CLASS-II CYTOPLASMIC AND TRANSMEMBRANE DOMAINS ARE NOT REQUIRED FOR CLASS II-MEDIATED B-CELL SPREADING

B cells cultured on immobilized anti-class II monoclonal antibody (nAb ) change from round to flattened cells, with lamellipodia and filopodi a. This change in cell morphology, termed 'spiders', occurs within 30 min upon culture and is mediated through either I-A or I-E molecules. Class II molecules that are defective in mediating protein kinase C (P KC) due to the deletions of both alpha and beta chain's cytoplasmic (C y) domain sequences can induce spider formation. B-cell transfectants that express chimeric MHC class II/class I molecules, where the ectodo mains are class II sequences and the transmembrane and Cy domains are class I sequences also form spiders when cultured on anti-class II mAb . The spider morphology is not induced by either anti-immunoglobulin ( Ig) or anti-MHC class I mAb. Treatment of B cells to increase intracel lular cAMP, a component of the class II signaling pathway also results in spider formation with the same kinetics and percent change in the responding population as that induced by anti-class II mAb. Cytochalas in A treatment which disrupts cytoskeletal actin filaments and the tyr osine kinase inhibitor, genistein, both inhibit spider formation. Acti n redistributes from a concentric ring in round cells to the ends of t he filopodia in the spiders. The mechanism of spider induction whether resultant from second messengers following class II signaling or from non-signaling-induced physical interactions of class II with intracel lular cytoskeletal components only requires the extracellular domains of class II. The biologic relevance of B-cell spiders is currently not known but has been reported to be associated with class II signal tra nsduction and efficient Ag presentation.