Abstract 1485: Deficiency of the Regulatory Molecules PD-L1 and PD-L2 Results in Increased Atherosclerosis in LDLR-null Mice
Background: T-cell-mediated immune responses to plaque antigens can promote proatherogenic inflammatory events. PD-L1 and PD-L2 are B7-family molecules expressed on several cell types that down-regulate T-cell activation via the inhibitory molecule PD-1 on T-cells. We hypothesized that PD-L1 and PD-L2 are important in regulating T cell responses and thereby limiting atherosclerosis.
Method: Triple knock-out (TKO) mice were bred from PD-L1/PD-L2 double knockout mice and LDL receptor (LDLR) deficient mice, all on a C57Bl/6 background. Atherosclerosis was analyzed after 10 weeks of high-cholesterol diet.
Results: PD-L1/PD-L2 deficiency significantly increased the atherosclerotic burden. TKO mice had a three-fold increase in lesion area in the aortic arch compared to LDLR KO controls: mean area 122 ± 16 x 103 μm2 (N=23) vs 41 ± 9 x 103 μm2 (N=14), P=0.002, as well as more than a two-fold increase in lipid positive area. The increased atherosclerotic burden was also evident in the descending aorta with a three fold increase in atherosclerotic fractional area: 5.5 ± 0.8 % (N=8) vs 1.7 ± 0.29 % (N=6), P=0.002. There was no difference in average weights or lipid levels between the groups. The TKO mice had a local hyperactive inflammatory/immune response as evident by enlarged draining iliac lymph nodes with increased mean total cells as well as increased numbers of activated CD4 T-cells. Serum levels of the pro-inflammatory cytokine TNF-α were increased in the TKO mice compared to controls. We analyzed CD4+ T cell activation by anti-CD3 or specific antigens in the presence of cholesterol loaded peritoneal-derived macrophages or splenic antigen presenting cells (APCs). The results demonstrated significantly increased activation by PD-L1/PD-L2 null cells compared to PD-L1/PD-L2 expressing control cells. Significantly, APCs from atherosclerotic TKO mice induced increased T cell responses to oxidized-LDL, a putative antigen important in propagating atherosclerosis, compared to APCs from control mice.
Conclusion: PD-L1/PD-L2 has an important role in down-regulating immune responses and limiting lesion development in the context of atherosclerosis. These effects are mediated through regulation of T cell responses to antigen presenting cells.