Raw data to: 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 IgM protease, designated IdeSsuis. Vaccination with recombinant IdeSsuis elicits protection against S. suis serotype 2. This study was initiated by the finding, that the isogenic deletion mutant ΔideSsuis shows a hypervirulent phenotype in 4-week-old piglets, presumably in association with maternally-derived antibodies against IdeSsuis. This prompted us to conduct a field study to test the working hypothesis that maternal αIdeSsuis IgG antibodies are biologically relevant. Investigations in one herd with autogenous S. suis vaccination pre farrowing showed a significant correlation of IgG levels against IdeSsuis 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 wt and ideSsuis mutants demonstrated that expression of functional or non-functional full-length IdeSsuis 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-models confirmed that serum IgG against IdeSsuis exerts a significant, time-independent effect on survival of S. suis wt and ΔideSsuis_C195S, expressing a non-functional variant of IdeSsuis, but not ΔideSsuis. Furthermore, IgM binding to S. suis wt influenced survival of S. suis wt and ΔideSsuis_C195S in a time-dependent manner. This is in line with the concept that αIdeSsuis IgG are protective through induction of Fc-mediated opsonophagocytosis rather than neutralisation of the IgM protease, at least in suckling piglets.