Vector Vaccines and Immunity

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Both cell-mediated and humoral immunity play a role in the protective immune response against avian viruses. Antibody usually appears within 6-10 days after infection not only in the blood but also locally. Their systemic level depends largely on the invasiveness of the strain but generally, immune response peaks at about 3-4 weeks. For instance, although high levels of systemic antibody have always been associated with protection against Newcastle Disease (ND), reports have suggested that antibody titres in serum measured by haemagglutination inhibition (HI) test are not directly correlated with the level of resistance of chickens to experimental NDV challenge.

In particular, local immunity is provided within the upper respiratory and intestinal tract of chicken by production of IgA after vaccination and it was shown that local antibody on the mucosal surface of the respiratory tracts play an important role not only in protection of the challenged birds but also in limitation of primary replication at the portal of entry and shedding of the virus.

Cell-mediated immune response may be detected as early as 2-3 days after vaccination with an attenuated vaccine, which is earlier and stronger than after vaccination with an inactivated one. Nevertheless, studies in bursectomized birds have shown that cell-mediated immunity is not sufficient by itself to protect against NDV but may be essential for virus clearance. Similar studies have shown the importance of both arms of the immune response to get satisfactory protection against highly pathogenic avian influenza (HPAI).

However, the continuous outbreaks of fatal ND or HPAI in commercial poultry flocks in many part of the world indicate that routine vaccination in the field often fails to induce sufficiently high levels of immunity to control the diseases. Especially, in regions where the viruses are enzootic and where there is high pressure from the field, the need for very early immunization is hampered by the interference of an important passive immunity. In addition, current vaccination strategies can be effective in preventing serious illness and death of infected birds, but do not stop (sub)clinical infection (e.g. egg drop) and shedding of virus.

In this context, the use of vector vaccines that are less sensitive to the interference of MDA, like the recombinant herpesvirus of turkey (HVT), or the in ovo application of vaccines, by priming at an early age, have been shown as very promising. Among advantages of recombinant HVT (rHVT), it is noteworthy to induce a strong cell-mediated immunity (CMI) and to be safe and nonpathogenic for administration as in ovo vaccine.

Additionally, the choice of the gene to be inserted in these recombinants is of great importance. For instance, among the NDV antigens, the fusion (F) and/or haemagglutinin-neuraminidase (HN) envelope glycoproteins have been used successfully, either as subunit or recombinant vaccine, to immunize experimentally and protect chickens against virulent NDV challenge. Nevertheless, although both F and HN glycoproteins elicit neutralizing antibodies, only the F protein can induce antibody production that prevents cell-to-cell spread of the virus. Moreover, the use of the F protein represents a potential method of differentiating infected from vaccinated animals (DIVA), as immunized birds develop no haemagglutination inhibition antibody.

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