Frontline Science: Tim-3-mediated dysfunctional engulfment of apoptotic cells in SLE.

T cell Ig and mucin domain-containing molecule 3 (Tim-3) has been found to play important roles in autoimmune diseases, but whether Tim-3-mediated engulfment of apoptotic cells is involved in systemic lupus erythematosus (SLE) remains to be elucidated. In this study, we verified the role of human Tim-3 (hTim-3) as the receptor of phosphatidylserine (PS) in human embryonic kidney (HEK)293 cells, which initiated the engulfment of apoptotic cells. Both IgV and the mucin domain of Tim-3 were crucial in the phagocytosis of apoptotic cells, and there existed the key cytoplasmic domain for signal transduction. Alanine at 111, locating around the FG-CC' loop of hTim-3, was necessary for its engulfment of apoptotic cells. In accordance, Tim-3 on CD14 cells negatively correlated with the percentage of peripheral apoptotic cells in control subjects. However, although Tim-3 was significantly increased on CD14 cells in SLE patients, peripheral apoptotic cells remained much higher than those in control subjects. Tim-3 on CD14 cells showed positive correlation with percentage of apoptotic cells and level of dsDNA, indicating the involvement of Tim-3 in SLE. Accordingly, soluble Tim-3 (sTim-3) was significantly increased in plasma of SLE patients, which might contribute to higher expression of a disintegrin and metalloproteinase (ADAM)-10. Pretreatment with both plasma from SLE patients and recombinant sTim-3 greatly inhibited hTim-3-initiated phagocytosis of apoptotic cells. Furthermore, anti-Tim-3 antibody depletion of plasma from SLE patients reversed the decreased phagocytosis of apoptotic cells. Collectively, our data suggest that sTim-3 might play inhibitory roles in impaired Tim-3-mediated clearance of apoptotic cells in SLE.