Reversal of tumor acidosis by systemic buffering reactivates NK cells to express IFN-γ and induces NK cell-dependent lymphoma control without other immunotherapies

Like other immune cells, natural killer (NK) cells show impaired effector functions in the microenvironment of tumors, but little is known on the underlying mechanisms. Since lactate acidosis, a hallmark of malignant tissue, was shown to contribute to suppression of effective antitumor immune responses, we investigated the impact of tissue pH and lactate concentration on NK-cell functions in an aggressive model of endogenously arising B-cell lymphoma. The progressive loss of IFN-gamma production by NK cells observed during development of this disease could be ascribed to decreased pH values and lactate accumulation in the microenvironment of growing tumors. Interestingly, IFN-gamma expression by lymphoma-derived NK cells could be restored by transfer of these cells into a normal micromilieu. Likewise, systemic alkalization by oral delivery of bicarbonate to lymphoma-developing mice was capable of enhancing IFN-gamma expression in NK cells and increasing the NK-cell numbers in the lymphoid organs where tumors were growing. By contrast, NK-cell cytotoxicity was dampened in vivo by tumor-dependent mechanisms that seemed to be different from lactate acidosis and could not be restored in a normal milieu. Most importantly, alkalization and the concomitant IFN-gamma upregulation in NK cells were sufficient to significantly delay tumor growth without any other immunotherapy. This effect was strictly dependent on NK cells.