T cell pathology in the skin leads to monocyte influx, but we have little understanding of the fate of recruited cells within the diseased niche, or the long-term impact on cutaneous immune homeostasis. By combining a murine model of acute graft-versus-host disease (aGVHD) with analysis of patient samples, we demonstrate that pathology initiates dermis-specific macrophage differentiation and show that aGVHD-primed macrophages continue to dominate the dermal compartment at the relative expense of quiescent MHCIIint cells. Exposure of the altered dermal niche to topical haptens after disease resolution results in hyper-activation of regulatory T cells (Treg), but local breakdown in tolerance. Disease-imprinted macrophages express increased IL-1β and are predicted to elicit altered TNF superfamily interactions with cutaneous Treg, and we demonstrate the direct loss of T cell regulation within the resolved skin. Thus, T cell pathology leaves an immunological scar in the skin marked by failure to re-set immune homeostasis.

ABSTRACT CD4 T-cells require T-cell receptor (TCR) signalling for their activation and differentiation. Foxp3+ regulatory T-cells (Treg) are dependent on TCR signals for their differentiation and suppressive function. However, it is not fully known how TCR signalling controls the differentiation of polyclonal CD4 T-cells upon antigen recognition at the single-cell level in vivo. In this study, using Nr4a3-Tocky ( T imer- o f- c ell- k inetics-and-activit y ), which analyses temporal changes of antigen-reactive T-cells following TCR signalling, we investigated T-cell response to Spike protein fragments (S1a, S1b, S2a, and S2b) upon immunisation. We show that S1a and S2a induced the differentiation of PD1 hi CXCR5 + T follicular helper (Tfh) cells, which is related to CD4 T-cell immunogenicity. In contrast, S1b induced CD25 hi GITR hi PD-1 int Treg, which intermittently received TCR signalling. Using Foxp3-Tocky, which analyses Foxp3 transcriptional dynamics, the S1b-reactive Treg sustained Foxp3 transcription over time, which is a hallmark of activated Treg. Foxp3 fate-mapping showed that the S1b-reactive Treg were derived not from pre-existing thymic Treg, suggesting Foxp3 induction in non-Treg cells. Thus, the current study reveals temporally dynamic differentiation of CD4 T-cells and Treg upon immunisation in the polyclonal TCR repertoire.

T-cells play key roles in immunity to COVID-19 as well as the development of severe disease. T-cell immunity to COVID-19 is mediated through differentiated CD4+ T-cells and cytotoxic CD8+ T-cells, although their differentiation is often atypical and ambiguous in COVID-19 and single cell dynamics of key genes need to be characterized. Notably, T-cells are dysregulated in severe COVID-19 patients, although their molecular features are still yet to be fully revealed. Importantly, it is not clear which T-cell activities are beneficial and protective and which ones can contribute to the development of severe COVID-19. In this article, we examine the latest evidence and discuss the key features of T-cell responses in COVID-19, showing how T-cells are dysregulated in severe COVID-19 patients. Particularly, we highlight the impairment of FOXP3 induction in CD4+ T-cells and how the impaired FOXP3 expression can lead to the differentiation of abnormally activated (hyperactivated) T-cells and the dysregulated T-cell responses in severe patients. Furthermore, we characterise the feature of hyperactivated T-cells, showing their potential contribution to T-cell dysregulation and immune-mediated tissue destruction (immunopathology) in COVID-19.

Severe COVID-19 patients show various immunological abnormalities including T-cell reduction and cytokine release syndrome, which can be fatal and is a major concern of the pandemic. However, it is poorly understood how T-cell dysregulation can contribute to the pathogenesis of severe COVID-19. Here we show single cell-level mechanisms for T-cell dysregulation in severe COVID-19, demonstrating new pathogenetic mechanisms of T-cell activation and differentiation underlying severe COVID-19. By in silico sorting CD4+ T-cells from a single cell RNA-seq dataset, we found that CD4+ T-cells were highly activated and showed unique differentiation pathways in the lung of severe COVID-19 patients. Notably, those T-cells in severe COVID-19 patients highly expressed immunoregulatory receptors and CD25, whilst repressing the expression of FOXP3. Furthermore, we show that CD25+ hyperactivated T-cells differentiate into multiple helper T-cell lineages, showing multifaceted effector T-cells with Th1 and Th2 characteristics. Lastly, we show that CD25- expressing hyperactivated T-cells produce the protease Furin, which facilitates the viral entry of SARS-CoV-2. Collectively, CD4+ T-cells from severe COVID-19 patients are hyperactivated and FOXP3-mediated negative feedback mechanisms are impaired in the lung, which may promote immunopathology. Therefore, our study proposes a new model of T-cell hyperactivation and paralysis that drives immunopathology in severe COVID-19.