210 B.M. Buddle et al.
TB protein vaccines in cattle has been the diffi-
culty of inducing strong cellular immune
responses with these vaccines, despite co-
administration with a range of adjuvants and
immunomodulators. The use of virally-vectored
TB vaccines has shown considerable promise
when applied in heterologous prime-boost
approaches in which the bovine immune
response is primed with BCG and then boosted
with virally-vectored vaccines developed for
use as human TB vaccines. Priming with BCG
Danish and boosting with a replication deficient
human adenovirus 5 expressing Ag85A
(Ad85A) resulted in protection superior to that
with BCG alone (Vordermeier et al., 2009). In
a recent study, BCG-vaccinated calves were
boosted with either Ad5 expressing Ag85A
(Ad5-85A) or Ag85A, Rv0287, Rv0288 and
Rv0251 (Ad5-TBF), but only those boosted with
Ad5-85A induced a significantly lower histo-
pathological lesion score than that for those
vaccinated with BCG alone (Dean et al., 2014a).
From an immunogenicity study, the optimal
dose and route of immunisation of the Ad5-85A
used as a boost following a BCG prime was deter-
mined to be 2 × 109 infectious units delivered
intradermally (Dean et al., 2014b).
It has also been shown that Ad85A boost-
ing of BCG delivered by mucosal (endo bronchial)
or systemic (intradermal) routes induced
comparable peripheral blood responses, and
bronchioalveolar lavage cells producing
antigen-specific IFN-γ (Whelan et al., 2012).
Furthermore, when calves were vaccinated at
the same time with BCG via the systemic route
and Ad85A via mucosal (endobronchial)
application, there was a trend towards better
protective efficacy than for vaccination with
BCG alone (Dean et al., 2015).
14.2.3 Correlates of protectionAn impediment in the development of improved
TB vaccines for animals is that no single corre-
late of protection has been identified for TB,
although a number have shown promise in this
regard. Currently, it is still necessary to assess
protection against TB by challenging animals
with virulent mycobacteria. Protection against
an intracellular pathogen such as M. bovis is
largely dependent on a T-cell-mediated immune
response, and the most promising correlates of
protection include the quality of the immune
response defined by the magnitude and profile of
the cytokine response and induction of memory
responses.
Early IFN-γ responses are required post-
vaccination, so the timing of testing is impor-
tant, but the magnitude of the IFN-γ responses
does not always correlate with protection
( Buddle et al., 2003; Wedlock et al., 2007). A
search has been initiated for alternative or
additional cytokines to serve as correlates for
protection. In this regard, IL-17 and IL-22
responses to mycobacterial antigens have looked
promising in cattle (Rizzi et al., 2012; Bhuju
et al., 2012; Waters et al., 2015). IL-17 can be
produced by a range of T cells (TH17, γ∧ and NK
cells) and is considered to be important for the
accumulation of protective memory cells in the
lungs and cross-regulation of T-cell subsets
(Waters et al., 2015). Although the role of IL-22
has not been defined, one potential effector
mechanism could be the production of beta-
defensins. In small animal models, the numbers
of polyfunctional T cells have been associated
with protection (Aagaard et al., 2009; McShane,
2009). However, in cattle, the presence of IFN-γ,
IL-2 and TNF-α or combinations of at least two
of these markers did not predict vaccine efficacy
when measured before challenge, but was asso-
ciated strongly with increased pathology post-
M. bovis challenge (Vordermeier et al., 2009;
Whelan et al., 2011a). Detection of a delayed-
type hypersensitivity response to tuberculin is
the primary screening test for diagnosis of TB
in cattle; however, it is not a consistent correlate
for protection post-vaccination, particularly for
the maintenance of protection (Whelan et al.,
2011b). A confusing aspect is that many of
these markers for protection also serve as
indicators of disease post-challenge.
Measurement of T-cell memory responses
in cattle using the cultured ELISPOT method has
been recently shown to be a promising predictor
of vaccine efficacy, with significantly elevated
responses in vaccinated/protected animals com-
pared to matched vaccinated/non-protected
animals (Vordermeier et al., 2009; Dean et al.,
2014a). In addition, maintenance of strong cul-
tured ELISPOT responses was associated with
the duration of immunity post-vaccination