Macrophages and pathophysiology of bone cancers

Introduction

Macrophages have been found to have a pivotal role in tumor growth, dissemination of cancer cells, and metastasis development. Macrophages have been demonstrated to be involved in the pathogenesis of human cancers often metastasizing to bone—like breast (BC) and prostate cancer (PC)—and primary bone cancers like osteosarcoma. Macrophages are multifaceted myeloid cells capable to internalize and destroy external material, present fragments from the external milieu structures to T cells, and secrete a large variety of cytokines and growth factors. Macrophages are one of the antigen-presenting cells and professional phagocytes. Depending on the surrounding stimulus, macrophages can select from a spectrum of activation stages to adopt. In a broad sense these can either promote inflammation and cytotoxic immune response or downregulate T-cell activity [ , ] and promote tissue vascularization [ ], both beneficial to the development of cancer, or rather act silently as it is the case when phagocytosing cellular debris related to the normal turnover of tissues.

Macrophage differentiation, polarization, and activation status

Macrophages, dendritic cells, and osteoclasts differentiate from circulating CD14 ⁺ monocytes which in turn originate from bone marrow (BM) myeloid stem cells [ ]. In addition, tissue-resident macrophages originate in part from the yolk sac and fetal liver during development and they are capable to proliferate and replenish their own population in situ [ ]. Monocyte extravasation can be mediated by soluble factors such as fractalkine and C-X-C motif chemokine 12, CXCL12 (also known as stromal-derived factor 1α, SDF-1α) [ ]. In tissues, macrophages adapt their functional phenotype to the location and microenvironment [ ]. Macrophages are found in numerous tissues, e.g., liver (Kupffer cells), lungs (alveolar macrophages), brain (microglia), and gut, where their population is remarkably heterogeneous [ ]. Furthermore, resident macrophages and dendritic cells are present, in the BM in addition to multinuclear osteoclasts [ ]. Secondary lymphoid organs such as spleen and lymph nodes have their distinct macrophage populations directly regulating the innate and adaptive immunity [ , ]. Tissue macrophage population can be replenished via extravasation and differentiation of circulating monocytes, which originate from BM hematopoietic stem cells (HSCs) [ ].

Several studies have demonstrated that the same activated macrophage population can change over time according to the environmental stimulus they receive. Macrophages may also recruit a new population of macrophages into the tissue [ ]. Within the diversity of macrophage's spectrum of activation status there are two main extremes: the classically activated macrophages, termed M1 macrophages (similar to the Th1/Th2 nomenclature of T cells), and the alternatively activated macrophages, M2 macrophages [ ]. Histologically, in humans the M2 designation contains several defined intermediate statuses [ ] ( Fig. 17.1 ). In this review, we use the terms M1 and M2 most importantly to conceptually describe the immunological and biological meaning of the macrophage actions.

The immunomodulatory/proinflammatory properties of M1 macrophages evoke a cytotoxic response by Th1-type T cells promoting their proliferation, activation, and differentiation [ ]. On the other hand, antiinflammatory, M2-type macrophages, especially M2c formed by the action of surrounding IL-10, suppress immune responses [ ] by directing T-cell differentiation to Th2-type T cells [ ]. Moreover, they promote tissue remodeling, repair [ ], and blood vessel formation by secreting vascular endothelial growth factor (VEGF) [ ]. Physiologically all these capabilities and processes are of uttermost importance in the maintenance of tissue homeostasis [ ] and wound healing [ ].

Tumor-associated macrophages

During the development of the primary tumor, a cell level process of evolution undergoes between the cancer cell, the immune system, and the tumor stroma. This notion that tumors are a complex ecosystem, with a variety of cell types that cooperate with cancer cells, led to addition of immune system escape, tumor-promoting inflammation, and deregulation of cellular energetics to the six hallmarks of cancer (resistance to cell death, sustained proliferative signaling, evasion from growth suppressors, activation of invasion and metastasis, replicative immortality, and induction of angiogenesis) [ , ]. Tumor stromal cells, including tumor-associated macrophages (TAMs), contribute to all these hallmarks of cancer [ , ].

Of the cells of the tumor tissue approximately 5%–50% consist of TAMs [ , ]. Increased intratumoral macrophage density has been shown to correlate with poor disease prognosis in more than 80% of published studies on the clinical role of TAMs [ , ]. In mice, TAMs differentiate mostly from infiltrating monocytes rather than propagate from local tissue macrophages [ ]. Even if initially cytotoxic (M1 type), TAMs eventually switch their phenotype to immune suppressive, tumor growth promoting M2-type macrophages [ , ]. This process involves complex interactions between macrophages, tumor cells, other cells of the immune system, and other stromal cells such as fibroblasts [ ]. Cancer cells educate human monocytes and macrophages, enriching their transcriptome for terms of cell migration, angiogenesis, cell communication, and apoptotic processes [ ]. TAM gene signature is enriched in highly aggressive BC and associated with shorter disease-specific survival [ ].

In tumor tissue most of the TAMs reside near the tumor margin and around tumor vasculature, forming a decreasing gradient toward the inner tumor [ , ]. TAMs actively work for the tumor by: producing several molecules that sustain malignant cell survival, modifying neoplastic extracellular matrix proteins, promoting the development of newly formed vessels, and assisting tumor cells in their progression. Moreover, TAMs significantly affect the adaptive immune responses by inducing unresponsiveness of naïve T cells; by exhausting of cytotoxic T cells via PD-L1, B7-H4, and PD-L2; by recruiting and stimulating regulatory and Th2-type T cells, which in turn inhibit cytotoxic Th1 cells [ ]; and by promoting immune exclusion via fibrosis induction [ ].

Monocytic myeloid-derived suppressor cells (Mo-MDSCs) share some cell surface markers and immunosuppressive functions with TAMs. Proinflammatory factors present at the tumor site strongly attract Mo-MDSCs (and also granulocytic PMN-MDSCs). Unlike TAMs, Mo-MDSCs simultaneously express some, but not all, of the macrophage M2-type markers and some M1-type markers [ ]. Mo-MDSCs can locally differentiate into TAMs upon inhibition of the transcription factor STAT3 activity by CD45 phosphatase and thus can be considered as immature TAM progenitors [ ]. MDSCs are strong immunosuppressors as they: (a) inhibit T-cell activation and function by producing ROS and RNS; this impairs T-cell receptor (TcR) function and antigen recognition and disrupts signaling through the IL-2c receptor; (b) deplete arginine and cysteine which are required for T-cell activation and proliferation; (c) disrupt T-cell migration by peroxynitrite modification of the CC chemokine ligand 2 (CCL2); (d) promote natural T regulatory cells through the production of IL-10, tumor growth factor-β (TGF-β), interferon-γ (IFN-γ), and CD40–CD40L interactions; (e) inhibit natural killer (NK) cell cytotoxicity and production of IFN-γ by NK cells. TAMs and MDSCs have been recognized to establish an immunoregulatory cross talk where tumor MDSCs secrete IL-10 to downregulate IL-12 secretion by TAMs, and TAMs reciprocally induce MDSCs to produce more IL-10 [ ].