The precursors of CD8+ tissue resident memory T cells: from lymphoid organs to infected tissues

CD8+ tissue resident memory T cells (TRM cells) are essential for immune defence against pathogens and malignancies, and the molecular processes that lead to TRM cell formation are therefore of substantial biomedical interest. Prior work has demonstrated that signals present in the inflamed tissue micro-environment can promote the differentiation of memory precursor cells into mature TRM cells, and it was therefore long assumed that TRM cell formation adheres to a ‘local divergence’ model, in which TRM cell lineage decisions are exclusively made within the tissue. However, a growing body of work provides evidence for a ‘systemic divergence’ model, in which circulating T cells already become preconditioned to preferentially give rise to the TRM cell lineage, resulting in the generation of a pool of TRM cell-poised T cells within the lymphoid compartment. Here, we review the emerging evidence that supports the existence of such a population of circulating TRM cell progenitors, discuss current insights into their formation and highlight open questions in the field.