Non-specific immunity of BCG vaccine: A perspective of BCG immunotherapy

BCG is a widely used vaccine worldwide for neonates including Pakistan. BCG has more than 90% coverage through the EPI program which was introduced in 1965 in Pakistan. BCG has limited efficacy against the transmissible form of pulmonary tuberculosis in high TB endemic countries. However, BCG vaccination continues in these countries because BCG confers protection against the disseminated form of TB in children. BCG has also shown some protection against leprosy and certain forms of cancers. One reason for such nonspecific protection may be that BCG activates APCs via PAMPS that interacts with TLRs (2, 4 & 8), which initiate the inflammatory cascade thereby recruiting inflammatory cells to the site of infection and providing maturation signals for neutrophils, macrophages and dendritic cells. Such activation may be crucial for restricting the infection at the initial site. Furthermore, activation of the proinflammatory cascade also results in expression of adhesion molecules, co-stimulatory molecules as well as MHC class II molecule. MHC class II molecules engage CD4+ cells via the TCR receptor while the adhesion and costimulatory molecules bind to their respective receptors on CD4+ T cells for additional high affinity binding for T cell activation. Although activation of the innate arm may not provide subsequent memory, activation of T cells may introduce a certain level of memory response and therefore, may form a rational basis for BCG immunotherapy. This review, therefore, focuses on the immune activation related to both the innate and adaptive arm of the immune response that has been reported and further explores the utility of BCG immunotherapy related to non TB conditions.