Assessing the relationship between tumor-infiltrating lymphocytes and PD-L1 expression in triple negative breast cancer: Identifying optimal TILs cut-off value for pathologic reporting.

ABSTRACT

BACKGROUND: Triple Negative Breast Cancer (TNBC) presents diagnostic complexities, particularly in evaluating Tumor-Infiltrating Lymphocytes (TILs) and Programmed Death-Ligand 1 (PD-L1) expression. This study aimed to identify optimal TILs percentage cut-offs predictive of PD-L1 expression and to investigate the relationship between TILs, PD-L1, and tertiary lymphoid structures (TLSs). METHOD: Analyzing 141 TNBC cases, we assessed TILs, PD-L1 expression (clones 22C3 and SP142), and TLS presence. RESULTS: We identified TILs cut-offs (<20 %, 20-60 %, ≥60 %) correlating with PD-L1 expression. TILs <20 % rarely express PD-L1 with either 22C3 or SP142 clones. TILs ≥60 % demonstrate PD-L1 expression across both clones. TILs within the 20-60 % range correlate with PD-L1 expression using the SP142 clone, but not 22C3. Evaluating TILs solely at the tumor edge led to inaccuracies, highlighting the need for overall assessment of TILs throughout the entire lesion. TLS presence correlated with higher TIL percentages and PD-L1 expression, particularly with SP142. Discrepancies between 22C3 and SP142 clones (15 % vs. 50 % positivity, respectively) underscored the variability in PD-L1 detection. CONCLUSION: This study identifies TILs cut-offs predictive of PD-L1 positivity, suggesting the need for institutions to tailor these thresholds based on the selected PD-L1 clone and treatment. Evaluating TILs solely at the tumor edge may overlook the complexity of tumor immune infiltration. While TLS presence correlates with higher PD-L1 expression, particularly with the SP142 clone, its exact predictive value for PD-L1 remains to be clarified. The SP142 clone exhibits higher positivity rates compared to 22C3.