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Revealing the Potential of
the Immune System in Cancer
Strategies that leverage the immune response may play a key role in the investigation of novel therapies
- Innate and adaptive immunity act as complementary networks of
self-defense against foreign threats such as pathogens and cancer1 - The immune system is able to recognize foreign threats (nonself) as distinct from normal cells (self)2-4
- In cancer, normal cells have mutated into tumor cells and are recognized as nonself by both the innate and adaptive immune systems5,6
Tumor Microenvironment
Innate immunity: Natural killer (NK) cells mediate a rapid, antigen-independent response against cancer
- The innate immune response is the body’s first line of defense against pathogens and cancer; it responds immediately and independently of antigens1,5,7
- It is constantly on alert and can rapidly identify and attack tumor cells:
- While antigen specificity does not limit the speed and scope of the innate immune response, it is still regulated to protect normal cells. It recognizes activating and inhibitory signals from target cells to distinguish self from nonself, allowing for the elimination of tumor cells while sparing normal cells8-10
- NK cells are the main effector cells of the innate immune system11,12
- By engaging activating receptors, NK cells
recognize tumor cells as nonself and target them for destruction9
Adaptive immunity: capacity cytotoxic T cells to mediate specific and sustained responses to tumor antigens
- The adaptive immune response is antigen-specific and produces durable responses1,7
- Although not immediate, once activated the adaptive immune response can be sustained through immune memory13
- Cytotoxic T cells
are effector
cellsof the adaptive immune system1
Innate and Adaptive Immune Response
- APCs
, such as dendritic cells, are components of innate immunity that act as central messengers between the innate and adaptive immune responses1
- In cancer, the innate immune system can initiate tumor cell death and the release of signaling molecules such as DNA, adenosine triphosphate (ATP), and proteins, causing APCs to initiate an adaptive immune response14-17
Proinflammatory cytokine production by APCs enhances antitumor response
- Dying tumor cells release DNA or ATP and stimulate APCs to produce proinflammatory cytokines, which activate T cells and promote tumor inflammation17,18
- One way APCs may produce these cytokines is by forming a protein complex known as the inflammasome19,20
- Inflammasome-activated cytokines can support antitumor function and survival in activated T cells involved in the adaptive immune response17,19
APC processing and presentation of tumor antigens facilitate T-cell recognition of tumor cells
- Dying tumor cells also release proteins that APCs process into tumor antigens21,22
- APCs present these antigens to T cells, priming them to recognize tumor cells1,22
Neoantigens –
A Class of Tumor Antigen
Neoantigens promote T-cell recognition of tumors as nonself
- Neoantigens are a class of tumor antigen derived from unique DNA mutations that distinguish tumors from normal tissue, thus allowing the immune system to recognize them as nonself 6,23
- The collective number of somatic (acquired) mutations within the tumor genome is known as the tumor mutational burden (TMB), and some tumors have a higher mutational burden than others24-27
- Infiltration of cytotoxic T cells in the tumor microenvironment may be closely linked to a high TMB: the more neoantigens produced, the greater the potential for T-cell activation and infiltration28,29
- In both the innate and the adaptive immune responses, immune cells have the potential to recognize and eliminate abnormal cells such as tumor cells
- There are three principal stages of immune response:
- Presentation: the innate immune system rapidly identifies and attacks tumor cells and causes tumor cell death. This process releases tumor antigens that can activate the cytotoxic T cells of the adaptive immune system15,30
- Infiltration: immune cells are attracted by tumor antigens and other factors to the tumor site, where they invade and attack30
- Elimination: activated cytotoxic T cells recognize tumor cells as the source of the antigen and target them for elimination30
The balance of activating and inhibitory pathways regulates antitumor immunity
- The antitumor activity of NK and cytotoxic T cells is regulated through a network of activating and inhibitory signaling pathways4,31,32
- Activating pathways trigger an immune response, whereas inhibitory pathways, such as immune checkpoint pathways, counterbalance immune activation, enabling the immune system to attack tumor cells while sparing healthy cells32
Tumor cells leverage activating and inhibitory networks to evade the antitumor response
- The complex network of activating and inhibitory pathways enables the antitumor immune response to detect and eliminate tumor cells at any point in tumor development16
- However, tumors seek to evade or suppress the body’s natural ability to fight cancer, and they can evolve at any phase of growth to “outsmart” the antitumor immune response16,33
Tumor cells recruit inhibitory immune and stromal cells that suppress the immune response and contribute to tumor immune evasion
- The tumor microenvironment consists of different cell types that help tumor cells evade antitumor immune activity. These include immune cells, such as effector and non-effector cells, as well as structural cells, known as stromal cells, that surround the tumor34,35
- Non-effector cells
suppress the antitumor immune response by inhibiting effector cell function34
- Stromal cells
can temper effector-cell antitumor activity by acting as a barrier to immune cell infiltration of the tumor34,36
- As tumors evolve, they can influence the activation and composition of cells within the tumor microenvironment33
- Different tumor types use varying strategies for immune evasion, and each strategy’s success determines immune cells’ ability to react to the tumor37
- Tumors are defined on a range from noninflamed (cold) to inflamed (hot) that is dependent upon their degree of immune cell infiltration37,38
Noninflamed tumors resist elimination by impairing tumor antigen presentation and T-cell infiltration
- Noninflamed tumors have a poor presence of immune cells—most notably cytotoxic T cells—in the tumor microenvironment35,37
- Noninflamed tumors have an impaired ability to present tumor antigens to T cells and to direct tumor-specific T cells to the tumor37,39
- These tumors may not express chemokines, key secreted factors that recruit immune cells to the tumors, reducing their ability to promote tumor-specific T-cell infiltration4
- Together, these factors limit cytotoxic T-cell activation and migration to the tumor, ultimately preventing tumor cell elimination41,42
- With few immune cells present and without a need to escape elimination, tumor cells have a low expression of inhibitory proteins41,42
In inflamed tumors, increased expression of inhibitory proteins can prevent elimination
- Inflamed tumors are marked by the presence of immune cells and can indicate a pre-existing immune response37,40,43-45
- A growing body of evidence suggests that a T-cell–inflamed tumor microenvironment exists in a major subset of advanced solid tumors43
- These cancers have a high TMB and produce many tumor antigens, thus recruiting diverse cytotoxic T cells29,37
- Antigen presentation, as well as T-cell activation, is a more active process in inflamed tumors compared with noninflamed tumors, and chemokine expression allows activated cytotoxic T cells to infiltrate the tumor site40,46-48
- To escape detection and destruction by these immune effector cells, tumor cells may increase their expression of inhibitory proteins42,49
- One mechanism for achieving this is to upregulate factors such as the bromodomain and extraterminal domain (BET) family of proteins, which regulates inhibitory protein expression50-52
- These inhibitory mechanisms can prevent cytotoxic T cells from eliminating tumor cells, allowing tumor and immune cells to coexist within the tumor microenvironment42,46
The tumor microenvironment can be converted from noninflamed to inflamed
- Reestablishing fundamental stages of immune response that are impaired within noninflamed tumors—presentation, infiltration, and elimination—is a key strategy to improve the broad potential of Immuno-Oncology
- Ongoing research aims to promote inflammation within tumors to increase susceptibility to antitumor immunity
Converting noninflamed to inflamed: enhancing tumor antigen presentation
- Tumor antigens, which may be sparse in noninflamed tumors, are required for initiating an adaptive antitumor immune response1,37
- In addition to mutated tumor-specific proteins, proteins that are highly expressed on tumor cells may also serve as tumor antigens that can potentially activate cytotoxic T cells53-55
- Preclinical data suggest that promoting tumor cell death by cytotoxic agents such as chemotherapeutics, irradiation, or infection with oncolytic viruses can stimulate tumor antigen release and can initiate an immune response56-62
- Other preclinical data suggest that vaccines may introduce tumor antigens to APCs and stimulate cytotoxic T-cell function63,64
- Finally, optimizing both tumor antigen presentation and adaptive immune response may further promote an inflamed tumor environment35
Converting noninflamed to inflamed: enhancing innate immune components
- Innate immune system components provide activating signals that can increase APC priming of T cells and immune cell infiltration of the tumor microenvironment18,35,65
- Preclinical studies suggest that stimulating these components of innate immunity can increase cytotoxic T-cell infiltration into noninflamed tumors66,67
Converting noninflamed to inflamed: enhancing chemokine production
- Noninflamed tumors also have little to no expression of chemokines, resulting in impaired T-cell recruitment40
- Preclinical data suggest that stimulating chemokine production, such as by irradiation exposure, can help restore cytotoxic T-cell recruitment and promote tumor infiltration58,68
- Furthermore, data suggests stimulating or inhibiting multiple pathways (eg, chemokine production and checkpoint inhibitors) may further promote an inflamed tumor microenvironment35
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REFERENCES–Revealing the potential of the immune system in cancer
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