Immune protection analysis
Next, the ability of the two mutants (A168N and D201G) to break through
the immune protection induced by the parent strain was analyzed by
avaccinal protection test. There were no unexpected deaths observed
during the study in all groups. There was also no infection in immunized
control chickens (Vaccine_control group) and blank control group. No
significant difference in the proportion of sick chickens with
respiratory signs were observed (including cough and mouth breathing)
among the 3 challenge control groups (CQY-2014, A168N and D201G group)
at 3 ~ 6 day-post-challenge (d.p.c), which were between
50% ~ 70%. At 14 d.p.c, the pathological lesions of
chickens in each of the 3 challenge and control groups were also
similar, and about 20% ~ 30% of chickens had
intestinal and tracheal congestion or hemorrhage. The results suggest
that the 2 key mutations (A168N and D201G) influencing antigenicity did
not exert a significant effect on H9N2-AIV pathogenicity.
Comparing the data between the 3 immune-challenge groups, it can be
observed that the D201G mutation [Vaccine_(D201G) group] has the
potential ability to break through the immune protection conferred by
the parental virus. Recovery of challenge virus remained at 60% and
20% at 5 and 7 d.p.c, respectively (Figure 4 B and C). During the study
30% of chickens presented with signs of respiratory disease in
Vaccine_(D201G) group. Groups challenged with other mutants had a
significant decrease in re-isolation rate (20% ~30%)
at 5 d.p.c. At 7 d.p.c., no virus was isolated and no clinical signs or
pathological impairments were observed (Figure 4). The results suggested
that the D201G mutation not only changes the antigenicity of H9N2-AIV,
but also confers the ability to escape the host immune responses.