Shifting immune-suppressed monocyte-derived macrophage programs into microglia-like reactive cells in Glioblastoma

Glioblastoma (GBM)is the most common primary brain tumour and is inevitably lethal. Given its poor response to standard treatment with surgery, radiation and chemotherapy, it is an attractive target for novel cures such as immunotherapy. In GBM, tumour-associated microglia/macrophages (TAMs),which constitute up to 30-40%of the bulk tumour mass, outnumber infiltratinglymphocytesand play key roles in immune escape mechanisms[1]. This scarcityof lymphocytesin the tumour microenvironment is in contrast with othertumour types, such as melanoma or lung cancer, therefore classifying GBM into the category of “cold tumours”.Hence, due to their substantialnumberand their ability to infiltrate the tumour, TAMs area key target for GBM immunotherapy.In GBM, TAMs are mainly composed of resident microglia, which colonize the brain early during development giving rise to a uniquecentral nervous system-specific immune population[2], and monocyte-derived macrophages, which extravasate the disrupted blood brain barrier due to the tumour-induced inflammatory environment[3]. It is currently emerging that these two myeloid cell populations possess different properties in the tumour microenvironment displayingtumour-supportive and/or anti-tumour roles [4]. Therefore, immunotherapies, which seeks to target TAMs indiscriminately,may be counterproductive. For example, as TAMs critically depend on colony-stimulating factor-1 (CSF-1)for their survival,differentiation and proliferation, murinegliomas can be regressed following CSF-1 receptor (CSF-1R) blockade [5]. However, clinical trials in GBM patients targeting CSF-1R have so far failed to increase overall survival[6].Evidence suggests that subsets of TAMs are resistant to CSF-1R inhibition, thus pointing towards TAMs heterogeneity[7]. Attemptsto specifically target peripheral macrophages, for example limiting monocyte infiltration viaCcl2genetic ablation,prolongs the survival of tumour-bearing mice [8], but these approaches have not been applied to patients yet.Notably, after CNS injuries, microglia-mediated phagocytosis and inflammation are suppressed by monocyte-derivedmacrophages, suggestingthat macrophages entering the CNS provide a regulatory mechanism that controls acute and long-term microglia-mediated inflammation[9], which in the GBM framework might promote microglia tumour-supportive features.Therefore, harnessingthe complexity and diversity of TAM adaptive features is critical to develop novel personalized immune therapeutic strategies for GBM patients.
2The present Action LIONSproposal resideson the transcriptionaland functional diversityof specific microglia-and monocyte-derived macrophage-like subsetsinGBM.Our hypothesisisthatspecific immune-suppressed peripheral macrophage subpopulations displayingdifferential adaptive features to the tumourmay bespecifically targeted and reprogrammedaccording to their ontogeny.We aim to identify genetic targets allowing shiftingimmune-suppressed monocyte-derived macrophage programs into microglia-like reactive cells in the tumour microenvironment,therefore steeringpro-tumorigenic immune-suppressed peripheral macrophages towards anti-tumour functions. Restoring their efficient immunological features, such as enhancing their phagocytic and antigen presenting cell properties to respectively engulf tumour cells and recruit lymphocytes, will be a crucial milestone for novel immunotherapeutic approaches. To achieve our goal, we willcombineexperimental and computationalanalyses in the framework of an interdisciplinary approach.We will take advantage of our recent single-cell transcriptomics analyses to characterizeTAMsheterogeneity in syngeneicand pre-clinical patient-derived orthotopic xenograft mouse models(see Preliminary results section). We willemployrecent developed computational analysis pipelinesto infer the underlying gene regulatory networksallowing targeted experimental perturbations fosteringanti-tumourfunctions(see Materials and Methods section).