Abstract
Porcine reproductive and respiratory syndrome virus (PRRSV) strain A2MC2 induces type I interferons in cultured cells. The objective of this study was to attenuate this strain by serial passaging in MARC-145 cells and assess its virulence and immunogenicity in pigs. The A2MC2 serially passaged 90 times (A2MC2-P90) retains the feature of interferon induction. The A2MC2-P90 replicates faster with a higher virus yield than wild type A2MC2 virus. Infection of primary pulmonary alveolar macrophages (PAMs) also induces interferons. Sequence analysis showed that the A2MC2-P90 has genomic nucleic acid identity of 99.8% to the wild type but has a deletion of 543 nucleotides in nsp2. The deletion occurred in passage 60. The A2MC2-P90 genome has a total of 35 nucleotide variations from the wild type, leading to 26 amino acid differences. Inoculation of three-week-old piglets showed that A2MC2-P90 is avirulent and elicits immune response. Compared with Ingelvac PRRS® MLV strain, A2MC2-P90 elicits higher virus neutralizing antibodies. The attenuated IFN-inducing A2MC2-P90 should be useful for development of an improved PRRSV vaccine.
Highlights
Porcine reproductive and respiratory syndrome (PRRS) is an economically important swine contagious disease across the world, which has resulted in an estimated $664 million loss per year to the swine industry in the United States alone[1]
Primary pulmonary alveolar macrophages (PAMs) cells were prepared from 4-8-week-old piglets and cultured in RPMI1640 medium supplemented with 10% FBS22
Porcine reproductive and respiratory syndrome virus (PRRSV) strain A2MC2 was subjected to serial passaging in MARC-145 cells to minimize previously observed moderate virulence[16]
Summary
Porcine reproductive and respiratory syndrome (PRRS) is an economically important swine contagious disease across the world, which has resulted in an estimated $664 million loss per year to the swine industry in the United States alone[1]. PRRSV appears to inhibit synthesis of type I interferons (IFNs) in pigs, whereas swine transmissible gastroenteritis virus (TGEV) and porcine respiratory coronavirus (PRCV) induce high levels of IFN-α6–8. PRRSV antagonizes induction of type I IFNs in both PAMs and MARC-145 cells as infection of the cells in vitro leads to very low level interferon-α(IFN-α) expression[6,9,10]. Adenovirus-mediated expression of IFN-αin pigs leads to reduction in disease signs when the animals were challenged with PRRSV13. ® tion of A2MC2 is needed for the IFN induction, whereas PRRSV strains VR-2332, Ingelvac PRRS MLV, NVSL. ® leads to earlier onset and higher levels of virus-neutralizing antibodies than the Ingelvac PRRS MLV vaccine strain[16]. Virus neutralizing antibodies against PRRSV confer protection of pigs against challenge with virulent strain[17]. Passive transfer of PRRSV-neutralizing antibodies in pregnant sows confers sterilizing immunity against
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