Normal lymphocyte development but delayed humoral immune response in CD81-null mice

CD81 is a cell surface molecule expressed on many cell types and associated with the CD19/CD21/Leu13 signal-transducing complex on B cells. A recent report implies that CD81 expression on thymic stromal cells is important in the maturation of thymocytes from CD4-CD8- to CD4+CD8+. However, we have produced CD81-null mice by gene targeting, and find that they undergo normal development of thymocytes and express normal numbers of T cells. B cells are also found in normal numbers in the spleen, blood, and peritoneal cavity of CD81-null mice, but they express a lower level of CD19 compared to heterozygous littermates. Finally, early antibody responses to the protein antigen ovalbumin are weaker in CD81-null mice compared to their heterozygous littermates. This is consistent with the proposed role of the CD19/CD21/CD81-signaling complex in lowering the threshold for B cell responses. These results show that CD81 is not required for maturation of T cells, but is important for optimal expression of CD19 on B cells and optimal stimulation of antibody production.