Variation in accessory cell requirements in human mixed lymphocyte response to leukaemic cell lines

Human leukaemia and lymphoma cell lines were investigated as stimulating cells in an allogeneic mixed lymphocyte response. Purified T cells and unfractionated mononuclear cells from normal donors were used as responders. The cell lines fell into three groups: (i) those which stimulated allogeneic responder T cells in the presence or absence of accessory (non-T) cells; (ii) those which stimulated T cells only in the presence of accessory cells; and (iii) those which failed to stimulate in either case. The accessory function was provided by adherent cells and non-adherent, non-T cells. There was no correlation between the stimulatory capacity of these cell lines and the presence of serologically defined HLA-DR determinants. These results are discussed in the context of the current two-signal hypothesis for T-cell activation.