1. Tumor Immunity: Exploring the Role of a Checkpoint
During the past decade, there has been an increased understanding of the mechanisms by which the immune system recognizes tumors, as well as how cancer evades that recognition. This increased understanding has resulted in the development of immune-based cancer therapeutics that are showing clinical benefit in randomized trials. Some of these agents fall into the category of “immune checkpoint inhibitors.” This presentation will provide an overview of what we have learned about how to generate successful antitumor immunity and describe the concept of immune checkpoints.
The numerous components of the immune system include antigen-specific cells, stimulatory or suppressive proteins expressed on those cells, and secreted substances such as cytokines and chemokines, as well as many other elements. All of these components have specific names and designations. Please see the glossary for a list of the terms, abbreviations, and definitions of the components of the immune system that are discussed in this presentation.

2. Many malignancies are infiltrated by immune system cells, particularly cells of the adaptive immune system, or T cells. The number, location, and phenotype of the infiltrating T cells affect prognosis.[1,2] T cells that are able to traffic and penetrate tumors have been shown to be associated with favorable outcomes, while T cells that never breach the tumor stroma have less beneficial impact. Because T cells are often already present within cancers, therapeutics directed toward increasing their function or number may provide clinical benefit.[3-5]

3. One of the most important immune system cells in the eradication of cancer is the CD8+ T cell. CD8+ T cells, also known as cytotoxic T cells (CTL) can kill tumor cells.[6,7] CD8+ T cells are supported by T helper cells (Th). The types of beneficial helper signals that CTL secretes are type I cytokines such as tumor necrosis factor (TNF)-α and interferon-γ. For optimal killing, the T helper type I (ThI) cells and CTL must reach the tumor and avoid regulatory mechanisms that impair their function.
This slide, the first of 3 to illustrate characteristics of an effective tumor-specific immune response, demonstrates adaptive immunity, in which lymphocytes (antigen specific) recognize immunogenic proteins. The antigen-specific CD4+ T cells secrete type I cytokines such as interferon and TNF-α (Th1), support generation of CD8+ CTL, and activate antigen presenting cells (APCs) that assist in T-cell recognition of tumors. CTL may directly kill tumor cells.

4. This illustration demonstrates how T cells penetrate tumor stroma
This illustration demonstrates how T cells penetrate tumor stroma. Here, antigen-specific T cells must traffic to the tumor, including all metastatic sites. They penetrate tumor stroma, which requires upregulation of specific receptors on the T cells. To eradicate cancer, T cells should be found in the tumor parenchyma in high density. Th1 cells and CTL modulate the microenvironment to support an ongoing effective immune reaction.

5. This slide demonstrates how natural mechanisms limit immunity and inflammation. Suppressive T regulatory cells (Tregs) are CD4+ T cells that secrete cytokines such as transforming growth factor (TGF)-β and interleukin (IL)-10 to limit CTL function. Other innate immune cells (APCs) can be recruited to the tumor to limit adaptive immunity via direct contact with T cells or cytokine release, which limits type I immunity. Tumors can upregulate ligands (eg, programmed cell death 1 ligand [PD-L1]), which inactivate T cells via receptor binding.

6. For the adaptive immune system to function properly, T cells have to receive certain signals. T cells have to recognize immunogenic proteins in the context of major histocompatibility complex (MHC) molecules, be stimulated appropriately by specific proteins expressed on their surface, and be exposed to cytokines in the environment that support their proliferation.

7. As outlined here, cancer evades immune recognition via multiple mechanisms.

8. Human tumors are immunogenic and often contain T cells
Human tumors are immunogenic and often contain T cells. Cancer immunotherapeutics can and are being developed that are aimed at either generating immunity in the minority of patients who have none or further stimulating the weak immune responses that are present in the cancer. Immune checkpoints, which are the natural mechanisms of our immune system to prevent autoimmunity or excessive inflammation, are one of the main ways cancer grows unchecked. Blocking these checkpoints may allow the native immune response to become “unleashed” and function to inhibit or eliminate cancer growth.

9. Our immune system is constantly seeking a balance
Our immune system is constantly seeking a balance. Every immune response, once initiated, must be dampened or turned off. This is accomplished via a series of proteins and their binding partners that are expressed on the surface of T cells and also by the cells that present protein antigens to T cells. Signaling via these proteins either stimulates or inhibits T-cell function. Co-stimulatory molecules and immune checkpoint proteins work in harmony to prevent excessive inflammation and autoimmunity while allowing the death of viruses and other pathogens.

10. T cells are present in tumors but are not capable of completely eradicating the cancer cells. However, co-stimulatory proteins and immune checkpoint proteins expressed on the surface of T cells are targets that can be exploited therapeutically.[8-10] If agents could be developed that turn on co-stimulatory molecules, T cells could be optimally activated. In contrast, if immune checkpoint proteins could be blocked, the immune response present in tumors would be enhanced. Drugs are being developed that target co-stimulatory or immune checkpoint proteins.
In this model, agonist monoclonal antibodies (eg, anti-CD137 monoclonal antibody) can be developed that activate positive co-stimulatory proteins, leading to enhanced T-cell function by supplying the necessary co-stimulatory signal. Antagonist monoclonal antibodies (eg, anti-cytotoxic T-lymphocyte antigen 4 [CTLA-4] monoclonal antibody and anti-programmed cell death 1 [PD-1] monoclonal antibody) can be developed that block checkpoint proteins, leading to enhanced T-cell function by preventing the inhibitory signal. Currently, immune checkpoint blockade is a standard of care for the treatment of metastatic melanoma (CTLA-4 blockade). Clinical trials of immune checkpoint blocking monoclonal antibodies are showing efficacy in other solid tumor types (eg, non-small cell lung cancer, colorectal cancer).

11. CTLA-4 is a protein expressed on the surface of T cells that binds to a ligand on APCs, as well as other cells.[11] Ligation of CTLA-4 will impair the function of the T-cell receptor (TCR) and inhibit signaling that would activate the T cells. CTLA-4 engages early in the process of T-cell activation and limits the magnitude of the immune response. Blocking CTLA-4 with monoclonal antibodies has resulted in durable clinical responses in metastatic melanoma. Ipilimumab (anti-CTLA-4 antibody) is a US FDA-approved therapy for this disease and the first to show survival benefit.[12,13]
As shown in this slide: (1) CTLA-4 and CD28 are both expressed on T cells. Both proteins bind CD80 and CD86 on APCs. If CD28 is the predominant ligand, T cells are activated. If CTLA-4 is the predominant ligand, T cells are not activated. (2) CTLA-4 balances CD28 by regulating the early stages of T-cell activation. The greater the signaling through CD28, the more CTLA-4 is transported to the cell surface to regulate the amplitude of the T-cell response. Signaling via CTLA-4 results in the recruitment of specific phosphatases that dephosphorylate the CD3-zeta chain of the TCR, limiting its activation and function. TCR signaling is required for optimal T-cell function. CTLA-4 is also expressed on CD4+ Tregs, and ligation activates Tregs to secrete type II cytokines to dampen the immune response. (3) CTLA-4 blockade enhances the function of Th1, increases recruitment and activation of CTL, and inactivates Tregs.

12. PD-1 and its ligands, PD-L1 and PD-L2, are also part of the immune checkpoint system. Ligation of these binding partners also affects the TCR and inhibits functional T-cell signaling. PD-1, however, becomes operative later in a T-cell response once inflammatory cytokines have been released. PD-1/PD-L1 interactions limit inflammation and inhibit CTL activity. Monoclonal antibody blockade of PD-1 and PD-L1 has shown promise in early-phase clinical studies, inducing prolonged response rates in patients with melanoma, non-small cell lung cancer, or colorectal cancer.[14-17]
As shown in this slide: (1) The initial response causes inflammation, with Th1 secreting type I cytokines, CTL being recruited and proliferating, and cell death and inflammation. (2) PD-1 and ligands PD-L1 and PD-L2 are expressed in response to inflammatory cytokines and T-cell activation. The PD-1/PD-L1/L2 axis exists to prevent autoimmunity and excessive inflammatory reactions. (Because PD-1 is expressed not only by T cells but also by NK cells and B cells, PD-1 blockade may also enhance the tumor-killing effect of NK cells.) PD-L1 is expressed on many cells, including APCs, cancer and endothelial cells, and T cells. PD-L2 has a more restricted expression, mostly APCs. When PD-1 binds to PD-L1, phosphatases are triggered that dephosphorylate the CD3-zeta chain, affecting the function of the TCR, and T cells are unable to respond further. (3) Blockade of PD-1 or PD-L1 restores T-cell function and allows destructive expansion of therapeutic antitumor immune response.

13. There are several agents under development that target a variety of immune checkpoints. Ongoing studies are exploring the combination of these new drugs with chemotherapy or even with each other, targeting multiple checkpoint proteins at one time. Immune checkpoint inhibitors are promising agents and have already become part of our therapeutic armamentarium in the fight against cancer.

14. A healthy immune system is one that can control T-cell stimulation and inflammation. Immune checkpoints are normal. Cancer behaves like a "self" protein, and immune checkpoints are activated to prevent inflammatory immunity developing against "self." Using the immune system, especially the adaptive immune system, to fight cancer provides specificity, the ability of tissue-destructive T cells to keep proliferating and killing until the tumor is gone, and immunologic memory (the ability of T cells to repopulate if cancer returns). Changing the balance of the immune system with checkpoint inhibition should allow the immunity to be fully armed against cancer.

15. Abbreviations

16. References

17. References (cont)

18. References (cont)