The cytoplasmic and the transmembrane domains are not sufficient for classI MHC signal transduction

Class I MHC molecules deliver activation signals to T cells. To analyze the role of the cytoplasmic and the transmembrane (TM) domains of class I MHC molecules in T cell activation, Jurkat cells were transfected with genes fo r truncated class I MHC molecules which had only four intracytoplasmic amin o acids and no potential phosphorylation sites or native molecules or both. Cross-linking either the native or the truncated molecules induced IL-2 pr oduction even under limiting stimulation conditions of low engagement of th e stimulating mAb. Moreover, direct comparison of transfected truncated and native class I MHC molecules expressed on the same cell revealed significa nt stimulation induced by cross-linking the truncated molecules, despite lo w expression. In addition, truncated class I MHC molecules were as able to synergize with CD3, CD2, or CD28 initiated IL-2 production as native molecu les. In further experiments, hybrid constructs made of the extracellular po rtion of the murine CDS ru chain and of the TM and the intracytoplasmic dom ains of H-2K(k) class I MHC molecule were transfected into Jurkat T cells. The expression of the transfected hybrid molecules was comparable to that o f the native HLA-B7 molecules. Cross-linking the intact monomorphic HLA-A,B ,C epitope or the polymorphic HLA-B7 epitope induced IL-2 production upon c ostimulation with PMA. In contrast, cross-linking the hybrid molecules gene rated neither an increase in intracellular calcium concentration ([Ca2+](i) ) nor stimulated IL-2 production. By contrast, cross-linking intact murine class I MHC molecules induced [Ca2+](i), signal and IL-2 production in tran sfected Jurkat cells. The data therefore indicate that unlike many other si gnaling molecules, signaling via class I MHC molecules does not involve the cytoplasmic and the TM portions of the molecule, but rather class I MHC si gnal transduction is likely to be mediated by the extracellular domain of t he molecule. (C) 1999 Academic Press.