Introduction
The pandemic caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has persisted since December 2019,1,2despite the widespread use of rapidly developed vaccines.3,4 Coronaviruses are classified into four genera (alpha-, beta-, gamma- and delta-coronaviruses) in theCoronaviridae family. Alpha and beta coronaviruses infect mammals, whereas gamma and delta coronaviruses primarily infect birds, though some can infect mammals.5 Certain coronaviruses are causative agents of important epidemics or pandemics in animals and humans, whereas others cause infection in their hosts in the absence of any apparent clinical manifestation.6 The oldest coronavirus pandemic reported in animals was caused by the avian coronavirus infectious bronchitis virus (AvCoV or IBV).7 This IBV pandemic has persisted in chicken flocks since 1931 despite mass or flock-based vaccination of the chickens globally with mostly live attenuated IBV strains by aerosol application.8
Vaccine implementers are exposed conjunctivally as well as through their respiratory tract almost on a daily basis during the application of live attenuated strains of IBV vaccines in hatcheries and poultry houses. This intensive vaccine virus exposure continues throughout their working lives. Poultry workers are in close contact only via vaccinated poultry with the IBV vaccine, but not with vaccine viruses directly. Therefore, their exposure to IBV vaccines seems to be significantly lower than that of vaccine implementers.
Like all coronaviruses, SARS-CoV-2 and IBV have a spherical pleomorphic electron-microscopic appearance, with an average size of 80 to 120 nm, similar genome organizations, and structural and non-structural proteins. IBV has seven genotypes and a number of recombinants based on the differences in nucleotide sequence of the S1 gene, which encodes the major immunological determinants.9,10 Thus, amino acid alterations (<5%) in the S1 protein in field viruses may influence the vaccine effectiveness and cross-protection.11,12 Hence, neutralizing antibodies against the S1 protein of each genotype by vaccination may not protect against infection with novel genotypes and recombinants. Clinical IBV cases persist unless vaccine strains homologous to the field IBV strains are used. On the surface of the envelope, S (spike), M (membrane), and E (envelope) proteins are found, while inside the envelope, the major protein present around the RNA of the virus is the nucleocapsid (N) protein.5 Concerning the spike sequence, a highly conserved region between SARS-CoV-2 and IBV can be found between the amino acids 807 and 830 (Figure S8).
During the course of an infection, the first contact of a coronavirus with its host is established by binding of the S protein to its specific receptor(s), which are located on the mucous surfaces of the respiratory tract.13,14 The S proteins of coronaviruses are formed by two subunits, S1 and S2. S1 contains a receptor binding domain (RBD) responsible for virus binding to its receptor and thus, determining the host and cell specificity of the virus.15 S1 is the most frequent mutation-developing site of coronaviruses, while the S2 region is relatively constant and rarely develops mutations.16,17 Besides the morphological and genomic similarities between coronaviruses infecting different animals and humans, there are large differences in the nucleotide and amino acid sequences of both structural and non-structural genes and proteins. However, there are also regions showing similarities. Despite their genetic and protein sequence differences, the topology and stereo-morphic structures of the antibody binding epitopes found in the S1 and N proteins of SARS-CoV-2 and IBV18 may result in cross-reactivity of antibodies developed during an infection or vaccination in one species against the other. Coronavirus-specific IgG and IgA antibodies in serum and mucosal surfaces can function as neutralizing antibodies.19,20 On the other hand, antibodies to other structural parts of the coronaviruses such as S2 and N can play an important role in antibody-dependent cell-mediated immunity, T cell immunity and in vivo viral neutralization although they do not directly affect RBD binding and thus epithelial cell infectivity.21-23 Cross-reactive antibodies generated in response to exposure to coronaviruses other than SARS-CoV-2 may therefore provide a certain level of protection against SARS-CoV-2 infection and/or severe disease.24 Conversely, non-neutralizing cross-reactive antibodies recognizing SARS-CoV-2 may also have detrimental effects if they facilitate antibody-dependent enhancement of viral entry into host cells.25
Although IBV and SARS-CoV-2 are in different genera of theCoronaviridae family, exposure to IBV may result in the development of cross-reactive antibodies to SARS-CoV-2. We hypothesized that poultry farm personnel, who are exposed to aerosolized IBV vaccines as a result of their occupation, will develop antibody responses to IBV, which may display cross-reactivity to SARS-CoV-2. Therefore, we measured serum IgG in three different cohorts: 1) poultry farm personnel, who had been previously frequently exposed to one of the IBV vaccine strains, 2) pre-pandemic controls, and 3) COVID-19 patients. IgG specific for the SARS-CoV-2 antigens S1, RBD, S2, and N antigen, including selected peptides from these antigens, were measured. In addition, we determined SARS-CoV-2 neutralization and IgG reactivity to the IBV S1 genotypes Massachusetts 41 (M41), Dutch 274 (D274), IS/1494/06 (Israel variant2), and 4/91.