Intratumoral CD103‐positive tumour‐infiltrating lymphocytes are associated with favourable prognosis in patients with triple‐negative breast cancer

Aims Cluster of differentiation 103 (CD103), a marker of tissue resident memory T cells, is expressed on subsets of CD8 + T lymphocytes. We investigated the prognostic significance of CD103 + intra‐epithelial tumour‐infiltrating lymphocytes (iTILs) in invasive breast cancer (IBC). Methods and results Immunohistochemistry was performed for CD103, CD8 and TGF‐β isoforms (1, 2 and 3) on tissue microarrays of 1187 IBC samples. CD103 + and CD8 + iTILs were present in 904 (76.2%) and 854 (74%) cases with an overall mean ± standard deviation of 38.2 ± 100.2/mm 2 and 30.4 ± 89.7/mm 2 , respectively. The numbers of CD103 + and CD8 + iTILs were positively correlated, and CD103 + iTILs outnumbered CD8 + iTILs in HER2‐positive and triple‐negative breast cancer (TNBC). CD103 + and CD8 + iTIL densities were significantly higher in tumours of histological grade 3, absence of lymphovascular invasion, high Ki‐67 index, high stromal TIL density or TGF‐β3 expression. High CD103 + iTIL density was associated with better disease‐free survival (DFS, P  = 0.007), but no significant association was observed for overall survival (OS). Subgroup analysis by cancer molecular subtype showed that CD103 + iTIL count was prognostic only for TNBC (OS, P  = 0.035; DFS, P  = 0.009). CD8 + iTIL density was significant for DFS, but not for OS, in the entire cohort and TNBC. In multivariate analysis, CD103 + iTIL density was an independent prognostic factor of OS ( P  = 0.02) and DFS ( P  = 0.007) in TNBC, while CD8 + iTIL density was not significant for survival. Conclusions CD103 iTIL density can serve as a predictor of good prognosis in patients with TNBC.