Decreased proliferation in stimulated T cells cultured in the absence of l-Arg. (A) Freshly isolated human T cells (5 × 10⁵) stimulated with anti-CD3 plus anti-CD28 were cultured in the presence or the absence of l-Arg, and uptake of [³H]-thymidine was measured at 24, 48, 72, and 96 hours. (B) Stimulated T cells (5 × 10⁵) were labeled with 1 μM CFSE and cultured in the presence or the absence of l-Arg; fluorescence was measured at 72 and 96 hours. Unstimulated (NS) control cells were CFSE-labeled T cells cultured in medium containing l-Arg. (C) Stimulated T cells (5 × 10⁵) cultured in the absence of l-Arg for 24 hours were replenished with 2 mM l-lysine, l-glutamine, l-citrulline, or l-Arg, and incorporation of [³H]-thymidine was measured at 72 hours.