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ANTIGENIC VARIATION IN MALARIA 107
host and parasite. Differences in the severity of
malaria infections can probably be attributed
to genetic variation affecting both the virulence
of the parasite and the susceptibility of the individual
human host.
Major variant antigens
Antigenic variation in P. falciparum manifests
itself in one well characterized family of cytoadherence
proteins, and probably in other so
far less characterized protein families, which
are synthesized by the parasite and inserted
into the surface membrane of the infected erythrocyte.
There is great diversity of these major
cytoadherence proteins within a single malaria
clone, within an infected individual, and
between different infections in the same or in
different people. The best characterized protein
responsible for antigenic variation in malaria is
called PfEMP1, for P. falciparum-infected erythrocyte
membrane protein 1, encoded by the
var gene family, discovered in 1995. There are
probably other variant EMPs, and corresponding
gene families, that contribute to cytoadherence
and antigenic variation. One additional
recognized family of variant erythrocytesurface
proteins is encoded by the rif genes.
Additional candidates will probably emerge
via the P. falciparum genome project. Unlike
VSGs, which are 50-kDa and GPI-anchored,
PfEMP1s are 200–350-kDa transmembrane
proteins. They are localized on the erythrocyte
surface to regions known as ‘knobs’, because
of their physical appearance in the electron
microscope. Additional proteins, on the cytoplasmic
face of the membrane, are responsible
for the characteristic ‘knobby’ appearance of
trophozoite and schizont-infected erythrocytes.
As with VSGs, some aspects of PfEMP1
structures are highly conserved, although there
is great sequence variation in the extracellular
domain, which contributes to their different
host-cell receptor specificities. Three major factors
delayed the identification of PfEMP1, relative
to the trypanosome VSG: P. falciparum
could not be cultivated in the laboratory until
the mid-1970s, the abundance of PfEMP1 is
very low, and the population of malariainfected
erythrocytes is antigenically heterogeneous,
although less so in culture-propagated
clones. By sequencing the DNA of subtelomeric
domains, a large family of variant
(var) genes was recently identified in P. vivax.
Working from the sequence information, but
still unable to culture the parasite, it was possible
to design probes to detect the corresponding
proteins on erythrocytes from infected
patients, and to demonstrate the presence of
corresponding circulating antibodies. There is
no sequence similarity between the major variant
erythrocyte surface proteins of P. falciparum,
P. vivax and P. knowlesi.
Regulation of PfEMP1 expression
Individual malaria parasites contain about
150 var genes but, as with the trypanosome
VSGs, the var gene repertoire of different isolates
is very diverse, so the total circulating
repertoire may be essentially infinite. In contrast
to trypanosomes, malaria has an obligatory
sexual stage in the mosquito phase of its
life cycle, which increases the diversification
of var genes. As with trypanosome telomeres,
most of the telomeres of the 12 chromosomes
of the haploid blood-stage P. falciparum have
a conserved structure and harbor telomereproximal
var genes. The remaining var genes
are dispersed around the genome, either singly
or in small clusters. Although many, and perhaps
all, var genes are transcribed by individual
parasites in the first hours following erythrocyte
invasion, all but one are subsequently
MOLECULAR BIOLOGY