Revealing the secrets of MARCO : a target for cancer immunotherapy

Our immune system protects us from infectious agents and sustains the homeostasis in our body. However, the immune system can also play a role in initiation and progression of diseases such as autoimmune diseases and cancer. Cancer is still one of the leading causes for death worldwide and inflammation became one of the hallmarks of cancer. Besides carrying out antitumor immune responses, the immune system also supports tumor growth through different aspects, such as promoting vascularization and suppressing other immune cells. Thus, targeting the immune system and harnessing its potential for anti-cancer therapy is a promising approach for treating cancer patients. Especially tumor-associated macrophages (TAMs) have shown to exert many tumor-supporting properties and represent a predominant immune cell population in most of the tumors. Therefore, the focus of this thesis was to evaluate the role of scavenger receptor MARCO on macrophages in immune responses and to reveal its potential as target for cancer immunotherapy. Paper I identifies a novel interaction between marginal zone macrophages (MZMs) and marginal zone B cells (MZBs) that regulates antigen (Ag) transport into the follicle and Ag deposition onto follicular dendritic cells (FDCs) in the spleen. This can be modulated by targeting MARCO on MZMs by monoclonal antibodies (Abs) which leads to less Ag shuttling into the follicle, decreased Ag deposition to FDCs and to a subsequent reduced adaptive immune response. Anti-MARCO Abs can also be found in human system lupus erythematosus patients and this study may clarify the reason for their increased susceptibility for infections. Paper II reveals MARCO as novel marker for a distinct tumor-promoting macrophage subtype in melanoma, breast and colon cancer mouse models. Anti-MARCO Ab treatment leads to reduced tumor growth and metastases and reprograms the MARCO immune suppressive TAMs towards a pro-inflammatory phenotype. The combination of anti-MARCO Abs with checkpoint inhibitor anti-CTLA-4 shows enhanced efficacy and suggests MARCO targeted therapy as a promising cancer immunotherapy approach. Moreover, MARCO expressing TAMs can also be found in human breast cancer and melanoma patients. Paper III investigates the mechanism of how anti-MARCO Ab treatment leads to reduced tumor growth in melanoma: Targeting MARCO by Abs leads to the activation of natural killer (NK) cells to increase their TRAIL-dependent tumor cell killing. MARCO TAMs display a perivascular macrophage phenotype and targeting MARCO not only leads to metabolic reprograming of the MARCO TAMs but results also in less confuse tumor vascularization. We confirmed enhanced efficacy of anti-MARCO Abs in combination with checkpoint inhibitors anti-PD-1 or anti-PD-L1 Abs and produced anti-human MARCO (hMARCO) Abs by immunizing mice with human MARCO protein to translate our findings to humans. Co-culture experiments with human MARCO macrophages and T cells or NK cells validate that targeting hMARCO by Abs re-activates the effector cells leading to increased activation, proliferation and human melanoma cell killing in vitro. Paper IV studies MARCO expression on immune suppressive myeloid cells in human pancreatic ductal adenocarcinoma patients, a cancer type with very poor prognosis. MARCO expression is induced by tumor-derived IL-10 and under hypoxic conditions not only found on immune suppressive TAMs but also on myeloid derived suppressor cells. Targeting MARCO myeloid cells by anti-human MARCO Ab restores cytotoxic anti-tumor activity of T cells and NK cells including increased activation, proliferation and human pancreatic cancer cell killing in vitro. Altogether, the work presented in this thesis gives us new insights into the biology of scavenger receptor MARCO, increases our understanding of the tumor microenvironment and how we can re-activate macrophages and modulate the immune suppressive tumor microenvironment by monoclonal Abs. Targeting MARCO enhances anti-tumor responses and the cytotoxicity of effector cells making it a promising approach for further immunotherapies. Our findings contribute to the design of new anti-cancer therapies including combining other immunotherapies with anti-MARCO Ab.