CD40, but not CD154, expression on B cells is necessary for optimal primary B cell responses.

CD40 is an important costimulatory molecule for B cells as well as dendritic cells, monocytes, and other APCs. The ligand for CD40, CD154, is expressed on activated T cells, NK cells, mast cells, basophils, and even activated B cells. Although both CD40(-/-) and CD154(-/-) mice have impaired ability to isotype switch, form germinal centers, make memory B cells, and produce Ab, it is not entirely clear whether these defects are intrinsic to B cells, to other APCs, or to T cells. Using bone marrow chimeric mice, we investigated whether CD40 or CD154 must be expressed on B cells for optimal B cell responses in vivo. We demonstrate that CD40 expression on B cells is required for the generation of germinal centers, isotype switching, and sustained Ab production, even when other APCs express CD40. In contrast, the expression of CD154 on B cells is not required for the generation of germinal centers, isotype switching, or sustained Ab production. In fact, B cell responses are completely normal when CD154 expression is limited exclusively to Ag-specific T cells. These results suggest that the interaction of CD154 expressed by activated CD4 T cells with CD40 expressed by B cells is the primary pathway necessary to achieve B cell activation and differentiation and that CD154 expression on B cells does not noticeably facilitate B cell activation and differentiation.

Specific immunosuppression by immunotoxins containing daunomycin.

Daunomycin, when conjugated with a targeting antigen by an acid-sensitive spacer, remains inactive at the intravascular pH of 7 but becomes active after cleavage within the acidic lysosomal environment of the target cell. This observation made it possible to construct cytocidal compounds that caused antigen-specific suppression of murine lymphocyte function. When daunomycin was coupled to the hapten conjugate of ovalbumin by an acid-sensitive cis-aconityl group, it caused hapten-specific impairment of immunocompetence in murine B lymphocytes in vitro and in vivo. Furthermore, the response by T lymphocytes to concanavalin A in vitro was selectively eliminated by a conjugate between daunomycin plus the acid-sensitive spacer and a monoclonal antibody specific for T cells.

LPS greatly enhances the antibody response to hapten-polysaccharide conjugates, but not to hapten-protein conjugates.

In confirmation of earlier findings, we observed that an injection of bacterial lipopolysaccharide (LPS) into mice caused a considerable increase in the serum concentrations of IgM and IgG (total Ig rose three- to four-fold in 7 days), and a corresponding increase in the concentrations of "natural" anti-(3-iodo-4-hydroxy-5-nitrophenyl) acetyl (NIP) and anti-trinitrophenol (TNP) antibodies. Our main purpose was to determine what effect LPS had on antigen-dependent responses. Hapten conjugates of a polysaccharide and of proteins were used as antigens. Hapten-protein conjugates induced a strong anti-hapten antibody response (up to 1 mg/ml of anti-hapten antibodies on day 7). Hapten-polysaccharide conjugates induced only a meagre increase in anti-hapten antibodies from the pre-immunization level (maximal concentration 65 micrograms/ml on day 7). LPS, when injected with the antigen, greatly enhanced the antibody response to the hapten-polysaccharide conjugates (up to 2.6 mg/ml of anti-hapten antibodies on day 7). It had little effect on antibody responses to hapten-protein conjugates. The combination treatment had the same effect on immunoglobulin concentrations as LPS alone.

The specificity of T lymphocyte responses to chemically defined antigens.

A system is described that allows the definition of T cell receptor specificity with some precision. It involves immunization of guinea pigs with hapten coupled to mycobacteria. The T cells of such animals respond to many but not all carriers modified by that hapten. Such T cells recognize neither hapten nor carrier alone, but rather determinants involving both the hapten and the carrier. No evidence for hapten-specific T cells was found. A model of the antigen binding site of the T cell receptor emerged from these experiments. According to this model, the T cell receptor consists of a single site of relatively large extent involving multiple subsites which are of low and roughly equal affinity. Thus, the haptenic group is not immunodominant for T cells as it is for B cells and for anti-hapten antibody. This suggests that the antigen binding receptor on T cells differs in some fundamental way from that on B cells. It is proposed that antigen recognition by T cells is mediated by an immunoglobulin heavy chain variable region that is not paired with an immunoglobulin light chain variable region.

Specifically cytotoxic human and mouse lymphoid cells induced with antibody or antigen-antibody complexes.

Human or mouse lymphoid cells could be "armed" with anti-NIP antibodies to become cytotoxic to NNP-conjugated fowl erythrocytes (NNP and NIP are closely related haptens). The arming factor was neutralized by a sufficient concentration of NIP-BSA (twice the concentration causing maximal precipitation) but low concentrations (e.g., 7% of the maximal precipitation concentration) increased the arming capacity.