An Ebola virus vaccine that leverages helper T cells induced by prior BCG immunization

Abstract

Abstract Protective antibody responses against the Ebola virus (EBOV) are dependent on the quality of CD4+ helper T (Th) cells. We propose that BCG specific Th cells may provide a source of Th cells that can be recruited to drive antibody responses against EBOV, given that the majority of individuals residing in EBOV endemic regions have been previously vaccinated with BCG. To take advantage of pre-existing BCG Th cells, a vaccine was developed that consists of the EBOV GP fused to the mycobacteria Th cell epitope (P25) of Ag85B (P25-EBOV GP). Presentation of the P25 epitope by B cells recruited cognate interactions with P25 Th cells to promote antibody responses against the EBOV GP. Experiments in mice showed BCG priming induced a broad range of Th cells including Th1, Th2, and Tfh subsets. These P25 Th subsets provided a significant dose sparing effect on the P25-EBOV GP vaccine in the induction of EBOV GP antibodies and resulted in increased titers of high affinity neutralizing IgG1 antibodies. The induction of IgG2c antibodies only occurred in BCG primed mice, where BCG vaccination is known to be a strong inducer of Th1 responses that drive class-switching to IgG2c. Microscopy of sections from secondary lymphoid organs of BCG primed mice showed that the majority of P25 Th cells remained in extrafollicular regions outside germinal centers and were most responsible for the induction of the IgG2c class-switched antibodies that were weakly neutralizing but capable of mediating antibody-dependent cellular toxicity (ADCC). BCG priming followed by vaccination with P25-EBOV GP protected mice against lethal EBOV infection, thus suggesting a potentially useful strategy for vaccination against EBOV in populations with high prevalence of previous BCG vaccination.