Adaptive Immunity 157
genome-wide analysis revealed that CD4+ T cells
in latently infected humans recognize a much
wider range of antigens (Tang et al., 2011; Lind-
estam Arlehamn et al., 2013; Commandeur
et al., 2013), many of which were cryptic epit-
opes. The majority of these were recognized by
CXCR3+CCR6+ IFN-γ- expressing Th1 cells
(Lindestam Arlehamn et al., 2013). Further
understanding the nature of the T-cell response
and its regulation by M. tuberculosis and M. bovis
is needed for the design of vaccines and regimes
that protect the lung effectively.
11.1.3 Regulation of CD4+ T-cell
responsesTissue-specific CD4+ T-cell responses are likely
limited by other cell populations or immunoreg-
ulatory cytokines such as IL-10 and TGF-β.
These pathways are important to limit immuno-
pathology, but they may also contribute to the
persistence of bacilli in bovine lymph nodes
(Widdison et al., 2006). Regulatory T cells
(Tregs) proliferate rapidly in M. tuberculosis-
infected lymph nodes (Shafiani et al., 2010) and
limit priming and activation of activated effec-
tors in the lung. Depletion of Tregs from mice
increased Th1 priming and reduced bacterial
burden (Scott-Browne et al., 2007). Tregs
induced by infection may also overcome the pro-
tective effects of BCG vaccination and reduce
recruitment of CD4+ and CD8+ T cells to the
lungs of mice (Ordway et al., 2011). While
FoxP3+ Tregs have not yet been described in
bovine TB, they were described to expand in
calves infected with M. avium subsp. paratubercu-
losis and hypothesized to limit secretion of IFN-γ
by Th1 CD4+ T cells (Bull et al., 2014).
11.1.4 Effector memory T-cell and central
memory T-cell subsetsImmune protection against a range of diseases
relies upon the induction and maintenance of
memory cells capable of responding rapidly and
efficiently upon subsequent infection. This
immunological memory may be induced follow-
ing natural infection or vaccines. Upon the first
encounter with antigen the number of specific T
lymphocytes increases over tenfold (Hou et al.,
1994; Murali-Krishna et al., 1998; Whitmire
et al., 1998; Pollock et al., 2001). This is associ-
ated with differentiation into effector cells which
express important molecules for pathogen con-
trol including TNF-α, IFN-γ, perforin and granu-
lysin. Following this, the majority of T cells
undergo apoptosis and only a few memory cells
will develop (Wilkinson et al., 2009; Totté et al.,
2010). In humans, memory T-cell subsets have
been defined based on cell surface antigen
expression. Central memory T cells (Tcm) have
the phenotype CD62L+CCR7+, preferentially
localize to lymphoid tissues and secrete princi-
pally IL-2. By contrast, effector memory T cells
(Tem) are CD62L–CCR7– and secrete only low
levels of IL-2 upon stimulation (Sallusto et al.,
1999; Champagne et al., 2001; Woodland and
Kohlmeier, 2009; Sallusto et al., 2010). In cattle,
populations of Tm have been similarly identified
by expression of CD45RO, CD62L and CCR7
(Blunt et al., 2015; Maggioli et al., 2015a), con-
trasting with some of the earlier observations
that CD62L is not a memory marker in cattle
(Howard et al., 1992), potentially indicating that
induced expression of CD62L is indicative of
antigen exposure.
The secretion of IFN-γ by effector T cells
and Tem in response to stimulation with myco-
bacterial antigens is the read-out of interferon-
gamma release assays, and this correlates with
infection with mycobacteria in both humans
and cattle. In addition, in a number of studies
the frequency of Tcm (measured by ‘cultured
ELISPOT’, which detects antigen-specific IFN-γ
release in long-term cultures of T cells; Maggioli
et al., 2015b) correlates with vaccine-induced
immunity measured as a reduction in bacterial
burden and tissue pathology (Vordermeier et al.,
2006, 2009; Hope et al., 2011). Studies with
samples from human patients have demon-
strated that the responding cells within these
long-term cultures (up to 14 days) are mainly
Tcm and that this response, in contrast to effec-
tor responses, correlates with protection from
infectious challenge (Todryk et al., 2009). In
cattle, Tcm (CD45RO+CCR7+CD62Lhi) are the
primary cell type responding in long-term cul-
tured IFN-γ ELISPOT responses to M. bovis infec-
tion (Blunt et al., 2015; Maggioli et al., 2015a).
The functional relevance of Tcm in driving natu-
ral or vaccine-induced protection remains to be