Abstract
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.