The BCG Vaccine: Pharmacological Biotechnology and Expanding Immunological Horizons

Tuberculosis (TB), caused by Mycobacterium tuberculosis, remains a major global health challenge, particularly in low- and middle-income countries. The Bacillus Calmette-Guérin (BCG) vaccine, developed from attenuated Mycobacterium bovis (M. bovis) and introduced in 1921, is the only licensed vaccine for TB prevention. Although BCG provides strong protection against morbid forms of pediatric TB, its efficacy against adult pulmonary TB is variable. Emerging evidence indicates that BCG exerts broader immunological effects beyond TB prevention, including non-specific immune protection mediated through trained immunity. BCG-induced trained immunity induces epigenetic reprogramming of innate immune cells which enhances host defense against heterologous pathogens. Advances in pharmacological biotechnology, including recombinant BCG platforms, genetic modification, and novel delivery systems, have demonstrated improved antigen presentation, durability of immune memory, and expanded clinical utility. Beyond TB, BCG remains a cornerstone immunotherapy for non-muscle-invasive bladder cancer and shows promising immunomodulatory potential in autoimmune diseases such as type 1 diabetes and multiple sclerosis. While adverse effects are generally mild, they tend to be more pronounced in immunocompromised individuals. This review examines the historical development, biotechnological production, immunological mechanisms, and expanding therapeutic applications of BCG. We synthesized findings on strain variability, manufacturing processes, and immune responses, highlighting the induction of T helper 1 (Th1)-mediated cellular immunity through interferon gamma (IFN-γ) and interleukin-12 (IL-12), macrophage activation, and granuloma formation. Overall, BCG represents both a critical public health intervention and a model system for advancing immune-based therapies.