Thrombospondin enhances erythroid colony formation in vitro

The thrombospondin (TSP) receptor CD36 is expressed on erythroid progenitors, but only at the erythroid colony-forming unit (CFU-E) stage. The roles of TSP and CD36 in erythroid development are not well understood. In order to investigate these roles, human bone marrow mononuclear cells (BMMC) were co-cultured in vitro with Chinese hamster ovary (CHO) cells expressing surface CD36. CFU-E colony formation was significantly decreased in the presence of CHO-CD36 cells compared to control CHO cells, suggesting that competition for TSP (or some other CD36 ligand) impairs CFU-E colony formation. In order to confirm the role of CD36 in this effect, BMMC were cultured with an antibody (FA6-152) which has been reported to activate CD36 signal transduction. FA6-152 significantly increased CFU-E colony formation. In order to confirm that this effect involved TSP and not some other CD36 ligand, a dose-response curve with exogenous TSP was performed. In serum-containing cultures, exogenous TSP did not alter CFU-E colony formation. However, in serum-free, TSP-deficient, cultures, CFU-E colony formation was enhanced by exogenous TSP. At low concentrations of exogenous TSP, the effects of TSP and FA6-152 on colony formation were additive. These findings indicate a contribution of TSP and CD36 to late erythroid progenitor development, and provide evidence that these effects are mediated through CD36.