Disrupted Lipid Homeostasis and Loss of IRF4 Impairs B-Cell and Plasma Cell Development in a Murine Model of Accelerated Aging

Abstract

The aging immune system is characterized by increased bone marrow (BM) adiposity, myeloid skewing, impaired lymphocyte maturation, and loss of hematopoietic stem and progenitor cell (HSPC) regenerative capacity. Human systemic aging studies have found alterations in lipid and fatty acid metabolism, such as increased saturated fatty acid accumulation and decays in n-3 polyunsaturated fatty acid (PUFA) species leading to cellular membrane rigidity. However, the extent to which impaired fatty acid metabolism contributes to immune system aging and HSPC development is largely unexplored. In this study, we performed multi-omics studies to characterize BM aging phenotypes in a recently described mouse model that exhibits signs of accelerating aging in brain and eye tissues. This robust aging model expresses a mutant inactive form of the very long chain (VLC) PUFA elongation enzyme, Elongation of very long chain fatty acids protein 2 (ELOVL2), which is responsible for the elongation of 22-carbon PUFAs to 24-carbon PUFAs. These VLC-PUFAs are synthesized in vivo and are not readily available through dietary means. Previous studies using this mouse model demonstrated that Elovl2 DNA methylation-dependent and mutation-associated loss of function is a key driver of age-related defects in eye and brain tissues, however the hematopoietic system has not been characterized to date in this model. To investigate the extent to which impaired VLC-PUFA production in the setting of ELOVL2 enzymatic deficiency might also accelerate blood stem cell aging phenotypes, multi-omics studies were performed in murine BM samples from young (3 months old) and aged (18 months old) wild-type mice, compared with age-matched Elovl2 mutant mice. Total BM samples were subjected to lipidomics analyses, whole transcriptome RNA-sequencing (RNA-seq), and ATAC-seq analyses. Lipidomic profiling showed an almost complete depletion of direct and secondary products of ELOVL2 in BM isolated from mutant animals, suggesting significant changes in the biophysical properties of membranes in the cells. Gene set enrichment analyses (Reactome) of RNA-seq data revealed that pathways involving B cell receptor (BCR) signalling and immunoregulatory interactions between lymphoid and non-lymphoid cells were among the most differentially regulated in Elovl2 mutant mouse BM compared to age-matched controls. Key lymphoid lineage maturation markers (Cd19, Cd22, and Slamf7, among others) were downregulated in the BM of aged wild-type mice, and to a greater extent in Elovl2 mutant animals. This was accompanied by a significant downregulation of the lymphoid and plasma cell transcription factor, interferon-regulatory factor-4 (Irf4), in aged and Elovl2 mutant groups compared with young mice. In addition, phenotypic analyses of B cell lineage marker protein expression were performed by flow cytometry to confirm depletion of lymphoid cell populations and Irf4 expression in this model. Finally, ATAC-seq data revealed changes in DNA accessibility related to Elovl2 mutation, suggesting significant epigenetic changes in nuclear programs. These findings suggest that Elovl2 loss of function in a murine model of accelerated aging promotes diverse lipidomic, transcriptomic and chromatin alterations targeting lymphoid populations in murine BM, which may be mechanistically driven by a downregulation of Irf4. Together, these results provide insights into a functional link between disrupted lipid homeostasis and aging-related defects in lymphocyte maturation typified by loss of B-cell and plasma cell populations. Together, modulation of lipid catabolism pathways may provide a novel avenue for reversing a variety of age-related immune deficits including enhanced susceptibility to infection.