Mounting evidence suggests a connection between human parvovirus B19 (B19) and autoimmune diseases, and especially an association between the B19-VP1 unique region (VP1u) and anti-phospholipid syndrome (APS). However, little is known about the antigenicity of B19-VP1u in the induction of APS-like syndrome. To elucidate the antigenicity of B19-VP1u in the induction of APS, N-terminal truncated B19-VP1u (tVP1u) proteins were prepared to immunize Balb/c mice to generate antibodies against B19-tVP1u proteins. The secreted phospholipase A2 (sPLA2) activities and binding specificity of mice anti-B19-tVP1u antibodies with cardiolipin (CL) and beta-2-glycoprotein I (β2GPI) were evaluated by performing immunoblot, ELISA and absorption experiments. A mice model of passively induced APS was adopted. Although sPLA2 activities were identified in all B19-tVP1u proteins, only amino acid residues 61–227 B19-tVP1u exhibited a higher sPLA2 activity. Autoantibodies against CL and β2GPI exhibited binding activities with all B19-tVP1u proteins. IgG that was purified from mice that had been immunized with amino acid residues 21–227 to 121–227 B19-tVP1u proteins exhibited significantly higher binding activity with CL. IgG that was purified from mice that had been immunized with amino acid residues 21–227, 31–227, 82–227 and 91–227 B19-tVP1u proteins exhibited significantly higher binding activity with β2GPI. Accordingly, significantly higher binding inhibition of CL was detected in the presence of amino acid residues 61–227 and 101–227 B19-tVP1u. Significantly higher binding inhibition of β2GPI was detected in the presence of amino acid residues 21–227, 31–227, 82–227 and 91–227 B19-tVP1u. The mice that received amino acid residues 31–227 or 61–227 anti-tB19-VP1u IgG revealed significant thrombocytopenia and those that received amino acid residues 21–227, 31–227, 61–227, 71–227, 82–227, 91–227, 101–227 or 114–227 anti-tB19-VP1u IgG exhibited significantly prolonged aPTT. These findings provide further information concerning the role of B19-VP1u antigenicity in APS-like autoimmunity.

Both human parvovirus B19 (B19V) and human bocavirus (HBoV) are known to be important human pathogens of the Parvoviridae family. Our earlier investigation demonstrated that both B19V-VP1u and HBoV-VP1u have a significantly disruptive effect on tight junctions (TJs) in A549 cells, implying the essential role of parvovirus in airway infection and lung injury. However, no direct evidence that B19V-VP1u and HBoV-VP1u induce lung injury exists. The present study further investigates the induction of lung injury by B19V-VP1u and HBoV-VP1u in naïve Balb/c mice following subcutaneous injection of PBS, recombinant B19V-VP1u or HBoV-VP1u. The experimental results reveal significantly increased activity, protein expression and ratio of matrix metalloproteinase-9 (MMP-9) to MMP-2 in Balb/c mice that received B19V-VP1u or HBoV-VP1u compared to those that received PBS. Significantly higher levels of inflammatory cytokines, including IL-6 and IL-1β, and greater lymphocyte infiltration in lung tissue sections were detected in mice that received B19V-VP1u or HBoV-VP1u. Additionally, significantly increased levels of phosphorylated p65 (NF-κB) and MAPK signaling proteins were observed in lung tissue of mice that received B19V-VP1u or HBoV-VP1u compared to those of mice that received PBS. These findings demonstrate for the first time that B19V-VP1u and HBoV-VP1u proteins induce lung inflammatory reactions through p65 (NF-κB) and MAPK signaling.