Boosting Live Malaria Vaccine with Cytomegalovirus Vector Can Prolong Immunity through Innate and Adaptive Mechanisms

Vaccines to persistent parasite infections have been challenging, and current iterations lack long-term protection. Cytomegalovirus (CMV) chronic vaccine vectors drive protection against SIV, tuberculosis and liver-stage malaria correlated with antigen-specific CD8 T cells with a Tem phenotype. This phenotype is likely driven by a combination of antigen-specific and innate adjuvanting effects of the vector, though these mechanisms are less well understood. Sterilizing immunity from live Plasmodium chabaudi vaccination lasts less than 200 days. While P. chabaudi-specific antibody levels remain stable after vaccination, the decay of parasite-specific T cells correlates with loss of challenge protection. Therefore, we enlisted murine CMV as a booster strategy to prolong T cell responses against malaria. To study induced T cell responses, we included P. chabaudi MSP-1 epitope B5 (MCMV-B5). We found that MCMV vector alone significantly protected against a challenge P. chabaudi infection 40-60 days later, and that MCMV-B5 was able to make B5-specific Teff, in addition to previously-reported Tem, that survive to the challenge timepoint. Used as a booster, MCMV-B5 prolonged protection from heterologous infection beyond day 200, and increased B5 TCR Tg T cell numbers, including both a highly-differentiated Tem phenotype and Teff, both previously reported to protect. B5 epitope expression was responsible for maintenance of Th1 and Tfh B5 T cells. In addition, the MCMV vector had adjuvant properties, contributing non-specifically through prolonged stimulation of IFN-γ. In vivo neutralization of IFN-γ, but not IL-12 and IL-18, late in the course of MCMV, led to loss of the adjuvant effect. Mechanistically, sustained IFN-γ from MCMV increased CD8α+ dendritic cell numbers, and led to increased IL-12 production upon Plasmodium challenge. In addition, neutralization of IFN-γ before challenge reduced the polyclonal Teff response to challenge. Our findings suggest that, as protective epitopes are defined, an MCMV vectored booster can prolong protection through the innate effects of IFN-γ. Authors’ summary Malaria represents a challenging target for vaccination. This is in part because of the requirement for CD4 T cell immunity in addition to the standard B cell responses current vaccines induce. However, human malaria vaccine approaches thus far have limited longevity of protection due to decay of T cell responses. This includes the most advanced malaria vaccine, a virus-like particle expressing one recombinant liver-stage antigen (RTS,S), and liver-stage parasites attenuated by radiation (PfSPZ), as well as live vaccination with drug-treatment. Our work seeks to prolong this protection using MCMV, a promising vaccine vector known to promote CD8 T cell responses. We observed that boosting the live malaria vaccine with MCMV including a Plasmodium antigen led to longer protection from P. chabaudi parasitemia, and can be used to promote maintenance of antigen-specific CD4 T cells. In investigating the mechanisms of the MCMV booster, we found that the cytokine IFN-γ is required for prolonged protection and enhances priming of the innate immune system for prolonged protection from malaria. Our research informs both the quest for a longer-lived malaria vaccine and that to understand mechanisms of protection from persistent infection.