BACKGROUND: High risk of recurrence/progression bladder tumours is treated with 
Bacillus Calmette-Guérin (BCG) immunotherapy after complete resection of the 
tumour. Approximately 75% of these tumours express the uncommon carbohydrate 
antigen sialyl-Tn (Tn), a surrogate biomarker of tumour aggressiveness. Such 
changes in the glycosylation of cell-surface proteins influence tumour 
microenvironment and immune responses that may modulate treatment outcome and 
the course of disease. The aim of this work is to determine the efficiency of 
BCG immunotherapy against tumours expressing sTn and sTn-related antigen 
sialyl-6-T (s6T).
METHODS: In a retrospective design, 94 tumours from patients treated with BCG 
were screened for sTn and s6T expression. In vitro studies were conducted to 
determine the interaction of BCG with high-grade bladder cancer cell line 
overexpressing sTn.
RESULTS: From the 94 cases evaluated, 36 had recurrence after BCG treatment 
(38.3%). Treatment outcome was influenced by age over 65 years (HR=2.668; 
(1.344-5.254); P=0.005), maintenance schedule (HR=0.480; (0.246-0.936); P=0.031) 
and multifocality (HR=2.065; (1.033-4.126); P=0.040). sTn or s6T expression was 
associated with BCG response (P=0.024; P<0.0001) and with increased 
recurrence-free survival (P=0.001). Multivariate analyses showed that sTn and/or 
s6T were independent predictive markers of recurrence after BCG immunotherapy 
(HR=0.296; (0.148-0.594); P=0.001). In vitro studies demonstrated higher 
adhesion and internalisation of the bacillus to cells expressing sTn, promoting 
cell death.
CONCLUSION: s6T is described for the first time in bladder tumours. Our data 
strongly suggest that BCG immunotherapy is efficient against sTn- and 
s6T-positive tumours. Furthermore, sTn and s6T expression are independent 
predictive markers of BCG treatment response and may be useful in the 
identification of patients who could benefit more from this immunotherapy.