Inhibitory Effects of Lactic Acid on Human Antigen-Specific CD8+ T-Cells.

A characteristic feature of inflammatory lesions or tumor sites is local acidosis, which is attributed to the local increase in lactic acid production. We studied the effect of such an acidic environment on the immune functions of antigen-specific CD8+ T-cells by incubating the cells in the presence of various concentrations of lactic acid for up to 48h. CD8+ T-cells were isolated from healthy donors and expanded by weekly stimulation with autologous dendritic cells pulsed with a mutated HLA-A2-binding Melan-A (ELAGIGILTV) peptide. The obtained T cell population consisted of at least 90% CD8+ and about 60% Melan-A specific T cells, as determined by Melan-A multimer staining. Incubation of CD8+ T cells with up to 20mM lactic acid for 24h did not cause T-cell apoptosis or cell death as determined by combined annexin/propidium iodide staining. However, the proliferative capacity of CD8+ T cells, as determined by 3H-thymidine uptake, was strongly inhibited. Similar results were obtained when we determined cytokine production and cytotoxic activity of the cells after a 24h culture period in 5-20 mM lactic acid. Production of both, IL-2 and IFN-gamma was strongly diminished in comparison to untreated cells, as determined by intracellular staining after stimulation with PMA/ionomycin for 5h in the presence of monensin. Analysis of the antigen-specific cytolytic capacity revealed that CD8+ T cells pre-cultured with lactic acid were less effective in killing antigen-loaded T2 target cells as compared to untreated CD8+ T cells. In parallel, the intracellular contents of the cytotoxic effector molecules granzyme-B and perforin was diminished. Re-adjusting the pH of the medium to a physiological value of pH7.4 could partially revert the effect of lactic acid. Treatment of CD8+ T cells with sodium lactate instead of lactic acid had no inhibitory effect. We conclude, that lactic acid is an important modulator of CD8+ T-cell function and may contribute, together with other factors, to immune escape mechanisms in the tumor environment.