Identification of the unique functional phenotype of iMATE-defining monocytes that drive hepatic T cell proliferation

Intrahepatic myeloid-cell aggregates form in response to Toll-like-receptor 9 (TLR9) signaling in a TNF-dependent fashion to provide a unique anatomic structure that drives local proliferation of cytotoxic CD8 T cells (iMATEs) and confers protection against viral infection. Yet, the identity of the iMATE-defining myeloid cell population remained elusive.

We systematically analyzed the phenotype of myeloid cells in the murine liver after TLR9 activation using a set of different methodologies. Initial flow cytometric phenotypic characterization and tSNE analysis revealed a complex composition of monocytes and newly differentiating macrophages that hinted towards a sequential replacement of liver-resident macrophages followed by repopulation through bone marrow derived inflammatory monocytes. Laser-capture microdissection and genome wide analysis of gene expression identified a set of marker proteins that were validated by flow cytometry and led to the definition of a particular phenotype of monocyte-derived macrophages exclusively found in iMATEs but not elsewhere.

Functional assays of these iMATE-defining monocyte-derived macrophages revealed a high potency in the induction of CD8 T cell proliferation, in the differentiation towards GzmB expression rendering them efficient killer cells and in cross-presenting soluble antigens to CD8 T cells. Liver macrophages, in contrast, failed to provide any support for T cell proliferation and did not show significant cross-presentation capacity.

The transient presence of iMATE-defining monocyte-derived macrophages in the liver indicates that protective hepatic T cell immunity is determined by the dynamics of the changes in inhibitory vs. stimulatory macrophage populations, which do not fall into the conventional M1/M2 categories but are related to iMATE formation.