Biology Reference
In-Depth Information
been associated with an increase in the frequency of intestinal mucosal
ASCs producing parasite-specific IgA
5
and with mast cell-mediated
immediate hypersensitivity responses to parasite antigens.
4,32
Non-
specific immunological mechanisms may also have a role: pigs previously
infected with transmissible gastroenteritis (TGE) virus were resistant to
infection.
33
Intestinal immunity to A. suum eggs does not appear to be
directly related to the number of adult worms in the host because removal of
adults by anthelmintic treatment or the direct transplantation into the
intestine did not affect protective immunity following challenge with A.
suum eggs.
17
Immunology of Ascariasis in Humans
Chronic infections with A. lumbricoides in humans are associatedwith the
production of high levels of specific and non-specific antibodies of all iso-
types and IgG subclasses
34,35
and a cytokine response characterized by the
production of Th2 cytokines (i.e. IL-4, IL-13, and IL-5) by peripheral blood
monocytes (PBMCs) and leukocytes (PBLs in whole blood cultures).
36
e
38
Other Th2 effector responses are also prominent during infection, reflected
by elevated numbers of peripheral blood eosinophils
39
and increased
expression of eosinophil degranulationproducts.
10
The production of IFN-
g
by PBMCs/PBLs stimulated with Ascaris antigens is not prominent in
ascariasis
37,38,40,41
but IL-10 production may be increased in infected indi-
viduals.
37,42
An increased production of IL-10 has been observed also to
occur spontaneously (i.e. in the absence of antigen stimulation) by PBMCs/
PBLs of individuals with chronic infections,
10,38
an observation previ-
ously made for other chronic helminth infections.
43
e
45
Albendazole treat-
ment of individuals co-infectedwithHIVand A. lumbricoides was associated
with a decline in plasma IL-10 compared to co-infected individuals
receiving placebo,
46
providing further evidence that IL-10 may be upregu-
lated non-specifically in infected individuals.
IL-10 is considered to be a key cytokine mediating immune regulation
during chronic helminth infections
10,38,43
e
45,47
and the combination of
elevated Th2 cytokines with IL-10 has been referred to as a modified or
regulated Th2 response.
48,49
Another cytokine that has been associated
with immune regulation during chronic helminth infections is TGF-
b
43
:
evidence for the increased expression of this cytokine during ascariasis is
inconsistent with some studies suggesting increased production or
expression of TGF-
b
1
34,47
but other authors have observed no such
effect.
10,41
It should be remembered that individuals living in endemic
areas are often infected with more than one soil-transmitted helminth
(STH) parasite and many children with A. lumbricoides, particularly those
with high parasite burdens, have a high probability of recent or current
STH co-infections (see Chapter 4). This makes it more difficult to attribute
