Molecular mechanisms of transmembrane signaling in B lymphocytes

Augmentation of mitogen-induced thymocyte proliferation by bacterial products is mediated by an Mr 36,000 monokine

The addition of picogram quantities of bacterial products to rat thymic cells in culture produced a doubling of the proliferative response to suboptimal levels of Concanavalin-A (Con-A). This effect was prevented by the depletion of adherent cells which comprised less than 0.1% of the total population. The response was restored by the addition of supernatants from peritoneal macrophages which had been stimulated 2 h previously with lipopolysaccharide. Treatment of these supernatants with phenylglyoxal, an inhibitor of interleukin-1 (IL-1), did not prevent the stimulatory effect. Augmentation of the thymocyte proliferative response could also be achieved by the addition of a partially purified monokine of relative molecular weight (Mr) 36,000 which is biochemically and functionally distinct from IL-1 and by a monoclonal antibody which binds to a common determinant on the Ia molecule. Fractionation of the thymic cells on a density gradient yielded a buoyant population which accounted for the majority of the proliferative activity and a dense fraction which was poorly responsive to the mitogen. The addition of the monokine to this latter fraction produced a significant increase in proliferation in response to Con-A. It is proposed that in the thymus, bacterial products stimulate thymic macrophages to release the Mr 36,000 monokine which in turn stimulates the thymic epithelial cells to release products which promote the survival and maturation of immature thymocytes. This work has implications for the regulation of thymocyte maturation.

Activation of protein kinase C from B lymphocytes by lipid A

The effect of lipid A, a residue of the lipopolysaccharide molecule, on protein kinase C from B lymphocytes has been studied. Lipid A activates and promotes the translocation of protein kinase C from the soluble to the particulate membrane fraction in a cell-free system reconstituted with purified enzyme and membranes isolated from B lymphocytes. These results demonstrate that the activating effect of lipopolysaccharide on protein kinase C from B cells is due to the lipid moieties of this molecule.

Properties of anti-Lyb-2-mediated B-cell activation and the relationship between Lyb-2 molecules and receptors for B-cell stimulatory factor-1 on murine B lymphocytes

The murine B-cell differentiation antigen Lyb-2 has been shown to be involved in B-lymphocyte activation and has been postulated by some to be related to a receptor for B-cell stimulatory factor I (BSF-1) (H. Yakura et al., J. Immunol. 137, 1475, 1986). Here we have demonstrated that monoclonal antibody (mAB) to Lyb-2 resembles BSF-1 in its ability to activate small resting B cells and enhancement of surface Ia. Anti-Lyb-2 antibodies bound B cells with very high avidity and were able to induce mobilization of cytosolic-free calcium. Anti-Lyb-2 mAB differs from BSF-1 in that BSF-1 but not anti-Lyb-2 is able to synergize with anti-mu in induction of B-cell proliferation. The relation between Lyb-2 molecules and BSF-1 receptors was tested in assays that measure binding of anti-Lyb-2 or BSF-1 in B cells and were found not to compete with each other. It appears that the two B-cell agonists anti-Lyb-2 and BSF-1 may exert their effects on B cells through different cell surface moieties as well as different intracellular pathways.

Transfected plasmacytoma cells do not transport the membrane form of IgM to the cell surface

Expression vectors coding for membrane-bound IgM antibodies were introduced into myeloma and B lymphoma cells. Only the lymphoma but not the myeloma cells were able to express the antibodies on the cell surface, although in both cases, complete antibodies were assembled intracellularly. In myeloma cells, the Ig molecules did not reach the Golgi compartment. Thus, the intracellular transport of membrane-bound antibodies is controlled in the B cell lineages in a developmentally ordered fashion.

Anti-mu antibody stimulates the phosphatidylinositol cycle and immunoglobulin secretion in a human lymphoblastoid B-cell line, LA350

Within 5 min of the binding of anti-mu antibody (anti-mu) to surface IgM on LA350, a human lymphoblastoid B-cell line, there was a significantly enhanced incorporation of 32P into the phosphatidic acid (PA) and phosphatidylinositol (PI) fractions of cellular phospholipids and the magnitude of the early increase in PA was twice as great as that in PI. This anti-mu-enhanced incorporation of 32P into PA and PI required the binding of a divalent form of antibody (IgG or F(ab')2), was blocked by coincubation with micromolar concentrations of soluble IgM, was decreased by incubation of cells at temperatures lower than 37 degrees C, and was inhibited by coincubation with millimolar concentrations of dibutyryl cyclic AMP and theophylline. Similar incorporation studies with [3H]inositol demonstrated a selective and significant increase in labeling of PI. In LA350 labeled with [3H]inositol for 30 hr (equilibrium) and acutely stimulated by anti-mu, specific hydrolysis of phosphorylated PI (PI 4,5-bisphosphate) was measured by the significantly increased release at 15 min of radioactive inositol 1,4,5-trisphosphate, inositol 1,4 bisphosphate, and inositol 1-phosphate. The release of these inositol phosphates was significantly augmented by coincubation with 0.01 M LiCl which prevented their simultaneous enzymatic degradation. All of these findings are consistent with an activation of a linked series of metabolic events known as the PI cycle. In similar cell cultures anti-mu significantly stimulated the secretion of IgM by LA350 as measured at 48 hr in a reverse hemolytic plaque assay. Two other IgM-bearing human lymphoblastoid B-cell lines which gave no evidence of turnover of 32P in PA and PI in response to binding by anti-mu likewise failed to enhance their secretion of IgM. We conclude that the binding of surface IgM on LA350 by anti-mu results in the generation of a transmembrane signal which causes a rapid activation of the PI cycle which itself may play a role in the subsequent increase in IgM secretion.

The induction of organ-specific antibodies during the graft-vs.-host reaction

During the parent (P) into F1 hybrid graft-vs.-host reaction (GVHR), nuclear, leukocyte and erythrocyte autoantibodies are commonly seen. The specificity of these autoantibodies is reminiscent of those found in systemic lupus erythematosus (SLE) patients and SLE-prone mice. Organ-specific antibodies, however, including thyro-globulin (Tg) antibodies do not arise spontaneously. There have been conflicting reports about the ability of exogenous Tg to induce an anti-Tg response during the GVHR. We have re-examined this question in greater detail. Using the murine P----F1 GVHR system, the results of this work demonstrate that mouse thyroglobulin (MTg)-specific antibodies can be induced during a GVHR. However, mice must both be undergoing a GVHR, and have received exogenous MTg. The highest autoantibody response occurs if mice are injected with mouse thyroid extract or purified MTg at the time of P----F1 cell transfer. The anti-MTg response is MTg dose dependent. The ability to induce anti-MTg antibody was not major histocompatibility complex restricted, for both the DBA/2----B6D2F1 (low responder H-2 haplotypes to MTg), and AKR or DBA/2----AKD2F1 (high/low responder----high responder haplotype) GVHR gave similar responses. The anti-MTg titers peaked between days 7-10 and declined thereafter. In contrast, antibodies to dsDNA were not present at this early time, but developed after several weeks. We conclude that organ-specific autoantibodies can be induced during a GVHR if the appropriate antigen(s) are presented near the time of GVHR induction.