CD40-DEFICIENT MICE GENERATED BY RECOMBINATION-ACTIVATING GENE-2-DEFICIENT BLASTOCYST COMPLEMENTATION

To study the role of the B-cell antigen CD40 in immune responses, mous e embryonic stem (ES) cells in which both copies of the gene encoding CD40 had been disrupted by homologous recombination were injected in R AG-2 (recombination-activating gene-2)-deficient blastocysts to genera te chimeras in which all mature lymphocytes are derived from the CD40- deficient ES cells. T- and B-cell number and phenotype were normal in the CD40(-/-) chimeras. However, B cells failed to proliferate and und ergo isotype switching in vitro in response to soluble CD40 ligand (sC D40L) with interleukin 4 (IL-4) but responded normally to lipopolysacc haride (LPS) with IL-4. CD40(-/-) chimeras completely failed to mount an antigen-specific antibody response or to develop germinal centers f ollowing immunization with the T cell-dependent (TD) antigen keyhole l impet hemocyanin. In contrast, CD40(-/-) mutant mice responded normall y to the T cell-independent (TI) antigens 2,4,6-trinitrophenyl (TNP)-L PS and TNP-Ficoll. The most noticeable alteration in the serum immunog lobulin levels of young (6-8 weeks old) CD40(-/-) animals was absence of IgE and severe decrease of IgG1 and IgG2a. These results confirm th e essential role of CD40-CD40L interactions in the antibody response t o TD antigens and in isotype switching.