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346 | Lupiani et al.

Table 12.1 Classification and representative strains of MDV
Species Virulence Oncogenic Representative strains
1
MDV-1 Mild (m) No (+/-) CVI988 , CU-2
(GaHV-2)
Virulent (v) Yes (+) GA, HPRS-16, JM
Very virulent (v) Yes (++) RB-1B, Md5
Very virulent plus (vv+) Yes (+++) 584A, 648A, 686
MDV-2 Avirulent No SB-1, HPRS-24
(GaHV-3)
HVT Avirulent No FC-126, HPRS-26
(MeHV-1)
1 CVI988 was attenuated by repeated passage in cell culture to generate vaccine.

HVT into virulent (v), very virulent (vv) and very virulent plus It consists of UL11, UL13, UL14, UL16, UL17, UL21, UL23,
(vv+) MDV (Witter, 1983, 1985) (Table 12.1). VP1/2 (UL36), UL37, UL41 (Virion host shut off protein, VHS),
VP11/12 (UL46), VP13/14 (UL47), VP16 (UL48), VP22
Virion structure (UL49), UL49.5, UL51, US2, US3, and ICP4 in HSV-1, while
MDV presents a virion organization similar to that of other pUL7, pUL11, pUL13, pUL14, pUL16, pUL21, pUL36, pUL37
alphaherpesviruses, which consists of a 100 nm icosahedral and pUL51 are conserved across all of the herpesvirus subfamilies
capsid consisting of 162 capsomers. The capsid contains the (Kelly et al., 2009). The tegument proteins play important roles
viral DNA and is surrounded by an amorphous proteinaceous not only during viral entry, but also in virion assembly and egress
layer, the tegument, which is coated by a lipid bilayer envelope into the host cell cytoplasm (Mettenleiter, 2002). During the ini-
with the viral glycoproteins (Table 12.2) (Honess, 1984). In cell tial phase of infection, the virus attaches to the host cell, the outer
culture, MDV enveloped particles are around 150–160 nm, while tegument is dissociated, and the nucleocapsids are then trans-
in feather follicle epithelium viral particles are significantly larger, ported and target the nuclear membrane with the help of pUL36.
273–400 nm (Calnek et al., 1970a). Viral immediate early (IE) gene expression is activated by VP16
(pUL48), and initiates viral transcription forming complexes
Morphogenesis with cellular proteins which bind IE promoters of other IE genes,
The capsid is the first virion structure which assembles after DNA such as infected cell proteins (ICP) 4, 22, and 27. Tegument pro-
replication. During virus replication, several viral particle types teins encoded by the UL46-UL49 gene cluster, which are found
are observed in different cell compartments. Three types of cap- only in the subfamily Alphaherpesvirinae, are major components
sids are found in the nucleus and in some cases in the cytoplasm: of the virion and are likely to play a structural role of MDV virion
type A, empty capsids lacking DNA and scaffolding proteins; formation (Jarosinski and Vautherot, 2015). pUL41 can degrade
type B, capsids lacking DNA but containing scaffolding proteins; mRNA and consequently suppress host gene expression thereof
and type C or nucleocapsids, capsids containing DNA and similar enhancing viral protein synthesis (Smiley, 2004). pUL23 and
to those present in infectious mature virions (Denesvre, 2013; pUL50, which are thymidine kinase and dUTPase enzymes,
Tandon et al., 2015). Both type A and B capsids are abortive cap- are involved in viral DNA replication. The pUS3 and pUL13
sids and are not infectious. The ratio of each of the capsid types are both serine/threonine protein kinases, and pUS3 (Benetti
varies during infection, but a specific capsid form will accumulate and Roizman, 2007) as well as pUL14 (Yamauchi et al., 2003)
if the virus fails to express one or more of the capsid proteins have anti-apoptotic functions. The pUL31 and pUL34 mediate
(Heming et al., 2017). The capsid consists of seven conserved primary envelopment in HSV-1 (Fuchs et al., 2002), and then
proteins in human herpesvirus 1 (HSV-1), which are pUL6, pUS3 functions de-envelopment at the outer nuclear membrane
VP23, VP5, VP21, VP24, VP26 and VP19C, encoded by genes of and forms primary virion and mature extracellular virus particles
UL6, UL18, UL19, UL26, UL26.5, UL35 and UL38. Homologue only by itself (Reynolds et al., 2002).The pUL36 and pUL37 are
capsid proteins encoding genes have been identified in the MDV essential for normal maturation of capsids into enveloped virions
genome, and MDV capsids were successfully assembled using and this forms the inner tegument of the secondary envelopment,
recombinant baculoviruses expressing MDV capsid proteins while pUL48 is important to form outer tegument (Mossman et
(Kut and Rasschaert, 2004). VP5 (UL19), the major capsid pro- al., 2000).
tein, and VP26 (UL35) form the capsomers; VP23 (UL18) and The MDV envelope is a lipid bilayer studded with viral glyco-
VP19c (UL38), form the triplexes which help stabilize adjacent proteins, which includes gL, gM, gH, gB, gC, gK, gD, gI, gE, and
capsomers; VP21 (UL26), VP24 (UL26) and VP22a (UL26.5) gN. They proved to be encoded by MDV by sequencing of the
are involved in the formation of the scaffold; and pUL6, forms the UL region of the genome (Lee et al., 2000). These proteins bind
portal through which the viral genome enters the capsid. specific cell surface molecule(s) and help the virus to enter host
The tegument layer is a network of proteins which, through cells. The gB and the gH–gL heterodimer, which are conserved
protein–protein interactions, connects the capsid and envelope. throughout the alphaherpesvirus family, are essential for virus