Plant Cell Culture-Derived Saponin Adjuvant Enhances Immune Response Against a Stabilized Human Metapneumovirus Pre-Fusion Vaccine Candidate

Abstract

Human metapneumovirus (HMPV) is a significant respiratory pathogen, particularly in vulnerable populations.

Background: No vaccine for the prevention of HMPV is currently licensed, although several subunit vaccines are in development. Saponin-based adjuvant systems (AS), including QS-21, have transformed the field of subunit vaccines by dramatically increasing their potency and efficacy, leading to the development of several licensed vaccines. However, naturally sourced tree bark-extracted QS-21 faces supply and manufacturing challenges, hindering vaccine development.

Objective: This study reports on an alternative plant cell culture system for the consistent production of highly pure QS-21.

Method: We evaluated the efficacy of cultured plant cell (cpc)-produced QS-21 in a novel HMPV vaccine, formulating a recombinant pre-fusion stabilized HMPV F protein (preF) with cpcQS-21 and a synthetic toll-like receptor 4 (TLR4) agonist adjuvant formulation.

Results: In mice, TLR4 agonist containing adjuvant formulations with plant cell-produced QS-21 performed equally to licensed adjuvant AS01 containing tree-bark-extracted QS-21 and demonstrated a significant increase in immunogenicity against HMPV preF compared to the unadjuvanted control.

Conclusion: Our findings pave the way for a reliable, scalable, and sustainable source of pure QS-21, enabling the development of highly effective HMPV and other vaccines with significant public health impact.

Keywords: AS01; HMPV; Pneumoviridae family viruses; QS-21; Saponin; adjuvant; cpcQS-21; human metapneumovirus; subunit vaccine.

Figure 1

Anti-F antibody titers following immunization. AS01 formulated with cpcQS-21 or beQS-21 elicited similar humoral immune responses in a prime-boost model of HMPV vaccination. Anti-F serum IgG response from BALB/c mice immunized with two doses (day 0 and 28) of 5 µg recombinant HMPV A2 PreF combined with AS01_B, AS01-SPQX, or AS100-SPQX. Anti-F antibody titers were measured by ELISA on day 27 (A) or 42 (B). Concentrations of TLR4 agonist (MPLA for AS01_B or PHAD^® for SPQX) and (be or cpc) QS-21 are indicated. Log10 relative potency (RP) titers are compared with a reference serum pool. Red horizontal bars indicate the median response per group, and the dotted line indicates the lower limit of detection (LLOD). Open symbols indicate that the response is at or below the LLOD. AS01B and AS01-SPQX were compared across doses by a t-test. AS100-SPQX was compared with 5:5 μg AS01B and 5:5 μg AS01-SPQX by a t-test. (ns, not significant; ** p ≤ 0.01).

Figure 2

IFN-γ-secretion by splenocytes stimulated with an HMPV A2 F peptide pool. HMPV vaccine formulations containing cpcQS-21 or beQS-21 induced HMPV-specific T cell-mediated immunity. Fourteen days following the second immunization (boost), IFN-γ secretion was assessed upon ex vivo stimulation with an F-pool for 18 h. The frequency of IFN-γ-secreting cells is depicted as the number of spot-forming units (SFU) per million splenocytes. Horizontal red bars denote group geometric means, and horizontal dashed lines indicate the lower limit of detection (LLOD) based on the 95th percentile of the background response. Open symbols indicate the response is at or below the LLOD. AS01B and AS01-SPQX adjuvant formulations were compared across doses by a Cochran–Mantel–Haenszel test. AS100-SPQX was compared with 5:5 μg AS01B and 5:5 μg AS01-SPQX with a Mann-Whitney U-test. No significant differences were found. ns, not significant.