Interactions of small B lymphocytes with unprimed noncytolytic T cells: dissociation between "presentation" and growth induction
The accessory cell requirements in lectin-dependent triggering and growth of unprimed Lyt-2-T lymphocytes were analyzed by quantitatively comparing the ability of small B cells and peritoneal macrophages to either induce reactivity to growth factors or support growth. Lightly or nonirradiated small B cells were 15 to 30-fold less efficient as compared to T cell-depleted peritoneal cell populations, in the support of lectin-stimulated Lyt-2-T cell proliferation. In contrast, lightly irradiated small B lymphocytes were quantitatively as efficient as macrophages in mediating lectin-driven Lyt-2-T cell proliferation, if relevant supernatants were added into culture. Finally, supernatants derived from cultures where T-small B cell ratios were optimal for growth of responder Lyt-2lymphocytes were two orders of magnitude less efficient than conditioned medium obtained from cultures containing optimal T-macrophage ratios, in their ability to support growth of activated T cells. We conclude from these experiments that: (a) in contrast to cytolytic T cell precursors, lectin-dependent induction of unprimed Lyt-2-T lymphocytes requires accessory cells; (b) small B cells and macrophages are equally competent in this respect; and (c) growth support by small B cell populations is due to contamination by macrophages which are the only cell type performing this function. We therefore interprete reports on Lyt-2-T cell proliferation upon stimulation with high numbers of small B cells as a two-step process: "presentation" and induction of T cells which is essentially B cell dependent, and factor production ensured by contaminating macrophages.