Humoral and cell-mediated immune responses of sheep infected
with Cytoecetes phagocytophila, the causative agent of tick-borne
fever, were studied. Previous immunological studies were hampered
by lack of reliable sources of antigen. Attempts, therefore, were
made to improve antigen yields. Two methods were used successfully.
The first was based on the properties of corticosteroids to induce
granulocytosis; sheep injected with betamethasone sodium phosphate
injection BP at the peak period of parasitaemia had a neutrophilia
with an increase of infected cells within six hours. The second
method involved the culture of infected blood overnight at 37°C
which increased the number of infected cells and the number of
organisms per infected cell.
A relationship between complement fixing antibodies and pro¬
tective immunity was found and a threshold of protective immunity
established. The kinetics of antibody responses to the organism
were studied by fractionating immunoglobulins of sera from immune
sheep by gel-filtration and ion-exchange chromatography and by
complement fixation test. Antibody response to _C. phagocytophila
was characterised by an initial production of IgM followed by IgG
but the IgM persisted for long periods.
The complement fixation test was also used to assess the
antigenic relationships between strains of Ch phagocytophila. All
strains tested showed strong antigenic relationships but quantitative
differentiation was possible.
A hitherto undescribed in vitro test for cell-mediated immune
response of sheep was developed. The cell-mediated immune
response appeared earlier than the humoral immune response.