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The Skaggs Institute
for Chemical Biology
Expression of Therapeutic Antibodies in Eukaryotic Algae
S.P. Mayfield, M.V. Beligni, A. Manuell, M. Muto, M. Tran, D. Siefker
With the advent
of genomics and proteomics technologies, the discovery of new protein therapeutics
has greatly accelerated during the past few years. Although these discoveries enable
us to treat more diseases, protein-based therapeutic agents are expensive, limiting
the development of these new drugs and reducing the number of patients with access
to them. Currently, most protein therapeutic agents, and all therapeutic antibodies
(i.e., monoclonal antibodies), are produced by culturing mammalian cells. Capital
costs for production facilities can run into the hundreds of millions of dollars,
and culture media are also expensive, making monoclonal antibodies the most expensive
drugs on the market.
To
expand our repertoire of protein-based therapeutic agents and to help reduce the
cost of these drugs, we have developed eukaryotic algae as a system for expression
of therapeutic proteins. Using this system, we have expressed a human antibody that
was developed to block the effects of anthrax toxins and reduce mortality due to
anthrax. This antibody is a human IgG1 that was identified in serum samples from
soldiers who had been vaccinated against anthrax. The antibody binds to anthrax
protective antigen 83 at high affinity and was previously shown to prevent anthrax
toxins from entering cells in both cell-based assays and animal models. The antibody,
83K7C, was developed at Alexion Antibody Technologies, San Diego, California, and
Alexion has provided 83K7C purified from mammalian cells so that we can directly
compare the activities of antibodies expressed in mammalian cells with those of
antibodies expressed in algae.
Genes that encode the heavy- and light-chain
proteins of 83K7C were transformed into the eukaryotic green alga Chlamydomonas
reinhardtii, and a transgenic line was produced in which both proteins accumulated.
Comparison of 83K7C proteins expressed in the mammalian system with those expressed
in the algal system indicated that both heavy- and light-chain proteins accumulate
at the expected sizes in both systems. We also found that heavy- and light-chain
proteins assembled into correct functional antibodies in algae and that the 83K7C
expressed in algae were able to bind their target antigen, protective antigen 83
of anthrax toxin, at affinities similar to those of the Alexion antibody. These
data establish that antibodies expressed in algae can form functional antigen-binding
molecules and that 83K7C expressed in algae binds with affinity similar to that
of 83K7C expressed in mammalian cells. The next phase of this project is to purify
enough 83K7C expressed in algae to begin cell-based and animal trials for anthrax
neutralization with the antibody. These tests should begin within the next year.
In addition to expressing the anthrax
antibody, we are also expressing other therapeutic antibodies, including those used
as anticancer agents. CD19 is a cell marker found on B-cell cancers, including those
related to childhood lymphomas. We have expressed an antibody to CD19 in our algal
system. This successful expression will enable us to engineer unique anticancer
antibodies, including antibody-toxin fusion proteins that can only be made in this
system.
Publications
Fletcher, S.P., Muto, M., Mayfield, S.P. Optimization of recombinant
protein expression in the chloroplast of green algae. In: Transgenic Microalgae
as Green Cell Factories. León, A., Gaván, A., Fernández, E. (Eds.).
Landes Bioscience, Austin, TX, in press.
Franklin, S.E., Mayfield, S.P. Recent developments in the production of human therapeutic proteins in eukaryotic algae.
Expert Opin. Biol. Ther. 5:225, 2005.
Manuell, A.L., Mayfield, S.P. A bright future for Chlamydomonas. Genome Biol. 7:327, 2006.
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