The ability of Ia and H‐2K^k‐bearing membranes to replace the antigen‐presenting cell in an H‐2K^kallogeneic cytotoxic T cell response

Induction of an allogeneic cytotoxic T lymphocyte (CTL) response is dependent, in part, on uptake and processing of the class I alloantigen by antigen‐presenting cells and subsequent Ia‐restricted recognition of the alloantigen by helper T cells, resulting in lymphokine production. The nature of the antigen‐processing event has been investigated using reconstituted membranes to replace the antigen‐presenting cells in the generation of a secondary allogeneic CTL response. Membranes were isolated from an Ia^d‐positive antigen presenting B cell lymphoma (D2N), detergent solubilized and then reconstituted together with affinity‐purified H‐2K^k antigen in the presence of protease inhibitors. These reconstituted vesicles, containing both syngeneic Ia and alloantigen, were able to induce the helper T cell arm of the CTL response in cultures depleted of antigen‐presenting cells. A variety of control experiments provided strong evidence that the helper T cells recognized the H‐2K^k, probably in its native form, in an Ia‐restricted manner on the vesicles, while the pre‐CTL can directly recognize H‐2K^k. Recognition was only effective if both the Ia and alloantigen were inserted into the same membrane bilayer. The results strongly suggest that the obligatory antigen processing event required for helper T cell recognition of alloantigen is simply the insertion of the alloantigen into the same membrane bilayer as the syngeneic Ia restricting element.