CD28 and infection: The lower proportion of infected EM cells in our study (~ 25% of total infected CD4+ T cells) can be ascribed to by (i) the separation of TM and EM cells using CD27 in our study and (ii) the stimulation of CD4+ T cells prior to infection with anti-CD3 and anti-CD28 in the Parrish et al. study, which is likely to create a more differentiated memory CD4+ T cell population with more cells expressing CCR5 [40, 58].