Background/Purpose:

Diffuse alveolar hemorrhage (DAH) is an unusual complication of lupus with over 50% mortality. In both humans and mice, DAH is associated with ANCA-negative pulmonary capillaritis and hemosiderin-laden lung macrophages (Mϕ). C57BL/6 (B6) mice develop lupus with severe DAH after intraperitoneal injection of pristane. We have shown that the pathogenesis of pristane-induced DAH requires Mϕ and the uptake of dead cells opsonized by immunoglobulin and complement via complement receptor 3 (CD11b/CD18). Serpins, such as the rabbit myxomavirus-derived Serp1 protein, regulate coagulation and inflammation by binding serine proteases. Serp1 inhibits both thrombolytic (e.g. urokinase plasminogen activator) and thrombotic (Factor Xa) proteases and downregulates inflammation in part via its effects on Mϕ. We asked if Serp-1 protects mice from pristane-induced DAH.

Methods:

Pristane-treated B6 mice (21 per group) were given recombinant Serp1 or PBS i.p. from day 1 until d-14. In some experiments, Serp1 treatment was delayed until d-4 or Serp1 was given on days 1-3 only. The severity of DAH was evaluated by gross pathology and histology. Peritoneal cells and alveolar Mϕ collected by bronchoalveolar lavage (BAL) were analyzed by flow cytometry.

Results: DAH developed in 60% of pristane-treated mice with a 14-d mortality of 15%. In mice treated with pristane+Serp1, DAH developed in 10% and mortality was 0% (P<0.001). The interferon signature and numbers of peritoneal Ly6C^hi inflammatory Mϕ were reduced by Serp1 treatment (P<0.0001). We recently identified a novel M2-like Mϕ expressing CD138 that promotes the resolution of inflammation. Serp1 significantly increased the number of pro-resolution (CD138⁺) Mϕ in both the peritoneum and the lung (P<0.01). When Serp1 treatment was delayed by 3-d, the mice were not protected from DAH, even though hemorrhage does not become apparent until 7-d after pristane treatment. Interestingly, Serp1 treatment from days 1-3 after pristane reduced the incidence of DAH to 20%. High Ly6C^hi inflammatory Mϕ and low CD138⁺ Mϕ numbers correlated with lung hemorrhage. In addition, Serp1 treatment up-regulated the activity of liver X receptor-α (LXRα), a transcription factor that promotes M2 Mϕ polarization, in both peritoneal and lung (alveolar) Mϕ. Mice that are resistant to DAH consistently exhibited higher levels of LXRα activity than susceptible mice.

Conclusion:

The myxomavirus-derived protein Serp1 (effective in Phase IIa clinical trials for treating acute coronary syndromes) protects mice from DAH and is a potential candidate for treating this severe complication of lupus. Its mechanism of action appears to involve activation of LXRα.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/myxomavirus-derived-serpin-serp-1-reduces-diffuse-alveolar-hemorrhage-in-a-murine-model-of-lupus/