PCV2 down-regulates tight junction protein levels in the lungs of piglets. A Viral loads in the serum of the PCV2-infected piglets. The piglets were continuously infected with PCV2. B Immunofluorescence staining of ZO-1 and occludin in the lungs. Scale bar, 50 μm. C Protein levels of ZO-1 and occludin in the lungs were determined by Western blotting and band intensities relative to control were analyzed. CT, the control group. D qRT-PCR analysis of ZO-1 and occludin in the lungs. The results are shown as mean ± SD of three times experiments. Significant differences were determined using Student t-test. *P < 0.05. **P < 0.01; ***P < 0.001.