Requirements for the stimulation of allogeneic T lymphocytes by acute non-lymphoblastic leukaemia cells.

Blast cell populations from 32 patients with acute non-lymphoblastic leukaemia of various morphological types have been examined for their ability to stimulate allogeneic T lymphocytes from normal donors in one-way mixed leucocyte culture (MLC). At the same time, these leukaemic cell populations were examined for the amounts of major histocompatibility complex Class I and Class II antigens they expressed, and their ability to release interleukin 1 (IL1) in culture both with and without stimulation by lipopolysaccharide. The abilities of the leukaemic cell populations to stimulate in MLC, and to produce IL1, were found to be associated with the expression of morphological characteristics of monocytic differentiation, and correlated significantly. In contrast, no correlation was observed between the extent of Class I or Class II expression and MLC stimulatory ability. Many myeloblast populations of immature phenotype were unable to stimulate allogeneic T cells despite their strong expression of these antigens. This lack of stimulatory ability was not overcome by the addition of exogenous IL1. We therefore conclude that the correlation between the production of IL1 and MLC stimulatory ability does not necessarily imply a cause/effect relationship, and that the interaction between allo-antigen and the T cell receptor together with a supply of lymphokine 'co-stimulator' is not sufficient to activate resting T lymphocytes. The failure of some Class I and II antigen positive leukaemic blasts to stimulate in MLC even in the presence of exogenous IL1 may be due to the lack of other differentiation-associated cell surface molecules necessary for stable cell-cell interaction.