Which tumor pathological biomarkers would indicate eligibility for immunotherapy?

Key Pathological Biomarkers for Immunotherapy Candidacy

The most critical pathological biomarkers that determine immunotherapy eligibility are microsatellite instability-high (MSI-H)/deficient mismatch repair (dMMR) status, PD-L1 expression, and tumor mutational burden (TMB), with MSI-H/dMMR representing the strongest pan-cancer predictor of immunotherapy response 1.

Primary Testing Algorithm

First-Tier Biomarkers (Essential for All Candidates)

MSI-H/dMMR Status is the most robust predictor across cancer types and should be assessed first:

• Immunohistochemistry (IHC) for mismatch repair proteins (MLH1, MSH2, MSH6, PMS2) is the recommended initial test 1
• Loss of expression of any of these proteins indicates dMMR status
• PCR-based testing using five microsatellite markers (including BAT-25 and BAT-26) serves as the second-line method 1
• MSI-H/dMMR status predicts response regardless of tumor type, making it the only FDA-approved tissue-agnostic biomarker for immunotherapy

PD-L1 Expression via immunohistochemistry:

• Combined Positive Score (CPS) is the preferred metric for most solid tumors 2, 3
• Critical caveat: PD-L1 expression shows spatial and temporal heterogeneity, and thresholds vary by tumor type and specific immunotherapy agent 2
• Despite limitations, remains essential for patient selection in lung cancer, gastric cancer, and other malignancies 4

Tumor Mutational Burden (TMB):

• High TMB (typically ≥10 mutations/megabase, though thresholds vary) correlates with increased neoantigen load 3, 4
• Next-generation sequencing (NGS) coupling MSI and TMB analysis represents the optimal approach for comprehensive assessment 1
• Important limitation: significant overlap exists between TMB-high and MSI-H biology 2

Second-Tier Biomarkers (Tumor-Specific Enhancement)

Tumor-Infiltrating Lymphocytes (TILs):

• High TIL density, particularly CD8+ T cells, indicates an inflamed tumor microenvironment favorable for immunotherapy response 5, 4
• Tertiary lymphoid structures within tumors further enhance predictive value 5

Specific Genetic Alterations:

• Favorable: EBV-associated tumors in gastric cancer 2
• Unfavorable: KEAP1, STK11, SMARCA4 mutations predict resistance in lung cancer 6
• Wnt/β-catenin pathway activation correlates with immune exclusion and poor response 5

Blood-Based Biomarkers (Emerging Clinical Utility)

Systemic Inflammatory Markers 3:

• Neutrophil-to-lymphocyte ratio (NLR) or systemic immune-inflammation index
• Serum lactate dehydrogenase (LDH)
• Serum albumin

These provide prognostic information and should be obtained as baseline measurements.

Critical Pitfalls to Avoid

1. Assay Variability: Different PD-L1 companion diagnostics use different antibodies and scoring systems—ensure the appropriate assay for the specific immunotherapy agent being considered 2, 4

2. Temporal Heterogeneity: PD-L1 expression can change with prior treatment; use the most recent biopsy when possible 2

3. Single Biomarker Reliance: No single biomarker perfectly predicts response; MSI-H/dMMR has the strongest evidence but combining biomarkers improves patient selection 1, 3

4. Tissue Adequacy: Ensure sufficient tumor tissue for comprehensive testing—NGS panels can simultaneously assess MSI, TMB, and actionable mutations 1

Practical Implementation Framework

For any patient being considered for immunotherapy:

1. Mandatory testing: MSI/dMMR status (IHC first-line), PD-L1 expression, baseline inflammatory markers 1, 3

2. Strongly recommended: TMB via NGS if resources permit, particularly for tumors outside Lynch syndrome spectrum 1

3. Tumor-specific additions:

   • Lung cancer: Add STK11, KEAP1, SMARCA4 mutation status 6
   • Gastric cancer: Add EBV status, HER2 for combination strategies 2
   • Hepatocellular carcinoma: Add TIL assessment, tertiary lymphoid structures 5

4. Consider in research settings: Circulating tumor DNA dynamics, microbiome analysis, AI-assisted multimodal prediction models 2, 6

The strongest candidates for immunotherapy demonstrate MSI-H/dMMR status OR high PD-L1 expression (CPS ≥10 in most contexts) OR high TMB, with the best outcomes when multiple favorable biomarkers align 1, 3.