An Improved Scheme For Infectious Bursal Disease Virus Genotype Classification And Development Of A Subunit Candidate Vaccine Against Novel Variant Infectious Bursal Disease Virus

Infectious bursal disease(IBD),caused by infectious bursal disease virus(IBDV),is an important immunosuppressive disease in poultry.Recently,the emerging novel variant IBDV(nVarIBDV)has become a new threaten to the health development of poultry industry.IBDV was traditionally divided into four phenotypes,including classic IBDV(cIBDV),variant IBDV(varIBDV),very virulent IBDV(vvIBDV),and attenuated IBDV(aIBDV).However,with the emergence of some novel mutated IBDV strains,it is increasingly difficult to define these new IBDV strains(including the nVarIBDV)using the traditional descriptive classification method.IBDV genome contain two genome segments of A and B,both of which play important roles in the genetic evolution of IBDV.However,previous sequence analysis of IBDV mainly focused on segment A,which could not fully reflect the molecular characteristics of IBDV.In this study,an improved scheme for IBDV genotype classification based on the molecular characteristics of both VP2(encoded by segment A)and VP1(encoded by segment B)was proposed for the first time.In this scheme,IBDV was classified into nine genogroups of A and five genogroups of B,respectively;the genogroup A2 was further divided into four lineages.Based on the molecular characteristics of two segments of IBDV genome,the commonly used phenotypic classifications of classic,variant,very virulent,and attenuated IBDVs correspond to the A1B1,A2B1,A3B2,and A8B1 genotypes.The n Var IBDV which widely prevalent in China,Japan,South Korea,and Malaysia were classified as belonging to genotype A2dB1.The flexibility and versatility of this improved classification scheme will allow the unambiguous identification of existing and emerging IBDV strains,which will greatly facilitate the molecular epidemiology studies of IBDV.In view of the severe epidemic situation of the n Var IBDV(genotype A2dB1),the epidemiological detection of IBDV were carried out in four representative regions of China(Northeast China,Central China,East China,and South China).The results showed that the proportion of new variant strain(genotype A2dB1),HLJ0504-like vv IBDV strain(genotype A3B3),classic strain(genotype A1B1),and attenuated strain(genotype A8B1)detected in this study were 57.1 %,17.9 %,3.6 %,and 3.6 %,respectively.In addition to broilers,the epidemic of the n Var IBDV(genotype A2dB1)in laying hens and local breed chickens was first identified in this study,indicating that the n Var IBDV(genotype A2dB1)is still spreading in flocks and has become an important dominant epidemic strain.Additionally,a novel segment-reassortant IBDV(genotype A2dB3)was first identified in this epidemical study.Phylogenic analysis showed that segments A and B of this reassortant strain were derived from the emerging n Var IBDV(genotype A2dB1)and the originally widespread HLJ0504-like vv IBDV strain(genotype A3B3),respectively.The reassortant strain can induce serious damages of bursa,which indicates the complexity of IBDV evolution and popularity.The epidemic situation of n Var IBDV(genotype A2dB1)is becoming more and more serious,however no commercial vaccine with fully matching antigenicity is available now.In this study,the virus like particles(VLP)targeting n Var IBDV were developed using Escherichia coli expression system.The VLP was 25-nm and could stably storage not less than 17 months at 4 ?.Subsequently,the VLP was preliminarily purified and then made into a vaccine(SHG19-VLP).The protective effect of the vaccine was further evaluated using animal experiments.The SHG19-VLP vaccine could efficiently induce neutralization antibody.The challenge experiments showed that the SHG19-VLP vaccine could provide 100 % immune protection against the homologous n Var IBDV(genotype A2dB1)as well as the heterologous vv IBDV.In this study,an improved scheme for IBDV genotype classification based on two segments of IBDV genome was proposed for the first time,in which n Var IBDV was classified into genotype A2dB1.The n Var IBDV(genotype A2dB1)was widely prevalent in broilers,layers and local breed chickens,and has become one of the important dominant prevalent strains in China.Additionally,we developed a n Var IBDV-specific subunit(SHG19-VLP)vaccine which could provide effective protection against two dominant prevalent strains(n Var IBDV and vv IBDV)of our country.This study is of great significance for the comprehensive prevention and control of IBD.