Heterogeneity of tumour-infiltrating lymphocytes in breast cancer and its prognostic significance

Background and aims Tumour‐infiltrating lymphocytes ( TIL s) in breast cancer ( BC ) provide prognostic and predictive information. The aim of this study was to assess the spatial and temporal heterogeneity of TIL s in BC and its relationship with immune cell subtypes. Methods and results Immunohistochemically defined immune cell subtypes, i.e. those expressing T‐cell markers ( CD 3, CD 8, and FOXP 3), a B‐cell marker ( CD 20) and a histiocytic marker ( CD 68), were evaluated in a large series ( n = 1165) of invasive BC s. A subset of full‐face haematoxylin and eosin (H&E)‐stained slides were examined for TIL s heterogeneity within primary tumours and the corresponding local recurrent carcinomas to investigate spatial and temporal TIL s heterogeneity. H&E‐stained sections from multiple tumour blocks (three or four blocks per case) representing different tumour areas were evaluated to assess TIL s interslide heterogeneity and intraslide heterogeneity. Both average stromal TIL s ( AV ‐ TIL s) and hotspot stromal TIL s ( HS ‐ TIL s) were assessed. AV ‐ TIL s showed associations with all immune cell subtypes; however, CD3+ cells constituted the main component (mean number of CD3+ was 55), whereas CD 20+ cells constituted the smallest component (mean number of CD20 was 13). There was no significant statistical difference between TIL s across tumour blocks of the same case ( P = 0.251 for AV ‐ TIL s and P = 0.162 for HS ‐ TIL s). Triple‐negative breast cancer ( TNBC ) showed higher TIL s percentage than other BC subtypes ( P < 0.001). High AV ‐ TIL s, CD 3+ cells, CD 8+ cells and CD 20+ cells were associated with longer survival in TNBC patients ( P < 0.05). High AV ‐ TIL s in recurrent tumours showed a significant association with shorter post‐recurrence survival ( P = 0.004). Conclusions Despite the heterogeneity of immune cell type components, average TIL s in one full‐face H&E‐stained section reliably represent TIL s in the whole tumour. TIL s were associated with outcome in TNBC patients, as well as having prognostic significance for recurrent tumours.