Major histocompatibility class I molecules present Urtica dioica agglutinin, a superantigen of vegetal origin, to T lymphocytes

The Urtica dioica agglutinin (UDA) shares with the superantigens the proper ty of activating T cell subsets bearing particular V beta segments of the T CR. However, UDA is a lectin capable of binding to many glycoproteins on ce ll membranes. The implication of MHC versus other glycoproteins in UDA pres entation was presently studied. Using mutant mice lacking MHC class I (MHC- I), MHC class II (MHC-II) or both MHC antigens, we provided evidence that M HC-I and MHC-II molecules serve as UDA receptors. Presentation by either on e of these molecules ensured similar T cell responses and co-stimulatory si gnals were mandatory for optimal T cell activation and proliferation both i n MHC-I and MHC-II contexts. Remarkably, in the absence of MHC molecules, U DA could not be efficiently presented to T cells by other glycosylated prot eins. Surface plasmon resonance studies were used to confirm the binding of UDA to MHC-I molecules using a fusion protein consisting of MHC-I domains and beta 2-microglobulin. The results indicated that the interaction betwee n UDA and MHC-I molecules implicated lectin-binding site(s) of UDA. Taken t ogether, our data demonstrate that, in addition to MHC-II antigens, MHC-I m olecules serve as an alternative ligand for UDA.