Llamas, new research has revealed, may be harbouring an immune trick that is a potent therapy for Covid-19...
Despite a huge global effort, an effective vaccine against SARS-CoV-2 is not expected for months. For this reason considerable effort is being focused on the development of potent antibody-based therapies that can be administered to people at the earliest stages of infection with the aim of neutralising virus infectivity.
The simplest approach is to take blood from people who have recovered from an infection of interest, separate off the plasma and infuse this ‘convalescent serum’ into currently-infected individuals. The rationale behind this approach is that the serum contains a broad range of antibodies against the virus, and for many infectious diseases - such as Ebola and rabies - this can prove an effective treatment. Nevertheless, this approach requires the collection of serum from infected individuals, the characterisation and pooling of serum only from those individuals who mount a strong neutralising antibody response, and the blood product must itself be safe and free of harmful pathogens.
In order to circumvent these issues, scientists have been exploring ways to manufacture highly specific antibodies that recognise regions of virus particles that play critical roles in the infection of cells. Such antibodies are often referred to as "potently neutralising monoclonal antibodies". In a recent paper, scientists have gone a step further and generated an antibody in a llama that recognises the spike proteins of both SARS-CoV-1 and SARS-CoV-2, the causative agents of SARS and Covid-19 respectively. Importantly, the antibody can neutralise both SARS-CoV-1 and SARS-CoV-2 viruses, suggesting that such llama-derived antibodies could be useful therapeutics during coronavirus outbreaks. But what makes llama antibodies so special and why are they receiving so much attention?
The answer is mainly due to their small size, which can afford them access to regions of proteins that are otherwise out of reach to larger "conventional" antibodies. Most animals produce antibodies that contain two identical "heavy" protein chains and two identical "light" protein chains. Each of these chains has a variable domain (called VH and VL respectively) which work together to recognise a specific antigen or protein structure. In contrast, llama’s produce a simpler antibody comprising two heavy chains joined together, together with the associated variable domain. These are so-called “heavy chain only” antibodies. Importantly, the single variable heavy chain can bind antigen at high affinity and the variable region is fully functional on its own. The single chain variable region - or “nanobody” - is only 10% the size of a conventional antibody. It's also remarkably stable and can be readily manufactured in microbial cells (such as bacteria or yeasts) or mammalian cells. These are ideal properties for pharmaceutical production.
In their recent paper, Wrapp and colleagues immunised a llama, not with SARS-CoV-2 but with the spike proteins of the related coronaviruses SARS-CoV-1 and Middle East Respiratory Syndrome MERS–CoV. Following immunisation, RNA was extracted from peripheral blood lymphocytes, which includes antibody-producing B cells, and DNA copies of the single chain variable regions were cloned and expressed in a bacterial virus (bacteriophage) such that the antibody variable region is presented on the bacteriophage surface. This enabled the rapid screening and selection of bacteriophages that would bind tightly to immobilized SARS-Cov-1 and MERS-CoV spike proteins respectively. Using this approach it was possible to isolate SARS-CoV-1 and MERS-CoV nanobodies that could neutralise the infectivity of viruses expressing the respective virus spike proteins.
Of significance, one antibody, termed VHH-72, was able to bind the ACE-2 receptor binding domain of the spike proteins of both SARS-CoV-1 and SARS-CoV-2. This indicates that this antibody was able to recognise a structurally conserved region of the receptor binding domain which may exist in all SARS-CoV-like viruses. Although its binding to SARS-CoV-2 was less efficient than that observed with SARS-CoV1, construction of a bivalent protein containing a dimerised version of VHH-72 resulted in potent neutralisation of SARS-CoV-2 pseudotyped viruses.
The therapeutic potential of this antibody has yet to be explored but given the ease of manufacture and small size, which could assist in rapid tissue penetration following in vivo administration, the stage is set for rapid clinical evaluation. Maybe a llama will truly become humanity's best friend!
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