Overview of tumor immunology

The clinical applications of oncology and immunology converged in the 1800s when a physician, by the name of William Coley, inadvertently discovered immune surveillance by injecting a colony of bacteria into a tumor, resulting in its shrinkage.1 Since this monumental discovery, scientists and clinicians alike have trudged down a path of research and development, leading to the therapeutic practices in immuno-oncology we use today. Some of which involve cytotoxic immune responses, where the immune system is reprogrammed to fight cancer in the body.1 There are several cell types that are crucial to cytotoxic immune responses against cancer:

  • CD8+ lymphocytes and CD4+ lymphocytes (Th1/Th2 subclasses): These are commonly known as cytotoxic T cells and helper T cells. In an immune response, they mediate the distinction between self and non-self-antigens.1
  • Natural Killer (NK) Cells: For these cells, antigen presentation is not required by the major histocompatibility complex for cytotoxic activity. However, they act on cells with relatively low MHC expression.1,2
  • M1 macrophages: These cells release gamma-interferon (IFN) and are responsible for phagocytosis.1,3
  • M2 macrophages: These cells release cytokines (IL-4 and IL-10), which are responsible for manipulating transforming growth factor beta (TGFB), mitigating inflammatory response.1,3

Mechanism of action of immunotherapeutic agents and the role of biomarkers in immunotherapy


Checkpoint Inhibitors


PD-1 and PD ligand 1/2

Programmed Cell Death 1 (PD-1) is a protein expressed on the membranes of several immune system cells, such as, B cells, T cells, and NK cells. PD-1 can be activated by one of its two binding ligands, PD-L1 and PD-L2, which can be produced by a number of cell types, including those of cancerous cells.1 This binding effectively allows the tumor cells to ‘hide’ from the cytotoxic immune response cells, by inducing regulatory T cells, which downregulate immune response from effector T cells.1,4,5 Downregulation of cytotoxic immune responses, in turn, leads to an increase in tumorigenesis.1 A number of therapeutic monoclonal antibodies (mAbs) have been developed to inhibit the binding of PD-1 and its ligands, including atezolizumab, avelumab, durvalumab, nivolumab and pembrolizumab. PD-1 and PD-L1/2 expression has shown tendencies of increased response rates to immune checkpoint blockade, however, usefulness as a predictive biomarker in response to immunotherapies across disease settings remains uncertain.



CLTA-4 is a receptor protein bound to the membrane of T cells. The binding of this protein with CD80 or CD26, which is often expressed on the surface of cancerous cells, downregulates T cell activation via feedback inhibition loops.1 In clinical trials involving mice, inhibition of CLTA-4 resulted in a degree of tumor shrinkage.1,6 As for approved anti-CLTA-4 therapies, ipilimumab was the first checkpoint inhibitor biologic, notable for its ability to extend overall survival in patients with metastatic melanoma.7


Manipulating T cells

Adoptive T cell transfer is the practice and process of manipulating patient-specific T cells ex vivo (outside of the body), to make them more reactive to a desired antigen. This practice results in the production of chimeric antigen receptor (CAR) T cells.1 The CAR T cells are genetically modified to express a specific antigen-binding domain from a B cell receptor, which also shares characteristics of the CD3 T cell receptor (TCR) intracellular domain.1 A CAR T cell has the ability to recognize a specific surface antigen and activate a T cell response, independent of MHC recognition. 1,8,9


Adverse Effects Associated with Immune Therapies

Immune-related adverse events (irAEs) often are short lived, but have the potential to be severe and even be fatal. With swift identification of irAEs and immediate initiation of treatment, patient symptoms can be quickly quelled.10 While irAEs can affect multiple organs of the body, some of the more common and notable toxicities are detailed below.


Generalized Toxicities

For anti-PD-1 and anti-PD-L1 agents, fatigue and infusion-related reaction are two of the most common adverse effects associated with immune therapies. It is estimated that fatigue occurs at a rate of 16% to 24% of patients treated with anti-PD-1 and anti-PD-L1 agents. Patients treated ipilimumab experience fatigue symptoms at a rate of approximately 40%.10-12 For infusion related reactions, mild reaction side effects are reported in 25% of patients treated with anti-PD-1 and anti-PD-L1. Severe and life-threatening infusion related reactions occur in less than 2% of patients.10


Dermatologic Toxicity

Dermatologic toxicity is the most common irAE associated with checkpoint inhibitors.13 Of patients treated with ipilimumab, more than 40% of patients develop a rash or pruritus.14 As for patients treated with pembrolizumab or nivolumab, the rate is 30% to 40%.13 In most cases, dermatologic complications will be the first irAE experienced after treatment initiation, generally occurring 3.6 weeks after first treatment.15 Typically, complications consist of reticular and maculopapular rash on the torso and extremities.13,16 Comparing PD-1 inhibition to CLTA-4 blockade, dry mouth and oral mucositis seem to be more frequent in the former.10



Many of these irAE rashes can treated with corticosteroid creams, and also in combination with oral antipruritics to combat itching.10,17 Management of higher grade rashes (Grade 3/4) utilizing oral corticosteroids with immunotherapy suspension may be indicated.13 In rare cases, toxic epidermal necrolysis has been reported resulting in hospitalization, intravenous corticosteroids, and fluid and electrolyte monitoring.10 Rashes that are not resolved by topical corticosteroid creams should be immediately evaluated by a dermatologist.10


Gastrointestinal Toxicity – Diarrhea/Colitis

Diarrhea is a common irAE for patients enrolled in checkpoint inhibition immunotherapies, often presenting six to eight weeks after treatment initiation.10,13,14 CLTA-4 blockade has been shown to be associated with higher rates of diarrhea and colitis than other checkpoint inhibition therapies.10 When examining patients for emergent diarrhea and colitis, it is imperative to rule out the possibility of bacterial and viral infections, which can present with similar symptoms.10



Grade 1 symptoms (fewer than four stools per day over baseline) can be managed with hydration and anti-motility agents.10 Symptoms persisting through the standard grade 1 treatment should be countered with the initiation of a steroid therapy, pending the root of the issue does not stem from infection.10 Grade 2 symptoms (four to six stools per day over baseline) could benefit from a colonoscopy in situations where the diagnosis is unclear. In cases where symptoms present as severe or life-threatening enterocolitis (Grade 3/4; seven or more stools per day over baseline), permanent checkpoint inhibitor discontinuation as well as initiation of high dose corticosteroids are indicated.10



Increased serum levels of hepatic enzymes aspartate aminotransferase (AST) and alanine aminotransferase (ALT) are both potential adverse effects of PD-1 and CTLA-4 inhibition.10 Onset of elevated levels typically occurs approximately 8 to 12 weeks after treatment initiation, however, earlier and later onset has been observed.18 While abnormal liver function tests are not usually accompanied by radiographic anomalies, computed tomography (CT) scans may show hepatomegaly, periportal edema and periportal lymphodenopathy.19



Monitoring of transaminases and bilirubin, indicative of hepatic function, are recommended before each dose of ipilimumab.20 Elevated AST and/or ALT may be indicative of viral or drug induced hepatitis, necessitating prompt assessment.10 Cross sectional imaging of the liver with CT and MRI should be obtained to dismiss potential new or progressive liver metastases, cholestatic pattern or grade 2+ transaminitis.21 In the absence of obvious alternative etiologies, a lengthy, tapered course of corticosteroids are indicated, usually for a minimum of three weeks.10,13



Pneumonitis, defined as focal or diffuse inflammation of the lung parenchyma, is an irAE that is relatively uncommon, yet still has the ability to cause unfavorable outcomes.10,13,22 Incidence is variable, ranging from 0-10%, however, an overall incidence based on meta-analysis of 20 PD-1 inhibition studies was 2.7%.22 Combination immunotherapy has demonstrated increased incidence (10%) over PD-(L)1 monotherapy (3%), with pneumonitis arising from anti-CTLA-4 inhibition observed at <1%.  The initial onset of pneumonitis had a variable range of 2-24 months with a median onset of ~ 3 months, although combination immunotherapy resulted in earlier onset.22  While asymptomatic presentations are possible, symptoms may include dyspnea, cough, wheeze, fever, chest pain and hypoxia, with variable radiological findings.22 Pathologically, no characteristic features or markers have been associated with immune-related pneumonitis.10,22



Treatment is dependent on grade severity, with recommendations to hold immune checkpoint inhibition at every grade, including grade 1 if there is radiographic evidence of pneumonitis progression.22  Assessment for infectious etiologies with grade 2 or higher toxicity should include respiratory, blood and urine culture and sensitivities.22 Empiric antibiotics, bronchoscopy and close monitoring, along with hospitalization for higher grade disease may be required, along with oral or IV high-dose corticosteroids tapered over 4-6 weeks.22 For grade 3 and 4 toxicity, permanent checkpoint inhibitor discontinuation is indicated, and additional immunosuppressants such as infliximab, myophenolate mofetil, IVIG or cyclophosphamide may be necessary if no initial improvement is demonstrated with IV corticosteroids and empiric antibiotic therapy.22



Checkpoint inhibition can result in inflammation of endocrine glands, including the pituitary, thyroid and adrenal glands and often presents with generic symptoms such as nausea, headache, fatigue, and vision changes.10,13 The direct cause of immune-related endocrinopathies has been difficulty to isolate, as methods of diagnosis and assessment have varied greatly among clinical trials.10 Recent meta-analysis of 38 randomized trials approximated the incidence of clinically significant immune checkpoint endocrinopathies at 10%.22 Hypothyroidism, hyperthyroidism, and hypophysitis are among the most common endocrinopathies, however, primary and secondary adrenal insufficiency, as well as in rare instances, development of type 1 diabetes (T1DM) have also been observed.10,23 Selected studies have shown some recovery of endocrine function along the gonadal and thyroid axes, however, some toxicity may be life-long.13 Despite the potential for permanence of endocrinopathies, most can be successfully treated with hormone replacement, allowing for the continued use of immunotherapy.22



Management of immune checkpoint-related endocrine toxicities is endocrinopathy-specific, with considerations to continue checkpoint inhibition in mild grade hypo- or hyperthyroidism with close follow-up and thyroid function montioring.22 Higher grade toxicities for all endocrinopathies generally entail checkpoint inhibitor discontinuation until toxicity severity has been resolved, along with appropriate hormone supplementation and in some instances, stress-dose corticosteroids.22 Autoimmune T1DM needs to be distinguished from insulin-resistance related type 2 diabetes, with insulin therapy utilized for any suspected or unclear diagnosis of T1DM due to acute risk for diabetic ketoacidosis.22


Cardiovascular Toxicity

Although rare ( < 0.1%) , cardiovacular toxicity related to immune checkpoint inhibition has been reported with rapidly fatal events and devestating sequelae.22 Risk for cardiac-related toxicity may be higher with combination therapy and can occur within a range of 2-32 weeks after therapy initiation, with a mean onset of 10 weeks.22 Myocarditis, cardiomyopathy, heart failure, arrythmias, acute coronary syndrome and cardiac arrest are just some of the manifestations of checkpoint inhibitor-induced cardiotoxcity, and symptoms can often be masked by those of other irAEs or symptoms related to disease.22



Due to the potential for cardiac compromise and fatal outcomes, all grades of cardiotoxicity should undergo diagnostic assessment with electrocardiogram, cardiac biomarkers including troponin and brain-natriuretic peptide (BNP), echocardiogram and chest x-ray.22 High-dose corticosteroids and management of cardiac symptoms such as arrythmia and heart failure as detailed by national cardiology guidelines are indicated, along with discontinuation at any grade (usually permanent) of immune checkpoint inhibition.22


Opportunistic Infection

In observational studies, the risk associated with developing an opportunistic infection is relatively low, approximately 2-7%.23,24 For most patients with an uncomplicated irAE receiving corticosteroids for less than six weeks with no pulmonary complications, Pneumocystis pneumonia (PCP) prophylaxis is not recommended.10 However, for those receiving corticosteroids for uncomplicated irAEs with existing pulmonary disease, complicated irAEs  (defined as unresponsive to corticosteroids, requiring more than six weeks of corticosteroids or additional immunosuppressive therapy), or during corticosteroid administration for irAEs occurring with combined chemo- and immunotherapy, PCP prophylaxis may be indicated.10


The Role of Biomarkers in Immune Therapy for Cancers25

Peripheral biomarkers for immunotherapy treatments in a clinical setting are becoming increasingly important, as well as rapidly emerging field. Several varying biomarkers have developed to correlate with different treatments and stages of disease. Many of these biomarkers are what specific therapies target, such as CTLA-4 targeted by ipilimumab or PD-1/PD-L1 targeted by pembrolizumab or nivolumab. Standardization of testing methodologies, besides the identification of new biomarkers that may predict sensitivity to immunotherapy are essential aspects of biomarker research.


COVID-19 Infections and Cancer Immunotherapy26

COVID-19 infections are characterized by inflammation of the lungs and other organs, ranging from mild symptoms congruent with the common cold, to severe symptoms resulting in hospitalization and potentially death. Immunocompromised patients appear to be particularly affected by COVID, as well as, patients with cardiopulmonary comorbidities. A major issue concerning cancer treatment and COVID is the management of patients needing to receive checkpoint inhibition while navigating a global pandemic. Determinations for the use or discontinuation of immunotherapy should be individualized, carefully considering factors such as metastatic disease, risk of COVID-19 transmission between patient and healthcare staff, and diagnostic conundrums such as the mimicking of common irAE clinical presentations by COVID-19, exemplified by symptomatology of shortness of breath and cough (seen in both pneumonitis and COVID-19) and indistinguishable radiologic features.




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