Tertiary lymphoid structures critical for prognosis in endometrial cancer patients - a TransPORTEC study

B-cells play a key role in cancer suppression, particularly when aggregated in tertiary lymphoid structures (TLS). Here, we investigated the role of B-cells and TLS in endometrial cancer (EC). Single cell RNA-sequencing of B-cells showed presence of activated/memory B-cells, cycling/germinal center B-cells and antibody-secreting cells. Differential gene expression analysis showed association of TLS with L1CAM overexpression. Immunohistochemistry and co-immunofluorescence showed L1CAM expression in mature TLS localized in the myometrial wall or at the tumor invasive border, independent of L1CAM expression the tumor. Using L1CAM as a marker, 378 of the 411 molecularly classified ECs from the PORTEC-3 biobank were evaluated. TLS were found in 19%, predominantly in mismatch-repair deficient and polymerase-epsilon mutant EC. Multivariable Cox regression analysis showed strong favorable prognostic impact of TLS, independent of clinicopathological and molecular factors. Our data suggests a pivotal role of TLS in outcome of EC patients, and establishes L1CAM as a simple biomarker.Statement of significance Tertiary lymphoid structures have a pivotal role in the immune response against endometrial cancer. Presence of mature tertiary lymphoid structures can be easily assessed using L1CAM immunohistochemistry and has independent favorable predictive value for recurrence and endometrial cancer-specific survival.