Expression of CD40 and CD43 during Activation of Human B Lymphocytes

Abstract

CD40 and CD43 are two cell-surface glycoproteins that appear to be functionally involved in the growth stimulation of human B cells. Whereas CD40 is structurally similar to the NGF receptor and is present on all resting B cells, CD43 displays no homology to other known proteins and is expressed only on a subpopulation of these cells. To further understand the extra- and intracellular signals regulating these molecules and in which stage of activation they may play a role, we used various activation strategies and studied their expression on tonsillar B cells. As expected, activation of protein kinase C by TPA increased both CD40 and CD43. In contrast, a rise in intracellular Ca2+, e.g. by ionomycin, did not influence the expression of these antigens. However, in the presence of TPA, ionomycin further up-regulated CD43 but not CD40. Anti-IgM behaved similarly to ionomycin suggesting that the effect of this reagent was due primarily to its ability to increase intracellular Ca2+. Of three interleukins (IL-2, IL-4 and IL-6) only IL-4 had a significant effect when used alone in that it up-regulated CD40 but not CD43. However, in the presence of anti-IgM, both IL-2 and IL-4 synergistically up-regulated the two antigens. Complementation of antigen receptor stimulation with TPA or IL-4 increased CD40 during the first 24 h, whereas up-regulation of CD43 did not occur until 24 to 48 h after stimulation. With regard both to up-regulation in response to different stimuli and to kinetics, CD40 expression paralleled that of the early activation antigen CD23, whereas CD43 was induced in parallel with the transferrin receptor (CD71). Taken together, our results suggests that the expression of CD40 and CD43 is regulated by different intracellular signals and that CD40 may be important during early activation, whereas CD43 may have its major function during later stages of B-cell differentiation. These assumptions are in line with the observations that CD40 antibodies can directly activate resting B cells and that CD43 are retained on plasma cells.

Disregulation of leukosialin (CD43, Ly48, sialophorin) expression in the B-cell lineage of transgenic mice increases splenic B-cell number and survival.

“…The use of TG mice to study the disregulation of a putative adhesion/cell signaling molecule within a lineage where it normally functions provides a powerful in vivo approach that in turn may reveal functions not predicted by in vitro studies. Here we have shown that mCD43, previously used as a cell surface developmental marker (27)(28)(29)(30)(47)(48)(49), is involved in programmed cell death in the B-cell lineage and results in increased B-cell numbers. Disregulation provides a complementary approach to gene "knockouts," achieved by homologous recombination, for understanding the function of developmentally regulated genes.…”

“…This strategy was based on the fact that the mCD43 promoter is already active in parts of the B-cell lineage (27)(28)(29)(30) and that the immunoglobulin heavy chain enhancer can support augmented expression even from heterologous promoters (42). Our finding that the mCD43-Ig heavy chain enhancer fusion gene is expressed at high levels in mature peripheral B cells indicates that the immunoglobulin enhancer sequence exerts a dominant influence on mCD43 transgene expression, with a developmental program consistent with that of the immunoglobulin heavy chain itself (31).…”

“…B cells demonstrate a unique developmental program for mCD43 expression that makes them particularly amenable for the in vivo manipulation of mCD43 within a physiologically relevant context. CD43 is present on the surface of early B-cell progenitors and on terminally differentiated plasma cells but is absent on mature B cells (24)(25)(26)(27)(28)(29)(30). We took advantage of this "ON-OFF-ON" expression pattern of mCD43 in the B-cell lineage and altered its temporal expression in transgenic (TG) mice by using the immunoglobulin heavy chain enhancer to drive mCD43 expression in mature B cells (31).…”

“…1). The hybrid construct was microinjected into fertilized mouse eggs, implanted into pseudopregnant females, and allowed to develop to term (36 (24,(26)(27)(28)(29)(30). In the three TG lines examined, however, >95% of B220+ B cells expressed mCD43, as indicated by the large double-positive population (Fig.…”

Expression of a Constitutively Active Stat6 In Vivo Alters Lymphocyte Homeostasis with Distinct Effects in T and B Cells

“…To determine whether CD2:Stat6VT B cells are activated or simply have increased expression of Stat6-inducible genes, we analyzed the expression of other cell surface markers that indicate activation or maturation of B lymphocytes and are not regulated by IL-4. FACS analysis of freshly isolated B220 ϩ cells from wildtype and CD2:Stat6VT mice showed that CD40 expression, which is up-regulated on activated or mature B lymphocytes (43)(44)(45), was increased on transgenic B cells compared with wild type (Fig. 5A).…”