Arginine metabolism may be one of the mechanisms that CD14⁺HLA-DR^low/− monocytes suppress T-cell functions. (A) Plasma concentration of arginase I is measured by ELISA (left panel: control, n = 7; lymphoma, n = 16). Increased lymphoma plasma concentration of arginase I is correlated with increased percentage of CD14⁺HLA-DR^low/− circulating monocytes (right panel). (B) Real-time PCR analysis of arginase I transcript levels in sorted CD14⁺ monocytes. CD14⁺ monocytes were stained for HLA-DR and sorted into high-expressing and low-expressing HLA-DR populations. Quantitative reverse-transcribed PCR was performed and normalized to GAPDH (reference gene). Results are reported as follows: − indicates baseline/nondetectable crossing point (Cp); +, positive expression of 2- to 32-fold induction (1-5 Cps lower than baseline Cp); and ++, expression higher than 32-fold increase (> 5 Cp difference) over baseline. (C) Lymphoma PBMC proliferation can be increased with the addition of L-arginine or removal of monocytes (one representative sample, bar represents mean ± SD of 8 replicates). Statistically significant P values are shown between the groups.