Tumor-associated macrophages: from mechanisms to therapy

Abstract

The tumor microenvironment is a complex ecology of cells that evolves with and provides support to tumor cells during the transition to malignancy. Among the innate and adaptive immune cells recruited to the tumor site, macrophages are particularly abundant and are present at all stages of tumor progression. Clinical studies and experimental mouse models indicate that these macrophages generally play a protumoral role. In the primary tumor, macrophages can stimulate angiogenesis and enhance tumor cell invasion, motility, and intravasation. During monocytes and/or metastasis, macrophages prime the premetastatic site and promote tumor cell extravasation, survival, and persistent growth. Macrophages are also immunosuppressive, preventing tumor cell attack by natural killer and T cells during tumor progression and after recovery from chemo- or immunotherapy. Therapeutic success in targeting these protumoral roles in preclinical models and in early clinical trials suggests that macrophages are attractive targets as part of combination therapy in cancer treatment.

Figure 1. Tumor-associated macrophages in the primary tumor promote malignancy

In the primary tumor microenvironment macrophages under the influence of IL-4 produced by CD4⁺ T cells and tumors and WNT7b promote tumor cell invasion. This invasion is mediated via a paracrine loop involving tumor-synthesized CSF1 and macrophage-produced EGF that drives migration of tumor cells in lock-step with macrophages along collagen fibres that act as highways towards blood vessels. This process also requires TGFβ that drives n epithelial-mesenchymal transition (EMT) in the tumor cells that promotes migration and matrix remodelling via Cathepsins and matrix adhesion of tumor cells via SPARC. This streaming of tumor cells results in their pileup on the vessels where macrophages promote their intravasation into the circulation through a structure named the “Tumor Microenvironment of Metastasis” (TMEN). In addition to effect on tumor cell migration and invasion TIE2⁺ macrophages produce VEGF and WNT7b that stimulates angiogenesis in the tumor. Thus there is an additive effect caused by macrophages of increased migration of tumor cells towards vessels and increased vascular targets that results in a large number of circulating tumor cells and thus increased malignancy. Macrophages also suppress cytotoxic T-cell responses through the mechanisms described in figure 2.

Figure 2. Pro-tumor macrophage mechanisms of effector cells inhibition

TAMs express an array of effector molecules that inhibit the anti-tumor immune responses; this includes cell surface receptors, cytokines, chemokines and enzymes. Inhibition of immune responses by direct cell-to cell-contact is based on the interaction of TAMs receptors ligands with their counterpart death/inhibitory receptors expressed by the target immune effector cells. TAMs express the ligand receptors for PD-1 and CTLA-4 that upon activation suppress cytotoxic functions of T- cell, NKT cells and NK cells. TAMs also express the ligand for the death receptors FAS and TRAIL that triggers in target cells caspase dependent cell death (apoptosis). TAMs also express the non-classical HLA-G that inhibits T cell function through interaction with the co-stimulatory signal of T cells ILT2, and HLA-E that inhibit NK cells through CD94 (also known as NKG2). TAMs secrete the cytokines IL-10 and TGFβ that inhibit T cells effector functions and induce regulatory functions, and chemokines CCL5, CCL20, CCL22 that recruit nTreg cells. TAMs secrete Arginase I that inhibit TCR ζ chain re-expression in activated T cells by the depletion of L-arginine.

Figure 3. Macrophages promote metastasis

Arrest of tumor cells in the vasculature of target organs through the formation of microclots (1) results in CCL2-mediated recruitment of CCR2-expressing circulating inflammatory monocytes (2). These monocytes differentiate into metastasis-associated macrophages (MAMs) that mediate tumor cell extravasation via VEGF that increases vascular permeability (3). MAMS under the influence of CSF-1 further promote tumor cell survival (4) and persistent growth associated by angiogenesis and may also prevent T cell cytotoxicity (5).

Figure 4. Re-programming Macrophages to be Anti-Tumoral

Macrophages in the tumor in general are immunoregulatory and suppress immune responses to tumor-derived antigens. However, in some circumstances particularly with appropriate therapeutic interventions, macrophages can be anti-tumoral by direct tumor cell killing, the removal of vital support such as inhibition of angiogenesis or by the activation T cells. This differential polarization is under the control of many stimuli as shown that alters the differentiated state of the macrophages. Some factors such as GM-CSF act in pro- or anti-tumoral fashion dependent on context (Pyonteck et al., 2013; Su et al., 2014). Therapeutic interventions can re-polarize these cells to become immunostimulatory macrophages that on their own can cause tumor regression or that enhance the activity of chemo-, anti-vascular or immuno-therapies. References to these polarizing agents can be found in (De Palma and Lewis, 2013; Sica and Mantovani, 2012).