Summary
Every year there are 3.7 million episodes of severe infection caused by Streptococcus pneumoniae (pneumococcus) equating to 1.2 million deaths globally in people of all ages. Pneumococcus is covered by a sugar capsule which is variable and classifies this bacterium into 96 immunologically distinct serotypes. Pneumococcus is commonly found in the microflora that colonise the nasopharynx of healthy adults and children. This harmless state called “carriage” is a requisite for disease and a source of transmission.
To reduce the global burden of pneumococcal disease, low-cost vaccines with wide-serotype coverage that protect against carriage and disease are urgently needed. Vaccine developers, exploring the potential of conserved protein antigens, have been severely hampered by the lack of a validated correlate of protection, and some novel vaccines have foundered because of this impasse. PnuBioVax (PBV) developed by ImmBio is a multi-protein vaccine candidate with demonstrated safety and immunogenicity in a Phase I trial. Pre-liminary data generated using historical samples from the experimental challenge model with pneumococcus (LSTM), from colonised and non-colonised individuals, have shown that experimental carriage increases antibody levels against PBV and that pre-existing PBV antibody levels correlate with a shorter carriage duration. We propose a new partnership with ImmBio and the Malawi-Liverpool Wellcome-Programme to further investigate these results in different cohorts from the UK and Malawi. Success will 1) show that anti-PBV antibodies provide a correlate of protection from colonisation with a variety of pneumococcal serotypes in two distinct populations and, 2) generate preliminary data to support a trial of PBV in the Malawian and UK challenge models.
Project Outcomes
Streptococcus pneumoniae (pneumococcus) causes lethal infections like pneumonia, sepsis and meningitis, affecting vulnerable populations. Pneumococcus is covered by a capsule that is variable and classifies the bacteria in serotypes, >95 different have been identified. Pneumococcus can be found in the nasopharyngeal microflora of healthy adults and children in an innocuous state called carriage. Carriage is a source of transmission and a first step to develop serious infections. Licenced vaccines are based on capsular components conferring protection against only few pneumococcal serotypes. Vaccines with wide-serotype coverage that protect against carriage and infection are needed to reduce the burden of disease caused by pneumococcus. Protein-based vaccine candidates are being explored as they could potentially confer universal protection. However, to develop successful protein-based vaccines we need to identify human immune mechanisms mediating protection against carriage and disease.
Using the multi-protein vaccinate candidate PnuBioVax (PBV) and samples from the UK (EHPC-LSTM) and Malawi (EHPC-MLW) pneumococcal challenge models, we aimed to identify antibody responses to PBV and their correlation with protection against carriage. We measured anti-PBV antibody levels before and after experimental colonisation in sera, nasal wash and bronchoalveolar lavage fluid (BAL). We used samples from challenge models with 2 different pneumococcal serotypes, SPN6B (EHPC-LSTM and EHPC-MLW) and SPN15B (EHPC-LSTM).
Results showed no difference in anti-PBV antibody levels at baseline between carriage negative and positive for any cohort. This indicates that baseline anti-PBV levels do not associate with experimental carriage acquisition. Results also showed that carriage with SPN6B (EHPC-LSTM) increases levels of anti-PBV antibodies in blood, nasal wash, and BAL after inoculation. A correlation between levels of anti-PBV antibodies in sera with those detected in BAL was observed, suggesting a migration of antibodies from the blood to the lungs. In addition, levels of anti-PBV antibody in BAL correlated with nasal pneumococcal density. Previous data has shown that nasal pneumococcal density associates with microaspiration. Therefore, is not surprising that higher nasal bacterial density will also associate with higher antibodies in the lung. As induction on memory B-cell responses may be a better predictor of long-term protection against carriage than antibody measurements, we conducted ELISpots with samples from SPN6B EHPC-LSTM to measure levels of anti-PBV antibody producing memory B cells in blood. Carriage increases levels of those memory B cells suggesting that those cells could protect against subsequent carriage.
The increased in anti-PBV antibody levels in sera and nasal wash samples observed for SPN6B LSTM was not present in carriage with SPN6B in Malawi or with SPN15B. For those models the latest follow-up sample available was 14 days after inoculation. Based on the data obtained for SPN6B EHPC-LSTM, 14 days is not enough time to detect carriage-induced differences in anti-PBV antibody levels as no differences at this timepoint were observed for this model either.
The data obtained with SPN6B (EHPC-LSTM) corroborate that carriage is an immunising event that not only elicits systemic and mucosal anti-polysaccharide antibodies as previously shown, but also antibodies against proteins contained in PBV. This confirms that PBV antigens are immunogenic and are presented to immune cells during pneumococcal carriage and therefore have potential as a protein-based vaccine.