IDO1 is up-regulated in the gut mucosa of SIV-infected macaques and is associated with a loss of NKp44⁺ NK cells. (A) Quantitative RT-PCR analysis of cytokine transcripts was performed on rectal biopsies collected from naive and SIV-infected macaques. Mann-Whitney U tests were used for naïve versus SIV comparisons (*P < .05, **P < .01, ***P < .001). (B) Relative expression of IDO1 in rectosigmoid biopsies was correlated with plasma viral loads (top left) and absolute numbers of NKp44⁺ and NKG2A⁺ NK and CD4⁺ and CD8⁺ T cells quantified in biopsy specimens of the same animals as shown in Figure 4B. P values < .05 are considered significant.