THU0003 CRYSTALLINE SILICA IMPAIRS EFFEROCYTOSIS CAPACITIES OF HUMAN MONOCYTE-DERIVED MACROPHAGES THROUGH RHOA-ROCK ACTIVATION

Abstract

Background: Inhalation of crystalline silica can lead to pulmonary diseases and systemic autoimmune disorders, such as systemic sclerosis (SSc), systemic lupus erythematosus or rheumatoid arthritis (1). A failure of apoptotic cell clearance, also called efferocytosis, is reported in autoimmune diseases and, impaired efferocytosis in macrophages from SSc patients has especially been described recently (2). However, the precise mechanisms linking crystalline silica exposure and autoimmune disorders is still to be determined. Objectives: This study explored the effects of crystalline silica on efferocytosis abilities of human macrophages. Methods: Monocyte-derived macrophages (MDM) were exposed in vitro to crystalline silica for 4 hours. Their ability to phagocyte CFSE-positive apoptotic and non-apoptotic Jurkat cells and their polarization profile after silica exposure were assessed by flow cytometry. Efferocytosis capacities of MDM from SSc were also evaluated using the same methods. Results: Crystalline silica exposure impaired efferocytosis capacities of human MDM in a specific and dose-dependent manner. This effect of silica required the expression of SR-B1 and, was associated with a decreased membrane expression of the M2 polarization markers CD206, CD204 and CD163. Their expressions after silica exposure were similar to those of M1 polarized MDM. Silica increased F-actin staining, RhoA activation and phosphorylation of myosin phosphatase subunit 1 (MYPT1), a known ROCK target. Y27632, a Rho kinase (ROCK) inhibitor, reversed the F-actin staining, the phosphorylation of MYPT1 and, at least in part, the silica-induced impairment of efferocytosis. Moreover, efferocytosis abilities of MDM from SSc patients were similar to those of silica-exposed MDM and, a treatment of SSc-MDM with Y27632 significantly increase their efferocytosis capacities, suggesting an activation of the RhoA/ROCK pathway in SSc MDM also. Conclusion: These findings demonstrate that silica impairs efferocytosis in MDM via an activation of RhoA/ROCK pathway (Figure 1). These results also suggest a therapeutic potential of drugs targeting this pathway and advance the hypothesis that silica exposure may contribute to the impaired efferocytosis capacities of macrophages from SSc patients, a silica-associated systemic disorder still without curative treatment to date.