How do you vaccinate against a master of immune evasion?
Posted on June 14, 2023 by Matthew Reeves
Matthew Reeves takes us behind the scenes of their latest publication 'A temperature-dependent virus-binding assay reveals the presence of neutralizing antibodies in human cytomegalovirus gB vaccine recipients’ sera' published in Journal of General Virology.
My name is Matthew Reeves and I am a Group Leader at the Institute of Immunity & Transplantation which forms part of the Division of Infection & Immunity at UCL in the UK.
My laboratory investigates viral pathogens that cause disease in vulnerable patient populations such as organ transplant patients. The pathogen that receives the most attention is Human cytomegalovirus (HCMV), a member of the herpes virus family. Like all herpes viruses, HCMV infects you for life and it is likely over 60% of the population are carrying the virus. Although our immune system cannot prevent HCMV infection (or even re-infection) we rarely see disease in patients with a functional immune system. In fact, our immune system dedicates a substantial proportion of our immune response to controlling this infection. The importance of our immune system for control of HCMV is demonstrated when the virus either infects an immune-naïve host (e.g. fetus), or an individual is immune-suppressed (e.g. transplant) or immune-compromised (e.g. AIDS) where in all populations we see severe HCMV-associated disease. This threat from infection is exacerbated by the capacity of the latent virus to reactivate in the host which is also a major cause of disease. All this has led to HCMV being designated highest priority for vaccine development.
Our research has two major focuses: Firstly, we want to understand the molecular mechanisms that control HCMV latency and reactivation in the host and secondly, what will a vaccine against HCMV need to look like to be effective against a virus considered a paradigm for immune evasion.
Our most recent paper concerns our ongoing attempts to understand the composition of the vaccine response in sera we have taken from a phase II vaccine trial of a HCMV vaccine. The vaccine was based on a protein called glycoprotein B (gB) which is on the surface of the virion and is essential for entry into all cells HCMV infects. The correlate of protection in multiple phase II studies of this vaccine was the concentration of anti-gB antibodies but the mechanism of protection has remained elusive. We are fortunate to have access to sera taken from gB vaccine recipients who were on the waiting list for organ transplantation and we are interested understanding what different activities are present in the antibody response. What we knew was the gB antibodies made in response to the vaccine lacked one key property – they did not efficiently bind and neutralise the virus in our standard assays in the lab. However, we wondered what would happen if we slowed the whole process of infection down – and thus infected cells at +4oC rather than 37oC. What this does is it allows the virus to bind to the cell but not enter. In order for entry to occur gB must change structure which happens rapidly at 37oC. At +4oC this is much slower and when we do this we can now see neutralising antibodies in vaccine recipients sera. This does not happen with all gB antibodies we tested and so suggested the presence of some antibodies in our vaccine patients that only recognise gB in certain situations and our modified assay was revealing them.
A lab photo (with our lab logo designed by Anastasia Lankina) which is from our Covid Christmas curry which took place in March!
It is fair to say that the driving force for investigating this was curiosity – the observation that none of the antibodies induced by the gB vaccine were potently neutralising (which is the classical way antibodies work) yet antibodies were the correlate of protection in patients puzzled us. Then when we saw in an earlier study that we could detect neutralising antibodies so rapidly post-transplant in our vaccinated patients it left us considering whether they were there pre-transplant but at very low concentrations and our lab assays were not capable of detecting them. Essentially, we considered that we were witnessing a prime:boost (vaccine:transplant) event. Thus we asked what happens if we slow everything down and also change the conditions to favour binding of high avidity but low concentration antibodies (in both cases by dropping the temperature). It was exciting when we saw evidence of neutralising antibodies – they were clearly there in some patient sera – but then we were perplexed because it looks like they were distinct from the ones we saw in the patients post-transplant!
Ideally we would look to really try and run multiple types of different assays and really take these observations apart but the patient sera is so valuable and we only have a limited amount and so every experiment really needs to work (in that the data are reliable not that there has to be an effect). Perhaps the standout experience for me is that I was pretty convinced when I saw the first data that it was the evidence for our prime:boost hypothesis but it appears from this study that this is perhaps not the case! My comment to all trainees in the laboratory is that you have to follow the data even when it creates more questions than answers and embrace the uncertainty!
Moving forward what we hope to do is understand which antibodies in the sera are responsible for the neutralisation we are seeing. The model suggests that the antibodies are binding to gB only when it is in certain conformations as it goes through the fusion process upon viral entry. By characterising the antibodies that are binding at certain times will give more clues about how the structure of gB changes and provide insight into how gB works. In essence it may be an example of going from bedside to bench. Afterall understanding how viruses infect cells is fundamental if we want to design therapeutics against them.
So why do I think this research is important? A vaccine against HCMV is the holy grail but the development of additional anti-virals that we can add to the portfolio of drugs we have to control this virus is not to be understated – and the identification of inhibitors of gB function could be a powerful addition to that. Furthermore, we have the possibility of using the immune response, and specifically understanding how it recognises gB in different stages of virus infection, to further understand the mechanism of HCMV entry into cells.
Thumbnail image designed by Anastasia Lankina
