Polyclonal hypergammaglobulinemia and formation of hydrophobic immune complexes in porcine reproductive and respiratory syndrome virus-infected and uninfected pigs

Infection of young conventional, domestic pigs with porcine reproductive and respiratory syndrome virus (PRRSV) strains VR2332 and JA142 resulted in a rapid, progressive increase in serum IgG reaching maximum levels of 20-30 mg/mL at about 3 weeks post infection (p.i.), which were maintained until at least 63 days p.i., whereas the level of serum IgG remained at 4-6 mg/mL in sham-infected pigs. In most of the VR2332 and JA142-infected pigs hypergammaglobelimenia was associated with the formation of hydrophobic, 150-300-kDa IgG-containing immune complexes that bound in the presence of 0.1% Tween 20 to ELISA plates that were not coated with any antigen. The ELISA plate-binding activity remained low in most infected pigs, but reached high levels in some JA142-infected pigs. Binding of the immune complexes was also observed, but at a lower level, to uncoated ELISA plates in the peptide ELISA for anti-PRRSV Abs. The immune complexes bound to uncoated ELISA plates with a much lower affinity than Abs to plates coated with peptides containing the appropriate epitopes. The immune complexes also bound to HerdChek ELISA plates, but because of low binding affinity for these plates, the bound complexes were removed by the repeated washes with Tween 20 solution. Overall the PRRSV-induced hypergaminaglobulinemia and generation of ELISA plate-binding immune complexes resembled those observed in mice infected with the closely related lactate dehydrogenase-elevating virus (LDV) and thus, like the latter, seem a result of a polyclonal activation of B cells. We also found that sera of a group of older sows possessed high levels of IgG as well as of ELISA plate-binding immune complexes, in spite of being PRRSV infection negative by all criteria presently available. On the other hand, sera from wild hogs contained no ELISA plate-binding IgG in spite of possessing high total serum IgG levels.