Characterization and spatial distribution of the immune cell infiltrate in triple-negative breast cancer: a novel classification based on plasma cells and CD8+ T cells.

Stromal tumor-infiltrating lymphocytes (sTILs) are a robust prognostic and predictive biomarker in triple-negative breast carcinoma. However, the sTIL compartment comprises different cell populations. The aim of the study is to characterize the distribution of T cells (CD3+ and CD8+), B cells, and plasma cells and explore their association with outcome in the surgical specimen of 62 patients. Furthermore, programmed death ligand 1 expression and the presence of tertiary lymphoid structures (TLSs) are explored. Patients with higher sTILs achieve better progression-free survival (PFS) (P = .0013), and tumors have more plasma cells in the infiltrate. Specifically, higher counts of T cells (both CD3+ and CD8+) have better PFS (P = .002 and P = .0086, respectively) as it is observed in tumors with higher infiltration of CD8+ T cells in the tumor core (P = .035). Higher infiltration by B cells and plasma cells shows a positive tendency toward increased PFS (P = .06 and P = .058). Programmed death ligand 1 (SP142) is positive in 56% of tumors. Tumors with at least 1 TLS (42%) show higher CD8+ T cell infiltration in the tumor core and the sTIL value doubles compared to tumors devoid of TLSs [sTIL mean: 36 ± 11% and 18 ± 5% (CI [Confidence Interval]: 95%), respectively]. Our study demonstrates that the characterization of the immune cell infiltration is as relevant as its distribution. Moreover, the importance of considering different immune cell types for classification is emphasized. Therefore, a new classification of triple-negative breast carcinoma immune infiltration with CD8+ T cell and plasma cell densities in the tumor core and infiltrative margin is proposed.