Abstract:
Background: Leading malaria vaccine, RTS,S, is based on the circumsporozoite protein (CSP) of sporozoites. RTS,S
confers partial protection against malaria in children, but efficacy wanes relatively quickly after primary
immunization. Vaccine efficacy has some association with anti-CSP IgG; however, it is unclear how these antibodies
function, and how functional antibodies are induced and maintained over time. Recent studies identified antibody-
complement interactions as a potentially important immune mechanism against sporozoites. Here, we investigated
whether RTS,S vaccine-induced antibodies could function by interacting with complement.
Methods: Serum samples were selected from children in a phase IIb trial of RTS,S/AS02 A conducted at two study
sites of high and low malaria transmission intensity in Manhiça, Mozambique. Samples following primary
immunization and 5-year post-immunization follow-up time points were included. Vaccine-induced antibodies were
characterized by isotype, subclass, and epitope specificity, and tested for the ability to fix and activate complement.
We additionally developed statistical methods to model the decay and determinants of functional antibodies after
vaccination.
Results: RTS,S vaccination induced anti-CSP antibodies that were mostly IgG1, with some IgG3, IgG2, and IgM.
Complement-fixing antibodies were effectively induced by vaccination, and targeted the central repeat and C-
terminal regions of CSP. Higher levels of complement-fixing antibodies were associated with IgG that equally
recognized both the central repeat and C-terminal regions of CSP. Older age and higher malaria exposure were
significantly associated with a poorer induction of functional antibodies. There was a marked decay in functional
complement-fixing antibodies within months after vaccination, as well as decays in IgG subclasses and IgM.
Statistical modeling suggested the decay in complement-fixing antibodies was mostly attributed to the waning of
anti-CSP IgG1, and to a lesser extent IgG3.Conclusions: We demonstrate for the first time that RTS,S can induce complement-fixing antibodies in young
malaria-exposed children. The short-lived nature of functional responses mirrors the declining vaccine efficacy of
RTS,S over time. The negative influence of age and malaria exposure on functional antibodies has implications for
understanding vaccine efficacy in different settings. These findings provide insights into the mechanisms and
longevity of vaccine-induced immunity that will help inform the future development of highly efficacious and long-
lasting malaria vaccines.
