Toll-like receptors: The role in bladder cancer development, progression and immunotherapy

Bladder cancer is one of the leading causes of death worldwide. The main immune mechanisms which lead to bladder cancer development or treatment outcomes have yet to be elucidated. Toll-like receptors (TLRs) play key roles against cancer. TLRs are expressed both on immune cells and on tumour cells and drive immune responses in progression as well as treatment of cancer. Identification of signalling pathways via TLRs could revolutionize further improvement of therapeutic strategies against cancers in the future. According to the recent studies, TLRs agonists are effective immunostimulants and have important role in induction of immune responses with immunotherapeutic potential against several diseases including cancer. They play an important role in the bladder urothelium as a part of immune defence against uropathogens. On the other hand, decreased TLRs expression was found in bladder tumours, particularly in non-muscle-invasive ones. Bacillus Calmette-Guerin (BCG) (agonist of TLR2 and TLR4) is approved by US FDA for immunotherapy of bladder cancer. Despite high efficiency, immunotherapy with BCG may cause toxicity and adverse effects. Nowadays, in vitro and in vivo studies have been conducted to find alternative options for non-responder patients. Studies on TLR agonists for bladder cancer treatment have shown promising results. In this review, we discuss recent data about mechanisms played by TLRs in bladder cancer developments as well as therapeutic application of TLR agonists in cancer treatment.