Differences in immunogenicity of TP53-mutated cancers with low tumor mutational burden (TMB) A study on TP53mut endometrial-, ovarian- and triple-negative breast cancer
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To explore why in large phase III randomized clinical trials TP53-mutated (TP53mut) endometrial cancer (EC) was the only tumor showing survival benefit to immune checkpoint inhibitors (ICIs) added to chemotherapy when compared with other low TMB TP53mut cancers, such as high-grade serous ovarian (HGSOC) and triple-negative breast cancer (TNBC). From 606 patients with one of the three mentioned cancers, "The Cancer Genome Atlas" data on clinical outcome, TMB and detailed composition of the tumor immune-microenvironment (TIME) (immune infiltrating cells, cytokines, and other immune-modulators) were compared using the Kruskal-Wallis test, followed by Pearson correlation. Prognostic value of studied variables was assessed by Kaplan-Meier and Cox-regression analyses. TMB was very low in all three TP53mut entities, being lowest in EC (median: 1.27 Mut/Mb; p < 0.001). Interestingly, high TMB was significantly associated with improved clinical outcome in every entity, whereby best discrimination for PFS was found in EC (HR: 0.52). Compared to EC, immune-suppressing regulatory T-cells were higher in HGSOC and TNBC (p < 0.001) and M2-like macrophages higher in HGSOC (p < 0.001). In contrast, immune-activating mDCs were more prominent in EC than in HGSOC (p < 0.001). Differential modulator expression analyses revealed highest discrimination for the immune-inhibiting FOXP3, C1QA and XBP1, which all exhibited lower levels in EC compared with HGSOC and TNBC (p < 0.001). Characteristics of TIME differ substantially among the assessed entities in terms that EC exhibits fewer immunosuppressive traits, expecting a higher likelihood for responding to ICIs, despite a very low TMB, whereas HGSOC and TNBC exhibit an immune hostile TIME.