Background: Tertiary lymphoid structures (TLSs) are transient ectopic lymphoid aggregates whose formation might be caused by chronic inflammation states, such as cancer. The presence of TLS is associated with a favorable prognosis in most solid malignancies. The recognition of the relevance of TLS to cancer has led to a growing interest in TLS as an immunomodulatory target to enhance tumor immunity, although how TLSs are induced in the tumor microenvironment (TME) and how they affect patient survival are not well understood. Methods: TLS distribution in relation to tumor infiltrating lymphocytes (TILs) and related gene expression were investigated in high grade serous ovarian cancer (HGSC) specimens. CXCL13 expression, which is strongly associated with TLS, and its localization in immune cells, were examined. We explored the tumor microenvironment for CXCL13 secretion by adding various inflammatory cytokines in vitro. The induction of TLS by CXCL13 was examined in a mouse model of ovarian cancer. Results: CXCL13 gene expression correlated with TLS formation and the infiltration of T cells and B cells, and was a favorable prognostic factor for HGSC patients. The coexistence of CD8+ T cells and B-cell lineages in the TME was associated with a better prognosis of HGSC and was closely related to the presence of TLSs. CXCL13 expression was predominantly coincident with CD4+ T cells in TLSs and CD8+ T cells in TILs, and shifted from CD4+ T cells to CD21+ follicular dendritic cells as the TLS matured. Although TGF-β was reported to stimulate CXCL13 production, our in vitro results revealed that CXCL13 secretion was promoted in CD4+ T cells under TGF-β + IL-2-restricted conditions and in CD8+ T cells under TGF-β + IL-12-rich conditions. In a mouse model of ovarian cancer, recombinant CXCL13 induced TLSs and enhanced survival by the infiltration of CD8+ T cells. Conclusions: TLS formation was promoted by CXCL13-producing CD4+ T cells and TLSs facilitated the coordinated antitumor responses of cellular and humoral immunity in ovarian cancer.

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