Antibodies to Murine CD40 Protect Normal and Malignant B Cells from Induced Growth Arrest

We have previously described the production of polyclonal anti-murine CD40 antibodies that specifically bind recombinant murine CD40 expressed on L cells and induce vigorous proliferation of normal murine B lymphocytes. The current study utilizes these antibodies to explore the distribution and function of CD40 in murine B cell development. Murine CD40 is expressed at high levels by normal splenic B cells and all Ig-positive B cell lymphomas tested to date. It is not expressed by the 70Z/3 pre-B cell line, BaF3 pre-B cell line, or by numerous T cell and myeloid cell lines. 70Z/3 pre-B cells can be induced to express CD40 by LPS stimulation of the cells. Stimulation of purified splenic B cells with anti-CD40 antibodies causes upregulation of class II MHC antigens, CD23, and ICAM-1 and results in extensive aggregation of the cells. Antibodies to murine CD40 are extremely effective at rescuing malignant and normal B cells from induced growth arrest. Anti-CD40 antibodies protect WEHI-231 and CH31 B lymphoma cells from growth arrest induced by soluble anti-IgM antibodies, TGFβ, or a combination of both stimulants. Similarly, anti-IgM preactivated normal splenic B cells which normally die rapidly from growth arrest after 1 or 2 days culture produce a vigorous proliferative response to subsequent stimulation with anti-CD40 antibodies plus IL-4. Interestingly, anti-CD40 antibodies provide little to no protection against B lymphoma growth arrest induced by immobilized anti-IgM antibodies. These data confirm and extend functional properties assigned previously to human CD40 and identify numerous defined murine model systems to explore the molecular basis of CD40-mediated protection from induced B cell growth arrest.