Many people have probably lately been thinking “I want my momma!” – and scientists have also been thinking “I want that llama!” – or at least it’s antibodies! Turns out from an animal found in a stable you can find antibodies that are incredibly stable! And coronavirus fighting this might enable! A llama named Winter was made famous recently because, after injecting her with coronavirus protein, scientists were able to isolate antibodies from her blood that, after a little tweaking, proved effective at blocking SARS-CoV-2, the novel coronavirus that causes the disease COVID-19, from infecting cells in a dish. This is just one of the strategies used to develop MONOCLONAL ANTIBODIES. You’ve probably gotten used to hearing about antibodies in the context of tests to see if people have had Covid-19. But antibodies might also be able to treat COVID-19 – or even prevent it all together, and recently this strategy went into a clinical trial.  

I’m going to go into more detail, so if these terms don’t make sense yet, don’t worry, but, for the impatient and/or less geeky, here’s the gist. Antibodies are little proteins made by B cells as part of the adaptive immune response to an infection. They specifically bind to parts of the invader (like viral proteins) and call for backup. Some antibodies (which are called neutralizing antibodies) can bind in such a way that they prevent the virus from infecting cells. But it takes your body a while to find “good antibodies” because they have to sift through millions of different ones (each B cell randomly edits its DNA to make a different one), see what works, and then make a ton of them.

Since this takes time, people typically don’t develop specific antibodies for an infection (i.e. seroconvert) until late in that infection. Good for preventing re-infection, but not so good at doing the early fighting-off, and definitely not good for preventing someone from getting infected in the first place. But if you could give a person those good antibodies without them having to find them themselves, you could potentially treat or even prevent infection (prophylaxis). 

But how do you get these antibodies? Without having to go through that whole finding process?

One way we’ve already seen antibodies used is with convalescent plasma – these aren’t purified antibodies, though – you just take the blood of people who’ve recovered from COVID-19, remove the non-cell parts to get the plasma, which contains antibodies, and give it to someone who needs it. But this is certainly far from efficient. That person’s plasma doesn’t only contain antibodies for SARS-CoV-2 – it also contains antibodies for a whole bunch of other invaders that person’s body has fought off throughout their lifetime. And – even when it comes to the SARS-CoV-2 antibodies present, there a whole bunch of different ones, because different B-cells experiment differently and can still make ones that work – there’s no one right solution – but there are better ones. So some of the antibodies are stronger than others, either because they bind tighter or they bind in a better blocking position. 

Some companies are looking to find the really good ones from the blood of recovered patients. Other companies are hoping that they can make lab animals do the finding process for them by injecting them with viral protein and isolating antibodies from their blood. Either way, whose bodies have done the initial “finding” but you still need to weed out these potential candidates to find the good ones. So, the next step is testing them to see if they can block the virus, isolating the DNA of really good ones, and then sticking that DNA into other cells to make a ton of it. Then purifying it and injecting it into people. 

terminology note: Since all of the copies of the antibody you’re injecting are identical – they all came from the same “parent” B cell, we call this a MONOCLONAL antibody. This differs from convalescent sera, which contains POLYCLONAL antibodies – antibodies from lots of different B cell parents. Also, since we’re on this term tangent, the thing the antibody recognizes (e.g. the coronavirus Spike protein) is called an ANTIGEN, the specific part of the antigen the antibody binds to is called the EPITOPE, and the specific part of the antibody it binds to is called the PARATOPE. 

The getting the lab animals to make antibodies part is a lot like how some vaccines work, with you being the human and getting to keep the antibodies you make. But there are really important differences between vaccines and antibody therapies. 

Vaccines are designed to induce ACTIVE IMMUNITY – their goal is to introduce someone’s body to parts of the virus or an inactivated or weakened version of the virus in a harmless form and get that person’s body to learn to make its own antibodies against it. This provides long-lasting protection, because your body will keep making it. 

This is different from passive immunity – passive immunity includes convalescent plasma and monoclonal antibody treatments – and it’s goal is to just give someone the antibodies (premade). This way you know they’re getting the good stuff, but they only get it when you give it to them – stop giving it and the body won’t have it because it doesn’t know how to make it itself (teach a man to fish thing…)

Both have usefulness, but both are very different and easily confused. I’ve talked a lot about vaccines, and touched on convalescent plasma, but today I want to talk more about monoclonal antibodies (mAbs). Monoclonal antibodies are used to treat cancer & autoimmune diseases (when you see the ending “-ab” on drug names, this is why. But they haven’t been used much for infectious diseases – except a few, most notably Ebola – and it was this Ebola treatment that outcompeted Remdesivir as an Ebola treatment, leading Remdesivir to be sidelined until recently. 

That same company (Regeneron) which made that Ebola drug is now working on a coronavirus one and I will tell you more about it – as well as some of the strategies being used by different companies to find & make good ones, but I think it helps to have a little background on antibody making and make-up, so…

When an animal is confronted with something unusual, like a protein from an invading virus or bacterium, it can mount an immune response to destroy the invader, but only if it knows that the thing is unusual, and the way it knows this thing is “foreign” is because it binds to proteins called antibodies that are made by a type of immune cell called B cells, which make antibodies and put them on display (sticking out of their membrane) to “keep watch.” Antibodies are made by a type of immune cell called B cells as part of the adaptive immune response when you first encounter the thing. So that next time you encounter the thing, alarms will ring!

All antibodies bind to the thing they “recognize” (often a viral protein) but if antibodies happen to bind to a virus part in such a way that they prevent that virus from infecting a cell, we call them neutralizing antibodies (they “neutralize” the viral threat). For SARS-CoV-2 (the novel coronavirus that causes the disease COVID-19) this usually means binding to the virus’ Spike protein (the one that juts out from the viral membrane) in its Receptor Binding Domain (RBD). This binding can be directly in the spot it uses to latch onto the cell’s ACE2 receptor (the Receptor Binding Motif), out-competing it – or it can bind in some way that prevents the Spike protein from undergoing the dramatic shape-shifting (conformational changes) required for the Spike protein to get cleaved by proteases, shoot out its inner parts, latch onto the cell membrane, and fuse it with the viral one. more on that here: 

So if B-cells are making these, how does anyone ever get sick? Problem is, the B-cells don’t know what they’re going to have to face in the future – they can’t predict that someone with COVID-19 is going to sneeze on you. And they don’t even know it’s out there (your cells aren’t tuned in to CNN). So they don’t know what to make. Instead of stocking up on every possible antibody, which is like limitless, they wait until an infection starts and then “experiment” to find what works – different B-cells make different antibodies (they have different variable regions) and then the ones that bind the virus (and not the host) get selected for and more of those get made. This process (called clonal selection because all of the cells made from that original experimenter are clones of it) takes time (which is why antibodies don’t show up until late in infections and your immune system relies on more generic first responses). 

Different B cells make different antibodies (having different variable regions) but each B cell can only make one type because the random “experimenting” they do to decide what to make involves actually changing their DNA in a process called somatic recombination – don’t worry – the “somatic” tells you that these genetic changes are happening in non-germline-cells – the changes are permanent for this B-cell, and all of the cells that arise from it (clonal cells), but the changes don’t effect any of the other cells in your body, like the ones that would get passed on to your children. 

How it works is that antibodies have a “generic” adapter region (constant region) and variable regions, which are unique to different antibodies and are where the experimentation occurs. B-cells “randomly” choose what to make by piecing together different options for different parts of the variable regions of antibodies through a process called V(D)J recombination – more on that here:  Immature B cells have a whole menu of options to choose from, but in their maturation process they go through a form of somatic recombination, where they choose from that menu – and remove the other choices so they (and all the cells that come from them) don’t have options anymore – they’re forced to specialize in making that specific antibody. 

Bottom line – different B-cells randomly specialize to make different antibodies. Those antibodies all, by chance, happen to recognize different things. These things can be different molecules all together (e.g. Antibody 1 recognizes Protein A and Antibody 2 recognizes Protein B) OR the things can be different parts of the same molecule (e.g. Antibody 1 & 2 both recognize Protein A, but different parts of it). 

When someone gets infected with a virus, immune cells called antigen presenting cells engulf the virus, cut its proteins up into little pieces, and then display those pieces on their surface. Then, B cells that happened to have made an antibody that can bind it do bind it, leading to those B cells being selected for. Those chosen B cells start making more copies of themselves, some of which (memory B cells) continue to display the antibody sticking out from their membrane, while others (effector or plasma B cells) secrete the antibody into the serum. This is why convalescent serum can be used as a therapy. 

As we mentioned before, this serum (the cell-less part of blood you get when you spin blood in a centrifuge to separate out the cells) contains a lot of stuff. It has antibodies to all sorts of invaders that person has fought off in the past. And, even the antibodies against the virus of interest (e.g. SARS-CoV-2) likely target multiple different viral proteins. In the case of a lab-injected animal, where you’re only injecting a single protein to serve as an antigen (typically the Spike protein here). You can purify out the antibodies that really do bind the antigen, but you will still have a polyclonal mixture of antibodies that bind to different parts of that protein with different “goodnesses.” 

You can test how good they are using things like an ELISA, which measures binding to a protein of interest (e.g. coat the wells of a plate with an antigen (e.g. Spike protein) -> add antibodies to test -> let them potentially bind and wash off non bound stuff -> add labeled antibody that binds to the generic part of the first antibody -> wash off non bound stuff -> see if you see signal -> shows you if there was antibody there that stuck) and neutralization assays to test if the virus can actually infect cells. 

But, even if you find great ones, without the B cells they came from, you have a limited supply. If you want to find the good ones AND make more, you need to go straight to the source – you need those B cells!

What companies are doing is taking bloods’ B cells, splitting them up so that each B cell gets its own well in a chip, letting that B cell secrete antibodies, and seeing if they make good ones (such as with antigen-coated beads in a smaller scale version of an ELISA). AbCellera Biologics, the Canadian company working with Eli Lilly, who started a clinical trial of their mAb LY-CoV555, has a microfluidics chip that can test 200,000 cells at once! 

In the figures, I show a typical workflow for the Berkeley Lights system used by the Vanderbilt team led by James Crowe. You can see a webinar about it here: 

They can then take the “top hits” and sequence their DNA’s antibody instructions and stick those instructions into a piece of DNA called a vector that contains signals and binding sites that will enable expression cells to make the antibody when you stick the vector in them. The sticking-in is called transduction, and, since you “recombined” DNA the antibodies produced are called recombinant antibodies. Scientists express proteins recombinantly all the time, so there are a lot of established protocols and good expression systems in place, but it can be labor/time/cost expensive, which is a downside to antibody therapies. 

For large-scale production, antibody makers often use CHO cells – a cell line that comes originally from Chinese Hampster Ovaries – these are good to use because, as mammalian cells, they’re more similar to human cells than bacteria or yeast, so they’re better at making more complicated proteins that require special folding helpers and modifiers, etc. 

But before you get to large-scale, you want to make sure you only waste all that time/money/etc. making the really good ones. So they start small, doing expressions on the really really small scale, in tiny wells in a dish and cells such as yeast or bacteria and doing further testing before they go big. 

Speaking of cell similarity, antibodies are used a lot in the lab – not to treat people, just to see if things are bound to membranes in a Western blot, etc. and for these we use antibodies made from bunnies, goats, mice – we can do this because we’re not sticking these into people. If we did, those peoples’ immune system would attack them as foreign. So if we want to make antibodies we stick into people, they “need” to be human antibodies. Even though we’re making antibodies in cells from hamsters, the proteins are human because the DNA you put in was. So that’s not a problem. But if you want to get a mouse to make antibodies  through the learning process (e.g. inject it with viral protein and see what it comes up with), you need to use a really special mouse – one whose immune system has been “humanized” so that the antibodies it makes are human ones. This is the strategy being pursued by the company Regeneron (they’re the group that made the Ebola antibody treatment) 

It’s not completely true that you can’t use antibodies from other animals, even if they aren’t humanized – the trick is that you can’t use the whole thing. Enter the llama. 

Llamas and other camelids have conventional antibodies like we do – those Y-shaped IgG antibodies with 2 heavy chains, 2 light chains, and constant regions (the straight-up base part of the Y) and variable regions (the arms of the Y) of each chain (VH & VL). But they also make a smaller antibody called a “heavy-chain-only antibody” (HCAb) which just has a single variable domain – the heavy one (VHH). This VHH part is stable on its own – super stable in fact – and, when alone, is referred to as a nanobody. Since it doesn’t have the llama-specific constant region, you can stick it into people without them rejecting it as foreign. But, on their own, they might not be sticky enough. 

This was the case with the nanobodies found from that llama Winter. The story was published in the journal Cell in the beginning of May 

The group, led by Xavier Saelens at Ghent University in Belgium, and Jason S. McLellan at the University of Texas at Austin, were able to get results so quickly because when they started the project, they weren’t even thinking about SARS-CoV-2, no one was – this was pre-this-pandemic time, but it wasn’t pre-pandemic. And they were looking to make nanobodies against other coronaviruses – SARS-CoV (the original SARS virus) and MERS – so they injected Winter with proteins from those viruses. 

But because SARS-CoV-2 and SARS-CoV have such similar Spike proteins, they decided to look if the nanobodies that they found against SARS-CoV would cross-neutralize SARS-CoV-2. They found that, on their own, the nanobodies could bind the SARS-CoV-2 Spike protein, but not very tightly and weren’t good at neutralizing it (blocking viral entry). It’s not that surprising since they weren’t looking for anti-SARS-CoV-2 antibodies. But they didn’t give up. 

The problem they faced was the individual VHHs weren’t sticky enough – they have a high off-rate constant meaning that, although they’d happily bind, they’d quickly fall off. So, in order to get a stronger grip, they fused a couple of them to the constant region of a human antibody (the Fc domain of human IgG1). This VHH-72-Fc construct did a much better job, and it’s still a lot smaller and stabler than a conventional human antibody. And maybe if people inject SARS-CoV-2 proteins into a llama instead of the original SARS version, they can get better nanobodies. 

Nanobodies are attractive for a few reasons – in addition to being more stable than conventional antibodies and thus easier to store, stockpile, and deliver, they’re a lot smaller, so it’s easier for them to move around in our body and get into deep protein grooves. And it’s possible that they could be inhaled instead of injected 

These are just experimental at this phase, and there’s a lot of experimenting going on. Over 50 companies working on antibody therapies: And here are some of the big names to watch: 

Another thing to note is that many of these companies are pursuing a mix of monoclonals – so like a polyclonal, but with just a couple of antibodies that you choose. This can have benefits including giving you backups in case one epitope of the virus mutates so that the antibody no longer binds.  You get these backups with active immunity as well because you get a polyclonal response – your body stocks up on lots of different antibodies, hopefully strong ones!

more Covid-19 resources:

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