Biology Reference
In-Depth Information
INTRODUCTION
Despite the global health impact posed by Ascaris lumbricoides, there
have been relatively few laboratories that have been devoted to under-
standing the biochemical, physiological, and immunological interactions
between the parasite and its host. The fact that there exists a closely
related species, Ascaris suum, in a similarly-sized host species, the pig, has
stimulated research on understanding the human infection, and has also
led to the advent of materials and reagents, and the development of ideas
that may be relevant to human populations exposed to both species.
Genomics, transcriptomics, and proteomics are set to enhance the power
of investigations into Ascaris and ascariasis enormously, and will hope-
fully extend the discoveries and associated questions that have arisen
already. This chapter is primarily intended to summarize findings on the
proteins that are produced by Ascaris in its interaction with its hosts, but
also to explore and extend ideas that may have relevance to this and other
nematodiases.
STAGE-SPECIFIC SURFACE
AND SECRETED ANTIGENS
The antigens of Ascaris and all parasitic nematodes could be crudely
placed into four categories
those presented on the surface of the
parasites, those that are secreted (excretory/secretory; ES), those released
during molting, and those usually confined to the interior of the parasites
and only released upon death of a worm. For species with tissue-parasitic
larval stages, such as A. suum and A. lumbricoides, all four classes are
potentially important to protective immune responses, evasion thereof,
and immunopathology.
The secreted proteins of the infective and later tissue-migratory larvae
are different from those of other genera of nematodes,
1,2
although they do
cross-react immunologically with the antigens of other ascaridids such as
Toxocara canis and Anisakis simplex.
3,4
The original work, some of which
used radio-labeled materials, showed that ES comprises a set of secreted
(glyco)proteins that change as the larvae migrate through the tissues to
arrive at the lungs.
1,5
Our unpublished work shows that the most abun-
dant secreted proteins are glycosylated, that some occur as disulfide-
bonded heterodimers (
Figure 3.1
), and that larvae in in vitro culture can
be made to synthesize and secrete the same set of antigens as observed by
radio-iodination of secreted materials (F. Qureshi and M.W. Kennedy,
unpublished). The surface antigens of Ascaris larvae have also been found
to change during their migration and development from infective larvae
(formerly designated second stage larvae, L2, now recognized to be third
e
