IL-10 expression correlates with HIV viremia and response to IL-10Rα blockade. The level of IL-10 protein in plasma and mRNA expression in bulk PBMCs were examined in 35 untreated HIV⁺ individuals. IL-10 protein concentration (A) and IL-10 mRNA expression values (B) correlated significantly with plasma viral load measurements (R = 0.6017 and R = 0.6754, respectively). Next, the impact of IL-10Rα blockade on HIV p24–specific CD4 T cells was correlated with IL-10 protein or mRNA results. The IL-10 proliferation index, defined as the ratio of the fraction of proliferating (%CD3⁺CD4⁺CFSE^low) cells in the presence of the IL-10Rα blocking antibody versus isotype control, indicated a significant correlation between an individual's responsiveness to IL-10 pathway blockade and both plasma IL-10 protein (R = 0.5324, P = .034) (C) or PBMC IL-10 mRNA (R = 0.6951, P < .001). (D) Correlation coefficients suggest that IL-10 mRNA expression in PBMCs is a better predictor of response to IL-10 blockade than plasma IL-10 protein concentration. All statistical analyses used the Spearman rank sum test.