| To evaluate the mucosal immune effect of the attenuated strain PRRSV CH-1R, the following examination was done using 6-week-old landrace piglets.The safey and the mucosal immune responses of the PRRSV CH-1R strain inoculated intranasally was determined by examination of clinical signs, pathomorphism, and antibody distributiong. Furthermore, we also detected the level of antibody IgA and IgG, and cytokine IFN and TGF of piglets inoculated intranasally with strain PRRSV CH-1R. Several immune indexes and the differences of pathological damage, and protective capability to the body which were induced by high-virulence strain attacked were also detected. The results suggested that some tissues and organs of piglets inoculated intranasally with strain PRRSV CH-1R changed their clinical signs and histopathological characters, and compared with the high-virulence strain immune group, the symptom was slighter. Indirect immunofluorescence test (IIFT) showed that the fluorescence reaction of the immune group was absolutely remarked stronger than that of control group and high-virulence strain immune group. In nasal and lung lavage fluid of piglets inoculated intranasally with strain PRRSV CH-1R, we found that the level of sIgA trended to increase as the time progress, then we infected virulent virus strain to the same group and found that the level of sIgA elevated in evidences. In blood serum of immune group, the level of cytokine TGF-β1 elevated in 14 days after immuned, and then decreased from 15th day. When attacked high-virulence strain, the level raised much higher in the first 7 days than earlier stage, but after 14 days it decreased. The level of cytokine IFN-γchanged in immune group was the same with which in control group. The level of antibody IgG also increased as the time went by, 14 days after immuned it had the trend to increase, and after 35 days it reached the plateau phase. When high-virulence strain was attacked, the level of IgG raised distinctly quicker than in earlier stage. Concluding all the test results we believed that immune injection of low virulent strain PRRSV CH-1R through intranasal way had good effect in stirring high level mucosa and humoral immune reaction .At the same time, we chose 10 virus strains of CH-1a and CH-1R in different generations to amplify the ORF5 gene, and compared the sequences with other 8 strains to analyze the inheritance and variation of CH-1R strains during the attenuation process. The mutation analysis of ORF5 showed that the attenuate strain has differences with CH-1R and another 8 strains, the homology of ORF5 range from 88.7% to 99.8%.Further more, phylogenetic analysis indicated that this kind of strain still belongs to the CH-1R subgroup. To compare its amino acid sequence of GP5 with other strains we found that the generation S70 of CH-1a strain and the generations S85, S100, S115, S120 and S130 of CH-1R strain changed their 38th amino acid from His to Gln; the generations S120, S125 and S130 of CH-1R strain changed their 146th amino acid from Leu to Gln; the generations S39, S55 and S65 of CH-1R strain changed none. Homology of the amino acid between the above-mentioned 10 generations and reference strains was in between 88.5% and 97.0%. RFLP analysis demonstrated a specific endonuclease restriction site: TspE I .This restriction site could be used to distinguish CH-1R,CH-1a from other wild-type and reference strains.
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