Lactic acid suppresses proliferation and partially induces cell death in CTLs. (A) ³H-thymidine uptake in CTLs during short-term (6.5 hours) incubation in 5 to 20 mM lactic acid or sodium lactate. Data are means ± SEM (n = 3). (B) CFSE-labeled CTLs were stimulated for 24 hours with PHA/ionomycin, and cell division was determined by FACS on day 6 in the absence or presence of 5, 10, and 20 mM lactic acid. Gated viable cells from a representative experiment are shown (n = 3). (C) CTLs were treated with 10 or 20 mM lactic acid or HCl (pH 6.5) for 6.5 hours or 24 hours, and viable cells were determined by flow cytometry as annexinV-FITC/PI-negative cells. Data are means ± SEM; n = 8; *P < .02; **P < .003.