Thursday, May 01, 2008

crazy camels

just two weeks to go until the release (in Europe) of "The birds, the bees, and the platypuses". As I mentioned before, the book has three parts, a crazy, a sexy, and a cool one. (although some of the stories may tick more than one of these boxes, and I don't like boxes anyway, so don't take the categories too seriously!)

Here's a chapter from the crazy section:

Magic bullets from the desert

This story began quite inconspicuously with a conversation I had in my office, back in the days when I was still doing research at the University of Oxford. A former colleague had come back to do some experiments in the course of a collaborative project that I didn’t know anything about. When I asked him what his experiments were about, he said he was studying the interaction between lysozyme and camel antibodies. “Lysozyme” was the boring part of that sentence, as everybody in this lab had some connection to this classic workhorse of protein and enzyme studies. But the other half was news to me, so I pricked up my ears and asked: “Camel? What’s special about camels?” So he told me, and I must have retold this story more than a dozen times in various formats. It’s still one of my favourites.

It all started with a mutiny in a university teaching lab, some time in the late 1980s. A bunch of biology students were told to do the immunology experiments that countless others had done before them, fishing antibodies from human blood serum, and separating them into different groups. They were not too keen, as the serum might contain HIV, and also because the results of the experiment were well known and already documented in their textbooks. Their tutors offered to sacrifice a few mice instead -- not a very popular choice either. Eventually, a few liters of serum leftovers were discovered in the freezers of the research labs -- they were from dromedaries. This exotic sample inspired the students sufficiently to give up the strike action and start working on the separation of the antibodies. They found the usual distribution of immunoglobulins that one expects to see, but they also discovered a group of smaller antibodies that did not correspond to anything known to science.

This episode happened at the Free University of Brussels, and it might have ended in obscurity, had not two researchers at this university, Raymond Hamers and Cecile Casterman investigated more deeply. They believed that that the smaller antibodies were not just degraded copies of the real ones, but that they were of a special kind. They repeated the students’ experiments with fresh samples from camels and llamas, and confirmed that all the animals in this group (the camelidae) produce some amounts of antibodies which are very different from the standard ones in that they are lacking the pair of protein molecules known as the light chains. They consist only of the heavy chains, which is why they are now referred to as heavy chain or HC antibodies. (In normal antibodies, a pair of heavy chains is arranged in a symmetrical Y shape, with one light chain attached to each of the branches.)

Ordinary antibodies are horrible things to deal with, as they are complicated molecular assemblies, very difficult to produce in bacteria, too bulky for many medical applications, and may trigger an unwanted immune response in a patient (yes, there are antibodies against antibodies!). Therefore, many labs have tried to find something simpler, to construct miniature antibodies combining the binding specificity of a real antibody with some extra user friendliness. Serge Muyldermans and Lode Wyns, also working at the Free University of Brussels, were pursuing research in this direction. They took up the trail of the camels, but it was quite a trek. In the beginning, it was far from clear that these molecules were functional antibodies with the high variability and specificity of the real thing. What they needed to do was to find a camel, immunize it with a specific antigen, wait a year, and see whether the camel had produced specific HC antibodies against that substance. A party of researchers travelled to Morocco, bought a camel, immunized it ... and had it stolen before they could get at the precious serum!

These practical problems were eventually overcome with a little royal help. His Highness, General Sheikh Maktoum Bin Rashid Al Maktoum, then the ruler of Dubai in the United Arab Emirates, supported the research by providing camel serum from his renowned veterinary research centre. What the researchers found out about the camel antibodies was even more promising for medical and biotechnological applications than they could have hoped for.

It turned out that the HC antibodies, like normal ones, recognize a wide range of antigens, but they interact with them in different places. Thus, HC antibodies raised against small enzymes such as lysozyme or ribonuclease can penetrate the active site and provide a potent inhibitor for the enzyme, while conventional antibodies would bind somewhere more accessible.

This all boils down to the fact that in a conventional antibody, the antigen recognition is provided by two sites (at the upper ends of the “Y”), each composed of two different molecules, a heavy chain and a light chain, resulting in a rather bulky arrangement. In HC antibodies, each binding site is contained in a narrowly defined region of one molecule, the variable domain of the heavy chain. This is why it reaches the parts that other antibodies can’t. This also means that it is a lot easier to further miniaturize this antibody. If you want to miniaturize a human antibody by cutting off all the parts that are not involved in binding, you get enormous difficulties trying to keep the binding domains of the heavy and of the light chain together. With the camel version, you can just genetically isolate the DNA for the binding domain, get bacteria to make it, and you’ve got your miniature antibody, known to scientists as a single domain antibody.

Single domain antibodies are the ideal tool for a range of applications from scientific research tools through to diagnostic kits to be used at home. One very promising field is imaging of living tissue, and especially cancer diagnosis. When trying to localize a tumour, you want a label that, apart from recognizing the particular molecules found on tumour cells, penetrates easily into the tumour. Once it has done that and bound to the target, you want to be able to wash out any unbound leftover material as easily, so that it won’t show up in the picture. Common antibodies fail on these accounts, but preliminary tests suggest that single domain binders derived from camel HC antibodies could be used. It also appears that they will not normally provoke an immune response as full-size (non-human) antibodies would. Furthermore, the small size of these molecules allows scientists to use them as building blocks for constructs which might contain two different binding sites, or even a binding site combined with an enzymatic or other activity. They could even be harnessed inside the cell, as so-called intrabodies.

A typical example of an antibody-based consumer product is the home pregnancy test kit which you dip into a urine sample, and then wait for a blue stripe to turn up. One version of this, designed to indicate the presence of a characteristic pregnancy hormone, consists of two different kinds of antibodies against this hormone. One set is glued to the solid support in the window area where you want the blue stripe to appear. When hormone molecules float by, they will get bound by these antibodies. A second set of antibodies, recognizing a different part of the hormone molecule, is charged with blue-colored particles. When this second set comes across the hormone molecules firmly bound to the first set, it will bind to them and thus make the blue color accumulate in the window. As this kind of test requires two kinds of antibodies which bind the target in different ways such that they not interfere with each others binding, a combination of conventional antibodies on the solid surface and camel-type antibodies in the liquid would be ideally suited.


Further reading

Muyldermans Serge, et al., Trends Biochem. Sci., 2001, 26, 230- 235,

What happened next

Since 2002, a spin-out company called Ablynx has begun to develop a number of products based on the advantages offered by camelidae antibodies. As of 2007, Ablynx has over 90 staff and contracts with several pharma giants. The first drug based on camel antibodies has just entered a phase I clinical trial. Watch this space.
T. N. Baral et al., Nature Med. 2006, 12, 580.

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