Abstract
Adjuvant combinations may enhance or broaden the expression of immune responses to vaccine antigens. Information on whether established Alum type adjuvants can be combined with experimental CD1d ligand adjuvants is currently lacking. In this study, we used a murine Clostridioides difficile immunization and challenge model to evaluate Alum (Alhydrogel™), α-galactosylceramide (α-GC), and one of its analogs 7DW8-5 singly and in combination as vaccine adjuvants. We observed that the Alum/α-GC combination caused modest enhancement of vaccine antigen-specific IgG1 and IgG2b responses, and a broadening to include IgG2c that did not significantly impact overall protection. Similar observations were made using the Alum/7DW8-5 combination. Examination of the impact of adjuvants on NKT cells revealed expansion of invariant NKT (iNKT) cells with modest expansion of their iNKTfh subset and little effect on diverse NKT (dNKT) cells. Side effects of the adjuvants was determined and revealed transient hepatotoxicity when Alum/α-GC was used in combination but not singly. In summary these results showed that the Alum/α-GC or the Alum/7DW8-5 combination could exert distinct effects on the NKT cell compartment and on isotype switch to produce Th1-driven IgG subclasses in addition to Alum/Th2-driven subclasses. While Alum alone was efficacious in stimulating IgG-mediated protection, and α-GC offered no apparent additional benefit in the C. difficile challenge model, the work herein reveals immune response features that could be optimized and harnessed in other vaccine contexts.
Highlights
The range of vaccine adjuvants currently deployed in the clinical setting is limited
Since a-GC and 7DW8-5 are well established to activate invariant Natural Killer T (NKT) (iNKT) cells and humoral immune responses to Alum Imject adjuvant have a partial dependence on diverse NKT (dNKT) cells, we examined these populations by flow cytometry 8 days after adjuvant administration (Figure 3 and Figure S2)
We have shown that the combination of Alum (AlhydrogelTM) with a-GC or 7DW8-5 did not confer substantial advantage with regards to protection against C. difficile
Summary
The range of vaccine adjuvants currently deployed in the clinical setting is limited. Commensurate with development of prototype adjuvants that operate through diverse mechanisms, there is interest in development of combination adjuvant platforms [1,2,3]. Such adjuvant combinations may enhance or broaden the expression of immune responses to vaccine efficacy or bring other benefits such as dose sparing. Alum has a positive track record for efficacy when a Th2driven humoral immune response is desired, but it does not stimulate good Th1-driven humoral (IgG2c) or cellular (CD8+ CTL) immunity [reviewed in [4, 5]]. Alum-enhanced humoral responses require damage-associated molecular patterns (DAMPs) such as uric acid to be released from Alum-damaged cells [6]. Alum leads to recruitment of numerous cell types including monocytes, eosinophils, neutrophils, Natural Killer (NK) cells, and Natural Killer T (NKT) cells to immunization sites [9]
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