The Tetraspanin CD82 Defines Erythroid Committment, Regulates Stem- and Progenitor Cell Survival and Mediates Adhesion.

CD82 belongs to the tetraspanin superfamily, members of which regulate multiple aspects of cell biology such as T-cell activation, proliferation, differentiation, and adhesion.We have previously shown expression of CD82 on hematopoietic progenitor cells. Here we were seeking to shed light on the functional role of CD82 in hematopoiesis. First, we found that CD82 expression is strongly associated with an erythroid committment as exclusively the CD82bright fraction but not the CD82medium or CD82dim fraction of CD34+ cells gave rise to erythroid colony forming units (CFU). In order to manipulate CD82 activity, we used as surrogate ligand, a previously described, CD82-activating monoclonal antibody (moAb), clone 50F11, to activate CD82 on hematopoietic precursors. We found that 50F11, but not another CD82 specific antibody clone, BL-2, specifically induced tyrosine phosphorylation of CD34+ cells. In Dexter-type long-term cultures (D-LTC) 50F11, and not IgG1 isotype control moAbs significantly inhibited myelopoiesis and the number of CD34+ clonogenic progenitors. Moreover, in long term culture initiating cell (LTC-IC) assays, 50F11as compared to isotype control antibodies substantially inhibited, but not entirely abolished, the number of 5 week-LTC-IC′s, indicating that CD82 activation inhibits progenitor - and stem cell proliferation or self renewal. Finally, plastic immobilized 50F11-antibodies caused a time-, and concentration dependent induction of adhesion of CD34+ cells, which was associated with the formation of F-actin and development of multipolar extensions. Finally, CD82 ligation by 50F11 caused a statistically significant down-regulation of the integrin CD49d (p=0,036) and CD62L (p=0,010).

Together, it is shown that CD34+/CD82high cells characterize an erythroid committment implying a role for this tetraspanin in erythroid hematopoiesis. Activation of CD82 induces adhesion and negatively regulates proliferation of adult stem- and progenitor cells. This implicates a so far unknown role for CD82 in the regulation of early hematopoiesis.