Abstract
This thesis describes studies that were undertaken to evaluate
in vitro cell-mediated and humoral immune mechanisms in bovine tropical theileriosis caused by Theileria annulata, and to identify
target cells infected by Theileria annulata and Theileria parva
sporozoites.
Bovine peripheral blood leucocytes (EBL) were fractionated into
B-enriched and T-enriched cell populations by nylon wool adherence,
plastic surface adherence or E-rosette sedimentation. Evaluation of
the infectibility of fractionated cell populations and EBL by Theileria annulata sporozoites in vitro showed that T. annulata selectively
infects and establishes in adherent cells, a significant proportion
of which exhibit Fc receptors, but these infected cells did not
appear to secrete immunoglobulin. Identification of the cell types
in T. parva and T. annulata cell lines using immunofluorescence and
rosette assays showed that T. parva cell lines exhibited T cell sur¬
face markers whereas T. annulata cell lines did not. Moreover, a
significant percentage of cells in T. annulata cell lines had Fc
receptors indicating that these parasites infect different lymphocyte subpopulations.
Optimal conditions for the production of bovine T cell growth
factor (TCGP) were established. A combination of Concanavalin A
(Con A) and bovine spleen cells yielded more potent TCGP than a
combination of Con A and EBL. The TCGP thus produced was used to
maintain T cell blasts in culture for about two months.
Cell-mediated immune responses in bovine tropical theileriosis
were studied using a cell culture technique. This technique
allowed for evaluation of cell-mediated responses in an autologous
system, in that cultures were initiated in vitro using EBL from
an animal before infection and these were later used as targets
for effectors from the same individual following infection.
Results of this preliminary study showed that cells whose cytotoxicity was restricted to autologous cell lines were generated
only in calves that had been successfully infected and immunized
against the parasite thus suggesting a role for geneticallyrestricted cytotoxicity in recovery from and immunity to theileriosis.
Non-genetically restricted cytotoxic cells were irregularly
generated in vitro following initiation and maintenance of an
autologous mixed lymphocyte reaction for five days in growth
medium. Addition of crude bovine TCGF to this culture system did
not enhance the generation of cytotoxic cells.
Sporozoite infectivity for bovine lymphocytes in vitro was
specifically inhibited by serum taken from cattle hyperimmunized
with stabilates of T. annulata sporozoites. There was no synergistic inhibitory effect when sporozoites were exposed to both
antisera and PBL from immune calves.
It was suggested that immunity against bovine theileriosis
is an interplay between humoral and cell-mediated .immune mechanisms.
Antibodies may inhibit entry of sporozoites into the host's target
cells and their subsequent transformation and cytotoxic cells
kill schizont-infected cells.
