LILRA3 promotes phagocytosis of U937 cells by upregulating CD36 expression
The phagocytosis efficiency was detected by FCM at 0.5, 6, 12 and 24 hours by analyzing the percentage of GFP+ Texas Red+ cells. As shown in Figure 4A and 4B, LILRA3-overexpressing U937 cells showed a strong ability to engulf beads beginning at 6 hours in comparison with cells expressing the null vector (5.23%±0.21% vs. 3.80%±0.35%, p < 0.01 at 6 hours; 26.00%±0.17% vs. 20.47%±0.42%, p < 0.001 at 12 hours; 30.03%±0.83% vs. 26.43%±0.64%, p < 0.01 at 24 hours). Pattern recognition receptors (PRRs), mainly including mannose receptor (MR) and scavenger receptor (SR), play important roles in phagocytosis. CD206/MRC1 (an important MR) and CD36/SCARB3 (an important class B SR) are two highly effective endocytic receptors expressed on monocytes[31,32]. qRT-PCR and western blotting were employed to further explore whether LILRA3 could alter expression of these two receptors on the plasma membrane (for primers used in the reaction, see Supplementary Table 2). We found that LILRA3 could significantly promote expression of CD36 yet had no influence on CD206 (Figure 4C, 4D, 4E). CD36 specific antibody (BD, San Jose, USA) was added to the culture of LILRA3-overexpressing cell lines, and the phagocytosis efficiency was detected again. The specific antibody could significantly decrease the phagocytosis ability of LILRA3-overexpression cell lines at a concentration dependent manner (p<0.01 for both groups at 12h culture, p<0.01 for 1mg/ml and p<0.001 for 10mg/ml at 24h culture) (Figure 4F). Taken together, our results reveal that LILRA3 enhances the phagocytosis ability of U937 cells by increasing CD36 expression.