The possible role of i region determined cell surface molecules in the regulation of immune responses

Expression of Ia antigens on hapten-specific B cells. I. Delineation of B-cell subpopulations

The nonimmune adult spleen contains at least two B-cell subpopulations. The majority of primary B cells express cell surface Ia determinants and have the capacity to give rise to IgG antibody-producing clones after T-cell dependent antigenic stimulation. There is also a small subpopulation of primary B cells which are, by definition, Ia negative, since their activity is not eliminated by negative selection with anti-Ia serum and complement. The Ia-negative B cells give rise to clones that produce only IgM antibody. These B-cell subsets may form a continuum in B-cell maturation, or they may exist as discrete B-cell lineages. Since the cellular expression of Ia antigens appears to correlate with the ability of the B cell to generate IgG-producing clones, it is speculated that Ia molecules may have a role in the IgM to IgG B-cell switch mechanism.

Inhibition of lymphocyte mitogenesis by immobilized antigen-antibody complexes

Mouse spleen cells, cultured on surfaces coated with antigen-antibody complexes, are inhibited from responding to the B-cell mitogens, lipopolysaccharide, lipid A, Pneumococcal polysaccharide SIII, and poly I:C. The response to the T-cell mitogen, concanavalin A, is also substantially inhibited by immobilized antigen-antibody complexes, but specific inhibition of the response to phytohemagglutinin is minimal. Control experiments showed that immobilized complexes prepared from IgG F(ab')2 fragments and IgA antibodies (both of which fail to bind to Fc receptors when complexed to antigen) did not show significant inhibitory activity when compared with the inhibition observed with complexes prepared from whole IgG. Suspensions of antigen-antibody complexes prepared from the same antigen and intact IgG antibody did not inhibit mitogenesis. None of the mitogens used could be demonstrated to compete with the binding of aggregated immunoglobulin to the B-cell Fc receptor. It appears that the interaction of Fc receptor-bearing lymphocytes and/or macrophages with immobilized complexes prevents lymphocyte activation by mitogens. It is suggested that the mechanism(s) involved may be relevant to antibody feedback control of the humoral immune response.

Detection of non-H2 antigen(s) which, like Ia antigens, are associated with the Fc receptor of B lymphocytes

Antibodies contained in an A/J anti-B10 antiserum, when tested on lymphocytes from B10.A mice, were shown to bind to both B and T cells and to inhibit the binding of Ig complexes to the Fc receptors of B lymphocytes. These antibodies could be removed by absorption with B10.D2 lymphocytes. Similar results were obtained with lymphocytes from two to six (B6A)F1 x A/J backcross mice which had H-2a/a genotype. These data indicate that alloantigens determined by at least one non-H-2 locus are associated with or a part of Fc receptors. These antigens may be similar in structure and function to Ia antigens.