Background Osteoarthritis (OA) is a leading cause of chronic pain and functional disability in elder people. It is not a homogeneous but diverse group of synovial joint diseases. This could stay behind the therapeutic inconsistency observed in clinical practice caused probably by different diseases and/or different stages of a single disease. The comprehensive immunophenotyping of immune cells and their cell counts in synovial fluid (SF) might therefore advance our understanding of a particular type/stage of OA. This could have diagnostic value as well as provide novel insights into the pathophysiology of OA.
Objectives To characterize immune cells present in SFs from OA patients in different stages of a disease.
Methods We performed immunophenotyping of SFs from 63 patients with OA and 10 SFs from control patients (non-OA) without clinical/radiographic signs of OA using flow cytometry. We were able to characterize the following immune cells in the sampled SFs: T helper lymphocytes (CD3+/CD4+), T cytotoxic lymphocytes (CD3+/CD8+), NK cells (CD3-/CD16+/CD56+), B lymphocytes (CD19+), T regulatory (Treg) cells (CD4+/CD25+/CD127-), mast cells (CD203c+/CD117+), M1- (CD14+/CD86+/HLA-DRhigh) and M2-polarized macrophages (CD14+/CD163+/CD206+), and neutrophils (CD15+/CD16+).
Results A comparison between OA and control (non-OA) SFs revealed phenotypic alterations mainly in T cells, NK cells, macrophages, and neutrophils. T cells were the predominant population in the SFs, with CD4+ T lymphocytes being more prevalent than CD8+ T cells in OA (increased CD4/CD8 ratio). The second largest cell population was macrophages. Despite the dominant mixed-polarized (M1-M2) macrophage subpopulations in both the studied groups, SFs from the OA patients displayed a tendency towards greater M1 activity comparing to the controls. A markedly increased percentage of neutrophils found in the OA group was associated with their activated state compared to the controls (increased CD11b). No difference was found in percentages of B, Treg and mast cells. Despite the similar numbers of NK cells in both the groups, the activation-associated marker CD69 was up-regulated in NK cells from the OA patients. Representative dot-plots (FCS-SSC) of inter-individual variability of the main immune cell populations in synovial fluids from osteoarthritic patients is shown in Figure 1.
Conclusions We were able to distinguish between the OA cases and controls in terms of their immune cells profiles (i.e. their numbers, activation status etc.). Additionally, we were able to follow specific immune cell patterns inside the OA group. Our study further emphasized the role of immune cells in the pathogenesis of OA. In particular, the results provide evidence suggesting ongoing activation of innate immunity as well as a shift towards T helper lymphocytes in fluids of the patients with OA. Our findings give a rational starting point to be addressed by the future research for clinically useful biomarkers associated with the OA development and progression.
Acknowledgements Grant support: MZ CR VES16–31852A, MZ CR VES15–27726A.
Disclosure of Interest None declared