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et al., 2012). Indirect immunofluorescence using gG-specific cleavage may occur. Deletion of gJ resulted in a reduction of viral
polyclonal antibodies localized this protein in the cytoplasm of titres during multi-step growth assays, but had no real effect on
infected cells (Helferich et al., 2007a). plaque sizes, suggesting gJ may have a role in viral replication but
The UL47 gene encodes a major ILTV tegument protein that not in cell-to-cell spread (Fuchs et al., 2005). Similarly, a differ-
is not essential for viral replication in vitro or in vivo (Helferich et ent gJ-deletion mutant generated from the USDAch ILTV strain
al., 2007b). Originally categorized as a late (γ2) gene by (Kong- did not show any impairment in regard to entry kinetics or viral
suwan et al., 1995; Helferich et al., 2007a) but as an early (EII) replication, but had impaired egress kinetics, as virions tended
gene by (Mahmoudian et al., 2012), the pUL47 was detected to accumulate inside cultured cells and were not released to the
in the cytoplasm and nucleus of infected cells (Helferich et al., allantoic fluid after inoculation of the CAM in embryonated hen
2007a), and its deletion resulted in only a minor delay in viral eggs (Mundt et al., 2011). Glycoprotein J-deletion mutants have
replication and a 10-fold reduction in viral titres, but no signifi- been shown to be attenuated in vivo and capable of conferring
cant effect was observed in plaque sizes when compared to the protective immunity to inoculated chickens (Fuchs et al., 2005;
wild-type parent strain. Ultrastructural studies determined that Mashchenko et al., 2013). Owing to the high levels of immuno-
UL47 gene-deletion resulted in significantly reduced numbers of genicity of gJ, recombinant ILTV deficient in gJ are attractive
virions inside and outside the infected cells, but no clear indica- vaccine candidates in DIVA control strategies (York et al., 1987;
tion of alterations in any of the virion maturation steps. Given its Fuchs et al., 2005; Mundt et al., 2011; Mashchenko et al., 2013).
nuclear localization, it has been postulated that pUL47 may also Glycoprotein D (US6) is a conserved alphaherpesviruses
be participating in gene regulation or particle assembly in the protein and the main viral ligand for cell-surface receptors. Upon
nucleus (Helferich et al., 2007b). The UL47-deficient virus was binding, conformational changes in gD trigger a cascade of molec-
attenuated in vivo, causing less severe pathology and clinical signs, ular events mediated by interactions between gD and gB as well
and resulting in a 100-fold reduction of tracheal viral shedding as gH/gL on the viral envelope, leading to the fusion of the viral
(Helferich et al., 2007b). envelope with the cell membrane that allows entry of capsids into
Glycoprotein J, encoded by US5, is an N- and O-glycosylated the cytoplasm (Spear et al., 2000; Spear and Longnecker, 2003;
polypeptide that is incorporated into the envelope of ILTV viri- Rey, 2006; Campadelli-Fiume et al., 2007). ILTV gD is expressed
ons (Veits et al., 2003a). Studies in HSV have determined that as two major peptides, with apparent masses of 65 and 70 kDa,
gJ is involved in preventing apoptosis in infected cells (Zhou which are not modified by N-glycosylation, despite several puta-
et al., 2000; Jerome et al., 2001; Aubert et al., 2008), but this tive sites of N-glycosylation existing in its amino acid sequence.
function has not been confirmed in ILTV; and given the very Only the larger form is incorporated into virions, while the 65 kDa
low level of amino acid identity between ILTV and HSV gJ form is commonly found in cell lysates and may represent an
isoforms (Kongsuwan et al., 1993b; Veits et al., 2003a; Fuchs immature, degraded or differently processed form of gD (Pavlova
et al., 2005) the function of this protein may not be the same. et al., 2013). In contrast to work performed using MDV (Parcells
US5 has been categorized as a late (γ2 or LIII) gene (Fuchs et et al., 1994), ILTV gD-deletion mutants could not be isolated,
al., 2005; Mahmoudian et al., 2012). Some debate exists about thus confirming the essential role that gD plays in ILTV replica-
the molecular mass of gJ as initial studies using specific mAb tion (Pavlova et al., 2013). A number of cell surface receptors
determined it to be expressed as a 60 kDa peptide, thus originally for mammalian alphaherpesvirus ligand gD, namely herpesvirus
denominated gp60 (Kongsuwan et al., 1993b); however, further entry mediator (HVEM or CD155), nectin-1 (formerly HveC)
study conclusively determined that ILTV gJ was expressed from and nectin-2 (formerly HveB), have been described (Spear et
spliced and non-spliced RNA species and was translated into al., 2000; Spear and Longnecker, 2003; Campadelli-Fiume et al.,
peptides with four different molecular masses (85, 115, 160 and 2007). It is unknown if ILTV uses the same ligand (gD) to inter-
200 kDa). Monoclonal antibody produced in different laborato- act with cell surface receptors. Notably, orthologues of the TNF
ries have consistently recognized a complex of glycoproteins with family receptor (HVEM) that participate in mammalian her-
similar sizes (York et al., 1987, 1990; Abbas et al., 1996; Veits et pesvirus entry have not been found in the chicken genome, nor
al., 2003a; Mundt et al., 2011). This complex of four peptides had have their cognate ligands (LIGHT and lymphotoxin-α) (Kaiser
previously been suggested to represent the gB complex (Kongsu- et al., 2005). On the other hand, nectins 1 and 2 (also known as
wan et al., 1991); however, immuno-precipitation studies using poliovirus receptor-related 1 and 2) have been predicted to exist
specific polyclonal antibody raised in rabbits determined that in the chicken genome (International chicken genome sequenc-
the gB complex was composed of peptides with quite different ing consortium, 2004). It would be interesting to ascertain which
molecular masses of 58, 100 and 110 kDa (Poulsen and Keeler, receptors are used by ILTV for entry.
1997) (see above). While the 160 kDa gJ product represents an Glycoproteins I (US7) and E (US8) are N-glycosylated poly-
immature form of gJ, the major 85 kDa form of gJ represents peptides that are incorporated into the virion, localized to the
the product translated from the spliced mRNA, and the 115 cytoplasm of infected CEK cells (Pavlova et al., 2013) and have
and 200 kDa products represent the translation of the unspliced been implicated in viral cell-to-cell spread (Devlin et al., 2006a;
mRNA species, all of which (except for the immature form) are Pavlova et al., 2013). Initial work reported that a double deletion
incorporated into the virion (Fuchs et al., 2005). Modifications mutant was incapable of replicating in the absence of the wild-
by N-glycosylation in the ER are required for further transport to type genotype, thus gI and gE were considered indispensable
the trans-Golgi network, where O-glycosylation and proteolytic (Devlin et al., 2006a). However, later work that utilized double