Differential macrophage requirements for T helper cell and T helper cell-induced B lymphocyte proliferation

Major histocompatibility complex-restricted helper T cell clones against "minor" antigens expressed on B cell and macrophage surfaces, when confronted with appropriate T cell-depleted spleen cells, are induced to proliferation and, in turn, activate "target-responder" B cells to polyclonal growth and maturation. Irradiation of helper cell populations, however, demonstrates that their effector functions (and B lymphocyte responses) are independent of proliferative activity. Adherent cell depletion on Sephadex G10 columns, while completely abrogating helper T cell proliferation, does not abolish helper cell- induced B cell responses, demonstrating a remarkable quantitative difference in macrophage requirements for the growth of these two cell types. Because significant B cell responses are detected upon interaction with primed helper T cells under conditions of extreme macrophage depletion, we conclude that the role of macrophages in T-B cell cooperation is limited to expansion of optimal numbers of helper T lymphocytes. It follows that activated helper cells can autonomously produce all B cell-specific growth and maturation factors mediating cooperative antibody responses. In contrast, the profound reduction of LPS-induced responses upon macrophage depletion suggests accessory cell production of such factors in thymus-independent B cell growth and/or maturation.