Protein antibody markers of pneumococcal carriage in support of protein and whole cell vaccine trial design using pan-proteomic antibody screening

Summary

Pneumococcal disease is a serious illness that can result in death. In many countries, children are given a vaccine to protect them against pneumococcal disease which is caused by a bacterial infection. There are more than 90 different types (‘serotypes’) of the bacteria. The vaccine only covers a 10-12 of these serotypes. New vaccines being developed may protect against all types of pneumococcal disease.

New vaccines are tested in clinical trials to show that they are effective. One way of doing this is to measure if the bacteria is present (‘carried’) in the nasal passages of children who have been vaccinated. The bacteria are commonly ‘carried’ in the nasal passages without causing disease, and this is the way it spreads to other people. If a vaccine can stop carriage, it will also stop the spread of disease.

Unfortunately, tests of nasal swabs for bacteria often miss detecting carriage. Carriage occurs only for a short period of time and the swab may not be taken at the time the bacteria were present. Thus, many carriage events are missed.

Another method of testing vaccines is to measure antibodies. However, the new vaccines (‘protein’ vaccines) produce different antibodies from the older (‘polysaccharide’) vaccines and so the old tests are no longer useful.

This study aims to use blood samples taken one month apart from children in a vaccine study in Nepal, and test for new protein antibodies. There are 2,200 different proteins that can be tested all at once using a new test called a protein microarray. The children in the study also had nasal swabs taken, so we will be able to see if those children carrying the bacteria also had high levels of antibodies. This study will enable us to know which protein antibodies could be used in future assessment of vaccines which contain pneumococcal proteins. These new vaccines could improve protection in both developing countries with the highest burden of disease, but could also be important in improving protection against meningitis and pneumonia in the U.K.

Project outcomes

Pneumonia is a serious illness that can result in death. In many countries, children are given a vaccine to protect them against infection by the pneumonia-causing bacteria Streptococcus pneumoniae, also known as the pneumococcus. New vaccines are tested in clinical trials to show that they are effective. Although one way of measuring effectiveness is to measure if the bacteria is present (“carried”) in the nasal passages of children, these tests can often miss detecting carriage. An alternative way to detect carriage is to develop a test that measures antibodies against pneumococcal proteins or polysaccharides that increase when the bacteria is carried. 

This study aimed to take blood samples from children in a vaccine study in Nepal and test for new protein antibodies that could be used in a test to detect carriage during a clinical trial. Among the Nepalese infants enrolled in a clinical trial of the PCV10 vaccine, a total of 15 non-carriers and 15 carriers (five serogroup 10 and/or 19 carriers, five serogroup 6 carriers and five serotype 34 carriers) were selected for this study. Carriage status was determined at the time point before a booster dose of the vaccine was given. Infant blood serum samples that were collected at the pre-booster time point and 1 month post-booster time point were included in the study. 

Samples were tested on S. pneumiae Pan-Genome Microarrays that contained 2,629 proteins and 24 pneumococcal capsular polysaccharides for measurement of serum immunoglobulin G (“IgG”) and A (“IgA”). Capsular polysaccharide responses 1 month post-booster dose of PCV10 were elevated levels for the serotypes contained within the PCV10 vaccine but not for non-vaccine serotypes, with the exception of serotypes 9N and 6A, both of which are serotypes from serogroups contained in PCV10 (9V and 6B, respectively). 

Both IgG and IgA responses were observed and provided validation of the microarray technology used with polysaccharides. IgG and IgA against carriage-specific capsular polysaccharides did not, however, associate with carriage status at the pre-booster time point or one month after booster. Significant anti-pneumococcal protein responses was observed in the infant samples for both IgG and IgA, which was consistent with observations made in healthy U.S. adults (Croucher et al., PNAS, 2017; Campo et al., eLIFE, 2018). 

IgG responses tended to be higher in infants with carriage of any serotype than in infants without carriage. Multiple pneumococcal proteins that are normally associated with the bacterial cell surface were identified as potential targets of antibody markers of carriage. 

Antibodies did not generally increase one month after carriage, irrespective of the carried serotypes. Thus, antibodies may have been maximal at time of carriage, pre-booster dose. Measuring sample pre- and post-carriage may more clearly identify carriage events. 

These encouraging results suggest that an antibody signature may serve as a sensitive measurement for detection of carriage events during clinical trials. Such a tool may simplify clinical trial design in LMICs and accelerate progression of vaccine candidates. Additionally, these data provide new knowledge of existing levels of immune responses in LMICs, particularly in infants.


Dr Joesph Campo 200x200pxDr Joseph Campo
Director of Operations
Antigen Discovery, Inc (ADI) (USA) 

Collaborators:
Professor Andrew Pollard, University of Oxford (UK)

Mrs Merryn Voysey, University of Oxford (UK)

Professor Shrijana Shrestha, Patan Academy of Health Sciences (Nepal)