Clinical and immunological features of PD-1 and Tim-3 expression on monocytes in axial spondyloarthritis

Regulation of inflammation events in inflammatory/autoimmune rheumatic diseases, particularly axial spondyloarthritis, is critically important for alleviating clinical symptoms. Recent studies have shown the key role of PD-1 and Tim-3 inhibitory receptors in regulating the functional phenotype of monocytes in inflammatory/autoimmune diseases. The aim of this study was to investigate the expression of the PD-1 and Tim-3 inhibitory receptors in different subsets of monocytes in patients with axial spondyloarthritis (axSpA), and to evaluate their association with clinical features, as well as with tolerogenic marker Mer tyrosine kinase (MerTK). The expression of PD-1 and Tim-3 in classical, intermediate, and non-classical monocytes was determined by means of multicolor flow cytometry in peripheral blood mononuclear cells in 60 patients with axial spondyloarthritis and 40 healthy donors. The analysis of circulating monocyte subpopulations revealed an increased proportion of classical monocytes in patients, which positively correlated with systemic inflammation markers: erythrocyte sedimentation rate and C-reactive protein. Like as in donor group, the highest expression of PD-1 and Tim-3 in axSpA patients was observed in intermediate and non-classical monocyte subsets. However, the relative proportions of PD-1 + and PD-1 + Tim-3 + cells in intermediate and non-classical monocytes were significantly decreased in axSpA patients, whereas Tim-3 expression levels did not differ from the donor values. Notably, the proportion of PD-1 + cells directly correlated with expression of MerTK, a factor mediating the anti-inflammatory activity of monocytes/macrophages in all monocyte subsets. A statistically significant decrease in PD-1 expression and co-expression of PD-1 and Tim-3 was detected in HLA-B27 antigen carriers, patients with advanced and late-stage disease, and those with peripheral axSpA, including hip joint involvement. On the contrary, in HLA-B27-negative patients and those with early/non-radiographic stage disease, PD-1 expression was comparable to donor levels, while Tim-3 + cell contents were increased among classical monocytes. In patients receiving first-line therapy, the decreased PD-1 expression and PD-1 + Tim-3 + co-expression directly correlated with disease activity. The obtained data that include a reduced PD-1 expression and PD-1 + Tim-3 + co-expression in intermediate and non-classical monocytes in patients with genetic predisposition, advanced disease stages, and peripheral joint involvement, as well as the association between PD-1 + cell number and MerTK expression in all monocyte subsets, suggest a dysregulation of inhibitory checkpoint receptor expression under inflammatory conditions. These findings could presume a diminished anti-inflammatory capacity of monocytes.