AN Ia - POSITIVE ALLOGENEIC EFFECT FACTOR

The immune response (I) region of the mouse 1-1-2 major histocompatibility complex regulates interaction between thymus-derived (T) and bone-marrow-derived (B) lympho-cytes that result in antibody synthesis (reviewed in references I and 2). Immune response (Ix) genes control specific immune responses to various synthetic and native antigens (3) but their cellular site of action (T cells and/or B cells) remains unresolved (reviewed in reference 4)./-region compatibility may (5, 6) or may not (2, 7-9) be required for optimal T cell-B cell collaboration during a secondary IgG antibody response. The/-region also determines a highly polymorphic series of membrane glycoproteins of approximately 30,000 mol wt denoted as/-region-associated (Ia) ~ antigens. These antigens are expressed predominantly on B cells (reviewed in reference 1), may serve as a B-cell differentiation marker (10), 2 may mediate certain steps of the switch from IgM to IgG antibody produc-tion° (10, 11), and are purported to be components of antigen-specific helper (12) and suppressor (13) factors as well as an antigen-nonspecific helper (14) factor. Implicit in these results is the involvement of Ia antigens in Ir gene function. However, the precise definition of this relationship as demonstrated by the cellular origin of these factors, their target and mechanism of action, and the role of their la molecules in lymphocyte interaction is still not yet clear. Allogeneic effect factor (AEF) has been identified as an antigen-nonspecific, soluble, T-cell-derived, helper factor that mediates in vitro B-cell activation in a manner similar to the in vivo allogeneic effect (14). The active components of this AEF have been reported to be la antigens (15) and B2-microglobulin (16). However, so far it has been difficult to demonstrate directly the presence of la antigens on helper T cells (10). In addition, AEF activity seems to reside with proteins of mol wt of 40-45,000 and 12,000 daltons, respec