Analysis of Single Nuclear Chromatin Accessibility Demonstrates Unique Myeloid Populations Differentially Influence Prognosis in Human Pancreatic Ductal Adenocarcinoma [snATAC]
Background: A better understanding of the pancreatic adenocarcinoma (PDAC) immune microenvironment is critical to improving outcomes. Myeloid cells are of particular importance for PDAC progression. Circulating monocytes are recruited and hijacked by the tumor to become immune suppressive tumor-associated macrophages. Meanwhile, tissue resident macrophages are innately anti-inflammatory and pro-fibrotic. Open chromatin and transcription factor activity provide source and function data for myeloid cells. Thus, we explore single nuclear assay for transposase accessible chromatin (ATAC) sequencing (snATAC-Seq), a method to analyze open gene promoters and transcription factor binding, as an important means for discerning the myeloid composition in human PDAC tumors. Methods: Frozen pancreatic tissues (benign or PDAC) were digested into single nuclei suspensions. snATAC-Seq and sn-Multiome (ATAC + RNA)  libraries were prepared using 10X Chromium technology and sequenced by an Illumina sequencer. Signac was used for preliminary analysis, clustering, and differentially accessible chromatin region identification. snMultiome data was used to validate cell annotations for the snATAC-Seq data. Results: Unique populations comprised the benign and tumor samples. Myeloid cell transcription factor activities were higher in tumor samples than benign pancreatic samples, enabling us to further stratify myeloid populations in PDAC tumors. GO enrichment analysis showed myeloid cells can activate immune response in tumors. Subcluster analysis revealed eight distinct myeloid populations. GO enrichment demonstrated unique functions of different myeloid populations, including IL-1b signaling (recruited monocytes) and intracellular protein transport (dendritic cells). Conclusions: These data suggest snATAC-Seq as a method for retrospective analysis of frozen human pancreatic tissues to identify myeloid populations. Understanding the source specific potential function of myeloid cells is important for patient prognosis in PDAC. Overall design: Nuclei were isolated from 50mg of frozen pancreatic tissue from patients undergoing pancreaticoduodenectomy (benign pancreas or PDAC). Four benign pancreata and 4 PDAC tumors were compared. None of the patients received neoadjuvant chemotherapy.