Page 72 - Avian Virology: Current Research and Future Trends

Newcastle Disease Virus | 65

NDV isolates have been classified into three major pathotypes to those of the African and Madagascar strains (R-R-R-K-RF) or
based on severity of disease produced in chickens: low virulence (R-R-R-R-RF). It was found that change of Q to R at position
(lentogenic), moderately virulent (mesogenic), and highly viru- 114 decreased virus replication and pathogenicity of the meso-
lent (velogenic). Lentogenic viruses produce a mild respiratory genic strain BC (Samal et al., 2011). These results suggest that
disease or subclinical infection. Mesogenic viruses produce more the natural aa sequence at the F protein cleavage site of an NDV
severe respiratory diseases with mortality only in very young strain is the optimal sequence for that strain for efficient cleavage
birds, while velogenic viruses produce severe disease with high by cellular protease.
mortality. Velogenic viruses are further divided into viscerotropic, The importance of F protein cleavage in fusion promotion
which cause mortality with haemorrhagic lesions in the intestines and viral virulence has been studied (Xiao et al., 2012; Kim et al.,
of dead birds and neurotropic, when neurological diseases pre- 2016, 2017; Manoharan et al., 2018). These studies showed that
dominate without haemorrhagic lesions in the intestine (Hanson efficient cleavage of the F protein is a prerequisite for syncytia
and Brandly, 1955). formation. But the cleavage may not necessarily induce syncytia
The viral determinants responsible for the virulence of NDV formation, suggesting that the sequence of the cleavage site also
are not completely understood. The aa sequence at the F protein determines whether the cleavage results in a conformation that
cleavage site has been identified as the primary determinant is biologically active. These studies also showed that cell-to-cell
of NDV virulence that differentiates virulent (mesogenic and fusion is not an absolute requirement for NDV replication. NDV
velogenic) strains from avirulent (lentogenic) strains (Panda et can replicate by single-cell infection in vitro and in vivo (Xiao et al.,
al., 2004a; de Leeuw et al., 2005; Römer-Oberdörfer et al., 2006; 2011; Kim et al., 2017). The sequence of the F protein cleavage
Samal et al., 2011). Virulent NDV strains contain multiple basic site can confer syncytia formation but not increased pathogenic-
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residues at the F protein cleavage site ( R/K-R-Q-R/K-RF ), ity (Manoharan et al., 2018).
which is a preferred recognition site for host protease furin, The HN protein, which possesses both the receptor recognition
‘R-X-(R/K)-R’ (X, any residue; underline indicates basic and neuraminidase activities of the virus, has also been evalu-
amino acid). The furin is a ubiquitous intracellular protease that ated for its role in virulence by using reverse genetics approach
is present in most cell types. In contrast, the F protein cleavage (Huang, Z. et al., 2004a; de Leeuw et al., 2005; Wakamatsu et
site of avirulent (lentogenic) NDV strains contains fewer basic al., 2006). However, the results of these studies have not always
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residues ( G/E-K/R-Q-G/E-RL ), lacks the furin motif, agreed. It was shown that the HN protein determines tropism
and is cleaved by trypsin-like extracellular proteases present in and contributes to virulence (Huang, Z. et al., 2004a; de Leeuw
secretions of the respiratory and enteric tracts. The presence of et al., 2005; Wakamatsu et al., 2006). However, in another study,
the furin motif at the F protein cleavage site of virulent strains replacement of the HN gene alone or both F and HN genes of a
confers the ability to replicate in a wide variety of tissues, whereas mesogenic NDV strain by the corresponding genes of velogenic
the dependence of avirulent strains on extracellular secretory pro- NDV strains failed to enhance the pathogenicity of the chimeric
tease restricts viral replication to the respiratory and enteric tracts. virus (Estevez et al., 2007). In a different study, the role of F and
However, NDV strains that contain identical F protein cleavage HN proteins was investigated by simultaneously exchanging
sites sometimes can differ substantially in virulence. For example, the F and HN ectodomains between NDV and APMV-2 (Kim
strains GB Texas (GBT) and BC have identical F protein cleav- et al., 2011). It was shown that the two contrasting phenotypes
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age sites ( R-R-Q-K-RF ), but GBT is a velogenic strain and correlated with the origin of the F and HN ectodomains when
BC is a mesogenic strain. Also, in some cases the sequence of the analysed for in vitro replication, syncytia formation, and patho-
cleavage site does not predict the true pathotype. For example, genicity in chickens. These results suggested that the F and HN
there are strains of NDV that have lentogenic cleavage site motifs proteins together determine cell fusion, tropism, and virulence
but are highly virulent in chickens (Tan et al., 2008), as well as phenotypes of NDV (Kim et al., 2011).
strains with velogenic motifs that do not appear to cause disease The M protein, which plays an important role in viral morpho-
(Servan de Almeida et al., 2009). These observations suggest that genesis, has also been analysed for its role in virulence. Exchange
viral factors, other than the F protein cleavage site, contribute to of M protein between PPMV-1 and NDV showed that it played a
the virulence of NDV. minor role in virulence (Dortmans et al., 2010). In another study,
The individual aa at the F protein cleavage site have also been it was shown that all three envelope-associated genes (M, F, and
evaluated for their role in virulence. It was found that F at position HN) were necessary for severe pathological changes of a virulent
117, R at 116, K or R at 115 and R at 113 are required for viru- NDV strain (Kai et al., 2015).
lence of NDV (de Leeuw et al., 2003). Interestingly, Glutamine The individual contribution of internal proteins N, P, and L
(Q) is present at position 114 of most avirulent and virulent NDV to the virulence of NDV has been examined using reverse genet-
strains, except some isolates from Africa (Servan de Almeida et al., ics methods. Results of a study, in which individual genes were
2009) and Madagascar (Maminiaina et al., 2010). These isolates exchanged between an avirulent NDV strain and a virulent NDV
contain R-R-R-K-RF or R-R-R-R-RF at their F protein cleavage strain belonging to the same phylogenetic lineage and genotype,
site but were isolated from unvaccinated apparently healthy chick- showed that the N and P proteins play a minimal role in NDV
ens. The role of Q at position 114 was analysed by changing the virulence, but the L protein is associated with virulence (Rout
F protein cleavage site of a mesogenic strain BC (R-R-Q-K-RF) and Samal, 2008). However, in another study, in which the genes