Tumour-Infiltrating Lymphocytes: Prognosis and Prediction

How TILs Are Assessed

The simplest method is a pathologist estimating the percentage of stroma occupied by lymphocytes on a haematoxylin and eosin slide, using published scoring guidelines. More detailed approaches use immunohistochemistry for specific subsets such as CD8 (cytotoxic), CD4, FOXP3 (regulatory) and CD20 (B cells).

The Immunoscore is a standardised, validated version for colorectal cancer that quantifies CD3 and CD8 cells in the tumour core and invasive margin and combines them into a score.

Prognostic Value

High TIL density is associated with better outcomes in colorectal cancer, triple-negative and HER2-positive breast cancer, melanoma, and several other tumour types, often independently of stage. The consensus Immunoscore was validated internationally as a prognostic classifier in stage I to III colon cancer.

Regulatory T cells and some myeloid populations can carry the opposite association, so the composition of the infiltrate matters, not just the total number.

Prognostic Is Not the Same as Predictive

A prognostic marker forecasts outcome regardless of treatment; a predictive marker forecasts benefit from a specific therapy. TILs are well established as prognostic. Their value in predicting checkpoint-inhibitor benefit is supported by biology but is less formally validated as a standalone test.

In practice TIL assessment is used routinely for prognosis in breast and colorectal cancer and as supportive context elsewhere.

Where TIL Scoring Is Used Today

In early triple-negative breast cancer, stromal TIL density is reported alongside grade and stage because higher TILs predict better outcomes and, in some analyses, better response to chemotherapy; standardised scoring guidelines from an international working group make this reproducible. In stage II to III colon cancer, the Immunoscore adds independent prognostic information to the TNM stage and can inform decisions about adjuvant chemotherapy.

Elsewhere, TIL assessment is supportive context rather than a decision tool. It is not used to select checkpoint inhibitors in lung cancer, where PD-L1 and tumour mutational burden are the operative markers, and a low TIL count does not by itself exclude a trial of immunotherapy.

Key Takeaways

• ·TILs are immune cells within and around a tumour, scorable on routine or stained sections.
• ·High TIL density predicts better outcomes in several cancers, and the Immunoscore formalises this in colon cancer.
• ·TILs are firmly prognostic; their predictive value for immunotherapy is biologically supported but less validated.