Ticks may immunosuppress their hosts during infestation to facilitate their survival. This suppression may predispose the host to severe concomitant infections by tick home micro-organisms. This thesis examined whether salivary gland extract (SGE) from the cattle tick, Boophilus microplus, suppresses the immune response of cattle to infection by the tick-bome protozoan parasite Babesia bovis.
The effect of SGE on T and B cell proliferation to stimulation with mitogens concanavalin A (ConA), phytohaemagglutinin (PHA), pokeweed mitogen (PWM) and lipopolysaccharide (LPS) was examined. In the absence of SGE, lymphocytes stimulated by PHA and ConA proliferated to the same extent. This response was inhibited by SGE of B. microplus by 39.0 % and 75.4 %, respectively. SGE of B. microplus stimulated the proliferation of B cells in the presence of LPS. Thus showing that subpopulations of lymphocytes are affected differently by SGE.
The effect of SGE from B. microplus on peripheral blood lymphocytes, neutrophils and monocytes was compared between two species of cattle, Brahman (Bos indicus) and Hereford (Bos taurus). SGE significantly inhibited the proliferative responses of lymphocytes to Con A in both Brahman and Hereford cattle (89% and 41%, respectively). Flow cytometric analysis of monocytes and neutrophils showed that SGE significantly reduced both the proportion of cells actively phagocytosing Escherichia coli labeled with fluorescein isothiocyanate (E. coli - FITC), as well as the numbers of E. coli-FITC taken up in Brahman cattle. In Hereford cattle, a significant depression in uptake was only observed in neufrophils. In both species of cattle, the proportion of monocytes and neutrophils with oxidative activity was significantly suppressed in the presence of SGE. These results indicate that peripheral blood leukocytes from different breeds of cattle respond differently to SGE.
The effect of SGE from Y and N strain of B. microplus on the proliferation of lymphocytes in the presence of ConA was examined. Ticks of each strain were collected from different animals as well as from the same animal. A difference in development was observed between the strains, with female ticks of the Y strain developing faster and reaching heavier engorgement weights than those of the N strain. SGE from each strain differentially inhibited lymphocytes in the presence of ConA, but this difference was not detected when the two strains of ticks were reared on the same host. It was concluded that the host has an influence on the composition of SGE, which may in turn influence the inhibitory effect of lymphocyte responses to ConA.
To test the hypothesis that B. microplus suppresses host immunity in vivo and enhances the transmission of B. bovis, tick-infested and tick-free cattle were inoculated with the virulent Calliungal strain of B. bovis at clinical threshold levels. The development and intensity of disease was monitored by measuring rectal temperature, packed cell volume (PCV) and parasitaemia. Host immune functions were assessed by assays for specific antibodies, IFN-γ, reactive nitrogen intermediates, phagocytosis and oxidative burst activity of neutrophils and monocytes, and proliferation of lymphocytes. Tick - infested animals had 8% higher PCV depression, higher maximum temperature and heavier Babesia burdens, and two more animals had to be treated for babesiosis than tick - free animals, although these differences were not significant. On certain days during infection, the tick - infested animals had significantly higher levels of interferon gamma, less intense phagocytosis of neutrophils and monocytes, more neutrophils displaying oxidative burst and more intense oxidative burst of neutrophils than tick - free animals. These results indicate that the tick has an influence on the immune response against Babesia.
Bioactive molecules from SGE responsible for suppression of T lymphocytes proliferation were fractionated by size exclusion high performance liquid chromatography (HPLC) and ion exchange HPLC. Resultant fractions were tested for inhibitory effects on lymphocytes. Three fractions were found to exert inhibitory effects (17.8%, 18.2 and 18.8% inhibition), but the activity could not be attributed to a specific molecule. Further studies are required to identify and purify those elements present in SGE to modulate host immune responses.