Levels of blood and intestinal CD4⁺IL-21⁺ T cells in chronic intravenous SIV infection of RMs. The absolute values (A-B) and percentages (C-D) of circulating (A,C) and intestinal (B,D) CD4⁺IL-21⁺ T cells were compared between intravenously SIV-infected (○) and uninfected (●) RMs. (D) In RBs, the absolute values of IL-21–producing CD4⁺ T cells are expressed as fraction of total CD3⁺ T cells. (E) Levels of IL-21 mRNA in the colon and jejunum of 4 RMs before (baseline) and at 10-12 weeks after SIV infection. IL-21 mRNA values are expressed relative to the values for GAPDH, and the ratios obtained with the baseline values were used to calculate the fold increases. (F) IL-21 protein levels were assessed in the colon by immunohistochemistry. IL-21⁺ cells within the lamina propria (top panel) as well as follicular aggregates (bottom panel) are shown in 2 (of 3) representative SIV-uninfected and 2 (of 5) representative SIV-infected RMs. Statistical analyses were performed to compare the levels of CD4⁺IL-21⁺ T cells (A-D) and of IL-21 mRNA (E) between SIV-infected and uninfected animals.