Circulating IgA, IgG, and IgM class antibody against Haemophilus parainfluenzae antigens in patients with IgA nephropathy.

We previously demonstrated a close relationship between the outer membranes of Haemophilus parainfluenzae (HP) antigens (OMHP) and IgA nephropathy (IgAN). Our objective was to clarify the relationship among IgA, IgG, and IgM class antibody against OMHP in the sera of 44 patients with IgAN and 62 patients with other glomerular diseases (OGD) by ELISA. Patients with IgAN showed a significantly higher level of IgA antibodies (P less than 0.0005) and IgG antibodies (P less than 0.001) against OHMP, than did patients with OGD. Positive correlations were observed between IgA and IgG antibodies, between IgA and IgM antibodies, and between IgG and IgM antibodies against OMHP in the sera of patients with IgAN. Immunoblotting showed that IgA, IgG, or IgM antibodies against OMHP in the sera of all patients with IgAN bound to components of OMHP. Amino acid sequences of three components of OMHP recognized by the sera from patients with IgAN revealed homology with those reported for outer membrane protein (OMP) P6 precursor, OMP P5, and P2 porin protein of H. influenzae. Results suggest that patients with IgAN have glomerular deposits of OMP P6 precursor, OMP P5, or P2 porin protein of HP, and a specific increase in the production of IgA antibodies against OMHP via polyclonal activation against these, with switching of production from one isotype to another, e.g. from IgM to IgA.