Tumor microenvironment is comprised of dynamic balances caused by interactions between the stimulatory or inhibitory receptors in the immune cells (including lymphocytes and various antigen presenting histiocytes), and their corresponding ligands present in the tumor cells as well as in some of the immune cells. Cancer cells may express antigens that differ from those of normal host cells due to genetic and epigenetic alterations occurring during carcinogenesis
]. To eliminate the cancer cells, the host immune system must recognize the tumor antigen first, and then present it to T cell receptor, which will lead to T cell activation and killing of the cancer cells. Several co-stimulatory and inhibitory receptor molecules are normally present on the surface of immune effector cells to regulate the T-cell mediated immune response. Known co-stimulatory receptors include CD27, CD28, CD40, CD137, GITR, and OX-40 (CD134) that will promote the T-cell response [5
]. Conversely, co-inhibitory receptors, including CTLA-4 (CD152), PD-1 (CD279), TIM3, LAG3 (CD223) and KIR, prevent immune response in order to protect against autoimmunity in physiologic status [1
]. For each of these stimulatory and inhibitory receptors, there are specific corresponding ligands that are normally present not only on antigen presenting cells
(APCs) and other immune cells, but also on non-immune cells including tumor cells or some normal cells of solid organs [1
]. Binding of inhibitory receptors on immune effector cells and their ligands on APCs, tumor cells, or non-immune normal cells can lead to immune tolerance: i.e. immune checkpoint. Cancer cells use these immune checkpoints to escape host immune system by creating an immunosuppressive microenvironment that down regulates T-cell activation and cell signaling. Immune checkpoint blocking agents disrupt this immune resistant mechanism by the tumor cells and establish a durable tumor control [24
It has been postulated that potential antigenic targets in lung cancer
can escape immune rejection by two very different mechanisms: 1. “editing” out particularly immunogenic neoepitopes, 2. induction of antigen-specific tolerance [25
]. Editing implies that T-cell recognition of a tumor neoantigen results in the selection of tumor cells lacking antigen, whereas tolerance induction implies that tumor specific T cells become incapable of attacking antigen-bearing cells. The relative importance of editing versus tolerance induction in human lung cancer remains to be determined [2
T-cell mediated immunity includes multiple sequential steps involving the clonal selection of antigen specific cells, their activation and proliferation in secondary lymphoid tissues, their trafficking to the sites of antigen and inflammation, the execution of direct effector functions and the provision of help through cytokines and membrane ligands for a multitude of effector immune cells [28
]. Each of these steps is regulated by counterbalancing stimulatory and inhibitory signals that fine tune the response. Although virtually all inhibitory signals in the immune response ultimately affect intracellular signaling pathways, many are initiated through membrane receptors, the ligands of which are either membrane bound or soluble (cytokine). Restifo et al. [29
] showed that suppressed antigen presenting molecules could be upregulated by IFNγ in the majority of lung cancer cell lines. This finding is highly relevant to immunotherapy because it suggests that suppression of tumor antigen presentation can be reversed in the majority of lung cancers if T cells or NK cells, the two major producers of IFNγ, could be activated within the tumor microenvironment.
PD-1 is expressed on activated T-and B-cells and its major ligand PD-L1 (B7-H1) is typically expressed on the subset of macrophages, but can be induced by inflammatory cytokines in a variety of tissue types [30
]. When activated T-cells expressing PD-1 encounter PD-L1, T-cell effector functions are diminished. PD-1 also binds PD-L2 (B7-DC), which is expressed selectively on macrophages and dendritic cells [31
]. These unique expression patterns suggest that PD-L1 promotes self-tolerance in peripheral tissues, while PD-L2 may function in lymphoid organs, although the role of PD-L2 in immunomodulation
is not as well understood. Shi et al. reported that PD-L2 protein is robustly expressed by the majority of primary mediastinal large B-cell lymphoma, but only rare diffuse large B-cell lymphomas and is often associated with PDCD1LG2
copy gain [32
In NSCLC, the most therapeutically relevant mechanism for immune resistance might be the expression of immune inhibitory receptor molecules in the tumor microenvironment. These molecules fall into a number of classes based on the nature of inhibitory ligand: cytokines, membrane ligands, and metabolites [2
]. The two inhibitory cytokines commonly expressed in lung cancers are IL-10 and TGF-β. Among the membrane inhibitory ligands, PD-L1 has been the most studied in NSCLC, though PD-L2, B7-H3, and B7-H4 have also been reported as upregulated in lung cancer [28
]. PD-L1 is expressed on tumor cells in approximately half of NSCLC but is sometimes expressed on myeloid cells in the stroma surrounding tumor cell nests.
Though not often considered, immune inhibitory metabolites are probably also important players in local immune resistance in lung cancer. Concentrations of adenosine, which binds to the inhibitory G-protein-coupled A2a receptor (A2aR) expressed on lymphocytes, have been shown to be extremely high in NSCLC tissue. Triggering of A2aR inhibits effector T-cell function and drives the development of Tregs, another inhibitory component of the tumor microenvironment [34
As a general rule, the stimulatory or inhibitory receptors and their ligands implicated in T cell activation are not necessarily overexpressed in cancers as compared to normal tissues. On the other hand, the inhibitory receptors and ligands that regulate T-cell effector functions are commonly overexpressed on tumor cells or on non-transformed cells in the tumor microenvironment [28
]. The soluble and membrane-bound receptor-ligand immune checkpoints have been found to be the most amenable to potential therapies via blocking antibodies
for inhibitory pathways. Thus, unlike the other currently used antibodies for cancer therapy, the antibodies that block immune checkpoints do not necessarily target tumor cells. Rather, they mainly target the inhibitory receptors on the immune cell surface or their ligands in order to enhance endogenous antitumor activity.