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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 property of activating T cell subsets bearing particular Vβ segments of the TCR. However, UDA is a lectin capable of binding to many glycoproteins on cell membranes. The implication of MHC versus other glycoproteins in UDA presentation 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 MHC‐I and MHC‐II molecules serve as UDA receptors. Presentation by either one of these molecules ensured similar T cell responses and co‐stimulatory signals were mandatory for optimal T cell activation and proliferation both in MHC‐I and MHC‐II contexts. Remarkably, in the absence of MHC molecules, UDA could not be efficiently presented to T cells by other glycosylated proteins. 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 β2‐microglobulin. The results indicated that the interaction between UDA and MHC‐I molecules implicated lectin‐binding site(s) of UDA. Taken together, our data demonstrate that, in addition to MHC‐II antigens, MHC‐I molecules serve as an alternative ligand for UDA.