Intermediate hematopoietic progenitors are increasingly permissive to HIV-1 infection in vitro with progressive development. (A) CD34+ cells were sorted to high purity by MACS from mononuclear cells purified from human fetal liver. Flow cytometry shows that of the live sorted cells, 98% express CD34 (left). From these, CD38+ cells were sorted to isolate each subpopulation of intermediate hematopoietic progenitors (center). The CMP, MEP, and GMP are all CD34+CD38+; the CMP expresses neither CD45RA nor CD110. Only the MEP expresses CD110; the GMP is CD45RA+CD110− (left). (B) Purity of the sorted CMP is indicated by CD110 expression (left) and CD45RA expression (center). Cells were infected with HIV_89.6 and assayed by qRT-PCR 48 hours later for full-length viral DNA (right) in the presence or absence of AZT. Each mark indicates a single donor; the data represent the mean ± SEM. (C) Purity of sorted GMP indicated by CD110 expression (left) and CD45RA expression (center). (D) Purity of the sorted MEP indicated by CD110 expression (left) and CD45RA expression (center). FSC, forward scatter; SSC, side scatter.