News and Publications
The Skaggs Institute for Chemical Biology
Scientific Report 1998-1999
Expression of Antibodies in Microalgae
S.P. Mayfield, E. Efuet, C. Fong, M. Gonzalez, D. Hogan, H.-C. Hung, B. Ngo,
We express antibodies in algae for several reasons. First, because of the
emergence of bacteria resistant to antibiotics, alternative treatments are essential
to maintain the present standards of health care. Treatment of bacterial infections
with antibodies is quite effective, but the cost of producing antibodies makes
such treatment prohibitively expensive. In addition, antibodies have the potential
to block viral infection, something that antibiotics, in general, cannot do.
Currently, a number of recombinant protein expression systems are available
for the production of antibody proteins. Each system has distinct advantages
in terms of protein yield, ease of manipulation, and cost of production. Microalgae
are one group of organisms that have received little attention concerning their
use as bioreactors for the production of recombinant proteins. Yet, use of microalgae
in such an endeavor has several advantages. These include relatively fast growth
rates; high cell densities; extremely large culture volumes, in excess of 500,000
L; low risk of contamination by viruses, prions, or endotoxin; low media costs;
the ability to transform chloroplast, nuclear, or mitochondrial genomes in a
variety of genetic backgrounds; and rapid progression from initial transformants
to the large-scale production of proteins.
For many recombinant antibodies, currently available methods of production
are inefficient or are simply too costly, a situation that has severely limited
the wider use of these proteins in a variety of applications. Functional antibodies,
for example, are hard to produce in prokaryotic expression systems, because these
complex proteins tend not to fold correctly in these systems. Although some antibodies
produced from mouse cell cultures are used as therapeutic agents in oncology,
antibodies produced in this manner are simply too expensive to be of practical
use in fields such as biocatalysis or bioseparation (purification of stereoisomers)
or as therapeutics for a number of nonlife-threatening human diseases.
The use of antibodies in these endeavors could be quite effective if the high
cost of production could be reduced.
We are engineering strains of the green alga Chlamydomonas reinhardtii to
express antibody genes in both the chloroplast and the cytoplasmic compartments
(Fig. 1). We have expressed several truncated versions of antibodies, known as
single-chain antibodies, in the chloroplast. These antibodies include ones that
recognize herpes simplex virus proteins and tetanus toxin and an antibody that
is a catalytic aldolase. We have also constructed vectors for the expression
of larger antibodies composed of both a light-chain protein and a heavy-chain
protein that will export these antibodies from the cell. The vectors designed
for the export of antibodies should direct the accumulation of functional antibodies
to the culture medium, allowing easy purification of the protein.
In general, antibodies expressed in algae have properties similar to those
of the original antibodies, which were expressed in either mouse or human cells.
Some of the single-chain antibodies we have expressed accumulate to relatively
high levels in transgenic strains and appear to be stable within the cells. These
studies indicate that pharmacologically important antibodies can be produced
in algae at high levels and that these antibodies act exactly as do antibodies
produced in more traditional and expensive systems. Antibodies expressed in algae
should allow easy purification and should lack many of the undesirable toxins
or mammalian viruses that can potentially contaminate antibodies expressed in
bacterial or eukaryotic cell cultures.
Bruick, R., Mayfield, S.P. Light-activated translation of chloroplast
mRNAs. Trends Plant Sci. 4:190, 1999.
Bruick, R., Mayfield, S.P. Processing of the psbA 5´ untranslated
region in Chlamydomonas reinhardtii depends upon factors mediating ribosome
association. J. Cell Biol. 143:1145, 1998.