Mechanisms of b cell triggering: studies with t cell-independent antigens

Specific in vitro antimannan-rich antigen of Candida albicans antibody production by sensitized human blood lymphocytes

We have developed a new antigenic system for the induction of specific in vitro antibody response in man. The antigen used was purified from the cell wall of Candida albicans strain A and contained greater than 96% polysaccharide mannan. Peripheral blood mononuclear cells from Candida-sensitized donors produced specific antimannan antibodies during a 7-d culture in the presence of mannan absorbed with methylated bovine serum albumin. Two methods were used to detect antimannan antibody responses. Antimannan antibody-producing cells were identified by radioautography with tritiated mannan. Antibody concentration in culture supernatants was measured by an enzyme-linked immunosorbent assay. In both methods, specific IgM and IgG (but not IgA) antibodies were detected. The antibody production to mannan was specific, since an antigenically unrelated polysaccharide (pneumococcal antigen S III) did not bind to methylated bovine serum albumin-mannan-induced blast cells and did not induce antimannan antibody-containing cells. Furthermore, a pulse with an excess of unlabeled mannan abolished [3H]mannan binding, whereas an excess of unlabeled S III did not. Similarly, no antimannan antibody was obtained in influenza virus-stimulated cultures and mannan-stimulated cultures were not inducing antiinfluenza antibodies. The antimannan antibody production was shown to be a T cell-dependent phenomenon. The T helper effect appeared to be radiosensitive. It was under a genetic restriction as it occurred only in autologous or semi-identical but not in allogeneic situations. This system is relatively simple, reproducible, and well suited for the study of specific secondary in vitro antibody responses to polysaccharide antigens in humans.

Role of B lymphocytes in cell-mediated immunity. I. Requirement for T cells or T-cell products for antigen-induced B-cell activation

Although B lymphocytes can be triggered by B-cell mitogens and by certain other molecules to produce lymphokines, they do not produce lymphokines when stimulated with specific soluble protein antigens. We have investigated whether T-cell help would enable B cells to produce lymphokines when activated by antigens. Addition of small numbers of T cells to B-cell cultures resulted in significant production of a monocyte chemotactic factor. T cells could be replaced by supernates of antigen-stimulated T cells, demonstrating both that the chemotactic factor was B-cell-dervied and that T-cell help was mediated by a soluble factor. Although the T-cell factor was nonantigen specific, B-cell activation required the presence of both antigen and T-cell factor. Thus, it appears that although dependent upon T cells, B lymphocytes may play an important role in amplification of cell-mediated immune responses.

The allogeneic bisection of carrier-specific enhancement of monoclonal B-cell responses

The ability of T cells to enhance the response of syngeneic and allogeneic B cells to thymus-dependent hapten-carrier conjugates was analyzed. This analysis was carried out on individual primary B cells in splenic fragment cultures derived from irradiated reconstituted mice. This system has several advantages: (a) the response of the B cells is entirely dependent on carrier priming of the irradiated recipient; (b) this B-cell response can be quantitated in terms of the number of responding cells; and (c) very small B-cell responses can be readily detected and analyzed. The results indicate that the majority of hapten-specific B cells were stimulated in allogeneic and syngeneic recipients only if these recipients were previously carrier primed. The number of B cells responding in carrier-primed allogeneic recipients was 60-70% of that in syngeneic carrier-primed recipients. The antibody-forming cell clones resulting from B cells stimulated in the allogeneic environment produced small amounts of antibody and antibody solely of the IgM immunoglobulin class, while the larger responses in syngeneic recipients were predominantly IgG1 or IgM plus IgG1. The capacity of collaborative interactions between carrier-primed T cells and primary B cells to yield IgG1 antibody-producing clones was shown to be dependent on syngeny between these cells in the H-2 gene complex. It is concluded that: (a) B cells can be triggered by T-dependent antigens to clone formation through collaboration with T cells which differ at the H-2 complex as long as these T cells recognize the antigen; (b) the immunoglobulin class produced by the progeny of stimulated B cells generally depends on the nature of the stimulatory event rather than the nature of the B cell itself; and (c) stimulation to IgG1 production is dependent on syngeny between the collaborating T and B cells probably within the Ir-1A region. The role of the Ia antigens in the formation of IgG1-producing clones is not yet clear; Ia identity could permit IgG1 production or, conversely, nonidentity of Ia could induce all allogeneic interactions which prohibit IgG1 production.