RP105 is associated with MD-1 and transmits an activation signal in human B cells.

RP105 was originally discovered as a mouse B-cell surface molecule that transmits an activation signal. The signal leads to resistance against irradiation-induced apoptosis and massive B-cell proliferation. Recently, we found that mouse RP105 is associated with another molecule, MD-1. We have isolated here the human MD-1 cDNA. We show that human MD-1 is also associated with human RP105 and has an important role in cell surface expression of RP105. We also describe a monoclonal antibody (MoAb) that recognizes human RP105. Expression of RP105 is restricted to CD19(+) B cells. Histological studies showed that RP105 is expressed mainly on mature B cells in mantle zones. Germinal center cells are either dull or negative. RP105 is thus a novel human B-cell marker that is preferentially expressed on mature B cells. Moreover, the anti-RP105 MoAb activates B cells, leading to increases in cell size, expression of a costimulatory molecule CD80, and DNA synthesis. The B-cell activation pathway using RP105 is conserved in humans.