Avian Paramyxovirus Type 1 from Wild Birds: Population Adaptation and Immunogenicity in Chickens

We examined the replication and adaptation of avian paramyxoviruses serotype 1 (APMV-1) using four isolates, Mallard/US(OH)/04-411/2004, Northern pintail/US(OH)/87-486/1987, Mottled duck/US(TX)/TX01-130/2001, and Mallard/US(MN)/MN00-39/2000, and assessed their potential as vaccine candidates for chickens. The adaptability of each virus was examined by serial passages in embryonated chicken eggs (ECE) and in Vero cells. All APMVs successfully replicated in ECE. In contrast, two isolates passaged in Vero cells showed successful replication and two showed a continuous decline in viral load during passages. Whole-genome sequencing analysis identified 14 genomic positions with significant variation in mean allele frequency. Changes of the predominant virus population were characterized by shifts of amino acid (aa) frequency at eight positions. Notably, four of these changes were located in the hemagglutinin-neuraminidase (HN) protein, one in matrix (M) protein, and two in the L-protein sequences. Remarkably, although the percentage of alternative amino acids in viral populations passaged in ECE showed limited variation-for example, at aa position 127 of HN, the frequency varied from 7.4% to 19.8% and at HN aa position 192 it varied from 5.1% to 43.5%-the variation of the viral populations passaged in Vero cells was significantly higher at the same positions (e.g., the frequency of the alternative amino acids at HN aa positions 127 and 192 changed from 20.8% to 95.2% and 7.2% to 91.2%, respectively). Isolate 2 passaged in Vero cells displayed a marked variation in alternative aa frequencies, specifically at positions 127 within the HN and 100 within the M proteins. Isolate 3, while showing no alterations at the same HN positions, showed a considerable change in alternative aa frequency in the L protein at position 1875, a change occurring only in the Vero cell environment. One-day-old specific-pathogen-free chickens inoculated with isolates passaged in ECE elicited serum antibody responses similar to those elicited by the LaSota reference strain. In contrast, APMVs passaged in Vero cells showed limited replication in chickens and reduced induction of systemic antibodies. Interestingly, one virus passaged in ECE and another in Vero cells elicited immunoglobulin A levels in lacrimal fluid comparable to the LaSota strain. We concluded that the four wild-bird APMV isolates tested demonstrated successful adaptation to ECE, with one isolate eliciting overall immune responses comparable to the LaSota virus, supporting their potential as vaccine candidates.