| Summary: | We studied the prognostic impact of tumor immunoglobulin kappa C (<i>IGKC</i>) mRNA expression as a marker of the humoral immune system in the FinHer trial patient population, where 1010 patients with early breast cancer were randomly allocated to either docetaxel-containing or vinorelbine-containing adjuvant chemotherapy. HER2-positive patients were additionally allocated to either trastuzumab or no trastuzumab. Hormone receptor-positive patients received tamoxifen. <i>IGKC</i> was evaluated in 909 tumors using quantitative real-time polymerase chain reaction, and the influence on distant disease-free survival (DDFS) was examined using univariable and multivariable Cox regression and Kaplan–Meier estimates. Interactions were analyzed using Cox regression. <i>IGKC</i> expression, included as continuous variable, was independently associated with DDFS in a multivariable analysis also including age, molecular subtype, grade, and pT and pN stage (HR 0.930, 95% CI 0.870–0.995, <i>p</i> = 0.034). An independent association with DDFS was also found in a subset analysis of triple-negative breast cancers (TNBC) (HR 0.843, 95% CI 0.724–0.983, <i>p</i> = 0.029), but not in luminal (HR 0.957, 95% CI 0.867–1.056, <i>p</i> = 0.383) or HER2-positive (HR 0.933, 95% CI 0.826–1.055, <i>p</i> = 0.271) cancers. No significant interaction between <i>IGKC</i> and chemotherapy or trastuzumab administration was detected (P<sub>interaction</sub> = 0.855 and 0.684, respectively). These results show that humoral immunity beneficially influences the DDFS of patients with early TNBC.
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