Background/Purpose: FLIP is an anti-apoptotic protein induced by chronic inflammation.  In this study a mouse line with FLIP deleted in CD11c+ dendritic cells (DCs), that spontaneously develops arthritis, was employed to define the potential role of tolerance induction and adaptive immune.

Methods:

Mice with FLIP deficient in DC (Flip ^f/f, CD11c ^cre/+ or KO) were generated by crossing Flip ^f/f with CD11c^cre transgenic mice.  Immune cells in lymphoid organs were analyzed by Flow-cytometry employing multi- fluorochrome-conjugated antibodies. Arthritis was evaluated by joint swelling and/or deformity. Pathology was analyzed by histologic HE staining. The levels of IgG and cytokines were determined by ELISA and serum rheumatoid factors (RF) by rabbit IgG-ELISA.  In vivo antigen presentation was determined by T cell proliferation to OVA, monitored by CFSE labeled CD4+ T cells from OTII mice. All data are analyzed comparing with gender matched littermate controls. 

Results:

The KO mice spontaneously developed progressive, erosive arthritis, starting at 6 weeks of age and reaching ≥ 80% incidence at ≥ 4 months.  For mice at ≥ 5 months, histological examination of knees, ankles, and paws demonstrated increased joint inflammation with neutrophils, macrophages and lymphocytes, bone erosion, pannus formation and cartilage destruction (p< 0.05-0.001).  Further, increased TNFα, IL-1β, IL-6, IL-10, CXCL5, INF-g and RANKL, but reduced OPG, were present in ankle homogenates (p< 0.001).  Old KO mice expressed increased circulating INF-γ, IgG1 and RF (p< 0.05-0.001).  All 5 month KO mice developed lymphadenopathy with increased B cells and plasmablasts.  Examination of 4 week old (young) KO mice, prior to the onset of arthritis, demonstrated reduction in the size and number of cells in the thymus.  The subset of DCs expressing high CD11c and MHCII and were CD8+ were significantly reduced in the thymus, as well as the spleen, and lymph nodes (LN) young mice (p< 0.05-0.001).  Further, single positive CD4+ or CD8+ T cells, the ratio of CD4:CD8 cells and CD4+/CD25+/Foxp3+ Treg cells were reduced (p< 0.05-0.001) in the thymus of young KO mice.  CD3+,CD4+ and CD3+,CD8+ cells were reduced and B cells were increased in the LNs of young mice.  In the LNs of old KO mice CD11c DC and CD3+,CD8+ cells normalized, while CD3+,CD4+ and B cells were increased. Finally, the young Flip ^f/f, CD11c ^cre/+ mice demonstrated reduced in vivo antigen presentation properties, identified by reduced CD4+ T proliferation in response to OVA. 

Conclusion: These observations suggest that reduction of FLIP in DC results in reduced positive and negative selection, reduced Tregs and increased B cells, IFN-g, and autoantibodies.  DCs expressing high levels of CD11c and MHCII are necessary for normal tolerance development and restraining the occurrence of inflammatory arthritis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/flip-deficiency-in-dendritic-cells-promotes-spontaneous-inflammatory-and-erosive-arthritis/