trans

IMMUNE RESPONSES TO TRYPANOSOME INFECTION 103
of ‘genome survey sequences’. With only one
exception reported so far, the genes of T. brucei
do not contain introns, so genomic sequences
are easy to interpret and utilize.
The number of ESs probably varies between
trypanosome isolates. In one clone, the number
was determined to be around 20, based on
DNA hybridization with an ES promoter probe
and the assumption that promoter duplications
occur in about half the ESs. The number
of metacyclic ESs has been estimated to be
20–30, based on a different criterion. The metacyclic
form is non-dividing and represents a
terminal differentiation stage in the tsetse salivary
gland, where the VSG repertoire was enumerated
using a library of monoclonal anti-VSG
antibodies. At present, it seems likely that the
total number of bloodstream and metacyclic
ESs will vary from strain to strain, owing to the
intrinsic instability of their telomeric location.
The fitness and virulence of individual strains
may depend partly upon the diversity of their
ESs, either at the metacyclic stage or in the
bloodstream, as discussed above in the context
of the transferrin receptor.
VSG evolution almost certainly occurs by
conventional processes of gene duplication
and mutation, although this has not been
specifically studied. The only way in which the
utility of mutations can be evaluated is for
the VSG to be expressed and exposed to the
immune system. Because expression involves
either ES switching, telomere exchange, or transposition
of a ‘chromosome-internal’ VSG to the
telomeric ES, which appears to be a unidirectional
event, this raises the question of how the
trypanosome retains a new VSG after ‘testing’
its value. One obvious way would be to use
reverse transcription of the VSG mRNA, to
recapture a copy of a useful new VSG into a
safer chromosomal environment. It seems likely
that trypanosomes did this in the past, even
if they do not appear to do it today. The
trypanosome genome contains hundreds of
copies of apparently dead retrotransposable
elements and probable reverse transcriptase
pseudogenes. Many copies of these elements
appear to be interspersed with VSG genes and
pseudogenes, often in the long and essentially
haploid regions that lie upstream of ESs, in the
regions that appear to account for a great deal
of the length variation between sister chromosomes
that is a feature of T. brucei. Some reports
have suggested that diversity could be generated
concomitantly with VSG duplication and transposition
to the ES, possibly by an error-prone
repair or replication mechanism, or perhaps
because mutations were introduced when
replicating silent ES-located VSGs that were
lightly modified by J. An alternative explanation
of these controversial data, which has
not been tested, is that these mutations were
not generated concomitantly with the overt
VSG expression event that was studied, but
that this VSG had been expressed at subliminal
levels during the infection and subjected
to mutation and immune selection over a
period of time. Simple calculations and computer
modeling suggest that undetectable
(by the usual techniques) low-level expression
of many variants will occur during the course
of a trypanosome infection, although each
peak of parasitemia contains only a few overt
variants.
IMMUNE RESPONSES TO
TRYPANOSOME INFECTION
There are several pathological manifestations
in trypanosomiasis, such as anemia and
hypocomplementemia, whose detailed discussion
is beyond the scope of this review,
save to mention in passing that some of the
pathology may be attributable to the escalating
antibody responses to the ever-changing
MOLECULAR BIOLOGY