Antigens presented to CD4+ T cells derive primarily from exogenous proteins that are processed into peptides capable of binding to class II major histocompatibility complex (MHC) molecules in an endocytic compartment. In contrast, antigens presented to CD8+ T cells derive mostly from proteins processed in the cytosol, and peptide loading onto class I MHC molecules in an early exocytic compartment is dependent on a transporter for antigen presentation encoded in the class II MHC region. Endogenous cytosolic antigen can also be presented by class II molecules. Here we show that, unlike class I-restricted recognition of antigen, HLA-DR1-restricted recognition of cytosolic antigen occurs in mutant cells without a transporter for antigen presentation. In contrast, DR1-restricted recognition of a short cytosolic peptide is dependent on such a transporter. Thus helper T-cell epitopes can be generated from cytosolic antigens by several mechanisms, one of which is distinct from the classical class I pathway.

SummaryWe have examined previously the peptide specificity of the T cell response to myelin basic protein (MBP) in patients with multiple sclerosis (MS) and healthy controls, and demonstrated that an epitope spanning amino acids 87-106 was frequently recognized. Because this region is encephalitogenic in some experimental animals, it has been postulated that the response to the epitope may have relevance to MS. In this study, the fine specificity of this response is studied using four well-characterized, monospecific T cell lines from three MS patients and an identical twin of a patient. Each of the lines recognized a peptide with the same core sequence, amino acids 89-99, although the responses were affected to various degrees by truncations at the 000H-or NH2 terminal ends of the 87-106 epitope. Importantly, the epitope was recognized in conjunction with four different HLA-DR molecules. Also, the T cell receptor /3 chain usage was heterogeneous, and each line expressed a different VDJ sequence. The four HLADR molecules restricting the response to this epitope have been shown to be overrepresented in MS populations in various geographic areas, suggesting that the response to this region of the MBP molecule may be relevant to the pathogenesis of MS. These findings may have important implications in designing therapeutic strategies for the disease .Although the cause of multiple sclerosis (MS)' is not 13. known, a T cell-mediated autoimmune process has been postulated. Myelin basic protein (MBP) is a potential target antigen because it induces experimental allergic encephalomyelitis (EAE) in susceptible animals. Encephalitogenic epitopes of MBP differ among susceptible strains and correlate with the MHC class II genotype (1) . The TCRs expressed by encephalitogenic T cells from PL/J and B10YL mice and Lewis rats use the same TCR V(3 chain, V08 .2, and have similarities in their VDJ regions, as reviewed in reference 2. Thus, the pathogenesis of EAE is related to the capacity of T cells with the appropriate TCRs to recognize epitopes of MBP 1 Abbreviations used in this paper. aa, amino acid ; EAE, experimental allergic encephalomyelitis ; HTC, homozygous typing cells; ICAM-1, intracellular adhesion molecule 1; MBP, myelin basic protein; MS, multiple sclerosis ; TCL, T cell line . 19presented in conjunction with class II MHC molecules . These requirements have provided rationales for therapeutic strategies that prevent or treat EAE (3-7).The analysis of the T cell response to MBP in patients with MS and in healthy controls has shown that several regions of the molecule are frequently recognized (8-10). One region, an epitope spanning amino acids (aa) 87-106, was recognized by >50% of T cell lines (TCL) derived from both MS patients and controls (9) . Our findings indicated that several HLADR molecules could serve as restriction elements for MBP or fragments of the molecule . In contrast, other investigators have reported that an epitope spanning as 84-102 is predominantly recognized by TCL from MS patients and large...

SummaryThe recognition of virus-infected cells by class I MHC-restricted cytotoxic T cells requires endogenous processing ofantigen for presentation . It is still unclear whether endogenous processing of antigen can be utilized by class 11 MHC molecules for presentation. To test this possibility, a human B cell line expressing HLAA2 and HLA-DR1 was infected with a recombinant vaccinia virus expressing the Influenza A virus M1 matrix protein (VAC-M1) and was assayed for lysis by different M1-specific cytolytic T cell lines, restricted by either HLA-A2 or by HLA-DR1 . Class II-restricted lysis ofVAC-M1-infected cells did occur. This lysis required de novo M1 synthesis and was not due to exogenous antigen . Several properties of the endogenous processing pathway for class 11-restricted presentation were different from those of the pathway utilized by class I molecules . First, class II-mediated recognition of VAC-M1 infected cells was less efficient, requiring higher doses of virus and longer infection times, than the class 1-mediated recognition . Second, chloroquine completely blocked presentation of endogenous M1 to class 11-restricted T cells but had no effect on the class I-restricted presentation . Third, the class II-restricted presentation of M1 was only mildly affected by Brefeldin A, a drug that prevents transport from the endoplasmic reticulum to the Golgi, whereas the class I-restricted presentation ofM1 was completely abrogated by this drug. These data demonstrate the existence of an endogenous processing pathway for the presentation of cytosolic antigen by class 11 molecules and show that this pathway is distinct from the one used for presentation by class I molecules .