A CCR5⁺ memory subset within HIV-1-infected primary resting CD4⁺ T cells is permissive for replication-competent, latently infected viruses in vitro

Objective: Resting CD4⁺ T cells are major reservoirs of latent HIV-1 infection, and may be formed during the early phase of the infection. Although CCR5-tropic (R5) HIV-1 is highly transmissible during the early phase, newly infected individuals have usually been exposed to a mixture of R5 and CXCR4-tropic (X4) viruses, and X4 viral DNA is also detectable in the host. Our aim was to identify which subsets of resting CD4⁺ T cells contribute to forming the latent reservoir in the presence of both X4 and R5 viruses. Results: Primary resting CD4⁺ naïve T (T_N) cells, CCR5^- memory T (T_M) cells, and CCR5⁺ T_M cells isolated by flow cytometry were infected simultaneously with X4 and R5 HIV-1, which harbored different reporter genes, and were cultured in the resting condition. Flow cytometry at 3 days post-infection demonstrated that X4 HIV-1⁺ cells were present in all three subsets of cells, whereas R5 HIV-1⁺ cells were present preferentially in CCR5⁺ T_M cells, but not in T_N cells. Following CD3/CD28-mediated activation at 3 days post-infection, numbers of R5 HIV-1⁺ cells and X4 HIV-1⁺ cells increased significantly only in the CCR5⁺ T_M subset, suggesting that it provides a major reservoir of replication-competent, latently infected viruses.