Innate Immune Response in Bovine Tuberculosis 145
Current studies have attempted to identify
how mycobacteria induce autophagy. It has
been demonstrated that autophagy is differen-
tially induced by mycobacterial species; M. smeg-
matis induced a higher autophagy response
than M. bovis BCG. In addition, it is postulated
that mycobacterial lipid components are respon-
sible for autophagy induction (Zullo and Lee,
2012). Cytosolic detection of bacterial products
plays a crucial role in initiating the innate
immune response, including autophagy activa-
tion. The M. bovis-induced AIM2 inflammasome
activation decreases autophagy in immortalized
and primary murine macrophages. This relies
on the inflammasome sensor AIM2 which
conjugates with cytosolic DNA to inhibit the
STING- dependent pathway involved in selective
autophagy. The DNA sensor of IFN-γ inducible
protein 204 (IFI204) plays an important role for
autophagy marker LC3 expression during
M. bovis infection (Liu et al., 2016, 2017).
Hence, a growing body of evidence points to
autophagy as an innate immunity mechanism
that plays an important role in the control of
mycobacterial, including M. bovis, infections.
10.4.2 ApoptosisApoptosis is a type of regulated cell death that
also has been reported to participate in the
innate immune response controlling the growth
of intracellular pathogens, including mycobac-
teria. This type of cell death is characterized by
not generating an inflammatory response.
Morphologically, apoptotic cells exhibit cellular
and nuclear shrinkage, chromatin condensa-
tion, DNA fragmentation and the formation of
apoptotic bodies. Two pathways are known by
which apoptosis is activated during physiologi-
cal or pathological conditions that are depen-
dent or independent of caspase activation; the
extrinsic pathway, which is initiated by ligand
binding, such as via TNF-α or FasL binding to its
respective receptor located on the cell surface,
and the intrinsic pathway activated by intra-
cellular death signals where the mitochondria
plays a fundamental role (Parandhaman and
Narayanan, 2014).
The participation of apoptosis in tuberculo-
sis has been shown in several studies where it
has been demonstrated that mycobacterial
species modulate apoptosis. Virulent strains of
M. tuberculosis and M. bovis were able to inhibit
apoptosis in a mouse model (Hinchey et al.,
2007; Rodrigues et al., 2009). In fact, the expres-
sion of several genes encoding for pro-apoptotic
proteins (e.g. CASP8, CASP7, IDB, CYCS) and
inhibitory for apoptosis (e.g. BCL2A1, CFLAR,
BCL2, BCL2L1, BIRC2, BIRC3, XIAP, MCL1 and
PRKX) was increased at 2, 6, 24 and 48 hours
after infecting bovine alveolar macrophages
with M. bovis, demonstrating the regulation of
genes involved in apoptosis during the early
stages of infection with M. bovis (Nalpas et al.,
2015). Moreover, microarray analysis of
M. bovis-infected macrophages identified an
increased number of downregulated genes com-
pared to uninfected controls, suggesting that
M. bovis infection is associated with host gene
repression (Widdison et al., 2011; Magee et al.,
2012; Nalpas et al., 2015). Macrophages from
healthy and infected animals can both be fully
activated by M. bovis infection, yet there are dif-
ferences between these macrophages: changes
(fold-changes) in global transcriptome induced
by in vitro challenge of M. bovis were higher in
healthy cows than in tuberculosis-positive cows,
suggesting that healthy macrophages responded
marginally better to in vitro infection (Lin et al.,
2015).
Results from different studies using an in
vitro model of macrophage infection have dem-
onstrated that virulent and avirulent strains of
M. bovis are capable of inducing macrophage
apoptosis (Gutiérrez-Pabello et al., 2002; Vega-
Manriquez et al., 2007; Castillo-Velázquez et al.,
2011; Esquivel-Solís et al., 2013). Macrophage
apoptosis was time and multiplicity of infection
(MOI) dependent. Rates of apoptosis, measured
via chromatin condensation and DNA fragmen-
tation, progressed rapidly after infection and
increased significantly post-infection. Also, the
number of bacteria per macrophage had a direct
effect on the apoptotic counts. Macrophages
infected with an MOI of 25 : 1 developed chro-
matin condensation and DNA fragmentation at
4 and 8 hours, respectively, whereas changes
in chromatin condensation induced by MOIs
of 10 : 1 and 1 : 1 required a longer time and
resulted in fewer apoptotic cells. In addition, it
was not only infected cells that underwent
apoptosis but also uninfected bystander cells,