Receptor-cytoskeleton interactions in the lymphocyte

Cellular mechanisms of adaptation of grafts to antibody

New, more effective, strategies of immunosuppression, including those recently designed to induce durable T cell tolerance (by grafting allogeneic or xenogeneic haematopoietic cells into T lymphocyte-depleted recipients), leave humoral rejection as the main barrier to transplantation of vascularized organs between different species. Recent experimental work indicates that hyperacute rejection can be prevented by manipulations of antibodies and complement. In this paper, we review the mechanisms governing the interaction of antibodies with cell surface antigens in vitro and in vivo, and their cellular consequences. Evidence is presented that, in appropriate conditions, antibodies can protect by effecting modification of graft antigenicity (adaptation or accommodation).

Organization of lymphocyte plasma membrane. Surface protein-membrane matrix interactions in B-cell lines of different stages of differentiation

Composition of surface proteins and their interactions with cytoskeleton or membrane matrix were compared in tumor B-cell lines of different stages of B-lymphocyte maturation. All studied B-cell lines were found to share a similar set of cell surface proteins, which are tightly associated with the cytoskeleton. The increase in amount of detergent-unextractable cell surface proteins with B-cell maturation suggested that differentiation of B lymphocytes was accompanied by development of specific interactions between surface proteins and elements of the cytoskeleton or membrane matrix. Using a recently developed procedure for lymphocyte plasma membrane fractionation we demonstrate changes in distribution of cell surface proteins in membrane matrix-rich and membrane matrix-poor plasma membrane fractions during B-lymphocyte maturation. Thus, cell surface proteins of the mature B-cell line MOPC-315 were predominantly found in the plasma membrane vesicles of a high buoyant density. These vesicles mostly contained plasma membrane proteins tightly associated with elements of the membrane matrix. In immature B cells (line 70Z3) virtually all surface proteins were detected in both low and high buoyant density membrane vesicles. The tendency to increased associations between surface proteins and cytoskeleton/membrane matrix with maturation of B cells could not be explained by increased amounts of filamentous actin, since no correlation was found between the amount of globular or filamentous actin and the degree of surface protein-cytoskeleton (membrane matrix) interactions.

CD5 and CD21 molecules are a functional unit in the cell/substrate adhesion of B-chronic lymphocytic leukemia cells

The modulation of surface molecules on B-chronic lymphocytic leukemia (B-CLL) cells was studied in vitro by incubation with CD19, CD20, CD21, CD5 and anti-IgM antibodies. The cytoplasmic changes were evaluated by analyzing the organization of cytoskeletal F-actin and the adhesive properties of stimulated B-CLL cells. The following observations were made: (a) CD5 antigen is capped on the surface of B-CLL cells (but not on normal CD5+ B and T lymphocytes). (b) On B-CLL cells, CD5 and CD21, the receptors for the C3d fraction of complement, co-cap and co-modulate while surface IgM as well as CD19 and CD20 do not cap. (c) B-CLL cells, after capping of CD5 or CD21 surface molecules, fail to organize F-actin into podosomes. (d) The pre-incubation of B-CLL cells with either CD5 or CD21 antibodies prevents their binding to purified C3d protein used to coat coverslips. These data indicate that an intimate spatial relationship exists between CD5 and CD21 molecules on the B-CLL surface. The CD5-CD21 complex is involved in the redistribution of cytoskeletal proteins which may control the adhesive properties of these malignant B cells.

Marmoset red blood cell receptor for membrane-associated complement components is not related to human CR1: partial characterization of the C3-binding proteins responsible for the spontaneous rosette formation between marmoset red blood cells and human leukocytes

Cells from all the human B-lymphoblastoid cell lines tested and most human monocytes form rosettes with marmoset red blood cells (MaRBC). Because previous reports suggested the involvement of complement components in this phenomenon, the mechanism of rosette formation and the eventual similarities between the MaRBC receptor and the CR1 receptor present on human erythrocytes have been analyzed herein. The binding of MaRBC to human leukocytes strongly differs from the immune adherence phenomenon: rabbit anti-human CR1 did not react with MaRBC and the MaRBC receptor-binding activity is Ca2+-dependent. Rosette formation required intact energy metabolism and cytoskeleton integrity of leukocytes. Our attempts to purify the receptor from MaRBC membranes revealed the absence of CR1. Nevertheless, C3-binding proteins were isolated by selective desorption by Sepharose iC3 column chromatography. A three-band pattern was observed under reduced conditions with 74,000, 70,000, and 53,000 molecular weights. It was not possible to further separate these components. This protein complex inhibited the rosette phenomenon between MaRBC and both Raji and U-937 cells, exhibited a very poor cofactor activity, and had no decay-accelerating activity toward the classical C3 convertase. This material did not cross-react with antibodies directed to human proteins. These results showed that erythrocytes from new world monkeys do not express a receptor analogous to the human CR1, but expressed C3-binding protein with low cofactor activity that could recognize membrane-associated complement components.

The role of immunoreceptors

The nature of the main types of immunoreceptors is discussed with reference to current investigations which are starting to indicate their function and structure. The immunoreceptors of the lymphoid cells, mast cells and basophils are reviewed.

A kinetic study of membrane immunoglobulin capping by flow cytometry

A flow cytometric procedure has been developed for performing kinetic studies on the capping of membrane immunoglobulin (mIg) on B lymphocytes. Mouse B cells were stained with fluorescein-conjugated antimouse-Ig antisera and subjected to pulse-shape (width, peak, and area) analyses prior to, during, and after ligand-induced redistribution of mIg. It was found that ring-stained, patched, and capped cells could be discriminated based on the width of the electronic signal curve generated as the cells passed through the laser beam. Additionally, endocytosis and or shedding of the cap could be correlated with a change in the area under the curve. Using these two parameters (width and area), the effects of temperature, cross-linking, and several pharmacological agents on the capping process were examined. Through the use of flow cytometry, the inhibitory effects of various perturbants could be localized to discrete stages of the capping process.