Two subsets of circulating Ly6Clo monocytes distinguished by CD138 (syndecan-1) expression and Nr4a1 dependence in pristane-treated mice

Chronic peritoneal inflammation following pristane injection induces lupus with diffuse alveolar hemorrhage (DAH) and pulmonary capillaritis in C57BL/6 mice. The pathogenesis involves pristane-induced microvascular lung injury. BALB/c mice are resistant to endothelial injury and DAH. Lung disease in C57BL/6 mice is abolished by depleting monocytes/macrophages. The objective of this study was to define the role of myeloid subsets in DAH. Hemorrhage and vasculitis were abolished in Ccr2-/- mice, indicating involvement of bone marrow-derived monocytes/macrophages. Along with Ly6Chi monocytes, we found two subsets of circulating Ly6Clo monocytes: one CD138- and a novel CD138+ subset. Nr4a1 -dependent patrolling Ly6Clo monocytes maintain vascular integrity after endothelial injury. Circulating Ly6CloCD138+ monocytes were associated with DAH and were absent in mice without DAH. They also were absent in Nr4a1-/- mice, whereas Ly6CloCD138- monocytes were unaffected. However, Nr4a1-/- mice were susceptible to pristane-induced DAH and lung vasculitis, suggesting that disease onset does not require Ly6CloCD138- monocytes. Peritoneal Ly6CloCD138+ M? were unchanged in Nr4a1-/- mice, indicating that they are not derived from Ly6CloCD138+ monocytes. We conclude that pristane-induced lung microvascular lung injury stimulates a wave of Nr4h1 -dependent Ly6CloCD138+ patrolling monocytes in an ineffectual effort to maintain vascular integrity in the face of ongoing endothelial damage. ### Competing Interest Statement The authors have declared no competing interest.