Stimulatory capacity of human T and B lymphocytes in the mixed leukocyte culture

LYMPHOCYTES can be separated into thymus-dependent (T) and thymus-independent (B) cells on the basis of ontogeny, surface membrane differentiation markers, and function. Thus, some functions can be attributed solely to T or B cells, whereas others require T/B cell cooperation 1,2 . When two populations of allogeneic lymphoid cells are cultured together in the mixed leukocyte culture assay (MLC), cellular interaction leads to blastic transformation and subsequent proliferation of a portion of the cultured cells. This proliferation response originates in T lymphocytes 3–5,30,31 . But secondary T cell-mediated B cell proliferation may in some species contribute to the overall proliferative response of T and B cells 6–9 . The question then arises: which cell type acts as stimulator in MLC? Is the stimulus to proliferate equally transferred by T and B cells, or are the antigenic determinants responsible for MLC activation represented exclusively or preferentially on the surface of only T or B lymphocytes? Studies in mice 9–11 indicated that T and B cells possess equal stimulatory capacities. However, in other reports the stimulatory capacity in mouse MLC was almost exclusively attributed to B lymphocytes (refs 3 and 12, and personal communication from D. H. Sachs).