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15Quantifying tissue-infiltrating immune and stromal cells provides clinically relevant information for 16 various diseases, notably cancer. While numerous methods allow to quantify immune or stromal cells 17 in human tissue samples based on transcriptomic data, very few are available for mouse studies.
18Here, we introduce murine Microenvironment Cell Population counter (mMCP-counter), a method 19 based on highly specific transcriptomic markers that allow to accurately quantify 12 immune and 4 20 stromal murine cell populations. We validated mMCP-counter with flow cytometry data. We also 21 showed that mMCP-counter outperforms existing methods. We showed in mouse models of 22 mesothelioma and kidney cancer that mMCP-counter quantification scores are predictive of response 23 to immune checkpoint blockade Finally, we illustrated mMCP-counter's potential to analyze immune 24 impacts of Alzheimer's disease. mMCP-counter is available as an R package from GitHub: 25 https://github.com/cit-bioinfo/mMCP-counter.26 3 Main text (2,967 words) 27 28 Introduction 29 30 For a large number of diseases, such as inflammatory diseases or cancer, it is often crucial to 31 accurately determine the cellular composition of the tissue where the pathology develops, in terms of 32 immune and stromal cell populations. An array of methods are available to obtain these data from 33 human samples, either by immunochemistry or cytometry, or computationally from transcriptomics 34 data 1 .
35The analysis of the immune and stromal composition of tissues is particularly critical in cancer studies.
36Indeed, tumors are highly heterogeneous tissues which are infiltrated by a variety of immune and 37 stromal cells 2 . It was shown that immune cell densities were associated with prognosis 3 : For instance,
38CD8 + T cells density correlates with prolonged patient survival in most cancers, whereas M2-polarized 39 macrophages are generally associated with a poor prognosis 3 .
40The transcriptome of a bulk tissue sample yields the averaged expression of genes across all the cells 41 present in the sample. As some genes are uniquely expressed in some specific cell populations, their 42 expression can be used to determine the abundance of the underlying cell populations. Using this 43 property, we have previously reported on MCP-counter, a method designed to quantify the immune 44 infiltrate of heterogeneous human tissues 4 , currently one of the best performing methods for this 45 purpose 5 .
46While murine models are widely used to decipher the pathophysiological mechanisms of various 47 diseases, including inflammatory diseases and cancer, the computational methods currently available 48 to measure the immune and stromal composition of murine tissues are few and limited, as compared 49 to what is available for human samples 6 .
50Here, we introduce murine Microenvironment Cell Populations counter (mMCP-counter), the 51 adaptation of the MCP-counter method to murine samples (Figure 1), which was made possible 52 thanks to the release o...