Infectious Bronchitis Virus Infection of Chicken: The Essential Role of Mucosal Immunity

Infection with either virulent or vaccine strains of infectious bronchitis virus (IBV) elicits a complex interaction of nonspecific, innate, mucosal, cellular, and humoral immunity, thereby mounting an optimal defensive response in chickens. Through this process, mucosal immunity plays an essential role in preventing infection and clearing the virus. It also assists in the development of longer lasting local immunity against IBV, mainly in the respiratory tract but also in the oviduct and gastrointestinal mucosal linings. The head-associated lymphoid tissues, particularly the Harderian gland, have an important role in the synthesis of immunoglobulin A (IgA). Levels of this immunoglobulin in lachrymal fluid often reflect the degree of protection against IBV challenge. Beyond the head, the importance of mucosal immunity has predominantly been studied in the trachea. Though IgA has been the major focus of IBV mucosal immunity investigations, the role of mucosa-associated nonspecific, innate, and cellular immune responses may also be significant. Ciliary movements and nonspecific substances in the mucosa, such as mucins and peptides, assist in the entrapment and removal of living and nonliving antigens. Mucosa-associated innate immune responses determine cascades of downstream cellular and humoral immunity against IBV. Cellular immunity, particularly involving CD4+, CD8+, and other T-cell subsets, have been studied in mucosa-associated sites, especially the trachea. The strength of cellular immunity at mucosal sites has been associated with protection against IBV. Recently, the evaluation of mucosal immunity has shifted from traditional methods to quantitative assays of mRNA transcription of immune genes. This and other molecular-based approaches will likely boost our understanding of chicken mucosal immunity against virulent and vaccine strains of IBV. It has been well accepted that mucosal immunity plays an important role in pathogenicity, vaccinal immunity, and protection conferred against virulent IBV.