Tertiary lymphoid structures in breast ductal carcinoma in situ correlate with adverse pathological parameters

Aims: To investigate tertiary lymphoid structures (TLSs) in ductal carcinoma in situ (DCIS) of the breast and their correlation with pathological features, immune cell markers and clinical outcomes. Methods and results: Morphological identification of TLSs in 198 DCIS cases incorporated B and T cell zones with high endothelial venules. TLS positivity was defined as ≥ 1 TLSs in lesional areas, while TLS area percentage was divided into two categories: low (TLSs < 5%) and high (TLSs ≥ 5%). Previously reported biomarkers included ER, PR, HER2, CD68, CD163, CD4, CD8 and PD-L1. TLSs were observed in 24.7% (49 of 198) of cases, with a mean diameter of 0.44 mm (median = 0.4 mm, range = 0.12–1.43 mm). TLSs were significantly associated with higher nuclear grade, presence of necrosis, hormone receptor negativity/HER2 positivity, triple negativity, tumour infiltrating lymphocytes (TILs) and immune related biomarkers such as FOXP3, CD163, CD4 and CD4/CD8 ratio (all P < 0.05). There were no significant associations between TLSs and recurrence, but a combination of TLSs^high with FOXP3⁺, CD4^high, CD4/CD8 ratio^high and CD68^high individually, compared with all other combinations, disclosed significantly poorer disease-free survival (DFS) for ipsilateral invasive recurrence (IIR) on both Kaplan–Meier and multivariable Cox regression analyses (all P < 0.05). Conclusions: TLSs in DCIS were associated with unfavourable prognostic features, TILs and immune cell markers in our study. TLSs^high/FoxP3⁺, TLSs^high/CD4^high, TLSs^high/(CD4/CD8) ratio^high and TLSs^high/CD68^high were independent factors for poorer DFS for IIR. Further exploration of the pathological significance of TLSs may provide a clinical basis for their recognition as an important structure and functional unit in the tumour immune microenvironment.