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Infectious Laryngotracheitis Virus | 331

(Fahey et al., 1983; Fahey and York, 1990). Furthermore, no dif- that different chicken breeds will have different susceptibilities to
ferences were observed in levels of ILTV infection of the trachea, ILTV infection (Loudovaris et al., 1991a,b; Poulsen et al., 1998),
detected by immunofluorescence, between vaccinated bursec- and that a larger proportion of monocytes/macrophages isolated
tomised and intact chickens, despite detectable differences in from chickens that are resistant to ILTV get infected with ILTV,
the levels of local IgA and IgY between both groups (Fahey and and vice versa.
York, 1990). Passive transfer of hyper-immune anti-ILTV serum Studies that have examined host-gene transcription during in
produced high levels of humoral antibody in the recipients but vitro (Lee, J. et al., 2010, 2012) and in vivo ILTV infections (Luo et
did not result in significant differences in terms of clinical disease al., 2014; Vagnozzi et al., 2016; Coppo et al., 2018; Vagnozzi et al.,
when recipient chickens were inoculated with virulent ILTV 2018) have revealed that a number of innate immune responses
compared with those that received serum from uninfected SPF are induced in primary sites of viral replication (in vitro), includ-
chickens prior to ILTV inoculation (Fahey et al., 1983). Taken ing induction of cellular defence mechanisms, cell cycle regulation
together, the results from all these studies indicated that humoral and matrix metalloproteinases (Lee et al., 2010; Coppo et al.,
or mucosal antibodies were not important for protection against 2018). In vivo, the responses measured at late time points in the
ILTV during first exposure or after vaccination and challenge. acute phase of infection (6 days pi) were more associated with the
Early studies using adoptive transfer of different types of orchestration of adaptive immune responses, with induction of
immune cells between histocompatible chickens, determined pathways involved in antigen processing and presentation, posi-
that splenocytes obtained from immunized chickens, but not tive regulation of immune processes, activation of leucocytes and
from naïve chickens, was capable of conferring protection against lymphocytes, T-cell activation and T-cell receptor signalling (Luo
virulent ILTV challenge. Similar results were obtained through et al., 2014). These findings are consistent with the histopatho-
the transfer of PBMCs from immunized chickens, but not cells logical characteristics of ILTV infection of the upper respiratory
+
obtained from the bursa of Fabricius or the thymus (Fahey et al., tract, where infiltration with CD4 , CD8 and B cells has been
+
1984). These results highlighted the importance of ILTV-specific observed at 4 days pi (Devlin et al., 2010).
cell-mediated immunity in the containment of ILTV infection, Recent studies have revisited the local immune responses
and, as the authors speculate, would suggest that non-specific at the trachea after CEO vaccination and subsequent challenge
immune cells present in the spleen of unsensitised chickens, (Vagnozzi et al., 2016), and have shown that a rapid interferon
such as NK cells or monocytes, would be less important. How- gamma response (within 6 hours after challenge) occurs concom-
ever, recent studies that have examined the anti-viral effects of itantly with a decrease in viral gene transcription. The effector
toll-like receptor agonists applied in vitro to primary monocytes cells involved in this immune response have not yet been iden-
or to monocyte/macrophage cell lines (MQ-NCSU or HD11), tified, but a similar response was not observed in unvaccinated
or in ovo, have demonstrated that non-specific stimulation of chickens, hinting that resident effector memory T-cells may be
the immune system induces anti-viral mechanisms, mediated involved in this response. More recent studies have determined
by monocytes/macrophages, that can reduce ILTV infection of cytokine transcription in a range of tissues of the upper respira-
cells and chicken embryos before hatch, and chicks after hatch tory tract following infection with ILTV and found that the peak
(Haddadi et al., 2013, 2015; Thapa et al., 2015; Abdul-Cader et al., of pro- (chCXCLi2, IL-1β, IFN-γ) and anti-inflammatory (IL-10,
2018; Ahmed-Hassan et al., 2018). The role of monocytes/mac- IL-13) cytokine expression occurred concomitantly with peak
rophages during in vivo ILTV infection has not been thoroughly leucocyte infiltration and tissue damage of the mucosa (Vagnozzi
investigated. et al., 2018). These findings are consistent with those by Coppo et
The study reported by Fahey et al. (1984) also indicated that al. (2018) who determined that inflammation and peak cytokine
ILTV infection of thymectomized chickens resulted in fulminant transcription were the main drivers of tissue damage in vivo
clinical disease and death, however, the data to support these (Coppo et al., 2018). Interestingly, Vagnozzi et al. (2018) also
observations was not presented. Further studies using bursec- found that a rapid and transient up-regulation of IFN-y occurred
tomised and thymectomized chickens by Honda et al. (1994) in the Harderian gland. This, combined with findings by Beltrán et
reached similar conclusions to those of Fahey et al. (1983, 1984) al (2017) who found that the Harderian gland may be a key player
and Fahey and York (1990), in that bursectomised chickens in the uptake of ILTV during acute infection, point towards this
were capable of mounting protective immune responses against gland having an important immunological role in the response
ILTV vaccine and/or subsequent challenge, and that thymec- against ILTV.
tomized chickens were less well protected after vaccination and
subsequent challenge than untreated birds (Honda et al., 1994).
Again, adoptive transfer of thymocytes or cells from the bursa of Clinical features, diagnosis and
Fabricius failed to protect chickens against ILTV but transfer of epizootiology
spleen or PBMCs from immunized birds did confer protection This section presents an update on recent advancements in the
against disease, and interestingly, one chicken (1/10) was also area of ILTV and on some aspects of ILT where a gap exists in
protected after transfer of splenocytes from unvaccinated chick- the current literature. Basic concepts of ILT can be found in
ens. Further support for the role of innate immune responses in other references (Tripathy and García, 2008; García et al., 2013a;
the containment of ILTV infections comes from the observations Menendez et al., 2014; Nair et al., 2017).