# Not Just Passing Through: Bone Marrow as a Home for Diverse Resident T Cells

## TL;DR

The bone marrow hosts diverse resident T cells, including CD69− memory T cells and Tr1-like cells, expanding our understanding of immune niches.

## Contribution

The study identifies new T cell subsets in bone marrow, challenging the assumption that only CD69+ cells are resident.

## Key findings

- CD69− CD4+ and CD8+ memory T cells in bone marrow have distinct transcriptional profiles and TCR repertoires.
- CD69+EOMES+GzmK+ Tr1-like cells in bone marrow are maintained by IL-15.
- Bone marrow contains a broader range of resident T cells than previously recognized.

## Abstract

Recent studies add to our understanding of T cell residency in the human bone marrow (BM). In the May 2025 issue of EJI, Schneider Revueltas et al. demonstrate that CD69− CD4+ and CD8+ memory T cells, presumed to be recirculating, have a distinct transcriptional cluster profile and a distinct TCR repertoire from their blood counterparts. These findings are in line with CD69− memory T cells taking up residence in the BM, similar to their CD69+ counterparts. In parallel, Pulvirenti et al. identify a subset of CD69+EOMES+GzmK+ Tr1‐like cells in the BM maintained by IL‐15. Together, these studies refine our understanding of the BM as a heterogeneous immune niche and suggest a broader definition of resident cells within this tissue.

CD8+CD69− memory T cells (Schneider Revueltas et al.) were identified to reside in bone marrow besides canonical CD8+CD69+ tissue‐resident memory T cells (TRM). In addition, CD4+CD69+ Tr1‐like cells (Pulvirenti et al.) were preferentially found in bone marrow. Together, these findings illustrate diversity in the T cell subsets that reside long‐term within the bone marrow.