FIGURE 1.

TREM2-Ig recognizes PS and other lipids on lipid arrays, but not on the surface of apoptotic cells. A, the TREM2-Ig fusion was used to probe an array spotted with various lipids as indicated in the legend on the right. Dark spots show lipids recognized by TREM2-Ig as detected by chemiluminescence. As shown by the left membrane, TREM2-Ig bound strongly to spots containing phosphatidic acid (PA), PS, and cardiolipin (CL). A human IgG1 isotype control exhibited no appreciable lipid binding. These results were reproducible with at least two different lots of arrays. PA and PS binding were also observed on sphingolipid strips (that do not include CL, not shown). B, Jurkat cells were treated for 12 h with actinomycin D1 (actD1) to induce apoptosis and expose PS on the cell surface. Cells were then stained with PS-binding reagents ANXAV and TIM1-Ig, with TREM2-Ig, or with a human IgG1 isotype control antibody. The membrane-impermeant vital stain propidium iodide (PI) was used to discriminate between dead cells (PI⁺) and apoptotic or viable cells (PI⁻). The vertical axis of each plot shows the relative intensity of ANXAV/Ig fusion staining and the horizontal axis shows the intensity of PI staining. Known PS-binding proteins ANXAV and TIM-1 labeled apoptotic cells (ANXAV⁺, PI⁻) as indicated in the bottom left plot. TREM2-Ig, however, failed to label apoptotic cells, indicating that it does not bind PS in the context of intact cellular membranes.