Sex-Stratified Aging Trajectories Reshape Cell-Type–Specific Gene Programs in Human Peripheral Immunity

Aging and sex are among the strongest determinants of immune phenotypes and multiple sclerosis (MS) risk, yet their joint transcriptional architecture in human blood cell subsets remains underdefined. Re-analyzing a sorted-cell bulk transcriptomic cohort (GSE137143) comprising CD4⁺ T cells, CD8+ T cells, and CD14+ monocytes from adults aged 19–72 years, we quantified gene-wise age-associated expression changes (age slopes), capturing the direction and magnitude of transcriptional change across adulthood, and constructed sex-stratified weighted gene co-expression networks (WGCNA) to group genes into coordinated biological programs. Module eigengenes (MEs), which summarize the overall activity of each co-expression module, were regressed on age with cell-composition covariates; functional interpretation was supported by dual Gene Ontology enrichment strategies (over-representation analysis and preranked GSEA) to link age-associated modules to immune-relevant pathways. We further developed a sample-resolved composite visualization that orders individuals by age and annotates per-module effect sizes (β) and statistical significance, making module activity trajectories interpretable at the individual-sample level. Cross-line validation demonstrated sign concordance between gene-level age slopes and ME-level age associations, supporting that the observed aging signals reflect coherent, program-level transcriptional trajectories rather than isolated gene-level fluctuations. At the gene level, significant aging-associated genes (FDR < 0.05) were most abundant in CD8⁺ T cells from females (329 genes) and CD4⁺ T cells from females (72 genes), with more modest signals in males (CD8_M: 10; CD4_M: 15) and none detected in monocytes. At the module level, aging was characterized by a robust age-associated down-regulation of cell-cycle and mitotic programs in T cells—most pronounced in CD8⁺ T cells (e.g., a representative cell-cycle module showed a strong negative age association, P = 1.48×10−8)—consistent with an age-related decline in proliferative capacity. In contrast, monocytes exhibited male-skewed activation of immune-receptor signaling pathways (including receptor-mediated signaling programs highlighted by GSEA) consistent with heightened innate immune activation in aging males. Female CD4+ T cells showed attenuation of nuclear-division programs (e.g., nuclear-division–related pathways showed significant negative age associations in females, P = 0.022), while several female monocyte/CD4 modules were annotated by cilium/cell-projection terms, suggesting sex-dependent remodeling of cellular structure and signaling with age. Together, these results delineate sex-specific aging trajectories across immune cell types and provide reproducible analytical outputs to facilitate reuse and independent validation.