Mucosal NK cells are not infected by SIV in vivo. (A) Cell surface expression of CD4 and CCR5 on mucosal NKG2A⁺ and NKp44⁺ NK cells. Bars represent means ± SEM of 8 animals per group. Mann-Whitney U tests were used for naïve versus SIV comparisons and Wilcoxon matched pairs tests were used to compare NKG2A⁺ and NKp44⁺ NK cells (*P < .05, **P < .01, ***P < .001). (B) RT-PCR analysis was used to quantify SIV RNA in NKG2A⁺ and NKp44⁺ NK cells and CD4⁺ T cells sorted from rectal mucosa of normal and SIV-infected macaques. The lower limit of detection of the assay was 3 SIV RNA copies/1000 cells and is indicated by the dashed bar.