Background/Purpose

Rheumatoid arthritis (RA) manifests in persistent synovial inflammation, cellular infiltration and pro-inflammatory cytokine production, and results in progressive joint destruction.  Both myeloid cells and dendritic cells (DCs) have been implicated in RA progression and persistence: DCs through either antigen presentation to autoreactive T cells or differentiation into synoviocytes and myeloid cells (including macrophages and neutrophils) through the production of degradative enzymes, cytokines, and chemokines.  However, the mechanisms underlying these activities are not fully elucidated.  We recently identified that caspase 8, an enzyme that initiates apoptosis and suppresses necroptosis by inhibition of RIPK1/3 signaling in a multitude of cells, is a novel DC and myeloid cell-specific inhibitor of inflammatory processes independent of its known role in cell survival.  Recently, a genome-wide association study identified an RA risk single nucleotide polymorphism within the locus that contains the gene that encodes for caspase 8.  However, the impact of DC and myeloid cell-specific loss of caspase 8 on arthritis progression has yet to be examined.

Methods Mice with caspase 8 flanked by loxP sites (Casp8^fl/fl, WT) were bred to mice expressing Cre under control of the lysozyme M gene promoter (Cre^LysM) expressed by myeloid cells or the CD11c gene promoter (Cre^CD11c) expressed by dendritic cells.  Cre^LysMCasp8^fl/fl and Cre^CD11cCasp8^fl/fl mice were crossed with RIPK3^-/-mice to determine the involvement of this signaling partner.  The K/BxN serum-transfer model of arthritis was utilized and clinical severity was assessed.  Ankle sections were stained with H&E and scored by a pathologist blinded to the study to assess pathology.  Flow cytometric analysis was used to characterize naïve joints.

Results The expression of CD206 (mannose receptor, involved in phagocytosis) was decreased on macrophages of naïve Cre^LysMCasp8^fl/fl joints, and CD86 was elevated on CD11b⁺ DCs from naïve Cre^CD11cCasp8^fl/fl joints.  Cre^LysMCasp8^fl/fl presented with accelerated resolution of arthritis, as assessed by reduced changes in ankle width on day 4, 7, 9 and 11 and clinical scores on day 7 and 9, as well as reduced pathology compared to WT.  In stark contrast, Cre^CD11cCasp8^fl/fl mice showed accelerated initiation of arthritis, as evidenced by increased changes in ankle width and clinical scores on day 2 and 4, as well as increased joint damage compared to WT. However, the in both cases, knockout of RIPK3 in Cre^LysMCasp8^fl/fl and Cre^CD11cCasp8^fl/flmice was sufficient to reverse the effect of caspase 8 deletion alone on arthritis initiation and resolution.

Conclusion While deletion of caspase 8 specifically in myeloid cells promotes the acceleration of arthritis resolution, DC-specific caspase 8-deletion exacerbates arthritis initiation.  In both situations, blockade of RIPK3 reverses the effect of caspase 8 deletion, indicating that the caspase 8/RIPK3 signaling axis plays an important cell-specific role in disease pathogenesis.  These data have implications for RA by elucidating previously unknown cell-specific functions of a potentially useful target for therapy.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-caspase-8ripk3-signaling-axis-has-opposing-roles-in-myeloid-and-dendritic-cells-during-progression-of-a-murine-model-of-acute-inflammatory-arthritis/