need for BSL3 experimentation, vaccine development programmes
could be significantly accelerated.
An initial step for the development of such a model requires
the establishment of a baseline for the consistent and reliable
recovery of mycobacteria from bovine tissues. Previous attempts in
this laboratory to recover BCG from cattle following s.c. challenge
proved inconsistent. It is thought that following s.c. inocula-tion mycobacteria would migrate to the lymph node draining
the site of inoculation; however, after inoculation, mycobacteria
could disperse within the subcutaneous area and it is possible that mycobacteria could migrate to more than one node. By
using intranodal inoculation, we have reduced the possibilities
of mycobacteria dispersing within the subcutaneous areas and
migrating to nodes other than the lymph node injected. To our
knowledge, the experiment described in Fig. 1 is the first time in
which a time curve, albeit partial to day 21, on the recovery of BCG
from cattle has been reported. Thus, this is the first report for the
relatively consistent recovery of BCG from cattle in quantifiable
numbers.
This protocol was then used to determine whether prior vaccination using BCG SSI would affect the recovery of BCG after
challenge compared to naïve animals in a manner similar to a
standard efficacy vaccine test where virulent M. bovis is used for the
challenge phase. Given the volume of literature and our previous
experience, we decided to use BCG SSI as the test vaccine in these
proof-of-principle experiments. We also decided to harvest lymph
nodes after 2 and 3 weeks as we reasoned that this would be suffi-cient time for immune responses induced by previous vaccination
to have an impact on the control of the BCG challenge and would
maximise our ability to detect differences between vaccinated and
non-vaccinated animals. On a group basis, prior BCG vaccination
did reduce the number of mycobacteria recovered from vaccinated
animals compared to non-vaccinated animals. However, from Fig. 4,
it is clear that there was animal to animal variation in both vaccinated and naïve animals following inoculation with BCG Tokyo. It
is also clear that not all BCG-vaccinated animals were protected
to the same extent. It is possible to divide the animals into pro-tected and not-protected by considering all BCG vaccinates with
cfu counts lower than the animal presenting the lowest cfu counts
in the non-vaccinated group as protected; all other BCG vacci-nates could be considered as not protected. Using this criterion,
4/12 animals would have been protected by BCG vaccination after
2 weeks; at 3 weeks, 6/12 animals would have been protected.
This outcome therefore parallels the outcome of vaccinated animals after challenge with M. bovis, with a proportion of animals
presenting with pathology not indistinct from naïve control animals, and another proportion of animals presenting without or with
significantly reduced pathology compared to naïve cattle