Phenotypic and functional characterisation of synovial fluid-derived neutrophils in knee osteoarthritis and knee infection.

OBJECTIVE: An increase in the number of neutrophils (NEUs) has long been associated with infections in the knee joints; however, their impact on knee osteoarthritis (KOA) pathophysiology remains largely unexplored. DESIGN: This study compared the phenotypic and functional characteristics of synovial fluid (SF)-derived NEUs in KOA and knee infection (INF). RESULTS: KOA NEUs were characterised by a lower expression of CD11b, CD54, and CD64 and higher expression of CD62L, TLR2, and TLR4 compared with INF NEUs. Except for CCL2, lower levels of inflammatory mediators and proteases were detected in KOA SF than in INF SF. Functionally, KOA NEUs displayed increased reactive oxygen species production and phagocytic activity compared with INF NEUs. Moreover, KOA and INF NEUs differed in cell sizes, histological characteristics of the surrounding synovial tissues, and their effects on the endothelial cells assessed by human umbilical vein endothelial cells. When KOA patients were subdivided based on the SF NEU abundance, patients with high NEUs (10%-60%) were characterised by i) elevated SF protein levels of TNF-α, IL-1RA, MMP-9, sTREM-1, VILIP-1 and ii) lower CD54, CD64, TLR2 and TLR4 expression compared to patients with low NEUs (<10%). Analysis of paired SF samples suggests that low or high NEU percentages, respectively, persist throughout the course of disease. CONCLUSIONS: Our findings suggest that NEU may play a significant role in KOA pathophysiology. Further studies should explore the mechanisms that contribute to the increased number of NEUs in SF and the clinical consequences of neutrophilic phenotype in KOA.

Knee osteoarthritis phenotypes based on synovial fluid immune cells correlate with clinical outcome trajectories.

BACKGROUND: Knee osteoarthritis (KOA) is a highly heterogeneous disease encompassing a wide range of clinical phenotypes. Phenotypes based on immune cells and protein pattern in synovial fluid (SF) and their relationship to clinical trajectories have not been described. OBJECTIVE: To assess phenotypes based on immune cells and protein pattern of SF in KOA. DESIGN: SF-derived immune cells were investigated in 119 patients with KOA using flow cytometry. Immune-phenotypes (iPhen) were determined by multivariate patient similarity network analysis and related to clinical trajectory (3-6 months post-sampling) along with protein pattern and macrophage chemokine receptors. RESULTS: Four iPhen were detected based on the distribution of T-lymphocytes, monocyte-macrophage lineage cells and activated CD8+ T-lymphocytes. The 'activated' phenotype (n = 17) had high T-lymphocytes but low monocyte-macrophage lineage cells and neutrophils, all highly activated, and showed improved symptoms in 70% patients. The 'lymphoid progressive' phenotype (n = 31) had high neutrophils, low lymphocytes and monocyte-macrophage lineage cells, low activation and was associated with lower pain levels. The 'myeloid progressive' phenotype (n = 35) had high NK and monocyte-macrophage lineage cells but low T-lymphocytes and activation. The 'aggressive' phenotype (n = 36) had high lymphocytes, macrophages, NK cells and neutrophils and high activation, and only 39% of patients improved during follow-up. Low CXCR4 and CCR7 expression on macrophages and high CXCL10 in SF were linked to improved clinical trajectory. CONCLUSION: We identified four immune-phenotypes that were associated with different clinical trajectories in KOA patients. How these phenotypes can be targeted therapeutically deserves further investigation.