Ammonia inhibits antitumor activity of NK cells by decreasing mature perforin

Immunotherapy revolutionized cancer treatment in the last decade. Natural killer (NK) cells are one of the key host immunity components against malignant cells. Thus, they are currently extensively investigated in the field of immunotherapy of cancer. Different approaches have been developed to improve the antitumor activity of NK cells. Nonetheless, tumor microenvironment remains an obstacle to effective NK cell-based therapies. Here, we demonstrated that a cancer-conditioned medium suppresses the anti-tumor activity of NK cells. Further, we found that ammonia, a by-product of cancer cell metabolism, accumulates in the cancer-conditioned medium and tumor microenvironment. We identified that ammonia impairs the cytotoxicity of NK cells as well as the effectiveness of antibody-based and chimeric antigen receptor (CAR)-NK-based therapies in vitro. Inhibited activity of NK cells was caused by decreased levels of perforin. This effect was dependent on the lysosomotropic features of ammonia and its ability to increase pH in acidic compartments. In consequence, upon contact with ammonia the mature form of perforin was decreased in NK cells leading to their dysfunction. Our findings demonstrate that in addition to its previously described role of promoting tumor growth as a nitrogen source for tumor biomass ammonia could promote tumor escape as an NK cells immune checkpoint. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=66 SRC="FIGDIR/small/567708v1_ufig1.gif" ALT="Figure 1"> View larger version (20K): org.highwire.dtl.DTLVardef@19db6a5org.highwire.dtl.DTLVardef@1424903org.highwire.dtl.DTLVardef@c795d7org.highwire.dtl.DTLVardef@13e7a8e_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LICancer-conditioned medium suppresses the antitumor activity of NK cells C_LIO_LIAmmonia accumulates in conditioned medium and in the tumor microenvironment C_LIO_LIImpaired cytotoxicity of NK cells is caused by ammonia that decreases perforin levels C_LIO_LIAmmonia causes NK cell dysfunction C_LI