Vaccine-induced responses to R21/Matrix-M - an analysis of samples from a phase 1b age de-escalation, dose-escalation trial

Abstract

Introduction: The pre-erythrocytic malaria vaccine R21 vaccine adjuvanted with Matrix-M reported good efficacy (75%) in an ongoing phase 3 trial and was recommended World Health Organization for use in children 5-36 months. Vaccine-induced antibodies against NANP are associated with protection, however, various factors such as age, pre-existing immunity, and vaccine dose have been shown to influence vaccine responses.

Methods: Samples from adults (n =18), children (n = 17), and infants (n = 51) vaccinated with R21/Matrix-M in a phase I trial were assayed for vaccine-specific antibody responses. We measured antibodies (quantity) by MSD and ELISA; and function (quality) by complement (C1q) fixation assay, inhibition of sporozoite invasion (ISI) assay, and avidity assay. Pre-existing malaria antibody exposure was assessed using an anti-3D7 Plasmodium falciparum crude parasite lysate ELISA.

Results: Vaccine-induced CSP antibodies (against full-length R21, NANP, and C terminus), exhibited complement fixation and inhibition of sporozoites. These were significantly lower in adults compared to children and infants. Additionally, children had a higher rate of decay of vaccine-induced antibodies compared to adults 2 years post-vaccination. Furthermore, a higher Matrix-M adjuvant dose resulted in significantly higher C1q fixation, and ISI than the low adjuvant dose in infants. Importantly, functional measures ISI and C1q-fixation were positively associated with the vaccine-induced antibodies overall, but avidity was not. Interestingly, in adults, previous malaria exposure was negatively associated with ISI but positively correlated with avidity and C1q fixation. At baseline, all the study participants were seropositive for anti-HBsAg IgG above the WHO-required protective threshold of 10 mIU/mL, and titers significantly increased post-vaccination.

Discussion: R21/Matrix-M was immunogenic across all age groups, with age and vaccine dose significantly affecting antibody magnitude and function. These findings emphasize the importance of evaluating the right adjuvant and vaccine dose for clinical development progression. This could thus inform the development of next-generation malaria vaccines. However, additional crucial factors need further exploration.

Keywords: R21; antibody; immunogenicity; malaria; past exposure; vaccines.

Figure 1

IgG responses post-vaccination with R21/Matrix-M. R21/Matrix-M vaccine-induced antibodies (A) Anti-full-length R21, (B) anti-NANP, (C) anti-C-term and (D) anti-HBsAg total IgG geometric mean (± 95% CI) as measured by MSD. The Y axis shows the geometric mean (± 95% CI) of antibody titers specified as log10 MSD antibody unit (AU). Kruskal-Wallis test with Dunn’s correction for multiple comparisons was used to compare mean antibody levels between more than two time points or groups, and determine the significance between the timepoints or groups. Sera were tested in triplicate. Comparisons between two time points or groups were conducted with Mann-Whitney tests. Only significance is indicated where p<0.05. The horizontal dotted line in panel (D) shows the 0.01 IU/mL WHO protective threshold for HBsAG. The x-axis shows the time course, D84 represents one-month post-primary vaccination, and D816 and D456 represent one-month post-booster dose for adults and infants respectively. The booster dose was administered at 1 year for infants and at 2 years for adults and children. Group 1A/B adults (10μg R21/50 μg Matrix-M), 2B children (10μg R21/50 μg Matrix-M), 3A/C infant (5μg R21/25 μg Matrix-M), 3B/D infants (10μg R21/50 μg Matrix-M), and 3E infants (5μg R21/50 μg Matrix-M). NANP-NANP6, (Asn-Ala-Asn-Pro) x 6, IgG, immunoglobulin G; HBsAg, Hepatitis B surface antigen; MSD, Meso-Scale Discovery multiplex assay.

Figure 2

Complement fixing capacity of the R21/Matrix-M vaccine-induced NANP antibodies. (A) The plots show anti-NANP complement fixing capacity (C1q) OD geometric mean (± 95% CI) at day 84 (1-month post-primary vaccination). (B) Correlation between the NANP C1q and NANP magnitude at day 84 (1-month post-primary vaccination). Kruskal-Wallis test with Dunn’s correction for multiple comparisons was used to compare C1q between more than two time points or groups, and determine the significance between the timepoints or groups. Only significance is indicated where p<0.05. The correlation was computed with Spearman rank correlation. Group 1A/B-10μg R21/50μg Matrix M, group 2B- 10μg R21/50μg Matrix M, group 3A/C 5μg R21/25μg Matrix M, group 3B/D10μg R21/50μg Matrix M and group 3E 5μg R21/50μg Matrix M. OD, optical density; MSD, Meso-Scale Discovery multiplex assay, NANP- NANP6, (Asn-Ala-Asn-Pro) x 6.

Figure 3

R21 induced immunity sporozoite inhibition capacity by age and vaccine dose post-vaccination. (A) The plots show the percentage of infection blocked as assayed by Inhibition of sporozoite (ISI) assay median (± 95% CI). (B) Correlation between the ISI and NANP magnitude at day 84 (1-month post-primary vaccination). Kruskal-Wallis test with Dunn’s correction for multiple comparisons was used to compare ISI between more than two time points or groups, and determine the significance between the timepoints or groups. Only significance is indicated where p<0.05. The correlation was computed with Spearman rank correlation. Group 1A/B-10μg R21/50μg Matrix M, group 2B- 10μg R21/50μg Matrix M, group 3A/C 5μg R21/25μg Matrix M, group 3B/D10μg R21/50μg Matrix M and group 3E 5μg R21/50μg Matrix M. OD, optical density, MSD, Meso-Scale Discovery multiplex assay, NANP-NANP6, (Asn-Ala-Asn-Pro) x 6.

Figure 4

Correlation matrix. Anti-schizont IgG antibodies were measured at baseline by ELISA; all the other vaccine-induced response was measured on Day 84 (a month post-primary vaccination). (A) Infants -group 3A/C (5mg R21/25mg Matrix M), group 3B/D (10mg R21/50mg Matrix M) and group 3E (5mg R21/50mg Matrix M) and Children- Group 2B (10mg R21/50mg Matrix M); (B) Adults- Group 1A/B (10mg R21/50mg Matrix M). The cross (X) indicates correlations that are not significant at 0.05 alpha level. The correlation was computed with Spearman rank correlation. OD, optical density; MSD, Meso-Scale Discovery multiplex assay, NANP-NANP6, (Asn-Ala-Asn-Pro) × 6.