This work is a study of the ability of three metacestode
species: Taenia crassiceps, Taenia taeniaeformis and secondary
infections of Echinococcus granulosus to interfere with the host's
immune response.Both mice and gerbils infected with secondary hydatidosis were
found to have a low antibody response to a somatic preparation of
Eh granulosus as detected by ELISA throughout infection despite, in
some cases, the presence of a large cyst burden. The possibility
that this was the result of suppression of the host's immune response
was investigated by studying the response of mice infected with
secondary hydatidosis and also the murine models of metacestode
disease T. crassiceps and T. taeniaeformis, to a subsequent infection
of the haemoprotozoan, Babesia microti. The host susceptibility to
the secondary infection was assessed by the percentage number of red
blood cells that were infected and the serological response to 13.
microti, as detected by IFAT, throughout the infection. All three
metacestode species were found to have enhanced parasitaemias and
consistently lower antibody titres to B. microti than the Babesia
only infected controls. The rate of decline in the parasitaemia
from peak was markedly slower in the concurrently infected mice
indicating that the suppression not only affected the development of
the infection but also the speed of the host's ability to resolve it.Metacestode extracts prepared from the surface of the parasites
have been shown to cause a degree of cytotoxicity when added to a
lymphosarcoma cell line culture. These same extracts and excretory secretory products of the metacestodes also depressed the normal
Con A blastic response of MLNC from both naive and infected donor
mice, to a significant extent. When living hydatid cysts are placed
in culture with MLNC the normal Con A blastic response is again
depressed. The MLNC from infected donors showed a greater depression
of the Con A response than the cells from naive donors. The longer
the period of culture of the MLNC with the hydatid cyst, the greater
the depression of the Con A response. The reverse situation was
found when T. crassiceps metacestodes were cultured with MLNC, as
a greater depression of the Con A blastic response was found when
the cells were exposed to the metacestodes for a shorter period of
culture. This result was difficult to account for and required
repetition.Various mechanisms were proposed for the generalised suppression
induced by metacestode disease. These include antigeniccompetition,
direct cytotoxic effects of parasite-derived factors and interference
by parasite secreted substances with lymphocyte function. It is
likely that several mechanisms account for the observed immuno¬
suppressive effects of metacestode infections on the host but without
further investigation of the nature of the suppressive factors, and
the target cells they act on, no defined interaction between host and
parasite can be postulated
