Regulation of intracellular glutathione levels and lymphocyte functions by lactate

The plasma concentration of lactate varies strongly under physiological and pathological conditions in the range of 1 to 30 mM. High but physiologically relevant lactate concentrations were previously shown to exert strong immunopotentiating effects and to augment the production of interleukin 2 (IL-2). However, the lactate derivative pyruvate can form covalent complexes with cysteine, suggesting the possibility that lactate may affect indirectly intracellular glutathione (GSH) levels and GSH-dependent lymphocyte functions. The experiments in this report now showed that 20-30 mM lactate had virtually no effect on the viability and intracellular protein content of mitogenically stimulated accessory cell-depleted splenic T cells or unfractionated spleen cells but indeed caused a marked decrease of the intracellular GSH level when compared with control cultures after 40-70 hr of incubation. The DNA synthesis of mitogenically stimulated splenic T cell cultures, i.e., a strongly GSH-dependent function, was also inhibited by lactate. This effect was overcome by high extracellular concentrations of GSH or cysteine. Lactate also inhibited the IL-2 consumption in mitogenically stimulated CD8+ T cell cultures and the activation of cytotoxic T lymphocytes in the late phase of mixed lymphocyte cultures. Additional experiments showed, finally, that lactate augments the incorporation of [14C]aspartate into 18 S and 28 S RNA, while incorporation of [14C]uridine is moderately inhibited, indicating that the de novo synthesis of pyrimidine nucleosides is markedly augmented in cells that are exposed to high extracellular lactate concentrations. Taken together, these studies show that high but physiologically relevant concentrations of lactate exert strong positive and negative effects on distinct aspects of T cell-mediated immune responses.