The mechanisms responsible for immune recognition and the control of subsequent effector cell functions are topics immunologists are beginning to investigate at the level of the gene. It appears that the cell-cell interactions required during each of these phases of the immune response are under direct control of genes that map in the major histocompatibility complex (MHC). I Thus, the role of the macrophage (Mob) in the processing and/or presentation of antigen to T and B lymphocytes has been reevaluated to include certain genetic restrictions observed among the interacting cell types (1). Primed guinea pig T cells, for example, become optimally stimulated to proliferate in vitro only when exposed to antigen-pulsed M~b sharing identical I region-associated (la) histocompatibility antigens (2, 3). Similar restrictions involving various loci within the H-2 complex of the mouse have been demonstrated for T-B cooperation resulting in in vivo antibody production (4, 5); Mcb-T cell interactions in in vitro secondary antibody responses (6); cytotoxic T-cell target recognition elicited by viral-infected (7) or chemically modified cells (8); successful transfer of delayed type hypersensitivity in vivo (9); and T-cell suppression of various functions (10-12).A unifying interpretation of these results, tested in our laboratory and a number of others, suggests that T lymphocytes initially recognize antigen together with MHC membrane components on the antigen-presenting cell (dual or associative recognition). In the course of subsequent interactions, the T cell must again "see" this same display of antigen + MHC product(s) in order to proliferate or effectively function.However, the exact nature of this genetic control, particularly for helper function, remains a controversial issue.

The ability of murine helper T cells primed to the antigen, sheep erythrocytes (SRBC) to cross-react with burro erythrocytes (BRBC) in the in vitro anti-trinitrophenol (TNP) response to TNP-RBC was shown to be under genetic control. Although non-H-2 genes were shown to influence the absolute level of helper activity assayed after SRBC priming, the extent of cross-reaction of SRBC-primed helpers with BRBC was shown to be controlled by an H-2-1inked Ir gene(s). H-2 haplotypes were identified which determined high, intermediate, or low response to the cross- reacting determinants and the gene(s) controlling the cross-reaction tentatively mapped to the K through I-E end of the H-2 complex. Helpers primed in F(1) mice of high x intermediate or high x low responder parents were tested for cross-reaction using B cells and macrophages (Mφ) of parental haplotypes. In each case the extent of cross-reaction was predicted by the H-2 haplotype of the B cells and Mφ, establishing the expression of the Ir gene(s) in B cells and/or Mφ a t least, but not ruling out its expression in T cells as well. The low cross-reaction seen when T cells from F(1) mice of high × low responder parents were tested on low responder B cells and Mφ was not increased by the presence of high responder Mφ, indicating the Ir gene(s) is expressed in the B cell a t least although it may be expressed in Mφ as well. These and our previously reported experiments are consistent with the hypothesis that helper T cells recognize antigen bound to the surface of B cells and Mφ in association with the product(s) of Ir gene(s) expressed on the B cell and Mφ.

Using lymph node T cells from poly-L(Tyr,Glu)-poly-D,L-Ala--poly-L-Lys[(TG)-A--L]-primed animals and B cells from animals primed with trinitrophenylated (TNP) protein or lipopolysaccharide, we have obtained anti-TNP-(TG)-A--L direct plaque-forming responses in vitro. Response to this antigen was shown to be controlled by the H-2 haplotype of the animal studied. The strain distribution of in vitro response was very similar to that previously reported by others for in vivo secondary IgG responses to (TG)-A--L. We investigated the cell types expressing the Ir gene(s) for (TG)-A--L in our cultures. F1, high responder x low responder mice were primed with (TG)-A--L. Their T cells were active in stimulating anti-TNP-(TG)-A--L responses of high responder but not low responder B cells and macrophages (MPHI), even though both preparations of B cells and Mphi were obtained from mice congenic at H-2 with one of the parents of the F1. For three low responder strains tested, of the H-2h2, H-2k, and H-2f haplotypes, the anti-TNP-(TG)-A--L response of low responder B cells and Mphis in the presence of high responder, F1 T cells could not be improved by the addition of high responder, antigen-bearing Mphis to the cultures. In one strain of the H-2a haplotype, it was shown that neither the B cells nor Mphis could be functional in anti-TNP-(TG)-A--L responses. Our results therefore suggested the Ir genes for anti-TNP-(TG)-A--L responses were expressed at least in B cells in all the low responder strains we studied, and, in mice of the H-2a haplotype, in Mphis too.

We have been interested in determining the mechanisms by which helper T cells collaborate with B cells in the generation of specific antibody responses. Based on a variety of experimental approaches we have proposed that the induction of B cell IgM plaque-forming cells (PFC) ~ responses to protein-bound haptens requires two types of helper T cell signals, the first both antigen specific and B cell Ia restricted and the second mediated by nonspecific factors, i.e. interleukins (1-6). Studies designed to test the synergistic effects of nonspecific factors and antigen-specific helper T cells have been complicated because the interleukins may have effects on T cell functions as well as, or instead of, the B cell response under study.We previously established the ability of a series ofT cell hybridomas to deliver antigen-specific, B cell Ia-restricted helper signals in the PFC response to soluble protein antigens (5). In addition to the obligate requirement for the antigenspecific helper T cell signal, the elicitation of optimal PFC responses was to varying degrees dependent on the addition of nonspecific factors provided by culture supernatants of concanavalin A-stimulated (Con A SN) spleen cells. This system offers an ideal opportunity to investigate the effects of nonspecific factors on B cell responses to soluble protein antigens under circumstances in which we are confident the effects of the factors are on the B cells themselves and not on the T cell hybridomas.Our primary approach to studying the interleukins involved in the generation of specific B cell antibody responses has been the primary in vitro PFC response to sheep erythrocytes (SRBC). Unlike the antibody response to soluble proteins, spleen Con A SN are sufficient to drive the anti-SRBC PFC response of T celldepleted splenic B cells (6). Using splenic B cells rigorously depleted of T cells and macrophages (MO) we have been able to replace the activity of spleen Con

We examined the expression of (TG)-A--L specific Ir genes in helper T cells using T cells from low responder leads to (B10, high responder x low responder) F1 chimeric mice. In this paper, the low responder strain studied was B10.M, H-2f. B10.M T cells from these chimeric animals do not help anti-TNP-(TG)-A--L responses, even though they have matured in a high responder thymus and been primed and challenged with antigen on high responder Mphi and B cells. These findings indicate that in the H-2f haplotype an Ir-gene controlling anti-(TG)-A--L activity is expressed in helper T cells. The findings are in contrast to those we have obtained and previously reported with T cells of another low responder haplotype, H-2a. Taken together with our previous findings that (TG)-A--L specific Ir genes are expressed by B cells and Mphi of both the H-2a and H-2f haplotypes, the results indicate two sites of action for Ir genes, and suggest two different gene products acting at different stages of the response, both of which are defective in H-2f cells, and only one of which is defective in H-2a cells.