Abstract
Infectious bronchitis virus (IBV) causes respiratory disease leading to loss of egg and meat production in chickens. Although it is known that macrophage numbers are elevated in the respiratory tract of IBV infected chickens, the role played by macrophages in IBV infection, particularly as a target cell for viral replication, is unknown. In this study, first, we investigated the ability of IBV to establish productive replication in macrophages in lungs and trachea in vivo and in macrophage cell cultures in vitro using two pathogenic IBV strains. Using a double immunofluorescent technique, we observed that both IBV Massachusetts-type 41 (M41) and Connecticut A5968 (Conn A5968) strains replicate in avian macrophages at a low level in vivo. This in vivo observation was substantiated by demonstrating IBV antigens in macrophages following in vitro IBV infection. Further, IBV productive infection in macrophages was confirmed by demonstrating corona viral particles in macrophages and IBV ribonucleic acid (RNA) in culture supernatants. Evaluation of the functions of macrophages following infection of macrophages with IBV M41 and Conn A5968 strains revealed that the production of antimicrobial molecule, nitric oxide (NO) is inhibited. It was also noted that replication of IBV M41 and Conn A5968 strains in macrophages does not interfere with the induction of type 1 IFN activity by macrophages. In conclusion, both M41 and Con A5968 IBV strains infect macrophages in vivo and in vitro resulting productive replications. During the replication of IBV in macrophages, their ability to produce NO can be affected without affecting the ability to induce type 1 IFN activity. Further studies are warranted to uncover the significance of macrophage infection of IBV in the pathogenesis of IBV infection in chickens.
Highlights
Infectious bronchitis (IB) is primarily a respiratory disease of chickens but with potential to cause more widespread infection in the urinary and reproductive tracts in chicken leading to significant production losses in commercial broiler and layer flocks worldwide [1]
Chickens infected with Massachusetts-type 41 (M41) or Conn A5968 strains of infectious bronchitis virus (IBV) showed transient and nonspecific clinical signs starting at 3 dpi that included huddling under the lamp, increased respiration and ruffled feathers
Higher macrophage numbers were found in chickens infected with IBV Conn A5968 strain compared to the IBV M41 strain (P
Summary
Infectious bronchitis (IB) is primarily a respiratory disease of chickens but with potential to cause more widespread infection in the urinary and reproductive tracts in chicken leading to significant production losses in commercial broiler and layer flocks worldwide [1]. Vaccines are available for commonly occurring serotypes and strains but they are not necessarily antigenically similar to the wild-type viral strains circulating in poultry barns These vaccine strains may provide some degree of protection for some related strains known as protectotypes [2], the commonly available vaccines may not elicit protective immune responses in a flock if the field strains are antigenically very different from the vaccine strains. Owing to this reason, vaccination against IBV is not currently considered to be a very effective control method and other biosecurity measures are necessary to prevent the introduction of IBV into poultry production facilities
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