Cellular responses to murine alloantigens of the major histocompatibility complex: the role of cell subpopulations that express different quantities of H-2 associated antigenic markers.

Subsets of lymphoid cells that function in the initiation and differentiation of cell-mediated responses to H-2-coded alloantigens were defined with an antiserum raised between congenic resistant lines of mice that differed for a limited number of components of the H-2 complex. Only those cells that express "Ia markers" can stimulate responses to H-2K, D and/or I region antigens in mixed lymphoid cell cultures, even though all lymphoid cells apparently express the H-2K and H-2D-coded private antigens. Ia markers, therefore, serve to distinguish the subset of cells which includes as its members the stimulating cells. The Ia marker(s) is expressed on the cell membrane of at least one of the two T cell subsets that collaborate in the development of T effector cells to H-2-associated alloantigens, i.e. precursors and helpers. The cells remaining after lysis with our antiserum plus complement no longer can respond in the MLR. Syngeneic non-T cells cannot reconstitute the response. Most activated and proliferating cells express the Ia determinant(s). A proportion of T effector or killer cells, however, does not express the Ia markers. We suggest, therefore, that the MLR-responsive cell in normal lymph nodes is an "activated" cell and the "Ia markers" are involved in the differentiation of T precursor to T effector cells. The end stage T effector probably is devoid of the Ia marker.