Infectious bronchitis virus (IBV) is a protectotype, enveloped and positive-strand RNAvirus of family coronaviridae.It is a highly contagious virus ofchicken, and producesrespiratory, urinary and reproductive tract infection called infectious bronchitis disease (IB).It enters in the body through theoral, nasal and ocular routes, replicates in the cytoplasm ofthe respiratory epithelial cells and spreads to all body tissues such as the ovaries, kidneys,testes, muscles, gastro-intestinal tract and proventriculus.It is excreted through therespiratory secretions and feces.Infected chicken show depression, decreased feed intake,nasal discharge, sneezing, tracheal tale and caceous plug formation in the trachea, leadingto asphyxia and death.IBV causes 100％ morbidity and 25-80％ mortality in young chicken.IBV has a high mutation rate and new strains always emerge by insertion, deletion andrecombination.The spike gene has major antigenic determinants which makes it attractiveas a vaccine candidate.Presently, vaccines available in the market donot provide completeprotection.There is dire need to prepare a new vaccine, which can protect circulating IBVstrains.In the present study, a new IBV XDC-2 was isolated from the vaccinated flock,pathogenicity and molecular characterization was analyzed.　　The contents of the study are as follows:　　1.Pathogenesis of an infectious bronchitis virus isolate in China　　In China, IBV is one of the most important viral pathogen of poultry since 1980.A newIBV XDC-2 strain was isolated and inoculated in 10-day-old chicken embryonated eggs.The pathological lesions in the developing embryos were detected.IBV was confirmedusing RT-PCR from the allantoic fluid.The virus titer of 50％ embryo infectious dose(EID50) was calculated 5 × 10-5.33/ml.In another experiment, one-day-old specific pathogenfree chickens were inoculated with a lethal dose of the same strain of IBV by differentroutes (oral, nasal and ocular), to observe the pathogenicity, morbidity and mortality rates. Eighty one-day-old SPF chicks were divided into four groups with20 chicks per group.The three groups were inoculated with 200μl egg albumin through the oral, nasal andocular routes while 20 chicks were inoculated with orally sterile PBS as a control group.Ininfected chicken, clinical signs observed were similar to the nephropathogenic IBV.Thedead or diseased chickens lungs did not show any prominent macroscopic lesions and thecontrolgroup also did not show morbidity and mortality.However, kidneys were highlyinflamed, visibly pale and distended with massive urate deposits.The histopathologicalexamination revealed nephritis, hyaline degeneration, tubular dilatation, necrosis of theepithelial cells and severe infiltration of the interstitial inflammatory monocyte cells.Thevirus was reisolated and detected from the chicken kidneys by RT-PCR.It confirmed thatIBV XDC-2 strain had much tropism to kidneys and predominantly nephropathogenicstrain isolates from China.　　2.Full length genome sequencing of an infectious bronchitis virus isolateXDC-2　　The full length genome sequence of IBV XDC-2 was obtained by RT-PCR.The genomesequence was 27714 nt in length, and had a similar genome organization to other IBVstrains.The S glycoprotein gene (Sland S2) had the highest nucleotide identity with thenephropathogenic BJ strain.However, S 1 gene amino acids compared with available IBVvaccine strains H120, H52 and Ma5 showed eight amino acid insertions (YSNGNSDV) at73-80 site and three amino acid deletions at (126-128).Moreover, the full length genomesequence compared pairwise, minimum divergence, phylogenetic analysis and distancematrix results showed that it had the maximum relationship to the indigenous BJ strain.　　Further, S gene recombination analysis confirmed that this isolate evolved by therecombination with previously circulating BJ strain and KM91 vaccine strain.All resultsindicated that XDC-2 was a new nephropathogenic IBV strain.　　3.Construction of recombinant plasmids containing IBV S1 and Ii-key　　segment of chicken major histocompatibility complex Ⅱ gene　　Nephropathogenic IBV XDC-2 strain S1 gene and Ii-key segment of chicken majorhistocompatibility complex (MHC-Ⅱ) gene tagged with Flag were amplified (S 1-F and Ii-F)with the expected band size (1684bp, 705bp) and confirmed through agarose gelelectrophoresis and purified.S1, Ii and DNA cloning vector pcDNA3.1 were digested byrestriction endonuclease enzymes and the DNA fragment S1-F and Ii-F were cloned intopcDNA3.1 vector.Recombinant plasmids were confirmed by colony-PCR, enzymedigestion and sequencing.Further, recombinant plasmids were transfected into BHK-21cells for protein expression.Cells lysate were used for immune-blot analysis, S1gene and Iigene were expressed with expected protein bands of 61KDa and 26KDa respectively.However, in this study, the primary animal experiment results showed that the humoral andcellular responses of pcDNA-S 1-F induced more significant responses than pcDNA-Ii-F orco-administration of both recombinant plasmids.The animal immune experiments shouldbe done in the future.