Eliciting an antihapten antibody response to vaccination
typically
requires the use of constructs where multiple copies of the hapten
are covalently attached to a larger carrier molecule. The carrier
is required to elicit T cell help via presentation of peptide epitopes
on major histocompatibility complex (MHC) class II molecules; as such,
attachment to full-sized proteins, alone or in a complex, is generally
used to account for the significant MHC diversity in humans. While
such carrier-based vaccines have proven extremely successful, particularly
in protecting against bacterial diseases, they can be challenging
to manufacture, and repeated use can be compromised by pre-existing
immunity against the carrier. One approach to reducing these complications
is to recruit help from type I natural killer T (NKT) cells, which
exhibit limited diversity in their antigen receptors and respond to
glycolipid antigens presented by the highly conserved presenting molecule
CD1d. Synthetic vaccines for universal use can, therefore, be prepared
by conjugating haptens to an NKT cell agonist such as α-galactosylceramide
(αGalCer, KRN7000). An additional advantage is that the quality
of NKT cell help is sufficient to overcome the need for an extra immune
adjuvant. However, while initial studies with αGalCer-hapten
conjugate vaccines report strong and rapid antihapten antibody responses,
they can fail to generate lasting memory. Here, we show that antibody
responses to the hapten 4-hydoxy-3-nitrophenyl acetyl (NP) can be
improved through additional attachment of a fusion peptide containing
a promiscuous helper T cell epitope (Pan DR epitope, PADRE) that binds
diverse MHC class II molecules. Such αGalCer-hapten-peptide
tricomponent vaccines generate strong and sustained anti-NP antibody
titers with increased hapten affinity compared to vaccines without
the helper epitope. The tricomponent vaccine platform is therefore
suitable for further exploration in the pursuit of efficacious antihapten
immunotherapies.