We studied proliferation and interleukin 2 production by B6 mouse spleen cells in response to stimulation by irradiated cynomolgus monkey spleen cells and compared the results with responses against whole MHC-disparate allogeneic controls (BALB/c). We found that (a) primary xenogeneic helper responses were absent, whereas primary allogeneic responses were brisk, (b) secondary xenogeneic helper responses were dependent on CD4+ T cells and responder antigen-presenting cells (APCs), whereas allogeneic responses could be mediated by either CD4+ or CD8+ T cells independently and were primarily dependent on the presence of stimulator APCs, and (c) secondary xenogeneic helper responses were blocked by an antibody directed against responder class II MHC molecules. These results suggest that mouse helper T cells recognize disparate xenoantigens as processed peptides in association with self class II MHC molecules, similar to the recognition of nominal antigens and unlike direct allo-recognition.

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