27Conventional methods for profiling the molecular content of biological samples fail to resolve 28 heterogeneity that is present at the level of single cells. these data represent a rich resource for future methods aimed at defining cell types and states. 47They will advance our understanding of developmental biology, and constitute a major step 48 towards a comprehensive, single-cell molecular atlas of a whole animal.

Single-cell analysis has become a powerful approach for the molecular characterization of complex tissues. Methods for quantifying gene expression and DNA accessibility of single cells are now well-established, but analysis of chromatin regions with specific histone modifications has been technically challenging. Here, we adapt the recently published CUT&Tag method to scalable single-cell platforms to profile chromatin landscapes in single cells (scCUT&Tag) from complex tissues. We focus on profiling Polycomb Group (PcG) silenced regions marked by H3K27 trimethylation (H3K27me3) in single cells as an orthogonal approach to chromatin accessibility for identifying cell states. We show that scCUT&Tag profiling of H3K27me3 distinguishes cell types in human blood and allows the generation of cell-type-specific PcG landscapes from heterogeneous tissues. Furthermore, we use scCUT&Tag to profile H3K27me3 in a brain tumor patient before and after treatment, identifying cell types in the tumor microenvironment and heterogeneity in PcG activity in the primary sample and after treatment.

One of the central challenges in the field of allo-immunity is deciphering the mechanisms driving T cells to infiltrate and subsequently occupy target organs to cause disease. The act of CD8-dominated T cell infiltration is critical to acute graft-versus-host disease (aGVHD), wherein donor T cells become activated, tissue-infiltrating and highly cytotoxic, causing wide-spread tissue damage after allogeneic hematopoietic stem cell transplant (allo-HCT). However, in human and non-human primate studies, deconvolving the transcriptional programs of newly recruited relative to resident memory T cells in the gastrointestinal (GI) tract has remained a challenge. In this study, we combined the novel technique of Serial Intravascular Staining (SIVS) with single-cell RNA-Seq (scRNA-seq) to enable detailed dissection of the tightly connected processes by which T cells first infiltrate tissues and then establish a pathogenic tissue residency program after allo-HCT in non-human primates. Our results have enabled the creation of a spatiotemporal map of the transcriptional drivers of CD8 T cell infiltration into the primary aGVHD target-organ, the GI tract. We identify the large and small intestines as the only two sites demonstrating allo-specific, rather than lymphdepletion-driven T cell infiltration. The donor CD8 T cells that infiltrate the GI tract demonstrate a highly activated, cytotoxic phenotype while simultaneously rapidly developing canonical tissue-resident memory (TRM) protein expression and transcriptional signatures, driven by IL-15/IL-21 signaling. Moreover, by combining SIVS and transcriptomic analysis, we have been able to work backwards from this pathogenic TRM programing, and, for the first time, identify a cluster of genes directly associated with tissue invasiveness, prominently including specific chemokines and adhesion molecules and their receptors, as well as a central cytoskeletal transcriptional node. The clinical relevance of this new tissue invasion signature was validated by its ability to discriminate the CD8 T cell transcriptome of patients with GI aGVHD. These results provide new insights into the mechanisms controlling tissue infiltration and pathogenic CD8 TRM transcriptional programing, uncovering critical transitions in allo-immune tissue invasion and destruction.One sentence summaryFlow cytometric and transcriptomic analysis reveals coordinated tissue-infiltration and tissue-residency programs driving gastrointestinal aGVHD.