Nonspecific “Lymphocyte Activating” Factors Produced by Macrophages

The complexity of macrophage interactions with lymphocytes (1, 2) has led to nomenclatural confusion in the recent immunologic literature. In addition to their role in “presenting” antigen to T cells in association with surface membrane components such as Ia (3, 4), macrophages release a battery of soluble factors that stimulate proliferation and differentiation of lymphoid cells, as well as other factors that are suppressive or cytotoxic (4–7). The general term “lymphocyte-activating factor” (LAF), first employed to denote the group of stimulatory macrophage factors (8–10) has tended to lose its usefulness because information has accumulated that factors acting on different target populations and affecting different cell functions may really be quite diverse. Several factors that are increasingly well defined act on T cells and their precursors (Table I) (reviews in 4, 7, 11, 12). The genetically related factor (GRF) contains macrophage Ia and a fragment of antigen (13, 14) and may be regarded simplistically as mediating in soluble form a recognition event similar to that which takes place at the surface of the macrophage.