Background/Purpose: Systemic lupus erythematosus (SLE) is a multi-organ, destructive autoimmune disease characterized by pathogenic autoantibodies.  Though it is accepted that dendritic cells (DCs) play an important role in disease initiation, only recently have studies implicated DCs as a major factor in SLE persistence.  DCs from SLE patients exhibit elevated expression of activation markers including co-stimulatory molecules and pro-inflammatory cytokines; however, the factors that are responsible for their aberrant activation is unknown.  We recently identified that caspase 8, a cysteine-aspartic acid enzyme known to function in death receptor signaling, is a novel DC-specific inhibitor of inflammatory processes.  While previous studies have shown that caspase 8 functions to initiate apoptosis and/or suppress necroptosis (through inhibition of RIPK1/3 signaling) in a multitude of cells, our data suggest that caspase 8 is not essential for the survival of DCs (apoptosis/necrosis), but rather inhibits the continuous activation of DCs, thereby preventing development of systemic autoimmunity.

Methods: Mice with caspase 8 flanked by loxP sites (Casp8^flox/flox. WT) were crossed with mice expressing Cre under control of the CD11c gene promoter (Cre^CD11c), which is expressed by dendritic cells.  Flow cytometric analysis was employed to characterize DC populations in both mixed bone marrow chimeras and BrdU pulse assays.  Bone marrow-derived DCs were cultured with TLR agonists +/- necrostatin-1 to block RIPK1 activity.  Luminex-based assays and ELISAs were used to detect cytokine and transcription factor DNA binding levels.

Results: Cre^CD11cCasp8^flox/flox develop splenomegaly, lymphadenopathy, autoantibodies, glomerulonephritis, immune complex deposition in the kidney, exacerbated proteinuria levels, heightened amounts of serum pro-inflammatory cytokines (IL-12, IL-1β, and IFNα/β), and early mortality.  Loss of caspase 8 in DCs does not affect their survival, as Cre^CD11cCasp8^flox/flox mice present with increased DC populations, there are equal numbers of Cre^CD11cCasp8^flox/flox and WT DCs in mixed bone marrow chimeras, there is no change in DC turnover rates using BrdU pulse assays, and bone marrow-derived DCs display similar levels of necrotic death independent of caspase 8 or RIPK1.  Loss of caspase 8 in DCs does not affect their survival, but they are highly activated, leading to elevated levels of activated lymphocytes in a paracrine manner.  The increased activation potential of Cre^CD11cCasp8^flox/flox DCs may be controlled by toll-like receptors 7 and 9 (TLR7/9) since caspase 8-deficient DCs display a hyper-responsiveness to TLR7/9 ligation with increased DNA binding activity of interferon regulatory factor (IRF) and STAT1.  Additionally, blocking RIPK1 signaling dampens the TLR7/9-induced secretion of pro-inflammatory cytokines in caspase 8-deficient DCs.

Conclusion: These results demonstrate that loss of caspase 8 in DCs initiates inflammatory phenotypes by a DC survival-independent novel mechanism.  These data have implications for autoimmunity by elucidating previously unknown functions of a potentially useful target for therapy.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/loss-of-caspase-8-in-dendritic-cells-induces-a-systemic-lupus-erythematosus-like-disease-that-is-independent-of-the-necroptosome/