Issue #74: The monoclonal antibody story part 5: humanisation

Written by Nobel Laureate Professor Peter Doherty

Very early in this Setting it Straight series (#5) I wrote about passive immunity which, as distinct from the ‘adaptive immunity’ that we prime in our own bodies by vaccination, involves the (generally intravenous, IV) injection of antibodies from another source (#17, #21). That form of passive immunotherapy (treatment) or immunoprophylaxis (prevention) began at the end of the 19th century with the administration of serum from horses that had been immunised to make antibodies (immunoglobulins, Igs) to the toxin produced by Corynebacterium diphtheriae.

For that time, the treatment was miraculous. Patients who were fighting for oxygen and life due to the toxin-induced inflammatory swelling and ‘diphtheritic membrane’ that blocks the respiratory passage could suddenly breathe again. Many of us may have been given a comparable tetanus antitoxin after, for example, standing on a rusty nail, though our main protection now against both tetanus and diphtheria is the active immunisation induced by vaccination. When it comes to antibody prophylaxis, the main current application is to give those who have a compromised immune system and may not benefit from vaccination regular doses of IVIg (Intragam in Australia), which is sourced and purified from pooled plasma drawn from donors at the blood bank. One strategy with COVID-19 would be to make that product from people who have been recently vaccinated and are themselves protected against developing severe disease.

Another possibility for both passive immune therapy and prophylaxis is, of course, to use monoclonal antibodies. Many of us first became aware of mAb therapy back in October 2020 when US President Donald Trump was infused by IV with eight grams of a ‘cocktail’ of two different ‘neutralising’ mAbs (made by the Regeneron company) that were specific for the SARS-CoV-2 spike protein. From what we know of his symptoms, together with his age, it’s likely that Trump was progressing to severe disease. The mAbs pulled him back from the edge. Until then, the only general use for an antiviral mAb in human medicine was to give Pallivizumab, a mAb specific for the fusion protein (#71) of Respiratory Syncytial Virus, to at risk infants (premature babies for instance) as a prophylactic. After his successful treatment, President Trump said that he would make the SARS-CoV-2-spike-specific mAbs that had saved him from serious illness available free to all American who need them. I don’t know how that has played out in practice, but they have certainly been widely used in the USA.

What the clinical use of these mAbs for immunotherapy reflects is that the technology had moved beyond the mouse mAbs made by Kohler and Milstein (#71) that proved so enormously valuable for researchers like me and for those involved in medical diagnostics, to develop humanised products that could be injected into people. Why was this necessary? While ‘mice and men’ – to borrow from John Steinbeck – have much of their biology in common, we can’t inject mouse mAbs into humans without running the risk that we will make an immune response against their ‘mousiness’. Just as vaccination protects us against virus, the human anti-mouse Ig response will both eliminate that particular mAb and any future mouse mAb we might want to give for some other purpose.

The ‘humanisation’ of mAbs was so medically important that one half of a second Nobel Prize (Chemistry, 2018) was shared between Sir Gregory Winter of the LMB, Cambridge – where Milstein also worked – and George Smith at the University of Missouri at Columbus. Smith was recognised for the development of phage display libraries, while Winter  received his accolade for first working out how to ‘engineer’ the mouse mAbs to make them look ‘human’ to our immune system, then adapting Smith’s phage display approach to find human Ig genes of the right specificity and bypass any need for mouse involvement. I won’t go into the technical details, which you can read in their respective Nobel Lectures. (Winter, Smith)

What this advance meant was that human mAbs could now be made in large culture volumes by expressing the relevant Ig genes in any of the established cell lines that companies use for producing ‘bucket loads’ of a particular protein. With regard to the medically important (as distinct from diagnostic) mAbs they must, as for any product that will be injected IV into humans, be produced and purified by trained staff working in scrupulously clean, GMP (Good Manufacturing Practice) certified facilities. Such sophisticated and appropriately licensed physical infrastructure tends to be in short supply, and we have been fortunate that CSL has such a facility at Broadmeadows in Melbourne. This can be used to make mAbs, but it has recently been dedicated to producing the AstraZeneca vaccine.

But while each vaccine shot contains 5x1010 virus particles – maybe a few milligrams of protein at most – just one mAb dose is likely to contain a gram or so. Vaccination that induces those of us who are immunocompetent to make our own persistent, polyclonal, antiviral antibody responses is, of course, an infinitely cheaper and more effective approach than passive mAb therapy (#5). Still, especially when it comes to protecting the immunocompromised, it’s clear that having therapeutic mAbs available is of great value for clinicians. Next week, we’ll complete this series on the mAb revolution by looking at their broader use in both COVID-19 and, generally, in medicine.