# Single‐Cell Profiling Across Immune Tissues and Organs Reveals Immunosenescence Signatures in Male Rhesus Monkeys

## TL;DR

This study maps immune aging in male rhesus monkeys using single-cell profiling, identifying key biomarkers like GZMB and BHLHE40 linked to immunosenescence.

## Contribution

The study provides a non-human primate single-cell atlas of immune aging and identifies novel biomarkers and regulatory factors specific to aging immune cells.

## Key findings

- GZMB expression is upregulated in aged monkeys across multiple cell types, suggesting a role in immunosenescence.
- BHLHE40 is a key transcription factor in CD8+ T cells during aging, regulating inflammation and exhaustion-related genes.
- PDCD4low naïve B cells decline with age in bone marrow, potentially impairing humoral immunity.

## Abstract

Aging gradually impairs immune system function, yet its systemic features across immune organs remain poorly characterized in primates. Here, we perform single‐cell transcriptomic profiling of bone marrow, spleen, mesenteric lymph nodes, and peripheral blood mononuclear cells from young and naturally aged male rhesus monkeys. Our study revealed extensive transcriptional remodeling across tissues, particularly the marked upregulation of GZMB expression across multiple cell types in aged monkeys, highlighting it as a candidate biomarker of immunosenescence. Gene regulatory network analysis identifies BHLHE40 as a key transcription factor enriched in multiple CD8+ T cell subtypes during aging, regulating pro‐inflammatory and exhaustion‐related genes. We also observe an age‐associated expansion of CD8+ central memory T cells with increased CCL5 and reduced IL7R expression, consistent with a shift toward a dysfunctional state. In the bone marrow, we discover a distinct naïve B cell population with low PDCD4 expression that declines with age, potentially compromising humoral immunity. These findings offer a comprehensive single‐cell atlas of immune aging in a non‐human primate model, providing novel insights into cell‐type‐specific and tissue‐dependent features of immunosenescence. Our work establishes a valuable resource for future translational studies and biomarker discovery in human aging.

Single‐cell profiling across bone marrow, spleen, mesenteric lymph, and blood in rhesus monkeys reveals organ Immunosenescence. GZMB rises with age, particularly in cytotoxic and terminally exhausted CD8+ T cells, and BHLHE40 emerges as a key transcription factor enriched across multiple CD8+ subsets, regulating pro‐inflammatory and exhaustion‐related genes; these patterns replicate in human bone marrow and PBMCs. PDCD4low naïve B cells decline in bone marrow, and Mesenteric lymph cDC2 cells show reduce antigen presentation. These findings highlight candidate biomarkers and regulatory nodes for intervention.