Intracytoplasmic domains of MHC class II molecules are essential for lipid-raft-dependent signaling

In addition to their role in antigen presentation, major histocompatibility complex (MHC) class II molecules have been widely described as signaling proteins in diverse antigen-presenting cells (APCs) including B cells and dendritic cells. By contrast, little is known of the signaling function of MHC class II molecules expressed in solid tumors. We describe the functional organization and signaling ability of I-A(k) expressed in a sarcoma, and report the recruitment of I-A(k) to lipid rafts after MHC class II engagement. Lipid raft integrity was required for I-A(k)-mediated reorganization of the actin cytoskeleton and translocation of protein kinase C-alpha (PKC-alpha) to the precise site of stimulation via I-A(k). Truncation of the intracytoplasmic domains of I-A(k) did not perturb I-A(k) recruitment to lipid rafts but abrogated PKC-a translocation and actin rearrangement. PKC-alpha was detected in lipid microdomains and enrichment of activated PKC-alpha in lipid rafts was induced by I-A(k) signaling. Ordering of the molecular events following engagement of the MHC class H molecules revealed that I-A(k) recruitment to lipid rafts precedes signaling. This is consistent with the absence of a requirement for the intracytoplasmic tails for localization to lipid rafts. These data reveal that lipid-rich microdomains play a key role in MHC class II-mediated signaling in a solid tumor.