Mechanisms of lymphocyte activation: the role of suppressor cells in the proliferative responses of chronic lymphatic leukemia lymphocytes.

Binding of (125)I-leukoagglutinin (LPHA) to lymphocyte membrane receptors at equilibrium generated similar curvilinear Scatchard plots in 20 patients with bursa-derived (B)-cell-type chronic lymphatic leukemia (CLL) and 15 controls. If biphasic plots are assumed, the two linear components show markedly diminished receptor capacity (15 and 137 ng/10(6) lymphocytes) in CLL as compared to controls (60 and 668 ng). In contrast, affinity was similar in patients (1.0 x 10(8) M(-1) and 2.1 x 10(6) M(-1)) and controls (1.8 x 10(8) M(-1) and 1.5 x 10(6) M(-1)). Highly purified B cells from patients and controls generated binding data comparable to that obtained from the mixed lymphocyte (ML) suspensions from which they originated. Maximal DNA synthesis of highly purified, normal, thymus-derived (T) and B cells in response to LPHA stimulation was comparable to that of ML (mitotic index [MI] 19.9, 20.1, and 23.4, respectively), though B-cell responses were slightly delayed. In CLL the markedly decreased and delayed DNA synthesis by ML (MI 2.3), and by highly purified T (MI 1.6) and B (MI 1.9) cells seemed out of proportion to their decreased receptor capacity for LPHA. The impaired mitogenic responses of leukemic cells from five patients were not enhanced when cocultured with normal lymphocytes. In contrast, cells from eight patients inhibited cocultured normal lymphocyte responses to LPHA by 94.3%. Sera from these patients and supernates from their cultured cells did not mediate this suppressor effect. These observations indicate that the decreased DNA synthesis observed in CLL is not an attribute of B cells and does not represent the expected response of a few residual normal T lymphocytes, but rather reflects impaired responses by all CLL cells. The defect does not relate to the density or function of membrane receptors for LPHA, to the presence of inhibitors in these patients' sera, or to depletion of helper T cells. Our data strongly suggest that one mechanism for the immunoincompetence observed in CLL reflects excessive suppressor-cell activity.