Background/Purpose: Previous studies implicate dendritic cells (DCs) in the initiation and persistence of systemic lupus erythematosus (SLE).  While DCs from SLE patients exhibit elevated activation, the factors responsible remain unknown.  To this end, we recently identified that caspase 8, an enzyme known to initiate apoptosis and/or suppress necroptosis (by inhibition of RIPK signaling) in a multitude of cells, is a novel DC-specific inhibitor of inflammatory processes independent of DC survival.

Methods: Mice lacking caspase 8 specifically in DCs were generated (Cre^CD11cCasp8^flox/flox).  Cre^CD11cCasp8^flox/flox mice were also crossed with MyD88^flox/flox, IRF3^-/- or IRF7^-/- mice to determine the role of these molecules in disease development.  Flow cytometric analysis was used to characterize DC populations in mixed bone marrow chimeras and BrdU pulse assays.  Bone marrow derived DCs were cultured with TLR agonists +/- necrostatin-1 (RIPK1 inhibitor), 1MT (indoleamine 2,3-dioxygenase (IDO) inhibitor), zIETD-FMK (caspase 8 enzymatic inhibitor).  Luminex based assays detected cytokine and transcription factor DNA binding levels.  

Results: Cre^CD11cCasp8^flox/flox develop a SLE-like disease characterized by splenomegaly, lymphadenopathy, autoantibodies, glomerulonephritis, immune complex deposition in the kidney, proteinuria, an interferon signature, and early mortality.  Loss of caspase 8 in DCs does not affect their survival, as there are equal numbers of Cre^CD11cCasp8^flox/flox and WT DCs in mixed bone marrow chimeras, there is no change in DC turnover rate using BrdU pulse assays, and bone marrow derived DCs display similar levels of death independent of caspase 8 or RIPK1.  However, caspase 8-deficient DCs are highly activated, leading to lymphocyte hyperactivation in a paracrine manner.  Immune complexes containing self nucleic acids activate TLR7/9, which require the adaptor MyD88 for subsequent up-regulation of proinflammatory gene expression.  The increased activation of Cre^CD11cCasp8^flox/flox DCs is controlled by a MyD88-dependent mechanism, as DC-specific loss of MyD88 in Cre^CD11cCasp8^flox/flox mice reduces disease, and caspase 8 deficient DCs display a hyperresponsiveness to TLR7/9 ligation with increased DNA binding activity of IRF independent of caspase 8’s enzymatic activity.  Additionally, blocking RIPK1, but not IDO, signaling dampens TLR7/9 induced secretion of proinflammatory cytokines in caspase 8 deficient DCs.  Despite caspase 8’s known inhibitory role on IRF3 activity, deletion of IRF3, but not IRF7, in Cre^CD11cCasp8^flox/flox mice exacerbates rather than ameliorates SLE-like disease, indicating that IRF3 transcribes inhibitory molecules that attempt to control the activation induced by the loss of caspase 8.

Conclusion: DC-specific loss of caspase 8 induces an SLE-like disease initiated by heightened DC activation via a MyD88- and RIPK1-dependent unknown mechanism.  This constitutive activation is dampened by IRF3-specific transcription of unknown molecules possessing inhibitory functions, thereby uncovering novel roles for caspase 8 and IRF3 and highlighting potentially useful targets for autoimmune disease therapy.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/loss-of-caspase-8-exacerbates-dendritic-cell-activation-in-a-myd88-and-ripk1-dependent-manner-that-is-controlled-by-inhibitory-molecules-transcribed-by-irf3/