings indicate that the self-antigen RMBP specifically induces accumulation of I-A/peptide complexes at signaling thresholds that inhibit pathogenic autoimmune responses. Overall, this study suggests a role for self-antigens in the generation of B7-deficient T-APC activity as a mechanism of tolerance in experimental autoimmune encephalomyelitis.

Partial agonism elicits an enduring phase of T-cell-medicated antigen presentation.

Previous studies have shown that the anti-CD4 mAb W3/25 strongly enhances T cell APC (T-APC) activity. In this study, single positive CD4+ and double negative (DN) (CD4-CD8-) T-helper cells specific for the 55-69 or 72-86 sequence of guinea pig (GP) myelin basic protein (GPMBP) were used to study CD4 regulation of T-APC activity. Clones were cultured with irradiated SPL and GPMBP or rat (R) MBP for 2-3 days, were propagated in IL-2 for another 1-3 days, were irradiated, and were used as T-APC. DN T cells specific for GP55-69 effectively presented GPMBP and were superior APC compared to other CD4+ T cells for presentation of this antigen. In contrast, DN T cells specific for the dominant encephalitogenic 72-86 determinant did not effectively present the agonist GPMBP but potently presented the partial agonist RMBP. The heightened APC activity of DN T cells reflected the lack of CD4 because the anti-CD4 mAb W3/25 promoted T-APC activity of CD4+ T cells to those levels expressed by DN T cells. Overall, T cells with potent reactivity to GPMBP or RMBP were subsequently unable to present that antigen, whereas T cells exhibiting partial or low antigen reactivities were highly effective APC for presentation of that antigen. The unrelated antigen conalbumin was presented by MBP-specific clones only when added to culture with a specific partial agonist. Together, these data indicate that partially agonistic MHC ligands promote prolonged expression of T-APC activity and that DN T cells may be specialized to mediate postactivational antigen presentation.

Class II MHC/peptide complexes on T cell antigen-presenting cells: agonistic antigen recognition inhibits subsequent antigen presentation.

Previous studies have shown that tolerogenic anti-CD4 (W3/25) and anti-LFA-1 mAb (LRTC1) which block T cell activation paradoxically enhance T cell-mediated antigen presentation. Lasting T cell APC (T-APC) activity requires and initial exposure of T cells to these mAb in the presence of professional APC and antigen. This study revealed a central mechanism regulating the duration of T-APC activity. T cell recognition of class II MHC complexes of T-APC catalyzed a rapid decay in the presentation of agonistic antigens, whereas partial agonistic signals decayed at a shower rate. Likewise, blockade of agonistic T-T cell autorecognition by these mAb led to the persistence of agonistic MHC/antigen on T-APC. The best predictor of T-APC activity was related to the ability of clonal T cells to respond to antigen presented by neighboring T cells. Strong responders were inefficient T-APC, whereas inefficient responders were strong T-APC. Addition of irradiated myelin basic protein (MBP0-specific responders to T-APC cultures specifically inhibited the subsequent presentation of MBP but not conalbumin, and vice versa. T-APC presentation of antigen to responder T cells also resulted in reduced surface expression of class II MHC I-A glycoproteins on T-APC. These findings indicate that agonistic recognition of antigen of T-APC specifically inhibits subsequent presentation of that antigen, whereas antagonistic MHC/antigen complexes are preserved for an enduring T-APC activity.

Acquired resistance to experimental autoimmune encephalomyelitis is independent of V beta usage.

In Lewis rats, activated encephalitogenic T-helper cells elicit a single bout of experimental autoimmune encephalomyelitis (EAE). Recovery from EAE is marked by reduced susceptibility to disease reinduction. The purpose of this study was to determine whether a dominant expression of V beta gene segments by encephalitogenic T cells was required for development of recovery-associated resistance. Several polyclonal and monoclonal T cell lines were derived from Lewis rats sensitized with R72-86, a synthetic peptide representing the 72- to 86-amino-acid sequence of rat myelin basic protein (RMBP). The results revealed broad heterogeneity among encephalitogenic T cells specific for R72-86 in regard to V beta expression and CDR3 sequence. Encephalitogenic clones exclusively bearing either V beta 4 or V beta 10 TCR or polyclonal T cells bearing heterogeneous TCR transferred EAE to recipient rats and elicited resistance to EAE as revealed by subsequent challenge with guinea pig (GP)MBP in complete Freund's adjuvant (CFA). Nonpathogenic V beta 3+ and V beta 8.6+ clones specific for the 68-86 and 55-66 regions of MBP, respectively, did not elicit effective protection from EAE. These data indicate that induction of postrecovery resistance to EAE does not depend upon a particular V beta usage.

T-helper lymphocytes specific for myelin basic protein: low-density activation prolongs a postactivation refractory phase marked by decreased pathogenicity and enhanced sensitivity to anergy.

Regulation of experimental autoimmune encephalomyelitis (EAE) in Lewis rats may involve activation-dependent negative feedback pathways of T-helper cells. Previous studies have shown that T-helper cells specific for myelin basic protein exhibit a postactivation refractory phase during which antigenic restimulation elicits proliferation without production of IL-2. Herein, we show that postactivation refractoriness inhibits regeneration of EAE transfer activity and is manifest by a lack of IL-2 mRNA accumulation despite induction of normal levels of IL-4 mRNA. Preactivated refractory T cells were substantially more susceptible than resting T cells to the induction of anergy. Low-density T cell activation or subcloning prolonged the duration of the refractory phase and engendered long-term desensitization of T cells marked by a blockade of IL-2 production and by enhanced susceptibility to anergy. Overall, these results support the concept that postactivation refractoriness controls the pathogenicity and differentiation of T-helper cells.

Anergy-associated T cell antigen presentation. A mechanism of infectious tolerance in experimental autoimmune encephalomyelitis.

CD4+ T cells promote immune responses against foreign Ags while actively suppressing responses against self Ags. To address how CD4+ T cells ensure self-tolerance, we focused on two CD4+ T helper cells specific for myelin basic protein (MBP). GP2.E5/R1 T cells recognized rat MBP (RMBP) as a partial agonist and mediated mild experimental autoimmune encephalomyelitis (EAE), whereas R2 T cells recognized RMBP with full efficacy and mediated severe EAE. GP2.E5/R1 T cells were more susceptible to anergy induction than R2 T cells. Anergic GP2.E5/R1 T cells lacked proliferative reactivity, but expressed both I-A glycoproteins and high levels of radioresistant APC activity. During induction of anergy, these T cells acquired the ability to present MBP. In a separate subsequent culture without further addition of Ag, anergic GP2.E5/R1 T cells elicited full proliferative and IL-2 production responses by R2 T cells. Unlike activations induced via irradiated splenocytes, irradiated anergic T cells elicited anergy in R2 T cells in the form of a postactivational phase of nonresponsiveness. Anergic GP2.E5/R1 T cells not only transferred anergy to pathogenic R2 T cells in vitro, but these anergic T cells also transferred resistance to EAE in Lewis rats subsequently challenged with guinea pig MBP in CFA. Antagonistic signaling by autologous RMBP was more tolerogenic than that of guinea pig MBP in both in vitro and in vivo models of infectious anergy. We conclude that in the presence of tolerogenic mAb, antagonistic signaling by a self protein elicited the coordinate expression of anergy and T cell-mediated APC activity as a mechanism for the genesis and spread of infectious tolerance.