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cell-mediated immune response of chickens is related to the IBDV of vaccination against the disease. Of course, vvIBDV or variant
strain. Currently our knowledge on T-cell response to IBDV IBDV strains transmitted into the vaccinated flocks from outside
infection is quite limited, and the viral components of IBDV that is always number 1 threat to poultry farms. Up to now, a variety of
carry dominant protective epitopes for T-cell activation have not vaccines have been developed and commercially available to prac-
determined so far. Overall, the role of cell-mediated immunity in tical use, such as attenuated live IBDV vaccines, inactivated IBDV
chicken response to IBDV infection is very important but needs vaccines, IBDV immune complex vaccines, genetic engineering
to be further elucidated. vaccines, and subunit vaccines. DNA vaccines used to hold a great
promise for the prevention and control of infectious diseases due
Suppression of chicken immune response to its convenience in storage and transportation, but its efficacy
by IBDV is far from expectation. Right now, it is clear that immunization
As described above, BF is the central immune organ of poultry of animals with DNA vaccines requires boosts, and one of the
that is responsible for the development and maturation of B cells boosts should be completed with protein antigens encoded by
and the generation of diverse antibody repertoire in young chick- the DNA vaccine. Priming with DNA vaccine and boosting with
ens, any damage to BF would affect humoral immune response inactivated vaccine conferred satisfactory protection of chickens
of chickens to antigens. Suppression of chicken humoral immu- against infectious bursal disease (Hsieh et al., 2007). However,
nity is the most distinctive feature of IBDV infection. As BF is DNA vaccination with VP2 gene fragment only conferred 75%
the target organ of IBDV, IBDV infection induces apoptosis in protection against virulent IBDV (vIBDV) challenge in chickens
proliferating B lymphocytes in BF, directly causing a rapid, pro- (Pradhan et al., 2014). Currently, no DNA vaccine is commer-
gressive loss of B lymphocytes as observed in the bursal cortex cially available for the prevention and control of chicken diseases.
and medulla, peripheral blood and thymic medulla, leading to the
suppression of humoral immunity, which makes chickens vulner- Attenuated live IBDV vaccines
able to secondary microbial infections. Live attenuated IBDV vaccines are commercially available and
There might be multiple mechanisms involved in IBDV- commonly used for the prevention and control of IBD. This type
induced suppression of immune response in chickens. During the of vaccines is generally developed by attenuating the virulence of
acute phase of IBDV infection, IBDV employs different strate- IBDV strains isolated from IBD outbreaks via serial passages of
gies to dampen antiviral response via suppressing type I IFN the field isolate in tissue culture or embryonated eggs. However,
expressions and MDA5-dependent signalling pathway, hijacking the timing for vaccination with a live vaccine is critical because
autophagic vacuoles, and inducing apoptosis in host cells, which maternally derived antibodies or circulating anti-IBDV antibod-
facilitates viral replication and spreading in BF. As a consequence, ies in chickens neutralize the vaccine. If the timing for vaccination
both innate and adaptive immune responses in infected chickens is inadequate, the titre of antibodies against IBDV cannot reach
are suppressed, and the immune system of survival chickens an optimal level, which might require revaccinations for safety. To
may not be fully functional even after recovery from the disease, be noted, vvIBDV can establish an infection in chickens with anti-
resulting in diminished Ab production following vaccination bodies after immunization with highly attenuated vaccine strains.
against other microbial diseases and increased susceptibility to While administration with less attenuated strains (‘hot vaccines’)
secondary microbial infections. Of note, vaccination of chickens can provide protection, this type of vaccine causes severer lesions
with live IBDV vaccines might also cause more or less damages to in the BF than highly attenuated vaccine strains (Müller et al.,
BF, which could diminish the expected titres of antibodies after 2003), leading to immunosuppression in vaccinated chickens to
administration with NDV or AIV vaccines. a certain degree, and affecting their immune response to other
vaccines, such as NDV or avian influenza vaccines. One of the
potential hazardous outcomes from administration of live attenu-
Control of IBD by vaccination ated vaccines might be the ‘revert to virulence’ or generation of
IBD remains a severe economic threat to the poultry industry a new viral strain via recombination between vaccine strain and
across the globe. IBDV is highly contagious and resistant to circulating wild IBDV strains.
inactivation with disinfectants. Once IBD occurs in flocks, the
virus persists and remains infective in poultry house for several Inactivated IBDV vaccines
months or longer even after the infected chickens are removed. The IBDV strains used for the development of inactivated IBDV
Vaccination is absolutely required for the prevention and control vaccines are usually isolated from diseased chickens in an out-
of this disease. On poultry farms with strict hygiene management, break of IBD. Inactivated IBD vaccine must have a high antigenic
immunization of chickens with vaccines for the control of this content (concentrated IBDV) in order to induce strong humoral
disease is often proven to be successful, but not always, because immunity in breeding adult chickens (maternally derived anti-
it is not uncommon to see some diseased chickens associated bodies) that helps protecting the progeny from IBDV infection.
with IBDV infection and even an outbreak of IBD in the flocks. As the maternally derived antibodies in newly hatched progeny
Although the reason for the unexpected outbreak of this disease chicks can neutralize the live IBDV vaccines when administered
in vaccinated flocks is not very clear, the genetic mutations in too early, the timing for vaccination of chicks with live IBDV vac-
the hypervariable region of circulating IBDV VP2 might be cines is a crucial factor affecting immune response. In contrast to
attributable under the immune pressure, resulting in the failure live IBDV vaccines, the efficacy of inactivated IBDV vaccines is