IntroductionC‐kit/SCF signaling plays a key role in regulating NK cell homeostasis, maturation, proliferation, and cytotoxicity. C‐kit‐deficiency in NK cells results in significant reduction of their number, suggesting an imperative role for c‐kit signaling in NK cell biology. We have recently showed that human NK cells express not only c‐kit‐receptor, but also both membrane‐bound and soluble forms of c‐kit ligand—Stem cell factor. The goal of this study was to characterize the c‐kit/SCF autocrine loop in peripheral blood NK cells obtained from patients with cancer.MethodsUsing Smart Flare and qRT‐PCR, we have characterized expression of c‐kit and two forms of SCF in patients’ NK cells and correlated these results with the expression of c‐myc and STAT3.ResultsOur results demonstrated that the expression of proto‐oncogenes c‐myc and c‐kit was significantly decreased in NK cells from all cancer patients. Expression of membrane‐bound SCF in NK cells correlated with the presence of remote metastases.ConclusionsWe suggest that the abnormal signaling and expression of c‐kit/SCF, c‐myc, and STAT3 in NK cells is responsible for the defect in their cytolytic activity in cancer and these defects at the gene expression level may be the cause rather than the result of tumor progression.

Natural killer (NK) cells have received a lot of attention in recent years for the roles they play in immunity and particularly in antitumor immune responses. Although defects in NK cell functions are recognized as important mechanisms for immune evasion of malignant cells, molecular pathways regulating NK cell dysfunction and exhaustion in cancer are largely unknown. Here we tested whether the c-myc proto-oncogene, known to promote cell proliferation, growth, differentiation, and apoptosis by regulating the expression of numerous target genes, may be involved in the mechanism of NK cell abnormalities in patients with lung and gastric cancer. Analysis of c-myc mRNA and protein expression in peripheral blood NK cells, mitogen-activated protein kinase (MAPK) activity, cell cycle, and cell longevity revealed a significantly decreased expression of c-myc mRNA and protein and mitotic arrest of NK cells in different phases of cell cycle. In addition, a significant decrease of NK cell death was also detected. These data allow the suggestion that defects of NK cell-mediated tumor surveillance may be associated with disturbed c-myc expression in NK cells in cancer patients. A better understanding of the mechanisms of NK cell dysfunction in cancer will help in the NK cell-mediated therapeutic eradication of primary and metastatic cancer cells and prolong patient survival.

The role of deregulated expression of oncogenes and tumor-suppressor genes in tumor development has been intensively investigated for decades. However, expression of oncogenes and their potential role in immune cell defects during carcinogenesis and tumor progression have not been thoroughly assessed. The defects in proto-oncogenes have been well documented and evaluated mostly in tumor cells, despite the fact that proto-oncogenes are expressed in all cells, including cells of the immune system. In this review, key studies from immune-mediated diseases that may be associated with oncogene signaling pathways are refocused to provide groundwork for beginning to understand the effects of oncogenes in and on the cancer-related immune system dysfunction.

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Expression of c-kit receptor on CD56bright subset of NK cells has been previously reported. Based on the fact that c-kit receptor and its ligand - stem cells factor (SCF), are co-expressed on many cell types, we have tested the hypothesis that c-kit and SCF might be co-expressed on human peripheral blood NK cells using RT-PCR, flow cytometry and DNA sequencing. NK cells were harvested using the immunomagnetic NK isolation kit and their purity was confirmed by flow cytometry. Analysis of RT-PCR data revealed the presence of two DNA fragments (about 330 bp and 420 bp) in NK cells. Identity of PCR products was directly confirmed by sequencing. To determine the specificity of SCF transcript formation only in c-kit-positive cell subset, NK cells were separated on c-kit-positive and c-kit-negative subsets using CD117 microbeads. No SCF mRNA expression was found in c-kit-negative NK cells even after increasing the quantity of total RNA to 2 μg and the number of amplification cycles to 40. Intracellular flow cytometry analysis with specific anti-SCF antibodies confirmed the expression of SCF protein in human NK cells. Thus, our results provide a definite proof of the existence of c-kit/SCF autocrine loop in c-kit-positive subset of human peripheral blood NK cells. The verification of the functional significance of this new phenomenon is in progress in our laboratory.