Effect of aging on rat testicular interstitial compartment: functional losses in endothelial and mesenchymal cells and activation of immune cells.

Human aging is associated with testosterone decline and spermatogenic defects, impacting fertility and health, yet the underlying cellular mechanisms remain poorly understood. This study aimed to establish a regulatory cell atlas of Leydig cells in aging rat testes and characterize intercellular interactions. Using single-cell RNA sequencing and immunohistochemistry, interstitial cells from young (3-month) and aged (21-month) BN rat testes were analyzed. Among 8 identified interstitial cell types, aging was not synchronous, but with remarkable heterogeneity and stochasticity. Mesenchymal cells exhibited reduced expression of genes involved in detoxification and extracellular matrix maintenance, alongside increased immune regulation genes. Endothelial cells, the most communicatively active population, showed decreased angiogenesis and cell adhesion/migration but enhanced immune responses and leukocyte chemotaxis. In immune populations, dendritic cells decreased while lymphocytes increased, with no significant change in macrophages. Transcriptomic shifts indicated a transition from innate to adaptive immunity. The loss of dendritic cells and gain of lymphocytes may be linked to CCL7 and CXCL9-12, as receptors Ccr1/Ccr5 and Cxcr3 were upregulated in lymphocytes but downregulated in dendritic cells. Overall, inflammatory activation and functional losses occurred in both immune and non-immune cells during testicular aging. This innate-to-adaptive immune shift may further compromise non-immune cell functions, accelerating testicular aging.