PD-1 prevents pathogenicity of effector CD8 T cells that infiltrate skin under homeostatic conditions
All healthy individuals have autoreactive T cells in their peripheral T cell pool, but peripheral tolerance mechanisms prevent these T cells from causing autoimmunity. Two primary mechanisms of peripheral tolerance are T cell anergy and clonal deletion, which occur after tolerogenic priming of T cells by dendritic cells (DCs). Induction of anergy and deletion are impaired in the absence of checkpoint receptors (like PD-1), but, once established, they are not reversed by blockade of checkpoint receptors. Thus, the occurrence of immune-related Adverse Events (irAEs) in up to 70% of patients treated with checkpoint inhibitor (CPI) immunotherapy has raised questions about how checkpoint receptors are functioning to maintain peripheral tolerance of autoreactive T cells under homeostatic conditions. To address this, we developed a novel mouse model for studying peripheral T cell tolerance in the skin in which the development of localized autoimmune-like pathology was dependent on skin-infiltrating CD8 T cells following skin-specific expression of T cell antigens (Ags) and administration of PD-1 blocking antibodies. Interestingly, Ag-specific CD8 T cells were found to infiltrate Ag-expressing skin even in the absence of PD-1 blockade, however they did not cause disease and in fact co-existed with the skin cells expressing their cognate Ag. At the protein level, non-pathogenic skin-infiltrating CD8 T cells produced lower amounts of many T cell effector proteins compared to their pathogenic counterparts, explaining the lack of cutaneous pathology, but non-pathogenic T cells were functional because they remodeled the local myeloid cell compartment. Moreover, and unlike in other models of tolerance, non-pathogenic and pathogenic skin-infiltrating CD8 T cells were nearly identical at the transcriptional level, indicating that both T cell populations were differentiating towards the same effector state. In this context, checkpoint receptors prevented the majority of the non-pathogenic CD8 T cells from reaching their terminal, fully effector and pathogenic state, thus maintaining tolerance in the skin. Finally, analysis of skin biopsies from two patients with cutaneous lichenoid irAEs showed presence of clonally expanded effector CD8 T cells with comparable transcriptional profiles in both lesional and non-lesional skin. Thus, our data are in line with a model of peripheral T cell tolerance where PD-1 allows peripheral Ag-specific effector CD8 T cells to co-exist in a tissue where their cognate Ag is expressed without causing pathology and provide a mechanism for the development of cutaneous lichenoid irAEs upon administration of CPI immunotherapy. Overall design: Prepared single cell suspensions from the 4-OH-Tam-treated skin area of for each mouse  and performed Chromium scRNAseq via 10x Genomics. A total of 21178 cells were sequenced  from the six experimental samples (two Ag OFF mice, two Ag ON mice, and 2 Ag ON/CPI mice)