Maternal antibodies against the immunoglobulin M-degrading enzyme of Streptococcus suis, IdeSsuis, play an important role in bactericidal immunity in young piglets in the field

Streptococcus suis ( S. suis ) expresses a specific immunoglobulin M (IgM) protease, designated Ide Ssuis . Vaccination with recombinant Ide Ssuis elicits protection against S. suis serotype 2. This study was initiated by the finding that the isogenic deletion mutant Δide Ssuis shows a hypervirulent phenotype in 4-week-old piglets, presumably in association with maternally derived antibodies against Ide Ssuis . This prompted us to conduct a field study to test the working hypothesis that maternal αIde Ssuis IgG antibodies are biologically relevant. Investigations in one herd with autogenous S. suis vaccination pre-farrowing showed a significant correlation of IgG levels against Ide Ssuis in serum of 2-week-old-piglets and levels in colostrum. These antibody levels declined over the subsequent weeks of life. Levels of IgM binding to S. suis in sera of 2-week-old piglets did not correlate with respective levels in colostrum but with the amount of colostrum taken up. Bactericidal assays with wild type (wt) and ide Ssuis mutants demonstrated that expression of functional or nonfunctional full-length Ide Ssuis results in increased killing of S. suis serotype 2 in the blood of 2-week-old piglets. This phenotype disappeared as piglets aged. Linear mixed-effects models confirmed that serum IgG against Ide Ssuis exerts a significant, time-independent effect on survival of S. suis wt and ∇ide Ssuis _C195S, expressing a nonfunctional variant of Ide Ssuis , but not Δide Ssuis . Furthermore, IgM binding to S. suis wt influenced survival of wt and ∇ide Ssuis _C195S in a time-dependent manner. This is in line with the concept that αIde Ssuis IgG are protective through induction of fragment crystallizable (Fc)-mediated opsonophagocytosis rather than neutralization of the IgM protease, at least in suckling piglets.