Finally, donor differences in the expression or phosphorylation/activity of cellular SAMHD1, which restricts HIV infection in resting CD4+ T-cells [41–43], may also impact the relative sensitivity or resistance of resting CD4+ T-cells to infection and the subsequent establishment of latency, and therefore the need for an additional stimulus such as exogenous chemokine such as CCL19. Here, CD4 is linked to infection.