Detection of replication-competent virus in resting CD4+ T lymphocytes in various compartments by virus culture assay. Purified CD4+/HLA-DR− T lymphocytes from SIV-infected macaques were cultured in fivefold limiting dilutions with CEMx174 cells to measure the frequency of cells harboring replication-competent virus. The results are expressed in terms of IUPM resting CD4+ T lymphocytes. (A) Resting CD4+ T cells from peripheral blood were examined at weeks 1, 7, and 13 p.i. and at necropsy (weeks 30, 31, 32, and 33 p.i. for animals 98P009, 97P021, 98P004, and 98P008, respectively; same for results shown in panel C). Lines show the plasma viral load at various time points (limit of detection, 100 copies/ml), and the bars show the IUPM values. (B) Lymph nodes were examined at weeks 3 and 4, 13, and 23 p.i. and at necropsy. (C) At necropsy, the frequencies of resting CD4+ T lymphocytes harboring replication-competent virus in spleen, peripheral blood, and mesenteric (Mes), axillary (Ax), inguinal (In), cervical (Ce), and retropharyngeal (Re) lymph nodes (LN) were determined. Axillary and inguinal lymph nodes (draining the arms and legs, respectively) were pooled. Cervical and retropharyngeal lymph nodes (draining different regions of the head and neck) were also pooled. CD4 single-positive (SP) and CD4/CD8 double-positive (DP) cells from the thymus were also examined. All tissues were examined from all four animals except for pooled axillary and inguinal lymph nodes from 98P004 due to insufficient numbers available (Δ).