Immunotherapy: Harnessing the Immune System Against Cancer

Immunotherapy represents one of the most consequential shifts in oncology practice over the past three decades, encompassing a diverse class of treatments that redirect or amplify the body's own immune defenses to identify and destroy cancer cells. This page covers the major categories of immunotherapy, the biological mechanisms underlying each, the cancer types where these approaches have the strongest evidence base, and the clinical decision factors that guide their use. Understanding immunotherapy is essential context for anyone navigating the broader landscape of oncology care.

Definition and Scope

Immunotherapy in oncology refers to any treatment strategy that modifies immune system activity to produce an anti-tumor effect. The U.S. Food and Drug Administration (FDA) has granted approval to immunotherapy agents across more than 15 distinct cancer indications since the first checkpoint inhibitor approval in 2011 (ipilimumab for metastatic melanoma). The National Cancer Institute (NCI) classifies immunotherapy into five primary categories:

  1. Checkpoint inhibitors — monoclonal antibodies that block inhibitory proteins (PD-1, PD-L1, CTLA-4) on immune cells or tumor cells
  2. CAR T-cell therapy — genetically engineered patient T cells redirected to recognize tumor-specific antigens
  3. Cancer vaccines — agents designed to stimulate tumor-specific immune responses, including both preventive and therapeutic forms
  4. Cytokines — proteins such as interleukin-2 (IL-2) and interferon-alpha that regulate immune cell activity
  5. Monoclonal antibodies (non-checkpoint) — including antibody-drug conjugates and bispecific antibodies that directly target tumor surface proteins

The scope of immunotherapy has expanded to include combination regimens, discussed further at /combination-therapy, and overlaps with targeted therapy when antibodies are engineered to block specific oncogenic drivers.

How It Works

Healthy immune systems rely on regulatory checkpoints to prevent autoimmune damage. Tumors exploit these same checkpoints — particularly the PD-1/PD-L1 axis — to evade immune surveillance. When a tumor cell overexpresses PD-L1, it binds PD-1 on cytotoxic T cells, effectively switching them off. Checkpoint inhibitors block this binding event, restoring T-cell cytotoxic activity.

The mechanism differs by agent class:

CAR T-cell therapy operates through a distinct mechanism: T cells are harvested from the patient, genetically modified to express a chimeric antigen receptor targeting a specific protein (most commonly CD19 in B-cell malignancies), expanded in culture, and reinfused. The FDA had approved 6 CAR T-cell products as of 2023, according to the (FDA CAR T-cell approvals page).

Biomarker testing is central to predicting response. Tumor mutational burden (TMB) and PD-L1 expression levels, evaluated through molecular profiling and biomarker testing, are FDA-recognized predictive biomarkers for checkpoint inhibitor eligibility across multiple tumor types. The regulatory framework governing biomarker-based prescribing decisions is detailed at /regulatory-context-for-oncology.

Common Scenarios

Immunotherapy is FDA-approved as first-line, second-line, or adjuvant therapy across a range of solid tumors and hematologic malignancies. The following represent high-evidence clinical settings:

Non-small cell lung cancer (NSCLC): Pembrolizumab monotherapy is approved for tumors with PD-L1 tumor proportion score (TPS) ≥ 50% and no EGFR/ALK alterations. In the KEYNOTE-024 trial (published in NEJM, 2016), pembrolizumab demonstrated a 10.3-month improvement in progression-free survival compared to platinum-based chemotherapy in this population.

Melanoma: Ipilimumab plus nivolumab combination therapy demonstrated a 5-year overall survival rate of 52% in the CheckMate 067 trial, compared to 26% with ipilimumab alone (NEJM, 2019).

Hematologic malignancies: CAR T-cell therapies dominate in relapsed/refractory large B-cell lymphoma and acute lymphoblastic leukemia (ALL). Tisagenlecleucel achieved an 81% overall remission rate in pediatric ALL in the ELIANA trial, as reported in the (FDA prescribing information for Kymriah).

Bladder, cervical, and head-and-neck cancers also carry FDA-approved checkpoint inhibitor indications, typically in platinum-ineligible or recurrent settings.

Decision Boundaries

Not all patients are candidates for immunotherapy, and the decision involves biomarker status, tumor histology, prior treatment history, and risk of immune-related adverse events (irAEs).

Checkpoint inhibitors vs. CAR T-cell therapy — key distinctions:

Factor Checkpoint Inhibitors CAR T-Cell Therapy
Administration IV infusion, outpatient Single infusion, inpatient monitoring required
Eligibility biomarker PD-L1 expression, TMB Tumor antigen expression (e.g., CD19, BCMA)
Primary toxicity Immune-related inflammation (colitis, pneumonitis, endocrinopathy) Cytokine release syndrome (CRS), neurotoxicity
Approval setting Solid tumors, some hematologic Predominantly hematologic malignancies

irAEs from checkpoint inhibitors are graded using the NCI Common Terminology Criteria for Adverse Events (CTCAE, Version 5.0). Grade 3–4 irAEs — affecting approximately 15–20% of patients receiving combination checkpoint blockade according to the NCI — may require high-dose corticosteroids or permanent treatment discontinuation.

Autoimmune conditions, organ transplant history, and active corticosteroid use above 10 mg prednisone equivalent per day are relative contraindications evaluated on a case-by-case basis. Patients are assessed for these factors through a clinical workup that may include genetic testing for cancer risk and pathology report review prior to initiation.

Managing toxicities that emerge during treatment, including irAEs and CRS, falls within the scope covered at managing side effects.

References

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