CD4 and HIV infectious disease: We recently demonstrated that although mucosal stromal fibroblasts are not susceptible to productive infection by HIV, they potently enhance HIV infection of CD4+ T cells.2 The mechanisms behind this enhancement are not fully elucidated, but seem to be mediated through a combination of trans-infection of CD4+ T cells by fibroblasts (whereby fibroblasts can capture HIV virions on their surface without becoming productively infected and transfer the virus to T cells2) and the fibroblasts’ ability to condition CD4+ T cells to become more permissive to infection.