links the antibody to the phage instead of theantibody being an integral part of one of thecoat proteins.This makes it easier to separatethe antibody at a later stage. MorphoSys alsooffers ‘Antibodies by design’, a service thatwill develop recombinant monoclonalHuCAL antibodies for individual targets.“This is especially interesting for researchersin industry and academia, as the turnaroundtime is only about two months and the coststarts at about E5,000 [US$6,000],” saysClaudia Gutjahr-Loeser of MorphoSys.

Rather than going for comprehensive-ness, a human-antibody library can bebiased in favour of particular targets, forexample by taking the initial antibody cod-ing sequences from individuals vaccinatedagainst a particular disease. Blood from sol-diers immunized against anthrax was thestarting point for a Fab antibody library that has allowed Katherine Bowdish and her

colleagues at Alexion Antibody Technologiesin San Diego, California, to identify high-affinity human monoclonal antibodies tocritical elements of the anthrax toxin (M. A.Wild et al. Nature Biotechnol. 21, 1305–1306;2003). The technology is also applicable todiseases such as smallpox, says Bowdish.

As the source of its library, Affitech inOslo, Norway, also uses blood from selecteddonors known to produce antibodies ofinterest, such as cancer patients, or peoplewho have been vaccinated or are recoveringfrom an infectious disease. Affitech’s chiefexecutive Martin Welschof says that this,combined with the company’s proprietaryscreening system, avoids the need for repeti-tive rounds of selection and means that it canmove from a 10-ml blood sample to a high-affinity human monoclonal antibody of thedesired specificity within four weeks.

Small is beautifulIf a therapeutic antibody has to penetratefar into body tissues, or into a tumour, forexample, smaller may well be better.Although some companies are concentrat-ing on scFv antibody fragments, UK-basedDomantis is producing even smaller humanantibodies from its phage libraries: theseconsist of just a single variable domain.

Like scFv fragments, these ‘domain anti-bodies’ can be mass-produced relativelycheaply in Escherichia coli or yeast. They arealso very stable, which opens up new optionsfor therapy, such as a powder that can beinhaled to treat asthma and other lung dis-eases, or a tablet to be taken orally for gutinflammation. “We’re only going after areaswhere domain antibodies have unique

NATURE | VOL 426 | 11 DECEMBER 2003 | www.nature.com/nature 729

advantages,”says Ian Tomlinson,co-founderand chief scientific officer of Domantis. Inpartnership with Peptech in North Ryde,New South Wales, Australia, Domantis isdeveloping a domain antibody against TNF-a for the treatment of rheumatoid arthritisand other inflammatory diseases.

Part of the inspiration for pursuingdomain antibodies was the natural domainantibodies found in camels, dromedaries,llamas — and sharks.These natural antibod-ies are also being explored for their potentialapplications by start-up biopharmaceuticalcompany Ablynx of Zwijnaarde,Belgium.

Mouse powerBoth Abgenix of Fremont, California, andMedarex of Princeton, New Jersey, take adifferent approach to making large numbersof different human antibodies for screening— they get mice to do it for them. InAbgenix’s XenoMouse and Medarex’sHuMab-Mouse, murine germ-line antibodygenes have been replaced with human ones.

antibodies technology feature

Antibodies are normally built inside cells, take uptheir final form as they are secreted from the cell,and function exclusively outside cells, binding tocell surfaces and extracellular molecules. But TerryRabbitts at the MRC Laboratory of MolecularBiology in Cambridge, UK, wants to use antibodies’highly specific binding properties inside the cellitself. He has engineered cells to make fragmentsconsisting of just a heavy-chain variable domainand hold them within the cell. These single-domainantibodies can interact with intracellular proteins atbinding affinities of 10 nanomolar or better. AtIclectus, an MRC spin-off company based inLondon, Rabbitts’ colleagues are developing these fragments as intracellularantibodies or ‘intrabodies’.

They became interested in intrabodies while they were unravelling therole of transcription factors involved in producing chromosomaltranslocations in tumour cells. Using intrabodies to bind and block thesetranscription factors inside the cell opened up a new range of researchpossibilities as well as pointing the way to new anticancer therapies.Transcription factors operate by binding other proteins, so “if you have an

intrabody that has an affinity close to or better thanthe affinity of the interacting partner, you should beable to block the interaction using the intrabody”,says Rabbitts.

Intrabodies offer the cell biologist the prospectof highly specific tools that can probeprotein–protein interactions in situ more delicatelythan techniques that simply remove one of theproteins from the scene by antisense or geneknockout. Because an intrabody binds to a specificsite on a protein, it can block a protein’s interactionwith one partner while allowing it to continue actingwith another.

Another use is for the validation of potential drug targets, one of the firstand vital steps in drug discovery. Most potential target proteins are modular,so intrabodies that bind to and block the function of individual modules couldhelp to determine which part of the protein is key to causing the disease.

Placing localization signals on an intrabody allows it to be directed tovarious parts of the cell, such as the nucleus. “With intrabodies you can bemore versatile in your experiments and ask more questions than, say, withRNA interference,” says Rabbitts. P.M.

ANTIBODIES WHERE YOU WANT THEM

Iclectus: the inside story.

Frozen phage: CAT’s library in store.

Katherine Bowdish: getting to grips with anthrax.

© 2003 Nature Publishing Group