Catalyzes the specific cleavage of porcine IgM bound to the bacterial surface. Can degrade only IgM but neither IgG nor IgA, and is host specific, as it exclusively cleaves porcine IgM but not IgM from six other species, including human, mouse and a closely related member of the Suidae family. Promotes survival in porcine blood. Is thus involved in a so-far-unknown mechanism of host-pathogen interaction at an early stage of the host immune response. IgM cleavage is inhibited by iodoacetamide but not by AEBSF, bestatin, E-64, Z-LVG-CHN(2), or EDTA. Is released into the extracellular space and remains at least transiently attached to the bacterial surface, likely via its C-terminal transmembrane domain. Domain encompassing residues 35-432 are sufficient for IgM cleavage. Cells lacking this gene lose IgM-cleaving activity and show an increase of surface-bound IgM antigen. They are also significantly attenuated in survival in blood of a piglet vaccinated once with a bacterin, in comparison to the wild-type. Belongs to the peptidase C66 family.