Ageing and interferon gamma response drive the phenotype of neutrophils in the inflamed joint

Objective Neutrophils are typically the most abundant leucocyte in arthritic synovial fluid. We sought to understand changes that occur in neutrophils as they migrate from blood to joint. Methods We performed RNA sequencing of neutrophils from healthy human blood, arthritic blood and arthritic synovial fluid, comparing transcriptional signatures with those from murine K/BxN serum transfer arthritis. We employed mass cytometry to quantify protein expression and sought to reproduce the synovial fluid phenotype ex vivo in cultured healthy blood neutrophils. Results Blood neutrophils from healthy donors and patients with active arthritis showed largely similar transcriptional signatures. By contrast, synovial fluid neutrophils exhibited more than 1600 differentially expressed genes. Gene signatures identified a prominent response to interferon gamma (IFN-gamma), as well as to tumour necrosis factor, interleukin-6 and hypoxia, in both humans and mice. Mass cytometry confirmed that healthy and arthritic donor blood neutrophils are largely indistinguishable but revealed a range of neutrophil phenotypes in synovial fluid defined by downregulation of CXCR1 and upregulation of Fc gamma RI, HLA-DR, PD-L1, ICAM-1 and CXCR4. Reproduction of key elements of this signature in cultured blood neutrophils required both IFN-gamma and prolonged culture. Conclusions Circulating neutrophils from patients with arthritis resemble those from healthy controls, but joint fluid cells exhibit a network of changes, conserved across species, that implicate IFN-gamma response and ageing as complementary drivers of the synovial fluid neutrophil phenotype.

Automated summary

Neutrophils in the blood of healthy individuals and patients with arthritis have similar transcriptional signatures, but those in synovial fluid exhibit significant differences, involving over 1600 differentially expressed genes. The gene signatures indicate a strong response to interferon gamma (IFN-gamma), as well as to tumor necrosis factor, interleukin-6, and hypoxia. Mass cytometry analysis confirmed that healthy and arthritic blood neutrophils are similar, but synovial fluid neutrophils show unique protein expression related to various factors such as CXCR1, Fc gamma RI, HLA-DR, PD-L1, ICAM-1, and CXCR4.