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Previous studies have provided evidence that myelin basic protein (MBP)-specific rat T cells acquire antigen via transfer of preformed peptide / MHC class II complexes from splenic antigen-presenting cells (APC). The purpose of the present study was to determine how T cells acquire peptide / MHC class II complexes from APC in vitro. Our results show that a MHC class II⁺ T cell line, R1-trans, released MHC class II-bearing vesicles that directly stimulated MBP-specific CD4⁺ T cells. Vesicles expressing complexes of MHC class II and MBP were also specifically cytotoxic to MBP-specific T cells. Surviving T cells acquired MHC class II / antigen complexes from these vesicles by a mechanism that did not require protein synthesis but depended on specific TCR interactions with peptide/self MHC complexes. Furthermore, MBP / MHC class II-bearing vesicles enabled T cells to present MBP to other T cell responders. These studies provide evidence that APC release vesicles expressing preformed peptide / MHC class II complexes that interact with clonotypic TCR, allowing MHC class II acquisition by T cells. Vesicular transport of antigen / MHC class II complexes from professional APC to T cells may represent an important mechanism of communication among cells of the immune system.

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 an 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 on T-APC catalyzed a rapid decay in the presentation of agonistic antigens, whereas partial agonistic signals decayed at a slower 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 (MBP)-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 on T-APC specifically inhibits subsequent presentation of that antigen, whereas antagonistic MHC/antigen complexes are preserved for an enduring T-APC activity.

Previous studies have shown that the anti-CD4 mAbW3/25 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 ofT-APC activity. Clones were cultured with irradiatedSPL and GPMBP or rat (R) MBP for 2–3 days, were propagated in IL-2 for another 1-3 days, were irradiated, andwere 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 dominantencephalitogenic 72–86 determinant did not effectively present the agonist GPMBP but potently presented thepartial agonist RMBP. The heightened APC activity ofDN 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.