Apolipoprotein E reduces the number and activation of non-invariant Natural Killer T cells

Abstract

Natural Killer T (NKT) cells, which modulate atherosclerosis, include two groups; invariant (iNKT) and variant (vNKT). These subsets differentially regulate the disease progression. Yet, the role of vNKTs in atherosclerosis remains unclear. We induced atherosclerosis by feeding high-fat diet (HFD) to Apoe-/- and analyzed the vNKTs in the liver and spleen. The vNKTs were termed non-iNKTs since they were negatively selected within the NKT population. Available literature suggests NKTs as lipid-recognizing cells; however, to our surprise, the non-iNKT numbers and phenotype remained unchanged between HFD-fed and chow-fed Apoe-/-. This was a blindsiding and unexpected outcome of the non-iNKTs being unaltered and unaffected with or without HFD, indicating no observable impact of atherosclerosis on these subsets. Albeit remaining unperturbed by atherosclerosis, these non-iNKTs demonstrated an identical but unique increase and upregulated activation in both the chow and HFD-fed Apoe-/-. These results instigated an investigation of the baseline correlation of the non-iNKTs between young C57BL/6 (WT) and Apoe-/-. Previously unknown and confounding results revealed upregulated activation and increased non-iNKT numbers but decreased IL-4+ non-iNKTs in the Apoe-/- compared to WT. Furthermore, HFD-fed WT that developed dyslipidemia, elucidated increased hepatic non-iNKTs and splenic IFN-gamma+ non-iNKTs compared to chow-fed WT controls. These results were not perceived in chow and HFD-fed Apoe-/-. Although lipid-responsive, non-iNKTs in Apoe-/- mice failed to respond to lipid stress, unlike those in dyslipidemic WT mice. These findings reveal that loss of Apoe, rather than atherosclerosis itself, drives altered non-iNKT biology. Thus, Apoe deficiency intrinsically dysregulates non-iNKTs, masking disease-associated immune changes. Apoe loss alters non-iNKT number and function, independent of atherosclerosis, and challenges the interpretation of immune responses by NKT subsets in Apoe-/- models. Therefore, this study warrants the use of Apoe null mice in studying NKT cells.