Expression, regulation, and function of B cell-expressed CD154 in germinalcenters

Activated B cells and T cells express CD154/CD40 ligand in vitro. The in vi vo expression and function of B cell CD154 remain unclear and therefore wer e examined, Tonsillar B and T cells expressed CD154 at a similar density bo th in situ and immediately ex vivo, whereas a significantly higher percenta ge of the former expressed CD154. CD154-expressing B cells were most freque nt in the CD38(positive)IgD(+) pre-germinal center (GC)/GC founder, CD38(po sitive) GC and CD38(-)IgD(-) memory populations, and were also found in the CD38(-)IgD(+) naive and CD38(bright)IgD(+) plasmablast subsets, but not in the CD38(bright)IgD(-) plasma cell subset. B cell expression of CD154 was induced by engaging surface fg or CD40 by signals that predominantly involv ed activation of AP-1/NF-AT and NF-kappa B, respectively. The functional im portance of CD154-mediated homotypic B cell interactions in vivo was indica ted by the Ending that mAb to CD154 inhibited differentiation of CD38(posit ive)IgD(-) GC B cells to CD38-IgD- memory cells. In addition, mAb to CD154 inhibited proliferation induced by engaging sig or CD40, indicating the rol e of up-regulation of this molecule in facilitating B cell responsiveness. Of note, CD154 itself not only functioned as a ligand but also as a direct signaling molecule as anti-CD154-conjugated Sepharose beads costimulated B cell responses induced by engaging surface Pg. These results indicate that CD154 is expressed by human B cells in vivo and plays an important role in mediating B cell responses.