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TITLE: CYTOPLASMIC TAIL TRUNCATION OF MHC CLASS II MOLECULES ABROGATES LIPID RAFT–DEPENDENT SIGNALING IN A MURINE SARCOMA

Stéphane Bécart,¹ Niclas Setterblad,¹ Suzanne Ostrand–Rosenberg,² Dominique Charron,¹ Nuala Mooney.<sup>1

1</sup>Institut Biomédical des Cordeliers, INSERM U396, Paris, France; ²Dpt of Biological Sciences, University of Maryland, Baltimore, Maryland

A role for MHC class II molecules in the anti–tumoral immune response has been clearly demonstrated in a murine immunotherapy model. Vaccination with sarcoma cells transfected to express the MHC class II isotype I–Ak, protected against subsequent wild–type tumor inoculation. MHC class II molecules present antigen and transmit signals leading to activation of the antigen presenting cell, we therefore examined the functional organization and signaling ability of I–Ak in this sarcoma. Engagement of I–Ak molecules led to rapid recruitment of peptide–bound I–Ak to lipid raft microdomains, followed by (i) PKC– &agr; recruitment and activation and (ii) PTK activation. I–Ak, PKC– &agr; and actin localization in rafts as well as tyrosine kinase activity depended on raft integrity. Reorganization of the actin cytoskeleton and recruitment of PKC– &agr; to the site of I–Ak stimulation were detected by confocal microscopy. In contrast, stimulation of truncated I–Ak molecules (I–Ak tr), which are incapable of providing anti–tumoral protection, did not mediate signal transduction. Despite I–Ak tr recruitment to rafts following engagement, they neither induced actin rearrangement nor PKC and PTK activation. This model provides compelling evidence that MHC class II molecules expressed in tumor cells transmit raft–dependent signals, mediated by the intracytoplasmic domains of I–Ak.