Dietary Pyridoxine Controls Efficacy of Vitamin B6-Auxotrophic Tuberculosis Vaccine Bacillus Calmette-Guerin ΔureC::hly Δpdx1 in Mice

The only tuberculosis (TB) vaccine in use today, bacillus Calmette-Guerin (BCG), provides insufficient protection and can cause adverse events in immunocompromised individuals, such as BCGosis in HIV+ newborns. We previously reported improved preclinical efficacy and safety of the recombinant vaccine candidate BCG Delta ureC::hly, which secretes the pore-forming listeriolysin O of Listeria monocytogenes. Here, we evaluate a second-generation construct, BCG Delta ureC::hly Delta pdx1, which is deficient in pyridoxine synthase, an enzyme that is required for biosynthesis of the essential cofactor vitamin B-6. This candidate was auxotrophic for vitamin B-6 in a concentration-dependent manner, as was its survival in vivo. BCG Delta ureC::hly Delta pdx1 showed markedly restricted dissemination in subcutaneously vaccinated mice, which was ameliorated by dietary supplementation with vitamin B-6. The construct was safer in severe combined immunodeficiency mice than the parental BCG Delta ureC::hly. A prompt innate immune response to vaccination, measured by secretion of interleukin-6, granulocyte colony-stimulating factor, keratinocyte cytokine, and macrophage inflammatory protein-1 alpha, remained independent of vitamin B-6 administration, while acquired immunity, notably stimulation of antigen-specific CD4 T cells, B cells, and memory T cells, was contingent on vitamin B-6 administration. The early protection provided by BCG Delta ureC::hly Delta pdx1 in a murine Mycobacterium tuberculosis aerosol challenge model consistently depended on vitamin B-6 supplementation. Prime-boost vaccination increased protection against the canonical M. tuberculosis H37Rv laboratory strain and a clinical isolate of the Beijing/W lineage. We demonstrate that the efficacy of a profoundly attenuated recombinant BCG vaccine construct can be modulated by external administration of a small molecule. This principle fosters the development of safer vaccines required for immunocompromised individuals, notably HIV+ infants. IMPORTANCE Mycobacterium tuberculosis can synthesize the essential cofactor vitamin B-6, while humans depend on dietary supplementation. Unlike the lipophilic vitamins A, D, and E, water-soluble vitamin B-6 is well tolerated at high doses. We generated a vitamin B-6 auxotroph of the phase II clinical tuberculosis vaccine candidate bacillus Calmette-Guerin Delta ureC::hly. The next-generation candidate was profoundly attenuated compared to the parental strain. Adaptive immunity and protection in mice consistently depended on increased dietary vitamin B-6 above the daily required dose. Control of vaccine efficacy via food supplements such as vitamin B-6 could provide a fast track toward improved safety. Safer vaccines are urgently needed for HIV-infected individuals at high risk of adverse events in response to live vaccines.