Avian paramyoxvirus-8 immunization reduces viral shedding after homologous APMV-8 challenge but fails to protect against Newcastle disease

Background Protection against infection by Newcastle disease virus (NDV), also designated as avian paramyxovirus subtype-1 (APMV-1), is mediated by immune responses to the two surface glycoproteins, hemagglutinin-neuraminidase (HN) and fusion (F) protein. Thus, a chimeric APMV-1 based vaccine that encodes APMV-8 HN- and F-proteins and expresses the hemagglutinin of avian influenza virus (AIV) H5N1, is able to protect against HPAIV H5N1 but fails to protect against NDV [PLoS One 8:e72530, 2013]. However, it is unclear whether avirulent APMV-subtypes, like APMV-8 can induce subtype-specific immunity and protect from a homologous challenge. Findings: APMV-8 infections of 3- and 6-weeks-old specific pathogen free (SPF)-chickens did not induce any clinical signs but was associated with virus shedding for up to 6 days. Viral replication was only detected in oropharyngeal- and never in cloacal swabs. Upon reinfection with homologous APMV-8, viral shedding was restricted to day 2 and in contrast to naive SPF-chickens, only RNA but no infectious virus was recovered. No protection was induced against virulent NDV challenge, although morbidity and mortality was delayed in APMV-8 primed chickens. This lack of protection is in line with a lack of reactivity of APMV-8 specific sera to APMV-1 HN-protein: Neither by hemagglutin-inhibition (HI) test nor immunoblot analyses, cross-reactivity was detected, despite reactivity to internal proteins. Conclusions Immune responses mounted during asymptomatic APMV-8 infection limit secondary infection against homologues reinfection and facilitates a delay in the onset of disease in a subtype independent manner but is unable to protect against Newcastle disease, a heterologous APMV-subtype.