Anti-CD20 therapeutic antibody rituximab modifies the functional organization of rafts/microdomains of B lymphoma cells.

Incubation of Burkitt lymphoma-derived Raji cells at physiological temperature with submicromolar concentrations of humanized anti-CD20 antibody rituximab (RTX) redistributes CD20 to liquid-ordered, plasma membrane rafts. This accumulation of the CD20 tetraspan protein in rafts does not change the existing lipid and phosphoprotein composition but makes sphingolipids and the Src regulator Cbp/PAG (Csk-binding protein/phosphoprotein associated with glycosphingolipid-enriched microdomain) transmembrane phosphoprotein more resistant to n-octyl-beta-pyranoside, a detergent that dissociates sphingolipid clusters. On the contrary, sphingolipids and Cbp/PAG are not protected by the presence of CD20 against the disruptive effects of methyl-beta-cyclodextrin, a cyclic carbohydrate that removes membrane cholesterol. After accumulation of CD20, the activity of the raft-associated Lyn kinase is down-regulated without apparent alteration of its relationship to substrates. Moreover, in rafts of lymphoblastoid cells that express lower amounts of Cbp/PAG, RTX redistributes CD20 to rafts but does not modulate the raft-associated protein tyrosine kinase activity, suggesting that the presence of Cbp/PAG protein in rafts is necessary for RTX to exert its transmembrane "signaling effects." Lastly, redistribution of CD20 in rafts renders the glycosylphosphatidyl inositol (GPI)-linked CD55 C'-defense protein hypersensitive to glycosylphosphatidyl inositol-specific phospholipases. By redistributing CD20 to rafts, RTX modifies their stability and organization and modulates the associated signaling pathways and C' defense capacity.

Differential involvement of CD40, CD80, and major histocompatibility complex class I molecules in cytotoxicity induction and interferon-gamma production by human natural killer effectors.

Natural killer (NK) cells are physiologically involved in the immune response against viruses, intracellular bacteria, and parasites as well as against malignant diseases. In addition to the cytotoxic activity, NK lymphocytes mediate a variety of homeostatic effects by producing cytokines. This study focused on the differential role of CD40 and CD80 costimulatory molecules and major histocompatibility complex class I (MHC-I) antigens in the regulation of cytotoxicity and of interferon (IFN)-gamma secretion of resting and interleukin (IL)-2-activated human NK cells. CD40 and CD80 molecules were observed to play a specific role in the induction of cytotoxic function but not in IFN-gamma production of IL-2-activated NK effectors. In addition, a critical role of CD94-dependent MHC-I recognition for the regulation of IFN-gamma production and target lysis was demonstrated. These data provide a possible mechanism underlying functional interactions between NK lymphocytes and CD40/CD80-expressing cell targets, as represented by dendritic cells.