Unlocking Immunity: TILs, Immunotherapy Biomarkers, and Toxicity Management in Oncology

Cancer immunotherapy has revolutionized oncologic care, shifting the paradigm from tumor-directed cytotoxic strategies to immune-based interventions. Agents such as immune checkpoint inhibitors (ICIs), adoptive T-cell therapies, and cancer vaccines harness the body’s immune machinery to target malignancies with unprecedented precision and durability. However, not all patients respond equally, and the risk of immune-related adverse events (irAEs) poses a critical management challenge. In this context, tumor-infiltrating lymphocytes, cancer immunotherapy biomarkers, and effective strategies for the management of immunotherapy toxicity have emerged as central elements in optimizing treatment outcomes.

Tumor-Infiltrating Lymphocytes: Gatekeepers of the Immune Response

Tumor-infiltrating lymphocytes (TILs) are lymphocytes, primarily T cells, that migrate into the tumor microenvironment. Their presence signals an ongoing immune response against tumor antigens and has significant prognostic and predictive implications. TILs were first recognized as a favorable prognostic factor in melanoma, breast, and colorectal cancers. More recently, their role in guiding immunotherapy decisions has gained traction.

The Biology of TILs

TILs are composed of heterogeneous populations, including CD8+ cytotoxic T cells, CD4+ helper T cells, and regulatory T cells (Tregs). CD8+ TILs mediate tumor cell killing through perforin and granzyme pathways, while CD4+ cells modulate the immune milieu. Tregs, conversely, may suppress anti-tumor immunity. The balance of these subpopulations influences therapeutic outcomes.

High TIL density correlates with improved responses to immune checkpoint inhibitors, particularly anti-PD-1 and anti-CTLA-4 therapies. In triple-negative breast cancer (TNBC) and non-small cell lung cancer (NSCLC), TIL presence has been linked to prolonged progression-free and overall survival. Additionally, adoptive cell therapy (ACT) utilizing expanded autologous TILs has shown remarkable efficacy in metastatic melanoma, supporting their role as both biomarkers and therapeutic agents.

TILs as Predictive Biomarkers

Quantifying and characterizing TILs offer predictive insights into immunotherapy response. Immunohistochemistry (IHC), gene expression profiling, and multiplex immunofluorescence are used to evaluate TILs. However, standardization remains a challenge. Ongoing efforts by the International Immuno-Oncology Biomarker Working Group aim to harmonize TIL assessment across tumor types.

Cancer Immunotherapy Biomarkers: Navigating the Path to Precision

While immunotherapy has transformed oncology, only a subset of patients derives a durable benefit. This heterogeneity underscores the need for cancer immunotherapy biomarkers that can guide patient selection, predict response, and monitor treatment.

Established Biomarkers

1. PD-L1 Expression

Programmed death-ligand 1 (PD-L1) expression on tumor or immune cells remains the most widely used biomarker for checkpoint inhibitors. High PD-L1 levels often correlate with better outcomes in NSCLC, urothelial carcinoma, and head and neck cancers. However, its utility is limited by dynamic expression, spatial heterogeneity, and assay variability.

2. Tumor Mutational Burden (TMB)

TMB quantifies the number of somatic mutations in a tumor genome. High TMB may generate neoantigens that enhance immunogenicity, making tumors more responsive to ICIs. While promising, TMB lacks standardized cut-offs, and its predictive value is cancer-type specific.

3. Microsatellite Instability (MSI)

MSI-high status, resulting from deficient mismatch repair (dMMR), is a robust predictor of response to ICIs. MSI-high tumors harbor increased neoantigen loads and are responsive across diverse malignancies, leading to the FDA’s first tissue-agnostic approval for pembrolizumab.

Emerging Biomarkers

• TIL density and phenotype: As discussed, high levels of activated CD8+ TILs are associated with favorable outcomes.

• Gene expression signatures: Interferon-γ-related gene sets and T-cell inflamed signatures are under investigation for their predictive utility.

• Peripheral immune cell profiles: Blood-based biomarkers such as circulating T cells, cytokines, and ctDNA offer non-invasive options for monitoring.

Integration of multi-omic data, combining genomics, transcriptomics, proteomics, and spatial analysis; promises a more holistic view of tumor-immune interactions, guiding personalized immunotherapy.

Immune-Related Adverse Events: A Double-Edged Sword

The efficacy of immune checkpoint inhibitors is rooted in immune activation. However, this activation can extend beyond tumors, leading to immune-related adverse events (irAEs) - a distinct spectrum of inflammatory side effects affecting virtually any organ system.

Epidemiology and Mechanisms

irAEs occur in up to 90% of patients receiving anti-CTLA-4 therapy and 70% of those on anti-PD-1/PD-L1 agents. Combination therapy further increases risk. These events arise from the breakdown of peripheral tolerance, leading to autoimmune-like responses.

Common irAEs include:

• Dermatologic: Rash, pruritus, vitiligo

• Gastrointestinal: Colitis, diarrhea

• Hepatic: Hepatitis, transaminitis

• Endocrine: Hypophysitis, thyroiditis, adrenalitis

• Pulmonary: Pneumonitis

Onset varies from days to months after therapy initiation. Some irAEs, particularly endocrine toxicities, may be irreversible.

Diagnosis and Grading

Early recognition is crucial. Diagnosis relies on clinical presentation, laboratory testing, and exclusion of alternative causes. The Common Terminology Criteria for Adverse Events (CTCAE) provides a standardized grading system, from mild (Grade 1) to life-threatening (Grade 4).

Management of Immunotherapy Toxicity: Striking the Balance

Effective management of immunotherapy toxicity is essential to maintain treatment efficacy while minimizing harm. A multidisciplinary approach involving oncologists, endocrinologists, gastroenterologists, and other specialists is often needed.

General Principles

• Grade 1: Continue therapy with close monitoring.

• Grade 2: Suspend immunotherapy; initiate low-dose corticosteroids (e.g., prednisone 0.5–1 mg/kg/day).

• Grade 3–4: Permanently discontinue therapy (depending on the toxicity); administer high-dose corticosteroids (1–2 mg/kg/day), followed by tapering over 4–6 weeks.

Refractory Cases and Second-Line Agents

Some patients do not respond to steroids. In these cases, immunosuppressants such as infliximab (for colitis), mycophenolate mofetil (for hepatitis), or IVIG (for neurologic toxicities) may be used. Biologic agents targeting specific immune pathways (e.g., tocilizumab for cytokine release syndrome) are emerging as targeted solutions.

Rechallenge Considerations

Reinitiating immunotherapy after irAEs is a nuanced decision. Factors include the severity and type of the initial event, the availability of alternative therapies, and the patient’s overall condition. Close monitoring and patient education are imperative if rechallenge is pursued.

The Future: Personalizing Immuno-Oncology

The integration of tumor-infiltrating lymphocytes and cancer immunotherapy biomarkers into routine practice is shaping a more personalized approach to immuno-oncology. As the field matures, combining predictive biomarkers with toxicity risk stratification may help balance efficacy and safety.

Artificial intelligence and machine learning are being leveraged to develop predictive models incorporating clinical, molecular, and radiologic data. Additionally, the development of “immunotherapy toxicity biomarkers” is an area of growing interest, aiming to identify patients at high risk for irAEs before therapy begins.

Prophylactic Strategies

Research is underway to evaluate prophylactic interventions such as microbiome modulation, immune profiling, and anti-inflammatory co-therapies, to prevent or mitigate irAEs without compromising efficacy.

Clinical Trials and Guidelines

Ongoing trials are refining biomarker-driven therapy, novel immunotherapy combinations, and optimized management of irAEs. Updated clinical guidelines from ASCO, ESMO, and NCCN provide frameworks for biomarker testing and irAE management.

Conclusion

The interplay between tumor-infiltrating lymphocytes, cancer immunotherapy biomarkers, and the management of immunotherapy toxicity underscores the complexity and promise of cancer immunotherapy. TILs offer insight into immune activation within the tumor microenvironment, while biomarkers help predict who will benefit and guide treatment decisions. Simultaneously, a deep understanding of irAEs and proactive toxicity management is essential to deliver safe, effective, and durable outcomes.

For oncologists, the future lies in refining this triad, harnessing the immune system, individualizing therapy, and managing its consequences with precision. As research evolves, these pillars will continue to redefine standards of care, offering renewed hope for patients across the cancer spectrum.