The endogenous repertoire harbors self-reactive CD4 T cell clones that adopt a follicular helper T cell-like phenotype at steady state.

The T cell repertoire of healthy mice and humans harbors self-reactive CD4 conventional T (T) cells capable of inducing autoimmunity. Using T cell receptor profiling paired with in vivo clonal analysis of T cell differentiation, we identified T cell clones that are recurrently enriched in non-lymphoid organs following ablation of Foxp3 regulatory T (T) cells. A subset of these clones was highly proliferative in the lymphoid organs at steady state and exhibited overt reactivity to self-ligands displayed by dendritic cells, yet were not purged by clonal deletion.

Altered selection on a single self-ligand promotes susceptibility to organ-specific T cell infiltration.

For the large array of self-peptide/MHC class II (pMHC-II) complexes displayed in the body, it is unclear whether CD4+ T cell tolerance must be imparted for each individual complex or whether pMHC-II-nonspecific bystander mechanisms are sufficient to confer tolerance by acting broadly on T cells reactive to multiple self-pMHC-II ligands. Here, via reconstitution of T cell-deficient mice, we demonstrate that altered T cell selection on a single prostate-specific self-pMHC-II ligand renders recipient mice susceptible to prostate-specific T cell infiltration. Mechanistically, this self-pMHC-II complex is required for directing antigen-specific cells into the Foxp3+ regulatory T cell lineage but does not induce clonal deletion to a measurable extent.

Eomes identifies thymic precursors of self-specific memory-phenotype CD8 T cells.

Unprimed mice harbor a substantial population of 'memory-phenotype' CD8 T cells (CD8-MP cells) that exhibit hallmarks of activation and innate-like functional properties. Due to the lack of faithful markers to distinguish CD8-MP cells from bona fide CD8 memory T cells, the developmental origins and antigen specificities of CD8-MP cells remain incompletely defined. Using deep T cell antigen receptor (TCR) sequencing, we found that the TCRs expressed by CD8-MP cells are highly recurrent and distinct from the TCRs expressed by naive-phenotype CD8 T cells.

Regulatory T Cell Development.

Foxp3-expressing CD4 regulatory T (Treg) cells play key roles in the prevention of autoimmunity and the maintenance of immune homeostasis and represent a major barrier to the induction of robust antitumor immune responses. Thus, a clear understanding of the mechanisms coordinating Treg cell differentiation is crucial for understanding numerous facets of health and disease and for developing approaches to modulate Treg cells for clinical benefit. Here, we discuss current knowledge of the signals that coordinate Treg cell development, the antigen-presenting cell types that direct Treg cell selection, and the nature of endogenous Treg cell ligands, focusing on evidence from studies in mice.