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Under some circumstances the HIV virus may infect cells that do not express receptors essential to HIV-entry. We hypothesized that platelet- and megakaryocyte-derived microparticles (MP) could play a role in such infections. MP are circular membrane fragments shed from the surface of eukaryotic cells. After adhesion to target cells, MP may transfer membrane-associated proteins to these cells. We found that peripheral blood platelet- (PMP) and megakaryocyte-derived MP (MegaMP) that highly express CXCR4 may transfer this receptor from the surface of platelets or megakaryocytes to the surface of CXCR4-null cells.Since this mechanism could potentially allow CD4+/CXCR4-null cells to become infected by T-tropic HIV, we incubated several human CD4+/CXCR4-null cells such as normal erythroblasts, glioblastomas U87, MAGI and hematopoietic cell lines UT-7, HEL and TF-1 with PMP or MegaMP. We found that these cells became CXCR4+. We next exposed these cells to X4-HIV (IIIB) and evaluated their susceptibility to infection by PCR, ELISA, and morphological analysis.We observed in all instances that after CD4+/CXCR4-null cell lines ‘acquired’ CXCR4 from PMP or MegaMP, they could became infected by X4 HIV.We postulate that both PMP and MegaMP may play a novel and important role in spreading HIV-1 infection by transferring the CXCR4 co-receptor to CD4+/CXCR4-null cells.