Fig. (1) Proposed mechanisms of exosome secretion and HIV-1 interaction. Exosomes can be secreted by means of a constitutive or regulated pathway. Exosomes can bind to HIV-1 via TIM-4, CD9, and/or CD81. This binding can facilitate exosome-mediated entry into CD4+ or CD4- cells. HIV-1 binds to exosomes released from the ‘donor’ cell and is shuttled to the ‘recipient’ cell.