Acidic pH inhibits non-MHC-restricted killer cell functions

Immunotherapeutic strategies in advanced stages of solid tumors have generally met with Little success. Various mechanisms have been discussed permitting the escape of tumor cells from an effective antitumoral immune response. Solid tumors are known to develop regions with acidic interstitial pH. In a recent study performed in the human system, we were able to demonstrate that non-MHC-restricted cytotoxicity is inhibited by an, acidic microenvironment. To get more insight into the mechanisms leading to this reduced cytotoxic activity, we have now investigated the influence of an acidic extracellular pH (pH(e)) on the killing process in detail. Unstimulated PBMC and LAK cells were used as effector cells. Both populations are able to kill tumor cells in a MHC-independent manner via perforin/granzymes or TNF alpha, whereas only IL-2-activated cells can use the killing pathway via Fas/FasL. We studied the influence of a declining pH(e) on the different killing pathways against TNF alpha-sensitive and -resistant, as well, as Fas-positive and -negative, target cells. Experiments in the absence of extracellular Ca2+ were used to discriminate the Ca2+-dependent perforin-mediated killing. Here we show that the release of perforin/granzyme-containing granules, the secretion of TNF alpha, and also the cytotoxic action of Fas/FasL interaction or of membrane-bound TNF alpha were considerably inhibited by declining pH(e). Furthermore, the secretion of the activating cytokine IFN gamma, as well as the release of the down-regulating cytokines IL-10 and TGF-beta(1), was strictly influenced by surrounding pH. As a pH(e) of 5.8 resulted in a nearly complete loss of cytotoxic effector cell functions without affecting their viability, we investigated the influence of pH(e) on basic cellular functions, e.g., mitochondrial activity and regulation of intracellular pH. We found an increasing inhibition of both functions with declining pH(e). Therefore, an acidic pH(e) obviously impairs fundamental cellular regulation, which finally prevents the billing process. In summary, our data show a strict pH(e) dependence of various killer cell functions. Thus, an acidic microenvironment within solid tumors may contribute to the observed immunosuppression in vivo compromising antitumoral defense and immunotherapy in general, respectively. (C) 2000 Academic Press.