Elucidating immune-regulatory features of the heterogeneous TAM population in brain metastasis

• Applicant: Dr. Michael Schulz
• Subject Area: Molecular and Cellular Neurology and Neuropathology
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 533752938

The central nervous system (CNS) is divided into distinct anatomical areas, each of which harbors different neural cell types. While the healthy brain parenchyma is populated by microglia (MG) as the sole immune cell type, adjacent areas including the skull bone marrow, and the meninges are populated by various immune cell types and own resident macrophage populations (CNS-associated macrophages, CAMs). Tissue-resident macrophages exist in different organs and play critical roles in tissue homeostasis. Cancers induce the recruitment of macrophages (tumor-associated macrophages, TAMs), which are often associated to pro-tumoral functions including immunosuppression within the tumor microenvironment (TME). Primary and secondary brain tumors (BrM) induce the recruitment of a variety of lymphoid and myeloid immune cells, with a predominant myeloid compartment. TAMs in BrM derive from MG (TAM-MG), and a mainly monocyte-derived macrophage (TAM-MDM) population. Both have been shown to manipulate the TME by direct or indirect interactions with other cell types, ultimately leading to disturbed anti-tumor functions of e.g. infiltrating lymphocytes. Our recent data and findings from others indicate, that TAM-MDMs form a highly heterogeneous population in mouse models of BrM, and patient samples. I hypothesize that local reservoirs of the skull bone marrow, the meninges and perivascular areas contribute cells to the TAM-MDM pool. Compared to TAM-MG, this non-MG TAM (nMG-TAM) population exhibits stronger immunosuppressive phenotypes. However, the underlying regulatory molecules remain unknown. In order to understand the cellular contribution of CNS-resident niches to the nMG-TAM population, I seek to perform in-depth analyses by employing high dimensional single cell RNA sequencing, and multiplexed histologic approaches. Experimental BrM will be induced in transgenic mice (Mrc1CreERT2xRosa26tdtomatoxCcr2-KO). In addition, a skull bone transplantation model (Ubc-GFP to wild-type) will be utilized. Together, both approaches will help to evaluate the contribution of resident monocyte/macrophage reservoirs to the nMG- TAM pool, thus increasing our understanding of BrM-TAM heterogeneity. Furthermore, I plan to examine the role of MS4A transmembrane molecules for establishing tumor-promoting functions in TAMs. Here, I want to employ another transgenic model (Cx3cr1-CreER/+ x Ms4a4afl/fl). Our expression data of TAMs from different BrM models suggest cell type-restricted co-expression of Ms4a genes. The corresponding proteins are implicated in regulating immune cell phenotypes and functions, hence representing molecules for future TME-targeted therapeutic approaches. Additionally, all findings will be complemented by analyzing human patient-derived tissue samples.

• DFG Programme: WBP Position