Cell surface antigen CD5 is a marker for activated human B cells

A minor subset of B cells which in vivo express the surface antigen CD5, has attracted much attention because of its involvement in autoimmune responses. On the basis of observations showing self‐renewal capacity of such cells in mice and also the absence of a substantial change of CD5 phenotype during B cell activation in vitro, the CD5+ B cells are now generally considered to represent a separate cell lineage. In the present study, CD5‐ B cells were isolated by cell sorter and then stimulated in vitro with mutagenized EL4 thymoma cells in the presence of T cell supernatant. About 70% of the B cells were CD5+ after 3 days. Thus, the CD5 antigen behaves as a B cell activation marker. In our system we found that the frequency of rheumatoid factor‐producing B cells was on average three times higher in CD5+ than in CD5‐ B cells isolated ex vivo from human peripheral blood. Most likely this reflects frequent activation of such autoreactive B cells in vivo.