Modulation of restricted class II T cell responses by peptides derived from self class II molecule

T cell reactivity to MHC class II-bound self peptides in systemic lupus erythematosus-prone MRL/lpr mice

The epitopes recognized by pathogenic T cells in systemic autoimmune disease remain poorly defined. Certain MHC class II-bound self peptides from autoimmune MRL/lpr mice are not found in eluates from class II molecules of MHC-identical C3H mice. Eleven of 16 such peptides elicited lymph node cell and spleen cell T cell proliferation in both MRL/lpr (stimulation index = 2.03-5.01) and C3H mice (stimulation index = 2.03-3.75). IL-2 and IFN-gamma production were detected, but not IL-4. In contrast to what was seen after immunization, four self peptides induced spleen cell proliferation of T cells from naive MRL/lpr, but not from C3H and C57BL/6.H2(k), mice. These peptides were derived from RNA splicing factor SRp20, histone H2A, beta(2)-microglobulin, and MHC class II I-A(k)beta. The first three peptides were isolated from I-E(k) molecules and the last peptide was bound to I-A(k). T cell responses, evident as early as 1 mo of age, depended on MHC class II binding motifs and were inhibited by anti-MHC class II Abs. Thus, although immunization can evoke peripheral self-reactive T cells in normal mice, the presence in MRL/lpr mice of spontaneous T cells reactive to certain MHC-bound self peptides suggests that these T cells actively participate in systemic autoimmunity. Peptides eluted from self MHC class II molecules may yield important clues to T cell epitopes in systemic autoimmunity.

Epitopic analysis of the Toxoplasma gondii major surface antigen SAG1

T and B cell epitopes of the major Toxoplasma gondii surface antigen SAG1 were studied following CNBr fragmentation. Three fragments, F1, F2 and F3, were obtained, of 19, 16.5 and 14 kDa, respectively. The positions of F1 F2 and F3 within the SAG1 protein were identified by N-terminal sequence determination. The F1 fragment located on residues 125-269 contains the C-terminus, and the fragment F2 (residues 1-124) is located at the N-terminal region. F3 is a C-terminal peptide about 40 amino acids shorter than the F1 fragment (residues 165-269). Polyclonal antibodies obtained from infected animals or humans and a monoclonal anti-SAG1 antibody did not recognize either the reduced protein or the reduced fragments on immunoblotting. The monoclonal antibody 1E5 did not recognize fragment F1. Mouse IgA and IgG antibodies from infected mouse sera and intestinal secretions, as well as human IgG antibodies, only recognized the whole protein and the F1 fragment. These results suggest that the fragment F1 encompasses all B cell epitopes recognized on the SAG1 protein after infection with the parasite and that the sequence 125-165 is essential for the structural integrity of these B cell epitopes. Murine anti-SAG1 T cell proliferation was observed in SAG1 immunized CBA/J mice (H-2k) and BALB/c mice (H-2d), but not in C57BL/6 mice (H-2b). The three fragments F1, F2 and F3 were able to induce specific proliferation of anti-SAG1 T cells from CBA/J mice, while only the F1 and F2 fragments induced specific blastogenesis of anti-SAG1 T cells from BALB/c mice.(ABSTRACT TRUNCATED AT 250 WORDS)

A peptide binding weakly to the major histocompatibility molecule augments T cell responses

An I-A(d)-derived peptide PB1 was found to enhance the reactivity of I-A(d)-restricted T cells. The augmentative effect was not due to the cross-reactivity of PB1 peptide with antigens. PB1 had no effect on T cells specific for I-A(b) and I-E(k), nor did PB1 increase the T cell responses to concanavalin A and staphylococcal enterotoxin B. The strict I-A(d) specificity suggests that PB1 enhances the recognition of antigen-I-A(d) complex by T cell receptor. PB1 bound to I-A(d) weakly. The augmentative effect could be found on other I-A(d)-binding peptides in appropriate conditions; however, PB1 was distinct in its prominently augmentative effect on all the I-A(d)-restricted T cells analyzed. A similar enhancing activity was demonstrated on a synthetic transferrin receptor peptide with minimum affinity for I-A(d). The unusual enhancing activity of PB1 may thus be attributed to the low I-A(d) binding affinity. It was postulated that the binding of low-affinity PB1 would not only stabilize I-A(d) structure, but also enhance the binding of other peptides. This was supported by the increased binding of OVA 323-339 and cI 84-98 to I-A(d) in the presence of PB1. The inclusion of PB1 in the immunization mixture also enhanced T cell responses in vivo, suggesting the possibility of using low-affinity peptide to promote specific immunity.