On the Molecular Basis of T Helper Cell Function

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Abstract
The differentiation of Ig+ B cells into plaque‐forming cells is dependent on antigen and factors produced by T cells and/or macrophages. We describe here the production of T‐cell factors termed lymphocyte promotor factors (LPF). A foetal calf serum‐specific T‐cell line and its clones synthesize LPF, which is defined as factors that polyclonally stimulate normal spenic T cells to differentiate into cytotoxic T lymphocytes (T‐LPF) and normal splenic B cells to differentiate into plaque‐forming cells into (PFC) (B‐LPF) in the apparent absence of specific antigen. The proliferation of and the B‐LPF production of all T‐cell clones tested were foetal calf serum‐specific and I‐Ab‐restricted. Some of these clones produced only T‐LPF, some clones produced only B‐LPF, and some clones produced both T‐LPF and B‐LPF. B‐LPF stimulate the polyclonal differentiation of Ig+ B cells into PFC without the apparent need for helper T cells, is different from T‐LPF, and induces almost exclusively IgM PFC. The B‐LPF described in the present paper are compared with previously described T‐cell factors, which stimulate antigen‐specific B‐cell responses or bystander B‐cell responses. The conclusion is that B‐LPF are probably different from B‐cell growth factors, T‐cell replacing factors, allogeneic effector factors, and interleukin 2.