Background/Purpose: Defective phagocytosis of dead cells is seen in SLE patients’ monocytes. Pristane-induced lupus causes accumulation of dead cells in tissues resembling that in SLE patients. However, the extent to which this reflects increased cell death vs. impaired removal of dead cells is unclear. The present studies addressed the clearance of dead cells in pristane-lupus

Methods: Clearance of dead cells from human bone marrow (BM) was examined by immunohistochemistry. The distribution of pristane in mouse tissues was detected by oil red staining and mass spectrometry. Murine macrophage (Mϕ) subsets were identified by flow cytometry (F4/80⁺CD11b⁺Ly6G^–). Gene expression in sorted cells was determined by real-time PCR. Phagocytosis of pHrodo red-labeled thymocytes by Mϕ from pristane-treated (develop lupus) vs. mineral oil (MO)-treated (do not develop lupus) mice was assessed in vitro and in vivo by flow cytometry. Autophagy was quantified by Cyto-ID green staining.

Results:

Tissues from SLE patients and pristane-treated mice contained numerous dead cells (activated caspase-3⁺). In non-SLE patients undergoing BM ablation and massive cell death prior to stem cell transplantation, dead cells were found only within resident BM Mϕ, not free as in lupus BM, consistent with a high capacity to clear dead cells. After i.p. injection in mice, pristane was transported to the lung and BM and could be detected by oil red staining and mass spectrometry along with numerous dead cells. We hypothesized that exposure of Mϕ to pristane causes a phagocytosis defect that predisposes to lupus. Consistent with that model, resident Tim4⁺ peritoneal Mϕ disappeared within hours of either pristane or MO injection and were replaced by BM-derived neutrophils and Mϕ. Peritoneal Mϕ from pristane-treated mice were poorly phagocytic vs. those from MO-treated controls and cells from pristane-treated mice were deficient in the class A scavenger receptor Marco. Marco is known to mediate phagocytosis of apoptotic cells, and the Marco⁺ Mϕ subset from control mice efficiently took up dead cells. The uptake of dead cells by peritoneal Mϕ from MO-treated mice was inhibited by anti-Marco antibodies. Expression of Marco mRNA is regulated by the transcription factor Nrf2 and was low in pristane- vs. MO- treated mice. Autophagy, which has been found to regulate phagocytosis by modulating Nrf2-mediated Marco expression, was increased in pristane-treated mice.

Conclusion:

Pristane may cause lupus by inducing a phagocytosis defect. The mechanism appears to involve modulation of phagocytosis due to increased autophagy, resulting in decreased activation of Nrf2, decreased scavenger receptor expression, and impaired clearance of dead cells.

« Back to 2015 ACR/ARHP Annual Meeting

ACR Meeting Abstracts - https://acrabstracts.org/abstract/impaired-phagocytosis-of-apoptotic-cells-in-pristane-induced-lupus-simulates-the-clearance-defect-in-human-sle/