Culture filtrate proteins from Bacillus Calmette–Guérin

Mycobacterium bovis Bacillus Calmette-Guérin (BCG) has long been used as a vaccine against tuberculosis and bladder cancer. BCG is also an effective adjuvant; it has long been known that its cell wall acts as an adjuvant. Recent research has revealed that various lipids and glycolipids are involved in the induction of innate immunity as ligands for C-type lectin receptors. However, the effects of the components of BCG used as adjuvants are not comparable to those of live BCG. On the other hand, since live bacteria are essential for the protective effect of BCG, a hypothesis has been proposed that proteins actively secreted by living bacteria are responsible for important immune responses, and various secreted proteins have been characterized. However, this hypothesis has not yet been verified. In this review, we discuss the purification, identification, and characterization of BCG-secreted proteins, namely culture filtrate proteins (CFP), and mention new possibilities for CFP.