The Tumor Microenvironment and Immune Checkpoint Inhibition

Tumor formation and progression revolves around two processes: genetic changes within the cells and rearrangement of the tumor microenvironment (TME). Conventional cancer therapies use a blunt force approach that interferes with genetic changes and/or the pathways they govern, or kills rapidly dividing cells directly. Appreciation for the role of the TME in cancer progression is a relatively recent development.

The tumor microenvironment (TME), or niche, consists of cellular (stromal cells) and non-cellular (extracellular matrix) components. Cellular components include tumor cells, tumor stromal cells (including stromal fibroblasts), endothelial cells, and a host of immune cells including, but not limited to, macrophages, microglia, and lymphocytes. The non-cellular niche, also quite complex, is composed of the structural proteins collagen, fibronectin, hyaluronan, laminin, and others.

Cancer cells, the "main act" in this model, direct cellular and non-cellular components to do the tumor's bidding through chemical signaling. These complex interactions confer cancer's unique ability to maintain its structure and function while continuing to expand, even after aggressive treatment.

Fertile ground

These components of the TME, together with signaling pathways that facilitate their cooperation, are fertile ground for drug developers. Chemical TME signals include the usual suspects (chemokines and cytokines) plus growth factors, non-cytokine mediators of inflammation, and enzymes that effect TME remodeling—all under the direct control of the tumor. Another set of bad actors, including circulating tumor cells, exosomes, cell-free DNA, and apoptotic bodies, help the tumor communicate with both normal and cancerous distant cells.

By no means have researchers "solved" even intensely studied TME components. What we know for certain is that the TME is a virtual bottomless bag of tricks that tumors dig into to avoid extermination.

Genetic, proteomic, and metabolomic studies all show significant, disease-altering changes, effected by the TME, to reprogram a cancer's initiation, growth, invasion, metastasis, and, last but not least, its response to treatment. Despite this understanding, and the strategic switch from therapies that target tumor cells specifically to those that disrupt the TME, successes with TME-targeting immunotherapies have been sporadic.

One such strategy involves immune checkpoint inhibition (CPI). Immune checkpoints are regulators of the immune response to cancer or infectious agents. Checkpoint mechanisms are finely tuned for self-tolerance, which allows them to distinguish entities that are "self" vs. "other”. They also may be inhibitory or stimulatory. CPI drugs block checkpoint pathways that prevent T cells from initiating a strong antitumor response. They achieve this by interfering in the normal binding between cancer cell antigens and the body's own checkpoint proteins.

CPIs are arguably the most promising drug category targeting the TME, having been used in a variety of cancers, including tumors of the breast, cervix, colon, head and neck, liver, lung, kidney, skin, stomach, rectum, and lymphatic system, plus additional solid tumors with a faulty DNA repair mechanism.

CPI drugs targeting cytotoxic T lymphocyte–associated antigen 4 (CTLA-4) and programmed cell death protein 1 pathway (PD-1/PD-L1), which have shown the most promise thus far, work by binding to either the cancer antigen or its immune cell receptor.

FDA has approved three anti-PD-1 antibodies: pembrolizumab (Keytruda), nivolumab (Opdivo), and cemiplimab (Libtayo), but response rates for these agents are mixed and associated with both immune-related adverse events and unusual tumor responses. Strategies targeting both CTLA-4 and PD-1 are currently under investigation to enhance efficacy and reduce the toxicity of these drugs. Additional checkpoint pathways are also under investigation, for example the lymphocyte-activation gene 3 (LAG-3, CD223) antagonists, and CD3 immunomodulators.

Target the process, not the tumor

CPI treatment failures are multi-factorial, but scientists believe that alterations in the TME, such as downregulation of immune checkpoint ligands, metabolic changes, and genetic alterations, all contribute. Following any of these categories leads deeper into the dizzying complexities of the tumor-immune response.

Immune dysregulation, for example, may involve downregulation of major histocompatibility complexes, predominance of T reg and/or myeloid-derived suppressor cells in the TME, activation of alternate checkpoint pathways, increases in inhibitory cytokines, and T cell exhaustion. Genetic changes include the epigenetic silencing of cytokine-coding genes, activation of the WNT/beta-catenin pathway, and mutations in JAK1 or JAK2 protein-coding genes (which also play a role in autoinflammation). TME-based metabolic changes can affect tryptophan catabolism and suppress glycolytic capacity.

All these mechanisms contribute, in rather unpredictable ways, to resistance to immune CPI agents. Potential intervention points for drug developers are numerous and include situations where the immune system correctly identifies the cancer but fails to eliminate it. Even when responses occur, in some instances they are unpredictable.

The approval of more effective CPI drugs will likely require targeting novel or under-utilized checkpoint ligand-receptor pairs, two or more CPI agents, or perhaps combining a CPI with anti-angiogenesis therapy, perhaps through bispecific antibodies. Using PD-1 or CTLA-4 agents with conventional chemotherapy, radiation, tumor vaccines, cell-based immunotherapies, or cytotoxic biologics are other possibilities. Combination therapies similar to these are already the standard of care for many antibody-based cancer therapies.

Targeting specific immune system cells within the TME is another obvious approach. It is highly unlikely that a "magic bullet" target will be uncovered in this manner but much can still be learned through drugs that enhance or inhibit the activity of dendritic cells, tumor-infiltrating lymphocytes, CD8+ and CD4+ T-cells, natural killer and natural killer T-cells, tumor-associated macrophages, granulocytes, T regulatory cells, myeloid-derived suppressor cells, cancer-associated adipocytes, tumor-associated endothelial cells and extracellular vesicles, and others.

Medicine is closer to the discovery stage for CPI drugs than for more mature treatments like chemotherapy or even monoclonal antibodies. Given the labyrinthine mechanisms involved in maintaining the TME, and its role in tumor-directed immunity, checkpoint inhibition is likely to retain the flavor of discovery for some time. A successful CPI approach, as opposed to therapies that indiscriminately kill dividing cells, will require understanding the many molecular and cellular entities within the TME that dictate the course of cancer. Specifically, these treatments must inhibit TME components that promote cancer progression and enhance those designed to fight tumors. That is, to turn "foes" that promote dysregulation of normal cancer-fighting pathways into allies.