BackgroundHIV-1 strains R5 and X4 can infect CD4 memory T cellsin vivo. Anti-CD3/28 stimulation induces β-chemokines and CCR5 down-regulation and renders these cells resistant to R5 HIV-1 infection. Here we describe an additional cellular mechanism that blocks productive R5 HIV-1 infection of CD4 memory T cells.MethodsBlood-derived CD4 memory T cells and CD4 T-cell clones were infected with primary R5 and X4 HIV-1 strains. Virus replication was correlated with CCR5 expression and β-chemokine production. Virus entry and infectivity were measured by PCR for early and late products of HIV reverse transcription respectively.ResultsR5 strains were up to 1000-fold less infectious than X4 viruses for CD4 memory T cells. This resistance was independent of CCR5 levels and of the Δ-32 mutation and theCCR2-V64I/CCR5-59653T linked mutations. Blocking endogenous β-chemokines relieved minimally this restriction. At the single cell level, CD4 memory cells were either permissive or non-permissive for R5 HIV-1 infection. R5 HIV titre was up to 10-fold lower than X4 virus titre even in a permissive clone. However, R5 viruses replicated as efficiently as X4 viruses in the permissive clone when neutralizing anti-β chemokine antibodies were added. Non-permissive cells blocked a post-entry step of the virus life-cycle and expressed early but not late HIV transcripts. Neutralizing anti-β chemokine antibodies promoted R5 virus replication marginally in the non-permissive clone.ConclusionSome blood memory CD4 T cells retard R5 HIV-1 replication via endogenous β-chemokines whereas others block productive R5 HIV-1 infection by an additional mechanism that interferes with a post-entry step of the virus life cycle. These natural barriers might contribute to lower pathogenicity of R5 HIV-1 strains for CD4 memory T cells than X4 viruses that emerge late in disease.