Initiation and regulation of immune responses to immunization with whole inactivated vaccines prepared from two genetically and antigenically distinct lineages of Egyptian influenza A virus subtype H5N1

Following the introduction of highly pathogenic avian influenza (HPAI) virus subtype H5N1, the Egyptian government implemented a massive poultry vaccination campaign as the cornerstone of its policies to control the virus. The efficacy of vaccination has been evaluated primarily by measuring titers of antibodies inhibiting the hemagglutinating activity of the viral hemagglutinin (HA). However, other aspects of the host response remain poorly understood. In the present study, in addition to hemagglutination inhibition (HI) titers, cytokine profiles were examined and IFNγ concentrations were measured in vivo after immunization with a whole inactivated virus (WIV) prepared from a classical strain of clade 2.2.1.2 (C121) and an antigenic drift variant of clade 2.2.1.1 (V1063). The results revealed an earlier response and higher HI titers and IFNγ levels in sera from chickens immunized with C121, accompanied by significantly higher expression of IL8, IL10, and IL18 in the spleen and IL6 and IL10 in the bursa, compared to those immunized with V1063. Furthermore, stimulation of the HD11 cell line with C121 induced gradual upregulation of pro-inflammatory cytokines, which was observed at 24 hours post-inoculation (hpi), and became more pronounced at 48 and 72 hpi, accompanied by upregulation of IFNα. Conversely, V1063 induced very early transient higher expression of pro-inflammatory cytokines at 3 and 6 hpi accompanied by upregulation of IL10, which then decreased at 24, 48 and 72 hpi. In summary, our results provide evidence of a correlation between adaptive immune responses induced by WIVs and higher expression of IL10 and IL18 in addition to early induction of IFNα. These findings could be used to improve immune responses induced by WIVs.