HIV-1 infection of human stromal cultures transduced by retroviral vectors. Duplicate cultures of primary, nontransduced human stroma (•), human stroma transduced with the L-RRE-Neo vector (▴), human stroma transduced with the LN vector (▪), and murine S17 stroma (X's) were infected with HIV-1_JR-FL at an MOI = 0.5. On day 8 after infection, human monocytic cells were added to one of each pair of cultures to amplify low levels of HIV-1 present. Samples of the culture supernatants were sampled every 4 days and analyzed for p24 gag protein by ELISA. (A) shows the results from the stromal layers infected with HIV-1, without subsequent addition of monocytic cells. (B) Shows the results from the stromal layers infected with HIV-1 (the duplicates from [A]), with monocytic cells added on day 8.