Abstract 5459: Unexpected Acceleration of Atherosclerosis in Ldlr-null Mice Reconstituted With Group X Secretory Phospholipase A2-Deficient Bone Marrow
Group X (GX) secretory phospholipase A2 (sPLA2) has the most potent hydrolyzing activity toward phosphatidylcholine and elicits a marked release of arachidonic acid among several types of sPLA2. GX is expressed by vascular cells and inflammatory cells like macrophages and CD4+ T lymphocytes and is therefore believed to play a pathogenic role in atherosclerosis. We evaluated the effects of GX deficiency on systemic immune response and the development of atherosclerosis, a vascular disease sustained by lipid modifications and inflammation. GX KO mice showed increased spleen size confirmed by higher splenocyte number compared to C57BL6 controls (P<0.01). LPS/IFN-γ-stimulated GX-deficient splenocytes produced more IL-12 than control splenocytes (3380 vs 1599 pg/ml, P<0.05), with no difference in IL-10. Purified splenic CD4+ T cells from GX KO mice showed enhanced proliferation and production of the Th1 cytokine IFN-γ (5575 vs 1443 pg/ml, respectively, p<0.05) compared with controls. GX-deficient blood mononuclear cells showed enhanced adhesion on activated HUVECs (8.8 vs 3.2 cell/field, P<0.01) and on LDLr KO carotid arteries (4.2 vs 6.4 cell/mm, P<0.01) ex vivo, compared to control cells. When challenged in vitro to test cell death susceptibility to a variety of apoptotic stimuli, bone marrow-derived GX-deficient macrophages displayed a significant increase of apoptosis compared to control cells. Finally, LDLr KO mice were lethally irradiated and reconstituted with either GX KO or wild type bone marrow. After 8 weeks of fat diet, mice reconstituted with GX KO bone marrow showed a ~100% increase of plaque size in the aortic sinus compared to mice with WT bone marrow (294 254 μm2 vs 142 304 μm2, respectively, P=0.001), despite similar plasma cholesterol level. This was associated with a more advanced plaque phenotype and marked accumulation of TUNEL-positive apoptotic cells (2.4 % vs 0.08 P<0.001). Immunologic analysis of splenocytes and splenic CD4+ T cells from GX/LDLr chimeric mice showed increased immune cell activation and Th1-related cytokine production. In conclusion, GX sPLA2 deficiency increases innate and adaptive immune responses, enhances macrophage susceptibility to apoptosis and accelerates atherosclerosis in mice.