Interactions of small B lymphocytes with unprimed noncytolytic T cells: dissociation between “presentation” and growth induction

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Abstract
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‐2− lymphocytes 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.

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