The Superantigen Streptococcal Pyrogenic Exotoxin C (SPE-C) Exhibits a Novel Mode of Action

Recombinant streptococcal pyrogenic exotoxin C (SPE-C) is a potent superantigen that stimulates Vβ2-bearing human T cells, but is inactive in mice. SPE-C binds with high affinity to both human HLA-DR and murine I-E molecules, but not to murine I-A molecules in a zinc-dependent fashion. Competition binding studies with other recombinant toxins revealed that SPE-C lacks the generic low affinity major histocompatibility complex (MHC) class II α-chain binding site common to all other bacterial superantigens. Despite this, SPE-C cross-links MHC class II to induce homotypic aggregation of class II–bearing B cells. Nondenaturing sodium dodecyl sulfate electrophoresis and size exclusion chromatography revealed that both wild-type and recombinant SPE-C exist in a stable dimer at neutral or alkaline pH. These data support a recent crystal structure of SPE-C and reveal yet another mechanism by which bacterial superantigens ligate and cross-link MHC class II.