Chronic SIV infection induces numeric and phenotypic alterations in gut-associated NK cells. (A) Frequencies of NKp44⁺ and NKG2A⁺ NK cells among live CD45⁺CD3⁻ mononuclear cells in rectal biopsies of naive and SIV-infected macaques. Horizontal lines indicate medians of 16-18 animals per group. (B) Absolute numbers of NKp44⁺, NKG2A⁺, and CD4⁺ and CD8⁺ T cells per milligram of rectal biopsy tissue in naive and chronically SIV-infected macaques were enumerated using a bead-normalized flow cytometry assay. (C) Expression of intracellular Ki67 and caspase-3 or cell surface annexin V binding was determined on NKp44⁺ and NKG2A⁺ NK cells. (D) Median fluorescence intensities (MFI) of cell surface molecules on NKG2A⁺ and NKp44⁺ NK cells in naive and SIV-infected macaques. Bars represent means ± SEM of 8-14 animals per group. Asterisks above columns reflect naïve versus SIV statistical comparisons, whereas those below the axes reflect NKG2A⁺ versus NKp44⁺ NK cells statistical comparisons in normal animals. 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).