Studies On Pathogenic Characteristics Of Infectious Bursal Disease Virus With Different Virulence And The Induced Transcriptome Changes In Chick Bursa

Infectious bursal disease(IBD)is an acute,highly contact and immunosuppressive Infectious disease caused mainly by infectious bursal disease virus(IBDV).IBDV mainly damages the Bursa of fabricius,which leads to severe immunosuppression and increases susceptibility to other pathogens.The Classic IBDV(c IBDV)strain was discovered in Gambro in 1957 and has since undergone two major mutations,,Variant IBDV(var IBDV)and Very virulent IBDV(vv IBDV)strains evolved in the late1980 s.vv IBDV swept the world with its high fatality rate and caused serious economic losses.vv IBDV has been an serious threat to the poultry industry in China for nearly 30 years.In recent years,new outbreaks of atypical IBD have emerged one after another in major poultry areas of our country.No obvious clinical symptoms have been observed in infected chickens,but the production performance has been significantly decreased,and the bursa of Farnsal has been seriously atrophy by dissecting.Our laboratory has taken the lead in isolating and identifying the pathogen as Novel variant IBDV(n Var IBDV).Further epidemiological investigation showed that vv IBDV and n Var IBDV have become the two predominant strains of IBDV in China.In this study,biological characters of vv IBDV(HLJ0504 strain),n Var IBDV(SHG19 strain)and IBDV attenuated strain(Gt strain)were systematically compared from the perspectives of clinical pathogenesis,micropathology,electron microscopy,viral replication ability and tissue distribution.vv IBDV causes acute death to chicks,and the peak of death is 3～5 days after infection(d p.i.).The fatality rate is as high as 80%.It affects various tissues and organs,especially the Bursa of fabricius,the central immune organ,and causes severe atrophy.Micropathological lesions such as acute lymphocyte necrosis and disintegration,and gross lesions such as spleen enlargement,thymus atrophy,spotted kidney,and glandular gastric hemorrhage occurred within 36 hours post infection(h p.i.).vv IBDV infection showed a variety of tissue distribution patterns,and the virus titer of vv IBDV was the highest in the Bursa of fabricius,followed by the caecal tonsil,spleen and thymus,as well as the non-lymphoid tissues such as kidney,heart,liver and lung.vv IBDV also had a high viral titer in the blood,and the period of hyperviremia was 60 hours to 4 days after infection,and obvious cloacal detoxification began at 36 hours after infection.n Var IBDV has no lethal effect on chickens,but it can cause severe acute lesions and atrophy of Bursa of fabricius,and it also affects a variety of tissues and organs.Although it also causes severe atrophy of the central immune organ,Bursa of fabricius,and necrosis and disintegration of acute lymphocytes,the gross lesions and atrophy of Bursa of fabricius of chickens infected with n Var IBDV are delayed compared with vv IBDV.The tissue distribution range of n Var IBDV is similar to that of vv IBDV,although the replication capacity of n Var IBDV is relatively low,the virus can still replicate rapidly in Bursa of fabricius and has the ability of external detoxification,which is one of the important reasons why Nvarib DV has become the dominant circulating strain.The IBDV attenuated strain was not pathogenic to chickens,and only had a very low titer in Bursa of fabricius at the later stage of infection,and could induce the body to produce a certain level of antibodies.Therefore,IBDV viruses with different virulence have significant differences in replication and pathogenicity.In order to further analyze the molecular mechanism of differences in biological characteristics of different virulence strains of IBDV,transcriptome sequencing was performed on Bursa of fabricius tissues of chickens infected with different strains of IBDV,and 15 differentially expressed genes were selected for q RT-PCR detection to verify the transcriptome sequencing results.At 24,48 and 72 h p.i.,8816,10238 and 4374 genes were differentially expressed in vv IBDV group,n Var IBDV group and IBDV attenuated strain group,respectively,compared with Mock blank control group.Compared with IBDV attenuated strain infected group,5124 and 5199 genes were differentially expressed in vv IBDV group and n Var IBDV group,respectively.Compared with vv IBDV group,3054 genes were differentially expressed in n Var IBDV group.GO functional cluster analysis,KEGG enrichment analysis and protein interaction network analysis showed that the differentially expressed genes of Bursa of fabricius infected chickens infected by different virulence strains mainly involved functional genes such as cytokines,chemokines,calcium ion binding proteins,ion channels,and cell receptor binding,which are enriched into MAPK signaling pathway,calciumion signaling pathway and cell cycle metabolism signaling pathway.It is speculated that the differential expression of these different genes in host may be closely related to the differences of biological characteristics of IBDV strains with different virulence.This study laid a foundation for in-depth understanding of the pathogenic mechanism of different virulence strains of IBDV,and has important reference value for the comprehensive prevention and control of different virulence strains of IBDV.