Background The molecular control of the “ebb and flow” of inflammatory cells into and out of the joint during the initiation, maintenance and resolution of inflammation in Rheumatoid Arthritis (RA) remains unclear.

Objectives To characterise the ingress and egress of immune cells into inflamed joints, and identify the key molecular pathways regulating their migration in real time in vivo.

Methods We developed a “breach of self tolerance” murine model of articular inflammation [1]. Using this model in conjunction with mice expressing the photoswitchable fluorescent protein kaede [2], we characterized the migration of cells from the inflamed joint (after photoswitching) to the draining lymph nodes. Flow cytometry of excised lymph nodes allowed the identification and phenotypic characterization of photoswitched cells that could only have originated from the inflamed joint. Using the same approach we identified and sorted cells that migrated to the draining lymph nodes and those that remained in the joint. RNA-seq analysis of these populations identified genes differentially regulating cellular retention in, and egress from, the inflamed joint to the draining lymph node.

Results The key populations identified exiting the inflamed joint and entering the draining lymph node were CD4⁺ and CD8⁺ T cells, FoxP3⁺ Treg, B cells and CD11c⁺ DCs. The populations present in the inflamed joint comprised mainly CD4⁺ T cells, CD11c⁺ DCs and Ly6G⁺ neutrophils. Analysis of RNA-seq data revealed pathways and genes previously highlighted as characteristic of RA in patient studies. More specifically focusing on cell migration, adhesion and movement pathways has indicated genes associated with the egress of immune cells from the inflamed joint.

Conclusions We have identified key gene expression profiles that may be important for the migration of cells from the inflamed joint, which may provide targets for therapeutic manipulation.

1. Maffia P, Brewer JM, Gracie JA, Ianaro A, Leung BP, Mitchell PJ, et al. Inducing experimental arthritis and breaking self-tolerance to joint-specific antigens with trackable, ovalbumin-specific T cells. J Immunol. 2004;173(1):151–6.

2. Tomura M, Yoshida N, Tanaka J, Karasawa S, Miwa Y, Miyawaki A, et al. Monitoring cellular movement in vivo with photoconvertible fluorescence protein “Kaede” transgenic mice. Proc Natl Acad Sci U S A. 2008;105(31):10871–6.

Acknowledgement This research is funded by Arthritis Research UK, and supported by the Arthritis Research UK Rheumatoid Arthritis Pathogenesis Centre of Excellence (RACE).

Disclosure of Interest None declared