Potentiating pneumococcal glycoconjugate vaccine PCV13 with saponin adjuvant VSA-1

Abstract

VSA-1 is a semisynthetic saponin adjuvant prepared from naturally occurring Momordica saponin and capable of stimulating antigen-specific humoral and cellular immune responses. Its immunostimulating activity in enhancing the immune responses induced by the clinical glycoconjugate pneumococcal vaccine PCV13 is compared with QS-21 in female BALB/c mice. Both VSA-1 and QS-21 boosted IgG and opsonic antibodies titers against seven selected serotypes, including serotypes 3, 14, and 19A that are involved in most PCV13 breakthroughs. Since VSA-1 is much more accessible and of lower toxicity than QS-21, it can be a practical saponin immunostimulant to be included in a new glycoconjugate pneumococcal vaccine formulation.

Keywords: Momordica saponin; VSA-1; adjuvant; immunostimulant; pneumococcal vaccine.

Scheme 1

Preparation of VSA adjuvants from natural Momordica saponins.

Figure 1

Serum antibody activity to seven serotypes on Day 42, (A) 3, (B) 4, (C) 6B, (D) 9V, (E)14, (F) 19A, and (G) 19F. BALB/c mice (8-10 weeks of age, six per group) were immunized via the subcutaneous route (s.c.) on days 0, 14 and 28. Serum samples were collected prior to the first immunization and at 2 weeks following the last immunization. The pooled serum samples of each group were analyzed by ELISA. The Y-axis shows the bound antibody (OD405-OD690) at various serum dilutions (X-axis). The serum pool from the mice immunized with saline had very small amounts of antibodies for all serotypes and its data were plotted close to the horizontal axis. Similarly, the data from pre-immune serum samples showed undetectable antibody for all serotypes (data not shown).