CCR2+ monocytes promote memory CD8+ T-cell differentiation via membrane-bound TGF-β.

Upon antigen recognition, CD8⁺ T cells undergo robust expansion and differentiation to give rise to effector and memory CD8⁺ T cells. The spatial determinants of the fate of effector and memory CD8⁺ T cells during acute infection are poorly understood. By integrating single-cell RNA sequencing (scRNA-seq) and spatially resolved transcriptomics, we revealed that naïve CD8⁺ T cells adopted a divergent trajectory in which they rapidly differentiated into memory precursor (MP) cells and IFN-responsive cells, with the latter representing the entry point of the effector T-cell lineage. In the spleen, monocytes largely colocalized with CD8⁺ MP cells following antigen stimulation. Specifically, compared with dendritic cells (DCs), the Ly6C^hiCCR2⁺ subset of monocytes promotes memory CD8⁺ T-cell differentiation. Mechanistically, monocytes express high levels of membrane-bound transforming growth factor-β (TGF-β), which is activated by thrombospondin-1 (TSP-1) to drive the memory CD8⁺ T-cell program through Smad signaling. Overall, our study reveals a novel spatial mechanism for CD8⁺ T-cell fate decisions, shedding light on the importance of monocytes in fostering memory CD8⁺ T-cell development in a cell‒cell contact- and TGF-β-dependent manner.