Purpose: CD8⁺ tumor-infiltrating lymphocytes (TIL) are key mediators of antitumor immunity and are strongly associated with survival in virtually all solid tumors. However, the prognostic effect of CD8⁺ TIL is markedly higher in the presence of CD20⁺ B cells, suggesting that cooperative interactions between these lymphocyte subsets lead to more potent antitumor immunity.

Experimental Design: We assessed the colocalization patterns, phenotypes, and gene expression profiles of tumor-associated T- and B-lineage cells in high-grade serous ovarian cancer (HGSC) by multicolor IHC, flow cytometry, and bioinformatic analysis of gene expression data from The Cancer Genome Atlas.

Results: T cells and B cells colocalized in four types of lymphoid aggregate, ranging from small, diffuse clusters to large, well-organized tertiary lymphoid structures (TLS) resembling activated lymph nodes. TLS were frequently surrounded by dense infiltrates of plasma cells (PC), which comprised up to 90% of tumor stroma. PCs expressed mature, oligoclonal IgG transcripts, indicative of antigen-specific responses. PCs were associated with the highest levels of CD8⁺, CD4⁺, and CD20⁺ TIL, as well as numerous cytotoxicity-related gene products. CD8⁺ TIL carried prognostic benefit only in the presence of PCs and these other TIL subsets. PCs were independent of mutation load, BRCA1/2 status, and differentiation antigens but positively associated with cancer–testis antigens.

Conclusions: PCs are associated with the most robust, prognostically favorable CD8⁺ TIL responses in HGSC. We propose that TLS facilitate coordinated antitumor responses involving the combined actions of cytolytic T cells and antibody-producing PCs. Clin Cancer Res; 22(12); 3005–15. ©2016 AACR.