The variable efficacy of Bacille Calmette Guerin (BCG) vaccination against tuberculosis has prompted efforts to improve the vaccine. In this study, we used autophagy to enhance vaccine efficacy against tuberculosis in a mouse model. We examined the effect of autophagy on the processing of the immunodominant mycobacterial antigen Ag85B by antigen presenting cells (APCs), macrophages and dendritic cells (DCs). We found that rapamycin-induced autophagy enhanced Ag85B presentation by APCs infected with wild-type Mycobacterium tuberculosis H37Rv, H37Rv-derived DeltafbpA attenuated candidate vaccine or BCG. Furthermore, rapamycin enhanced localization of mycobacteria with autophagosomes and lysosomes. Rapamycin-enhanced antigen presentation was attenuated when autophagy was suppressed by 3-methyladenine or by small interfering RNA against beclin-1. Notably, mice immunized with rapamycin-treated DCs infected with either DeltafbpA or BCG showed enhanced T helper type 1-mediated protection when challenged with virulent Mycobacterium tuberculosis. Finally, overexpression of Ag85B in BCG induced autophagy in APCs and enhanced immunogenicity in mice, suggesting that vaccine efficacy can be enhanced by augmenting autophagy-mediated antigen presentation.

Summary Mycobacterium tuberculosis

Complement C5-deficient (C5−/−) macrophages derived from B.10 congenic mice were found to be defective in killing intracellular Mycobacterium tuberculosis (MTB). They were bacteriostatic after activation with IFN-γ alone but bactericidal in the combined presence of IFN-γ and C5-derived C5a anaphylatoxin that was deficient among these macrophages. Reduced killing correlated with a decreased production of reactive oxygen species (ROS) in the C5−/− macrophages measured using fluorescent probes. Furthermore, a lack of colocalization of p47phox protein of the NADPH oxidase (phox) complex with GFP-expressing MTB (gfpMTB) indicated a defective assembly of the phox complex on phagosomes. Reconstitution with C5a, a known ROS activator, enhanced the assembly of phox complex on the phagosomes as well as the production of ROS that inhibited the growth of MTB. Protein kinase C (PKC) isoforms are involved in the phosphorylation and translocation of p47phox onto bacterial phagosomes. Western blot analysis demonstrated a defective phosphorylation of PKC (α, β, δ) and PKC-ζ in the cytosol of C5−/− macrophages compared with C5 intact (C5+/+) macrophages. Furthermore, in situ fluorescent labeling of phagosomes indicated that PKC-β and PKC-ζ were the isoforms that are not phosphorylated in C5−/− macrophages. Because Fc receptor-mediated phox assembly was normal in both C5−/− and C5+/+ macrophages, the defect in phox assembly around MTB phagosomes was specific to C5 deficiency. Reduced bactericidal function of C5−/− macrophages thus appears to be due to a defective assembly and production of ROS that prevents effective killing of intracellular MTB.

During acquired immunity to Mycobacterium bovis bacillus Calmette-Guerin (BCG) infection in mice, dendritic cells (DCs) present mycobacterial antigens to naive T cells to prime an immune response. Complement C5a (anaphylatoxin) secreted by mycobacteria-infected macrophages regulates IL-12p70 production. As IL-12p70 regulates Th1 immunity against mycobacteria in mice, we examined the effects of C5a on IL-12p70 secretion by murine DCs and Th1 immunity. DCs cultured from C5-deficient (C5(-/-)) and -sufficient (C5(+/+)) mice were infected with BCG in the presence or absence of the C5a peptide. ELISA showed that C5(-/-) DCs secreted less IL-12p70 (600 pg/mL vs. 100 pg/mL) than C5(+/+) DCs, and they secreted more IL-10. Using immunophenotyping, reduced CD40 expression was found on C5(-/-) DCs after BCG infection. BCG-primed DCs were then cocultured with naive or BCG-immune T cells to differentiate them into IFN-gamma-secreting Th1 T cells. Coincident with increased IL-12p70 levels, BCG-primed C5(+/+) DCs cocultured with naive or immune C5(+/+) T cells showed a larger increase in CD4+ IFN-gamma/CD8+ IFN-gamma+ T cells compared with cocultured DCs and T cells from C5(-/-) mice. Thus, BCG-primed C5(+/+) DCs were better able to drive a Th1 response. Furthermore, BCG aerosol-infected C5(-/-) mice showed reduced CD4 and CD8 IFN-gamma-secreting T cells in the lungs, concurrent with an increased growth of BCG. Thus, C5a, an innate peptide, appears to play an important role in the generation of acquired immune responses in mice by regulating the Th1 response through modulation of IL-12p70 secretion from DCs.