Heterogeneity in Plasmodium falciparum whole sporozoite vaccine induced humoral immune responses and protection in African volunteers: The role of age, human pegivirus and human immunodeficiency virus co-infections.

Malaria, a vector borne disease caused by Plasmodium species remains a major public health problem especially in sub-Saharan Africa. In 2018 alone, there were an estimated 228 million clinical cases and 405,000 deaths attributed to malaria. New tools such as efficacious vaccines, better drugs and diagnostics are needed to supplement the current malaria control tools that rest mainly on vector control measures. Clinical trials in malaria naïve volunteers have demonstrated high level of sterile vaccine induced protection in healthy individuals immunized with live, metabolically active, irradiation attenuated purified sporozoites (PfSPZ Vaccine) or live non-attenuated purified sporozoites given under chloroquine chemoprophylaxis (PfSPZ-CVac). Immunogenicity and protective efficacy against malaria induced by these whole sporozoite based vaccines varies widely between European/US versus African volunteers. Interestingly, variations in protection and immunogenicity of these malaria vaccines have been observed among African volunteers residing in different malaria endemic regions of East and West Africa. These different outcomes could be linked to levels of malaria pre-exposure and co-infections at the time of vaccination. Until recently safety, tolerability, immunogenicity and protective efficacy of PfSPZ Vaccine in immunocompromised populations including individuals infected with the Human Immunodeficiency Virus (HIV) was unknown. Given the high geographical overlap of HIV and malaria, an effective malaria vaccine deployed in this population could possibly support the long-term objective of regional elimination of malaria by using mass vaccination. Furthermore, the role of asymptomatic, under researched, highly prevalent viruses like Human Pegivirus (HPgV-1) circulating in Sub-Saharan Africa on malaria pathogenesis and vaccination outcomes remains elusive.
The aims of this PhD thesis include: 1) Evaluate the safety, tolerability, efficacy, and immunogenicity of the PfSPZ Vaccine in different populations and age groups residing in malaria endemic countries. 2) Compare the safety, tolerability, efficacy, and immunogenicity of two different whole sporozoite based vaccination approaches, PfSPZ Vaccine and PfSPZ-CVac in Equatorial Guinean adults. 3) The unbiased assessment of PfSPZ Vaccine induced humoral immunity using protein microarrays probed with serum samples of HIV positive and HIV negative volunteers to understand immune status before vaccination, immuno-dominance of vaccine induced antibody targets, and distinct antibody profiles that might be associated with vaccine-induced protection. 4) Investigate the interaction of chronic HPgV-1 co-infection on PfSPZ Vaccine induced humoral immunity and protection against homologous CHMI. These aims are structured around 6 manuscripts presented in this PhD thesis.
Manuscript 1-3: Safety, immunogenicity, and efficacy of radiation attenuated whole sporozoite vaccine (PfSPZ Vaccine) in African populations of different ages
In this chapter, we include the outcomes of clinical trials conducted in Bagamoyo, Tanzania. These trials for the first time i) evaluated the safety, immunogenicity and efficacy against homologous CHMI of irradiation attenuated purified Plasmodium falciparum sporozoites (PfSPZ Vaccine) in adult volunteers; ii) tested increasing dosages of PfSPZ Vaccine in different age groups including adults, adolescents, children and infants. We found PfSPZ vaccine to be safe and well tolerated and that vaccine inoculation by intravenous inoculation is well accepted even in younger age groups. Protective efficacy varied in the different trials leading to the identification of a vaccine regimen of 9x105PfSPZ per dose as suitable for further development. PfSPZ Vaccine induce immune responses, both cellular and humoral, were age dependent with infants mounting no measurable malaria specific cellular immunity in peripheral blood. Surprisingly, compared to other age groups, older children and adolescents mounted higher cellular and humoral immune responses. These findings are relevant for further optimization of PfSPZ vaccine regimen that might need to be adapted to different age groups to optimize vaccine induced protection. As an extension of these trials, we have compared PfSPZ Vaccine safety, immunogenicity and efficacy in HIV positive versus HIV negative volunteers. We observed marked differences in PfSPZ Vaccine induced efficacy between HIV positive (0%) and HIV negative individuals (80%) undergoing homologous CHMI(Manuscript in preparation).
Manuscript 4: Immunogenicity and protective efficacy of radiation-attenuated and chemo-attenuated PfSPZ vaccines in Equatoguinean adults
This work describes the outcome of a first time side-by-side comparison of two whole sporozoite based vaccine approaches (PfSPZ Vaccine and PfSPZCVac) in malaria pre-exposed individuals of Equatorial Guinea. We evaluated PfSPZ Vaccine dosages (2.7 X106) given three times at 8-week interval and PfSPZ CVac dose (1X105) given three times at 4 weeks interval. Homologous CHMI was employed for assessment of vaccine efficacy. Both approaches were safe and well tolerated in malaria pre-exposed individuals but the immunogenicity and protective efficacy differed. Vaccine efficacy was lower in the PfSPZ Vaccine group (27%) compared to the PfSPZ CVac group (55%), despite induction of about 2.9 times higher antibody titres against the circumsporozoite protein in the PfSPZ Vaccine group prior to CHMI. These results highlight the potential involvement of different protective immune mechanisms induced by each of the two whole sporozoite vaccines approaches and the effect of malaria pre-exposure on pfSPZ CVac vaccine induced efficacy in comparison to malaria naïve volunteers. We show that induction of high antibody titres against the circumsporozoite protein does not correlate with protection since no difference was observed between CHMI protected and non-protected volunteers.
Manuscript 5: HIV-1 positive and HIV-1 negative Tanzanian adults undergoing whole irradiation attenuated Plasmodium falciparum sporozoite vaccination mount antibody responses targeting the circumsporozoite protein and merozoite surface protein 5
In this manuscript, we investigated antibody profiles binding to 262 pre-selected antigens of Pf before and after vaccination as well as after homologus CHMI. We aimed to identify antibody profiles that might explain the observed poor vaccine induced protection in HIV positive individuals. We found a lower - albeit not statistically significant - antigen breadth in HIV positive volunteers at baseline before first vaccine inoculation. Immunization with PfSPZ Vaccine induced IgG and IgM isotypes specific for the Merozoite surface protein 5 (PfMSP 5) and the circumsporozoite protein (PfCSP) regardless of HIV infection status. Interestingly, volunteers displayed a highly personalized IgG and IgM immune profiles targeting Pf antigens before vaccination and these remained unchanged after PfSPZ vaccination confirming our previous results of antigenic imprinting in malaria.
Manuscript 6: Role of Pegivirus infections in whole Plasmodium falciparum sporozoite vaccine induced humoral immunity and controlled human malaria infections in African volunteers
In this study, we wanted to understand the role of human pegivirus infections in East and Western African adult volunteers and its impact on PfSPZ Vaccine induced humoral immune responses and homologous CHMI. We found HPgV-1 to be highly prevalent in our volunteers (29.2%) with circulating genotypes 1, 2 and 5 as described in other African settings. HPgV-1 infection did not alter PfSPZ vaccine induced antibody responses and parasite multiplication rates during CHMI. However, a higher proportion of individuals were protected against homologous CHMI that had ongoing, active human pegivirus infections. Significantly higher serum concentrations of IL-2 and IL-17A were measured in HPgV-1 positive volunteers likely indicating chronic activation of the immune system. CHMI was safe and well tolerated in HPgV-1 positive individuals since the viremia did not change upon acute asexual blood stage parasitemia. These results highlight the potential impact of chronic, asymptomatic viral infections on PfSPZ vaccine efficacy that needs confirmation in larger cohorts and in field studies of naturally occurring malaria infections.