# Sex-associated transcriptional changes to synovial macrophages in the aging joint

## TL;DR

This study shows how aging affects synovial macrophages differently in male and female mice, leading to changes in immune function and inflammation in joints.

## Contribution

The paper reveals sex-specific transcriptional changes in synovial macrophages during aging using single-cell RNA sequencing.

## Key findings

- Ly6C+ macrophages increase in old mice of both sexes, while CX3CR1+ lining macrophages decrease.
- MHCII+ macrophage proportions differ between sexes, with Arg1-expressing cells increasing in females and decreasing in males with age.
- Aging leads to a depletion of transitional macrophage cells, suggesting a differentiation defect in the synovial macrophage compartment.

## Abstract

Synovial macrophages are critical to tissue maintenance and immune homeostasis in the joints. However, the function of synovial macrophages is compromised with age, leading to increased susceptibility to chronic inflammation and arthritis. Here, we compare the transcriptional heterogeneity of synovial macrophages in young and old joints from male and female mice to better understand the impact of aging and the role of sex.

We compared synovial macrophage composition and transcriptional profiles in young vs. old joints from male and female mice using single-cell RNA sequencing with cell-surface protein detection (CITE-seq).

We defined five major synovial macrophage subpopulations: CX3CR1+ lining, CD163+ interstitial, MHCII+ monocyte-derived, Ly6C+ infiltrating, and Ctsk-expressing osteoclast-like cells, across age and sex. Ly6C+ macrophages were expanded in old mice of both sexes compared to young, while CX3CR1+ lining macrophages were reduced. MHCII+ macrophage proportion differed between sexes, with Arg1-expressing cells driving an increase in females and a decrease in males with age. Age-associated differential expression was positively correlated between sexes in the CX3CR1+ and CD163+ subpopulations and negatively correlated in the MHCII+ subpopulations. Significantly enriched pathways included upregulated electron transport chain signaling in interstitial macrophages in both sexes and significantly downregulated MAPK signaling in Ctsk+ macrophages in females. Trajectory analysis suggested that the aging synovial macrophage compartment was depleted for transitional cells, which may indicate a macrophage differentiation defect.

In summary, we report on both conserved age-related changes and those that differed between males and females. Our results provide insights on how macrophage heterogeneity changes with age in a sex-dimorphic manner and lays the foundation for research into their role in age-associated diseases, such as arthritis.