Background/Purpose:

Flip is well known as an anti-apoptotic protein induced by chronic inflammation that protects against death receptor-mediated apoptosis. Employing a FLIP myeloid lineage knock-out mouse line (Flip ^d/f, LysM^cre/+), we identified that FLIP is essential for macrophage differentiation and survival and for granulocyte homeostasis.  This study was performed to define the role of FLIP in myeloid cells in K/BxN anti-GPI serum transfer-induced arthritis.  

Methods:

Arthritis was induced by introducing 300ml anti-GPI serum collected from K/BxN mice into Flip ^f/f, LysM^cre/+ mice and the age and gender matched littermate controls.  Cell types and differential in peripheral blood were determined by complete blood count.  Cell types present in various organs were determined employing multi-color fluorochrome-conjugated cell marker antibodies, analyzed by flow cytometry.  The mouse ankles harvested at the indicated time points were fixed in 10% formalin for H&E histological staining, or frozen in -80^oc for ELISA cytokines analysis.

Results:

Similar to the Flip ^f/d, LysM^cre/+ mice reported earlier, Flip ^f/f, LysM^cre/+ mice exhibited a significant increase of circulating neutrophils and monocytes, multi-organ neutrophil infiltration, and significantly reduced mature macrophages in multi-effector organs, such as peritoneal cavity, although it was less severe.  The Flip ^f/f, LysM^cre/+ developed a significantly more severe of acute phase arthritis on days 2 and 4 post-induction compared to the controls.  The development of arthritis stopped after day 4 and began to improve in Flip ^f/f, LysM^cre/+ mice, while the controls continued to progressive, peaking at day 9.  The arthritis was significantly reduced in the Flip ^f/f, LysM^cre/+ mice between days 7 to day 14 compared with the controls.  Histological analysis demonstrated more articular and extra-articular inflammation and neutrophils in the Flip ^f/f, LysM^cre/+ ankles collected in day 2 and 4.  In contrast, inflammation, cartilage destruction, erosion and pannus were significantly reduced on day 10 and/or 14 in the Flip ^f/f, LysM^cre/+ mice.  Further, IL-1b in the ankle joints was significantly higher in Flip ^f/f, LysM^cre/+ ankles collected in day 0, 2 and 4, but not day 10 of arthritis compared with the controls.  However, IL-1b in the ankle ankles positively correlated with arthritis in the control group, but no correlation was observed in the Flip ^f/f, LysM^cre/+ mice.  In preliminary experiments, the infusion of wild type macrophages into the Flip ^f/f, LysM^cre/+ mice just after the injection of the anti-GPI serum resulted in less severe arthritis initially but more severe arthritis later in the course. 

Conclusion:

These studies demonstrate that FLIP in macrophages is essential for regulating neutrophil homeostasis and acute inflammation and that it promotes the progression of the effector phase of arthritis.  The lack of association between inflammation and IL-1b in the Flip ^f/f, LysM^cre/+ mice is consistent with diminished IL-1b signaling.  These observations suggest that FLIP in macrophages is a potential target for the therapy in patients with rheumatoid arthritis.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/flip-in-macrophages-promotes-the-progression-of-serum-transfer-induced-arthritis/