While tumor cell-intrinsic factors including tumor heterogeneity are known to drive invasion and metastasis, extrinsic factors within the tumor microenvironment (TME) have been shown to play critical roles in determining the fate of cancer progression
[15]. The stromal components of the TME are composed of several cell populations, including immune cells, fibroblasts, and endothelial cells
[16]. The interaction between these cell types and the ECM, vascular distribution, oxygen (hypoxia), nutrients, and chemokines further complicates the tumor milieu
[17].
The immune system has been demonstrated to shape the local TME
[18] and contributes not only to the elimination of tumor cells, but may also be subverted to support their invasion and metastasis
[19][20]. Under normal conditions, the Cancer-Immune cycle involving the dynamic communication of both innate and adaptive immune systems is activated to remove malignant cells in a process known as immune surveillance
[21]. This cycle initiates as neo-antigens are taken up by antigen-presenting cells (APCs) of the innate immune system and presented to the adaptive immune system to acquire specific recognition of tumor cells. These activated T cells then migrate to the tumor site allowing them to target the tumor cells expressing these neo-antigens. Effector molecules such as interferon gamma (IFN-γ), tumor-necrosis factor alpha (TNF-α), as well as recognizing molecules (e.g., NKG2D) have been shown to be important in this process
[22][23]. Functional immune surveillance results in the elimination of tumor cells; however, this is often disrupted or hijacked by cancer cells at multiple different points resulting in immune escape
[21][24]. Possible routes include, but are not limited to, (A) downregulation or impairment of antigen presentation ability
[25], (B) recruitment of pro-tumorigenic immune cells and secretion of inflammation-regulating cytokines by cells in the TME
[26], and (C) upregulation of the expression of immune inhibitory molecules to suppress immune cell cytotoxicity
[27]. Approaches that aim to restore an effective Cancer-Immune cycle are being investigated and are referred to as immune checkpoints
[21].
Innate immunity (e.g., dendritic cells, macrophages, neutrophils, and natural killer cells) represents the first line of defense and rapid response to foreign substances and in anti-tumor activities
[28]. However, these immune cells have also been shown to drive cancer-associated inflammation
[1] and to participate in several steps of the metastatic cascade from reorganizing ECM
[29], modulating vessel formation and permeability
[9], and influencing cancer cell motility and states
[30][31] (
Figure 2). This remainder of this review summarizes known characteristics of innate immune cell populations on tumor invasion and metastasis and discusses recent advances in the field.
Figure 2. Role of innate immune cells during the metastatic cascade. Different populations of innate immune cells are involved in different stages of the metastatic cascade both locally and in distant organs/tissues, including regulating the survival and proliferation of tumor cells and assisting in the establishment of a permissive tissue environment enabling tumor cell invasion and migration. DC: dendritic cell. TAM: tumor-associated macrophage. TAN: tumor-associated neutrophil. M- and G-MDSC: monocytic and granulocytic-myeloid-derived suppressor cells. MC: mast cell. NK: natural killer. ILC: innate lymphoid cell. EMT: epithelial-mesenchymal transition. ECM: extracellular matrix.