Induction of CD5 antigen on human CD5- B cells by stimulation with Staphylococcus aureus Cowan strain I.

We have examined whether the CD5 phenotype could be induced on human B cell surfaces by the polyclonal B cell stimulator, Staphylococcus aureus Cowan strain I (SAC). Fresh tonsillar B cells were prepared by Percoll density gradient from E- cells. The proportion of CD5+ B cells in the 50/60% and 60/70% interface high-density fractions varied between 1.2 and 10.2% depending on the tonsil preparations when they were placed on the in vitro culture 12-60 h prior to flow cytometric analysis. The expression of CD5 antigen obviously increased in the presence of SAC (1:10(5) v/v). The percentage of CD5+ B cells varied from tonsil to tonsil, from 25.1 to 65.9% in a series of experiments. The CD5+ B cells were found both among CD23+CD25+CD71+ and CD23-CD25-CD71- B cells. The level of CD5 expression was related to the cell size enlargement. The addition of anti-CD5 antibody in the culture blocked the CD5 induction by SAC without interfering with the expression of other activation markers. A time-course study showed that CD5 antigen appeared to be induced on the cell surface during the G0 to G1 phase transition in the cell cycle. When CD5+ and CD5- B cells were separated by magnetic isolation, the CD5- B cells showed DNA synthesis to the stimulation by SAC and expressed CD5 antigen on their cell surface. These results suggest that human CD5- B cells can express the CD5 phenotype by stimulation with the polyclonal B cell stimulator, SAC.