Page 119 - Avian Virology: Current Research and Future Trends

112 | Paldurai and Samal

pathogenesis (please refer the chapter on NDV for development Evaluation as vaccine vectors
of reverse genetic system for APMV-1). The basic methodology Reverse genetic techniques have made NDV an attractive vac-
for producing an infectious APMV from cloned cDNA by reverse cine vector for both human and animal diseases (Kim and Samal,
genetics involves co-transfection of plasmids encoding a copy of 2016). NDV is particularly an ideal vaccine vector for avian dis-
the antigenome as well as the proteins necessary for formation eases because it can be used as a bivalent vaccine. It can also be
of nucleocapsid and polymerase complex. In case of APMVs, it applied by being sprayed into the eyes or can be delivered in the
was found that N, P and L proteins were sufficient for recovering drinking water. Furthermore, a live NDV vaccine can elicit both
infectious virus. local and systemic immune responses. However, the efficacy of
To date, reverse genetic systems have been developed for NDV vector vaccines could be affected in chickens less than three
APMV-2 (Subbiah et al., 2011; Tsunekuni et al., 2017), APMV-3 weeks of age due to presence of maternal antibodies to NDV. This
(Kumar et al., 2011), APMV-4 (Kim et al., 2013), APMV-6 disadvantage can be overcome by vaccine vectors based on other
(Tsunekuni et al., 2017), APMV-7 (Xiao et al., 2012), and APMV serotypes. In order to be effective as a vaccine vector for
APMV-10 (Tsunekuni et al., 2017). These systems have applica- chickens, the candidate recombinant virus should be avirulent
tions in basic research as well as for developing vaccine vectors. to chickens and be able to replicate efficiently in chickens and be
able to carry and express a foreign gene.
The potential of APMV-2, APMV-3, APMV-4 and APMV-7 as
Role of F protein cleavage site sequence vaccine vectors has been evaluated (Kumar et al., 2011 and our
on APMV pathogenesis unpublished results). It was found that these recombinant APMVs
The reverse genetic systems of APMV-2, APMV-4 and APMV-7 (except APMV-3) were growth retarded after addition of a for-
have been used to understand the viral factors responsible for eign gene. Therefore, they may not be suitable as vaccine vectors.
tissue tropism and virulence of APMVs (Subbiah et al., 2011; Xiao APMV-3 strain Netherlands was found to be an alternative vac-
et al., 2012; Kim et al., 2013). Before these studies, our knowledge cine vector to NDV. One disadvantage of APMV-3 vector is that it
of avian paramyxovirus pathogenesis was based mainly on studies has some serological cross-reaction with NDV by HI test. There-
with NDV in chickens. The aa sequence at the F protein cleav- fore, it is thought that it may not be very effective in the presence
age site has been identified as the primary determinant of NDV of high levels of NDV antibodies. However, we have observed
pathogenicity in chickens (Peeters et al., 1999; Panda et al., 2004). that pre-existing antibodies to NDV in commercial chickens
The putative F protein cleavage site sequences of APMV-2 does not inhibit replication of APMV-3 strain Netherlands (Our
(K-P-A-S-RF), APMV-4 (D-I-Q-P-RF), and APMV-7 unpublished results). Advantages of APMV-3 vector over NDV
(L-P-S-S-RF) contain one or two basic amino acids. These vector, are that it is less virulent than the highly attenuated vaccine
sequences resemble the cleavage site sequences of avirulent NDV strains of NDV and it replicates systematically, thereby inducing a
strains. All these three APMV serotypes replicate in cell culture robust immune response. APMV-3 vector has been used to evalu-
without addition of exogenous protease and inclusion of protease ate the role of NDV F and HN protein in protective immunity
does not improve their efficacy of replication (Nayak et al., 2008; (Kumar et al., 2011). APMV-3 vector expressing the Ebola virus
Subbiah et al., 2008; Xiao et al., 2009). These viruses produce glycoprotein was found to elicit mucosal and humoral immune
single-cell infection and do not cause syncytia formation that responses in Guinea pigs (Yoshida et al., 2019). It was shown that
typically is a hallmark of paramyxovirus CPE. Also, these viruses the P-M gene junction is the optimal insertion site in the genome
are highly attenuated in chickens, which is incongruent with their of APMV-3 for GFP gene expression (Yoshida and Samal, 2017),
independence from exogenous protease. Thus, the importance of but the N-P gene junction is optimal for Ebola G virus gene
the F protein cleavage site in syncytia formation and pathogenic- expression (Yoshida et al., 2019). APMV-3 strain Netherlands has
ity in these viruses was unknown. great potential as a vaccine vector for avian pathogens.
To investigate the role of the F protein cleavage site in rep- The potential of APMV-2, APMV-6, and APMV-10 as vaccine
lication and pathogenesis of APMV-2, APMV-4 and APMV-7, vectors was evaluated by Tsunekuni et al. (2017). Recombinant
reverse genetic systems were used to generate mutant viruses vectors expressing the HA protein of highly pathogenic AIV were
whose F protein cleavage sites were derived from virulent NDV examined for their vaccine efficacy in chickens pre-vaccinated
strains and contained various numbers of basic residues (Sub- against NDV. The results suggest that recombinant vectors based
biah et al., 2011; Xiao et al., 2012; Kim et al., 2013). The mutant on APMV-2, APMV-6, and APMV-10 have potential as vaccines
viruses containing multibasic F cleavage sites exhibited protease for commercial chickens, which are routinely vaccinated against
independence, syncytia formation, and increased replication NDV (Tsunekuni et al., 2017).
in vitro. However, the mutations did not change the avirulent
nature of these viruses in chickens. These results showed that
the F protein cleavage site sequence is not the major determi- Perspectives and future directions in
nant for pathogenicity and virulence of these APMV serotypes research
in chickens. Further these results suggest that the structure of The rapid progress in discovery and characterization of novel
the F protein in addition to the cleavage site sequence plays a APMVs in the last eight years has broadened our understanding
role in the cleavability of the F protein and the activities of the of the genome diversity and host range of this group of viruses.
cleaved protein. Full genome sequence analysis of new and previously identified