Activated invariant Natural Killer-like T (NKT) cells can provide stimulatory factors to enhance antigen presenting cell (APC) priming of CD8⁺ T cell responses.  Targeting both an NKT cell ligand and CD8⁺ T cell-specific peptide to the same APC can increase the benefit of this NKT cell adjuvant effect.  To utilise the NKT cells as cellular adjuvants, we developed a variety of synthetic conjugate vaccines consisting of an α-galactosylceramide (α-GalCer) pro-drug and tumour peptides linked via a cathepsin-cleavable linker.  These conjugates can be processed by APCs to present α-GalCer via CD1d to NKT cells and present peptide via MHC class I to CD8⁺ T cells on the same APC, with the aim of increasing antigen-specific CD8⁺ T cell responses and improving anti-tumour activity.

When intravenously administered in vivo in mice, the conjugates were capable of eliciting a similar APC activation profile to unconjugated α-GalCer, and in a CD1d-dependent manner.  However, the conjugates were capable of inducing larger CD8⁺ T cell responses with improved cytotoxic function and anti-tumour activity than unconjugated α-GalCer admixed with tumour peptide. Interestingly, repeated doses of conjugates did not induce a high production of IL-10 from NKT cells, which could be seen from repeated administration of unconjugated α-GalCer.  We conclude that conjugating the glycolipid and peptide components together improves the antigen-specific in vivo immune response.  This has great potential for future immunotherapeutic vaccination strategies against a variety of diseases, including cancer, and will be further developed for clinical application.