Mechanisms of resident T cell-driven tissue responses during the onset and recurrence of human skin inflammation

Long-lived tissue-resident memory T cells (TRM) reside in nonlymphoid organs and can drive direct cytotoxicity, focal cytokine release and potent tissue-wide anti-infectious responses upon antigenic challenge. TRM cells poised to pathogenic responses have been identified in active and resolved psoriasis and mice models of allergic contact dermatitis (ACD). It is challenging to investigate the interactions between TRM cells and the local microenvironment in human tissues, and whether these cells promote disease in the absence of circulating T cells is less studied. This thesis focuses on the functional consequences of TRM cell activation inside the skin. PAPER I: TRM cells can provide protection from infections. The retention marker CD49a was correlated to both the epidermal location of skin TRM cells and their cytotoxicity. IL-15 unleashed the killing capacities of CD8+CD103+CD49a+ T cells. The expression of CD49a in CD8+CD103+ skin T cells was associated with more IFN-γ release compared to CD8+CD103+CD49a-, which were conversely better IL-17 producers. CD49a expression delineated a CD8+ TRM cell specialization that was conserved in two inflammatory skin diseases psoriasis and vitiligo that were respectively enriched for CD8+CD103+CD49a- and CD8+CD103+CD49a+ TRM cells. PAPER II: Psoriasis is linked to overproduction of IL-17 and Type 1 interferon is implicated as a disease trigger in the early events transforming never-lesional psoriasis (NLP) into full-blown psoriasis. CCR6+CD49a- TRM cells poised towards IL-17 production were enriched in NLP, possibly due to microbe-induced epidermal chemotaxis. Activation of skin-resident T cells in NLP skin triggered Type 1 interferon tissue responses, potentially via IFN-γ-induced release of IFN-α in keratinocytes. As IFN-γ-potent TRM cells accumulate in NLP epidermis, our findings suggest that Type 1 interferon release in NLP could be driven by TRM cells. PAPER III: TRM cells poised to IL-17 and IL-22 production are retained in the epidermis in resolved psoriasis. In healthy and diseased skin, the activation of T cells within skin explants using the pan-T cell-activating antibody OKT-3 led to interferon-driven core-response CXCL10 and CXCL9 expression. Additionally, IL-17-specific transcriptional signature was induced in resolved and active psoriasis sample, and the magnitude of this response was correlated with relapse shortly upon withdrawal of UVB treatment. PAPER IV: An upregulation of S100As transcripts persisted in the long-term in the epidermis of patients with resolved allergic contact dermatitis (ACD), indicating a disease scar. While inflammatory transcripts CXCL10, GZMB, and MMP12 were normalized after antigen exclusion for two months and two years, they were quickly induced in resolved epidermis upon exposure to the allergen. MMP12 was specifically upregulated in the epidermal compartment and codes for a protein capable of degrading the collagen IV that is a constituent of the skin basement membrane. In conclusion, the TRM-driven tissue responses in human healthy and inflamed skin are highly compartmentalized and disease-specific. This concords with the functional heterogeneity of TRM cells themselves but also relies on their interplay with the stromal cells, which can help unveil pathogenic mechanisms in these relapsing-remitting inflammatory skin diseases. Preventing the TRM cell establishment or favoring their displacing by topical treatments could lead to significant improvements in the care of patients suffering from inflammatory diseases