The tumor bulk is composed of a highly heterogeneous population of cancer cells, as well as a large variety of resident and infiltrating host cells, extracellular matrix proteins, and secreted proteins, collectively known as the tumor microenvironment (TME). The TME is essential for driving tumor development by promoting cancer cell survival, migration, metastasis, chemoresistance, and the ability to evade the immune system responses. Therapeutically targeting tumor-associated macrophages (TAMs), cancer-associated fibroblasts (CAFs), regulatory T-cells (T-regs), and mesenchymal stromal/stem cells (MSCs) is likely to have an impact in cancer treatment. In this review, we focus on describing the normal physiological functions of each of these cell types and their behavior in the cancer setting. Relying on the specific surface markers and secreted molecules in this context, we review the potential targeting of these cells inducing their depletion, reprogramming, or differentiation, or inhibiting their pro-tumor functions or recruitment. Different approaches were developed for this targeting, namely, immunotherapies, vaccines, small interfering RNA, or small molecules.

Interactions between malignant cells and neighboring stromal and immune cells profoundly shape cancer progression. New forms of therapies targeting these cells have revolutionized the treatment of cancer. However, in order to specifically address each population, it was essential to identify and understand their individual roles in interaction between malignant cells, and the formation of the tumor microenvironment (TME). In this review, we focus on the myeloid cell compartment, a prominent, and heterogeneous group populating TME, which can initially exert an anti-tumoral effect, but with time actively participate in disease progression. Macrophages, dendritic cells, neutrophils, myeloid-derived suppressor cells, mast cells, eosinophils, and basophils act alone or in concert to shape tumor cells resistance through cellular interaction and/or release of soluble factors favoring survival, proliferation, and migration of tumor cells, but also immune-escape and therapy resistance.

Tumor-associated macrophages (TAMs) in chronic lymphocytic leukemia (CLL) are also called nurse-like cells (NLC), and confer survival signals through the release of soluble factors and cellular contacts. While in most patient samples the presence of NLC in co-cultures guarantees high viability of leukemic cells in vitro, in some cases this protective effect is absent. These macrophages are characterized by an “M1-like phenotype”. We show here that their reprogramming towards an M2-like phenotype (tumor-supportive) with IL-10 leads to an increase in leukemic cell survival. Inflammatory cytokines, such as TNF, are also able to depolarize M2-type protective NLC (decreasing CLL cell viability), an effect which is countered by IL-10 or blocking antibodies. Interestingly, both IL-10 and TNF are implied in the pathophysiology of CLL and their elevated level is associated with bad prognosis. We propose that the molecular balance between these two cytokines in CLL niches plays an important role in the maintenance of the protective phenotype of NLCs, and therefore in the survival of CLL cells.

The tumour microenvironment is the surrounding of a tumour, including blood vessels, fibroblasts, signaling molecules, the extracellular matrix and immune cells, especially neutrophils and monocyte-derived macrophages. In a tumour setting, macrophages encompass a spectrum between a tumour-suppressive (M1) or tumour-promoting (M2) state. The biology of macrophages found in tumours (Tumour Associated Macrophages) remains unclear, but understanding their impact on tumour progression is highly important. In this paper, we perform a comprehensive analysis of a macrophage polarization network, following two lines of enquiry: (i) we reconstruct the macrophage polarization network based on literature, extending it to include important stimuli in a tumour setting, and (ii) we build a dynamical model able to reproduce macrophage polarization in the presence of different stimuli, including the contact with cancer cells. Our simulations recapitulate the documented macrophage phenotypes and their dependencies on specific receptors and transcription factors, while also unravelling the formation of a special type of tumour associated macrophages in an in vitro model of chronic lymphocytic leukaemia. This model constitutes the first step towards elucidating the cross-talk between immune and cancer cells inside tumours, with the ultimate goal of identifying new therapeutic targets that could control the formation of tumour associated macrophages in patients.

Communication with the lymphoid microenvironment is crucial for survival and proliferation of neoplastic B cells in chronic lymphocytic leukemia (CLL). In this study, we examined nurse-like cells (NLCs), CLL-specific macrophages, strongly implicated in CLL pathogenesis, and their interaction with leukemic cells. Using primary patient cells, we demonstrated that NLCs express high levels of folate receptor beta (FOLR2), which correlates with increased survival of cancer cells in vitro. Furthermore, we discovered that CLL cells acquire functional FOLR2 from NLCs via trogocytosis, enhancing their folate uptake. By mimicking the CLL microenvironment with soluble factors, CD40L and IL-15, we observed a strong NLC-dependent CLL cell activation and proliferation. Moreover, we linked this phenomenon with trogocytosis, by demonstrating that FOLR2+ CLL cells are the predominant population of actively cycling cancer cells. By multiplex immunofluorescence analysis of CLL patient lymph nodes, we confirmed the presence of FOLR2⁺ NLCs, and observed their enrichment in more aggressive and proliferative CLL cases. Finally, we detected FOLR2⁺ cancer cells, providing evidence of trogocytosis in situ. Taken together, we propose FOLR2 as a novel marker of protective NLCs, and highlight the importance of trogocytosis in improved CLL cell adaptation, including NLC-mediated activation and proliferation. Scheme 1. Schematic representation of trogocytosis-mediated acquisition of NLC-derived FOLR2 by CLL cells, and its impact on promoting cancer cell adaptability to folate deprivation. 1) CLL cells enter the lymph node (LN) and engage in contact with FOLR2 ⁺ NLCs. 2) The presence of T cell- and stromal cell-derived cytokines, CD40L and IL-15 promotes the activation and trogocytosis-mediated acquisition of FOLR2 by CLL cells. 3) FOLR2⁺ trogocytic CLL cells have an advantage in folate acquisition and show increased proliferation compared to the remaining population of cancer cells SUMMARY: CLL cells acquire functional FOLR2 from NLCs via trogocytosis, a process that is closely linked with enhanced CLL cell activation and proliferation in vitro Increased Frequency of FOLR2 expressing macrophages in more aggressive and proliferative cases of CLL. FOLR2 is a marker of M2-like protective NLCs in vitro

Monocyte-derived macrophages help maintain tissue homeostasis and defend the organism against pathogens. In tumors, recent studies have uncovered complex macrophage populations, including tumor-associated macrophages, which support tumorigenesis through cancer hallmarks such as immunosuppression, angiogenesis or matrix remodeling. In the case of chronic lymphocytic leukemia, these macrophages are known as nurse-like cells and they protect leukemic cells from spontaneous apoptosis contributing to their chemoresistance. We propose an agent-based model of monocyte differentiation into nurse-like cells upon contact with leukemic B cells in vitro . We performed patient-specific model calibrations using cultures of peripheral blood mononuclear cells from patients. Using our model, we were able to reproduce temporal survival dynamics of cancer cells in a patient-specific manner and to identify patient groups related to distinct macrophage phenotypes. Our results show a potentially important role of phagocytosis in the polarization process of nurse-like cells and in promoting cancer cells’ enhanced survival.