Lupus and proliferative nephritis are PAD4 independent in murine models

Abstract

Though recent reports suggest that neutrophil extracellular traps (NETs) are a source of antigenic nucleic acids in systemic lupus erythematosus (SLE), we recently showed that inhibition of NETs by targeting the NADPH oxidase complex via cytochrome b-245, β polypeptide (cybb) deletion exacerbated disease in the MRL.Faslpr lupus mouse model. While these data challenge the paradigm that NETs promote lupus, it is conceivable that global regulatory properties of cybb and cybb-independent NETs confound these findings. Furthermore, recent reports indicate that inhibitors of peptidyl arginine deiminase, type IV (Padi4), a distal mediator of NET formation, improve lupus in murine models. Here, to clarify the contribution of NETs to SLE, we employed a genetic approach to delete Padi4 in the MRL.Faslpr model and used a pharmacological approach to inhibit PADs in both the anti-glomerular basement membrane model of proliferative nephritis and a human-serum-transfer model of SLE. In contrast to prior inhibitor studies, we found that deletion of Padi4 did not ameliorate any aspect of nephritis, loss of tolerance, or immune activation. Pharmacological inhibition of PAD activity had no effect on end-organ damage in inducible models of glomerulonephritis. These data provide a direct challenge to the concept that NETs promote autoimmunity and target organ injury in SLE.

Murine lupus is Neutrophil Elastase-independent in the MRL.Faslpr model

“…Here, we show that genetic deletion of elane did not have any impact on clinical or immunological parameters of SLE in MRL.Fas lpr mice. Taken together with earlier work [22, 23, 26], these findings add additional evidence that challenges the concept that neutrophils and NETs, to the degree that NET generation and neutrophil effector function relies on PADI4 [7–13], ELANE [10, 14–17], or CYBB [4–6], critically drives lupus.…”

“…This point, which is punctuated by the current data, along with the lack of a role for neutrophils per se in driving lupus, heralds a juncture at which the field should seriously reevaluate whether the “NETs drive lupus” hypothesis remains valid and whether blocking NET formation or neutrophil function makes sense as a therapeutic strategy. In contrast, multiple studies have shown regulatory roles in various systems for CYBB, and to an extent PAD4 and MPO, such that disease is exacerbated in their absence [22, 23, 26, 27, 59]. We would thus posit that neutrophils and their effector mechanisms, such as NET generation, may have evolved to protect us from autoimmune diseases, rather than function as vectors to promote them.…”

Review: Cell Death, Nucleic Acids, and Immunity

“…These observations have been supported by mouse studies in which pharmacologic inhibition of NET formation in lupus-prone mice, either by inhibition of PAD4 activity or by inhibition of mitochondrial ROS, ameliorates disease (34,35). However, an argument refuting the notion that NET formation is required for disease development has been made in genetic studies of MRL/lpr mice in which deficiency of either PAD4 or NOX-2 did not protect against disease (36,37).…”

“…These observations have been supported by mouse studies in which pharmacologic inhibition of NET formation in lupus-prone mice, either by inhibiting PAD4 activity or by inhibiting mitochondrial ROS, ameliorates disease, (34, 35). However, an argument against the requirement for NETs has been made by genetic studies in MRL.lpr mice where deficiency of either PAD4 or Nox2 did not protect against disease (36, 37). ROS generation is necessary for NET formation in most contexts and we and others recently demonstrated that ROS generation by mitochondria, rather than Nox2, is key to oxidation of DNA and in particular, mitochondrial DNA (mtDNA) (34, 38).…”

Missing in action—The meaning of cell death in tissue damage and inflammation

“…Similar to Ncf1** mice, PAD4‐deficient mice exhibited higher autoantibody levels and enhanced proteinuria . In a parallel study by the Shlomchik lab, PAD4‐deficiency or pharmacological inhibition of PAD4 did not have any effect on tolerance loss, immune activation, or nephritis in the MRL. Fas lpr lupus model.…”