Properties of mouse CD40: Cellular distribution of CD40 and B cell activation by monoclonal anti‐mouse CD40 antibodies

Abstract

We describe here the derivation of a rat monoclonal antibody (mAb) against mouse CD40 (designated 3/23), which stains 45–50% of spleen cells of adult mice, approximately 90% of which are B cells. Interestingly, some 5–10% of both CD4⁺ and CD8⁺ T cells in the spleens of (some, but not all) adult, unimmunized mice are also CD40⁺, whereas CD40⁺ cells were not detectable in the thymus, even following collagenase digestion. Some 35–40% of lymphoid cells in the bone marrow of adult mice are CD40⁺ and virtually all of these are B220⁺, and hence of the B cell lineage: triple-color flow cytometry showed that CD40 is expressed at low levels on some 30% of pre-B cells, at intermediate levels on 80% of immature B cells and on essentially all mature B cells in the bone marrow. These results, therefore, suggest that in the mouse CD40 is expressed relatively late during the process of B cell differentiation. The mAb induced marked up-regulation of major histocompatibility complex class II molecules, CD23 and B7.2 antigens on mature B cells. It also stimulated modest levels of DNA synthesis in mature B cells by itself: this was markedly enhanced by suboptimal concentrations of mitogenic (but not non-mitogenic) anti-μ and anti-δ mAb, and moderately enhanced by co-stimulation with interleukin-4. Hyper-cross-linking of CD40 (using biotinylated mAb and avidin) also enhanced the proliferative response to anti-CD40.