Epigenomic programming of peripheral monocytes determines their transcriptional response to the tumor microenvironment
Classical monocytes are recruited to tumors and undergo transcriptional reprogramming resulting in tumor-promoting functions. Epigenomic features, such as post-translational modification of histones and chromatin accessibility, are key determinants of transcription factor binding and thereby play an important role in determining transcriptional responses to the tissue environment. It is unknown to what extent the epigenetic landscape of peripheral monocytes is rewired by cancer and how this could shape their transcriptional response upon recruitment to the tumor microenvironment. Here we used a combination of genome-wide assays for mRNA expression, chromatin accessibility and multiple histone modifications (H3K4me1, H3K4me3, H3K27ac) in a mouse model to investigate changes in the epigenomic landscape of peripheral monocytes. We then linked these epigenetic alterations to gene expression changes in monocytes occurring in the periphery or during tumor infiltration. We found that the distal tumor caused extensive remodeling of both H3K4me3+ promoters and H3K4me1+ enhancers in peripheral monocytes. Specifically, this involved the repression of interferon-responsive promoters and enhancers as well as the establishment of enhancers harboring binding motifs for transcription factors downstream of inflammatory and cytokine signaling pathways. The enhancers altered in the periphery could be linked to sustained gene expression changes which were less likely to be reversed in the tumor microenvironment. In addition, genes activated upon tumor infiltration showed prior epigenetic priming in peripheral monocytes. Overall, these results indicate that the epigenomic landscape of peripheral monocytes is altered in response to a distal tumor, and this could shape the transcriptional response of monocytes when they encounter microenvironmental signals upon infiltrating the tumor.