CD4 and HIV-1 infection: Platelet supernatants and recombinant CXCL4 exerted antiviral activity against HIV-1 without interfering with CD4 and coreceptor expression (Additional file 7: Figure S6) and neutralization of CXCL4 largely prevented HIV-1 inhibition by platelet supernatants, demonstrating that inhibition of HIV-1 infection by activated platelets is mainly due to release of CXCL4.