Soluble forms of the NKG2D receptor and its ligand NKG2DL1 in patients with colorectal cancer

Relevance. Natural killer (NK) cells are considered to be effective cytotoxic cells of the tumor microenvironment, whose function is to effectively eliminate tumor cells. Their activation can occur when the NKG2D receptor interacts with the NKG2DL ligand expressed, for example, on tumor cells, which leads to the death of the latter.

The aim of the study was a comparative study of the content of soluble forms of the sNKG2D receptor and its ligand sNKG2DL1 in the blood serum of healthy donors and patients with colorectal cancer (CRC), taking into account the clinical and morphological characteristics of the disease and prognosis.

Material and methods. We examined 65 patients with CRC (32 women and 33 men), who were treated at the “National Medical Research Center of Oncology named after N.N. Blokhin” of the Ministry of Health of Russia aged (median 60 years) and 30 healthy donors (20 women and 10 men) aged (median 47 years). The clinical diagnosis in all patients was confirmed by the data of the morphological study of the tumor according to the International Histological Classification of Tumors of the Digestive System (WHO, 2019), all had adenocarcinoma. The concentration of NKG2D and NKG2DL proteins was determined by ELISA in blood serum before treatment with Human B7-H3 Quantikine ELISA Kit reagents firm R&D (USA) in accordance with the manufacturer's instructions. The measurements were carried out on an automatic enzyme immunoassay analyzer BEP 2000 Advance (Siemens Healthcare Diagnostics, Germany). Statistical analysis of the obtained results was carried out using GraphPad Prizm v. 9. Overall survival analysis was performed by constructing survival curves using the Kaplan-Meier method.

Results. In patients with colorectal cancer, a significant increase in the content of sNKG2DL in blood serum was found compared to the control group. sNKG2D concentrations did not differ between controls and CRC patients. No significant associations were found between the content of sNKG2D and sNKG2DL1 proteins in the blood serum of CRC patients and the main clinical and morphological characteristics of the disease. A decrease in the concentration of sNKG2D was noted with the progression of the disease, and the highest content of sNKG2D was found in the localization of the tumor in the caecum. For the ligand sNKG2DL1, the opposite pattern was observed, namely, its lowest concentration in blood serum in tumors located in the caecum. For the ligand sNKG2DL1, the opposite pattern was observed, namely, its lowest concentration in blood serum in tumors located in the caecum. At the same time, the levels of sNKG2D and sNKG2DL1 in the control group directly and significantly correlated with each other (r=0.453; p=0.014), while no such pattern was found in the group of CRC patients (r=-0.014; p=0.935). sNKG2DL1 concentrations are not a prognostic factor in CRC, while high levels of sNKG2D tended to lead to a favorable prognosis of the disease, which indirectly confirms the positive significance of tumor infiltration by NK cells.

Conclusions. The mechanisms of the relationship between CRC avoidance of immune surveillance and the expression of the sNKG2D receptor and the sNKG2DL ligand are discussed as one of the directions in the study of immune checkpoints. We believe that the effect of CRC immunotherapy can be achieved by increasing the expression of NKG2D on immune cells and inducing the expression of NKG2DL in tumor cells, which is the basis of the concept of this line of research.