Comparative study of the soluble forms of immune checkpoint proteins VISTA, PD-1, PD-l1, and bone homeostasis regulator RANKL in bone tumor patients

Background. Low sensitivity of bone tumors to various schemes of standard chemotherapy stimulated the search of new targets for their medicamentous treatment. Among such targets are the so-called “immune checkpoints” promoting the escape of the tumor from the immune response of the organism, and some proteins directly affecting the remodeling and homeostasis of bone tissue.

The aim of this study was the comparative evaluation of the content of the soluble forms of VISTA, PD-1, PD-L1, and RANKL in blood serum of patients with malignant and borderline bone tumors, and practically healthy persons; analysis of the associations between these markers and the key clinical and pathological features of bone tumors.

Material and methods. The study enclosed 82 patients with malignant bone tumors (osteosarcoma – 50, chondrosarcoma – 16, chordoma – 12, Ewing sarcoma – 4), 18 patients with borderline giant-cell bone tumor (GCBT), and 29 practically healthy persons. Proteins’ content was measured in blood serum with standard ELISA kits: «Human VISTA/B7-H5/PD-1H ELISA Кit» (RayBiotech,), «Human PD-L1 Platinum ELISA», «Human PD-1 ELISA kit» (Affimetrix, eBioscience), «ampli-sRANKL» (Biomedica Medizinprodukte).

Results. In patients with bone sarcomas, a statistically significant increase in the level of sPD-L1 was revealed, as well as a pronounced trend towards a decrease in the levels of sVISTA and an increase in sRANKL compared with the control, while the level of sPD-1 practically did not differ from the control. A weak positive correlation between sVISTA and sPD-1 serum levels most prominent in chordoma patients (r_s=0.63) was observed. Significant positive correlation was also found between sPD-1 and sRANKL concentrations. sVISTA and sRANKL levels did not differ significantly between histological types of bone sarcomas. More than 20-fold increase of sVISTA level in Ewing sarcoma patients as compared to other malignant bone neoplasms did not reach the statistical significance level. The highest sPD-1 level was also observed in Ewing sarcoma, the difference with chordoma and chondrosarcoma being statistically significant. In GCBT patients both sPD-L1 and sPD-1 levels were significantly increased as compared to control, sVISTA level did not differ from control, while sRANKL was significantly higher than both in control, and in bone sarcoma patients. Negative correlation between sVISTA and sRANKL (r_s=-0.67) was also observed in this patients group.

Conclusion. Multidirectional changes in the levels of soluble forms of the proteins affected by targeted immunotherapy in peripheral blood of malignant and borderline bone tumor patients were demonstrated. The most prominent changes were found in the neoplasms with specific origin – GCBT and Ewing sarcoma that has a neuroectodermal derivation. Presumably, these diseases are the most plausible candidates for immune modulatory therapies.