Avian infectious laryngotracheitis virus (ILTV), named as a Gallid herpesvirus 1, is a member of the Iltovirus genus, Alphaherpesvirinae subfamily, and Herpesviridae family. ILTV has a linearized dsDNA genome of approximately 150 kb in size which contains unique long (UL) and unique short (US) sequences flanked by inverted repeat (IR) and terminal repeat (TR) sequences [1, 2]. The genome encodes 80 predicted viral protein open reading frames (ORFs). ILTV infection causes respiratory disease symptoms in chickens, pheasants, partridges, and peafowl [3, 4]. Clinical signs include extension of the neck, gasping, gurgling, rattling, and coughing of clotted blood . ILTV usually causes a reduction in egg production and variable mortality ranging from 5 to 70%, and can cause severe economic losses in the poultry industry .
Two types of commercial live attenuated vaccines, chicken embryo origin (CEO) and tissue culture origin (TCO), have been widely used to immunize chicken flocks against ILTV [6, 7]. However, it was found that live vaccines infect the nervous system similarly to virulent ILTV infections, and could possibly induce vaccinal laryngotracheitis (VLT) by transmission to unvaccinated birds [8–10]. Moreover, global ILTV outbreaks are mostly associated with CEO vaccines [11–13] and the genomic- and antigenic characteristics between virulent and vaccine ILTV are very similar .
Microarray analysis has become popular, along with the recent development of a RNA-seq (RNA sequencing) technique using next-generation sequencing, to analyze comprehensive gene expression in different biological conditions. Microarrays have been performed intensively to investigate host gene transcriptional responses to infection by various viruses such as hepatitis C virus (HCV) , rice dwarf virus (RDV) , influenza virus , herpesvirus saimiri (HVS) , human immunodeficiency virus (HIV) [18, 19], Japanese encephalitis virus (JEV) [20, 21], chicken anemia virus (CAV) , human cytomegalovirus (hCMV) , Epstein-Barr virus (EBV) , infectious laryngotracheitis virus (ILTV) , varicella-zoster virus (VZV) , alphaherpesvirus , Marek's disease virus (MDV) , herpes simplex virus type 1 (HSV-1) , even in vaccine strains including recombinant flavivirus , west nile/dengue 4 virus , and dengue virus .
Previously, we studied the differential gene expression of host responses against virulent ILTV infection in cultured primary chicken embryo lung cells using microarray analysis . To compare and contrast host responses to infection of vaccine ILTV to those of virulent ILTV infection, primary chicken embryo lung cells were infected with live attenuated CEO vaccines and host gene expression during a four day period post infection was determined using 44 K chicken oligo microarrays in the present study.