We conclude from the data that resting T cells, lacking the appropriate receptor, are probably unaffected by MLR suppressor factor. Instead, upon activation by allogeneic cells, and perhaps concomitantly with the development of a suppressor T-cell subpopulation, a receptor for MLR suppressor factor is expressed on the surface of responder T cells. Since this receptor is not associated nonspecifically with blast transformation (e.g., it is not detected on lipopolysaccharide or phytohemagglutinin-induced blast cells), it may represent a unique control mechanism for regulation of alloantigen-induced T-cell responses.
