ABSTRACT
Objectives Neutrophils are typically the most abundant leukocyte 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 exhibited largely similar transcriptional signatures. By contrast, synovial fluid neutrophils exhibited more than 1,600 differentially expressed genes. Gene signatures identified a prominent response to interferon gamma (IFNγ), as well as to tumor necrosis factor, interleukin 6, and hypoxia, in both humans and mice. Mass cytometry also found healthy and arthritic donor blood neutrophils largely indistinguishable but revealed a range of neutrophil phenotypes in synovial fluid defined by downregulation of CXCR1 and upregulation of FcγRI, HLA-DR, PD-L1, ICAM-1 and CXCR4. Reproduction of key elements of this signature in cultured blood neutrophils required both IFNγ and prolonged culture.
Conclusions Circulating neutrophils from arthritis patients resemble those from healthy controls, but joint fluid cells exhibit a network of changes, conserved across species, that implicate IFNγ response and aging as complementary drivers of the synovial neutrophil phenotype.
What is already known about this subject?
Neutrophils are central in the effector phase of inflammatory arthritis but their phenotypic heterogeneity in inflamed synovial fluid is poorly understood.
What does this study add?
RNA-seq and mass cytometry identify a hallmark phenotype of neutrophils in synovial fluid consisting of upregulated ICAM-1, HLA-DR, PD-L1, Fc receptors and CXCR4.
Transcriptomics highlight an IFNγ response signature conserved across humans and mice.
In vitro experiments implicate aging and IFNγ as complementary factors orchestrating the synovial fluid neutrophil phenotype.
How might this impact on clinical practice or future developments?
Understanding the specific features of neutrophils in the arthritic joint may disclose opportunities for safe therapeutic targeting of this lineage.
Competing Interest Statement
R.G.-B. received research support from Gilead. S.A.J. and A.H. are employees of Pfizer, Inc. H.-M.L. received research grants from Abbvie, Pfizer, Novartis, Sobi, Roche/Chugai, Gilead, Galapagos, GSK, UCB, MSD, and BMS. P.A.N. has received investigator-initiated research grants from AbbVie, BMS, Novartis, Pfizer and Sobi; consulting fees from BMS, Cerecor, Miach Orthopedics, Novartis, Pfizer, Quench Bio, Sigilon, Simcere, Sobi, XBiotech, and Exo Therapeutics; royalties from UpToDate Inc. and the American Academy of Pediatrics; and salary support from the Childhood Arthritis and Rheumatology Research Alliance.