PROPERTIES OF MOUSE CD40 - CELLULAR-DISTRIBUTION OF CD40 AND B-CELL ACTIVATION BY MONOCLONAL ANTI-MOUSE CD40 ANTIBODIES

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-mu and anti-delta mAb, and moderately enhanced by co-stimulation with interleukin-4. Hypercross-linking of CD40 (using biotinylated mAb and avidin) also enhanced the proliferative response to anti-CD40.