Regulation of IgM and IgD synthesis in B lymphocytes. I. Changes in biosynthesis of mRNA for mu- and delta-chains

Although IgD is expressed on the surface of resting B cells at a higher density than IgM, we determined that both the steady-state level and the biosynthetic rate of mu m-mRNA is higher than that of delta m-mRNA, suggesting that translational or post-translational processing of the Ig heavy chains may modulate the expression of cell surface Ig. After B cell activation by LPS, delta m-mRNA decreases drastically, accounting for the observed decrease in expression of IgD. Concomitantly, a dramatic increase in the level of mu s- and gamma s-mRNA corresponds to the observed increase in secretion of these Ig isotypes in LPS-stimulated cells.     

Structure and biologic functions of human IgD. XI. Identification and ontogeny of a rat lymphocyte immunoglobulin having antigenic cross-reactivity with human IgD.

Chicken anti-human IgD antiserum (anti-delta) has demonstrated an antigenically cross-reactive homologue on rat lymphocytes. IgD and IgM are the only cell surface immunoglobulins detectable by the lactoperoxidase radiolabeling technique employed. The results indicate that, although rat surface IgD is antigenically distinct from rat IgM, the respective H chains co-electrophorese in 10% polyacrylamide-SDS gels. Rat delta-chain has an apparent m.w. of 73,000 daltons and exhibits a minor 65,000 dalton component which probably represents a partially degraded delta-chain. The ontogenic emergence of rat IgD occurs approximately 3.5 weeks after birth whereas IgM, in contrast, is apparent by 6 days of age. Thus, as in the human, IgM develops before IgD. IgD receptors are undetectable in the thymus but are present in increasing levels in spleen, blood, lymph nodes, and Peyer's patches.     

A selective defect in IgM antigen receptor synthesis and transport causes loss of cell surface IgM expression on tolerant B lymphocytes.

To explore the biochemical basis for maintaining immunological tolerance by functional inactivation of self-reactive B lymphocytes, transgenic mice carrying rearranged anti-lysozyme immunoglobulin transgenes and a lysozyme transgene were used as a source of large numbers of tolerant self-reactive B cells. Antigen receptors of the IgD isotype were expressed at normal levels on tolerant B cells, contained the heterodimeric MB1/B29 signalling component of the receptor complex and were structurally indistinguishable from IgD on nontolerant B cells. In contrast, cell surface expression of IgM receptor complexes on tolerant B cells was greatly reduced, despite normal expression of mRNA encoding the receptor components. Three-fold fewer immunoreactive mu heavy chains were detectable after a short period of biosynthetic labelling and the immunoreactive mu chains produced were paired with kappa light chains and assembled normally into intact receptor complexes containing the MB1/B29 heterodimer. Nascent IgM receptor complexes nevertheless failed to be processed into an endoglycosidase H-resistant form in the tolerant B cells and thus appeared to be selectively blocked in their transport from the endoplasmic reticulum to the medial Golgi. These findings demonstrate that intracellular trafficking of antigen receptor complexes is regulated by exposure to receptor stimuli at the cell surface causing a long-lasting decrease in surface receptor expression on tolerant B cells.     

Evidence for an IgD homologue on chicken lymphocytes

Chicken lymphocyte membrane immunoglobulins (Ig), were precipitated with mouse monoclonal antibodies specific for heavy and light chain isotypes and analyzed by polyacrylamide gel electrophoresis. Very little or no membrane-bound IgG and IgA was detected. After sequential precipitation and removal of IgM reactive with any of three monoclonal anti-mu antibodies, anti-light chain antibody precipitated residual Ig with a relative electrophoretic mobility similar to that of IgM. Under reducing conditions, these surface Ig molecules had a heavy chain that appeared slightly larger (approximately 81,000 daltons) than mu-chain (approximately 79,000 daltons), and light chains of approximately 25,000 daltons. Complete clearance of membrane-bound IgM reactive with an anti-mu allotype antiserum left similar molecules precipitate by monoclonal anti-light chain antibody. These non-IgM molecules could be detected on the surface of lymphocytes from blood, spleen, bursa and the B cell line RAV-1, but not from thymus or blood from an agammaglobulinemic chicken. After capping of B cell surface IgM with anti-mu, immunofluorescent staining with anti-light chain antibody revealed residual Ig molecules disturbed across the surface of more than 90% of the IgM-bearing cells. The data suggest the existence of an avian homologue of mammalian IgD. Affinity-purified goat anti-mu antibodies and a fourth monoclonal anti-mu antibody reacted with both IgM and the putative IgD molecules, which suggests that the IgD homologue shares at least one common determinant with chicken IgM.     

Cell surface immunoglobulin XXI. appearance of IgD on murine lymphocytes during differentiation.

The differentiation of Ig-bearing lymphocytes in adult mice was studied by monitoring the appearance of IgD relative to IgM on the surface of splenocytes obtained from lethally irradiated animals reconstituted for various periods of time with adult bone marrow cells, neonatal splenocytes, or Ig- adult splenocytes. It was found that IgM appears before IgD on differentiating lymphocytes. Furthermore, the rate of appearance of IgD during differentiation of adult cells is similar to that observed with neonatal cells.     

Molecular components of the B cell antigen receptor complex of class IgD differ partly from those of IgM

Two classes of immunoglobulin, IgM and IgD, are present as antigen receptors on the surface of mature B lymphocytes. We show here that IgD molecules are noncovalently associated in the B cell membrane with a heterodimer consisting of two proteins of 35 kd (IgD-alpha) and 39 kd (Ig-beta), respectively. The two novel proteins are not found in the IgD-expressing myeloma J558L delta m, which fails to bring IgD antigen receptor onto the cell surface. In a surface IgD positive variant line of this myeloma, however, membrane-bound IgD molecules are associated with the heterodimer, suggesting that the formation of an antigen receptor complex is required for surface IgD expression. We further demonstrate that the IgD-associated heterodimer differs partly from that of the IgM antigen receptor and that its binding to the heavy chain only requires the presence of the last constant domain and the transmembrane part of the delta m chain.     

Differential endocytosis of IgM and IgD in murine B cell lines

The internalization of surface immunoglobulin (Ig) by B lymphocytes is the first step in the antigen-presenting function performed by these cells. Mature B cells coexpress on their surface IgM and IgD. At this time, there is controversy over whether these two isotypes serve different functions in the antigen-presenting process. The results presented here show that the intracellular pattern of distribution of IgM and IgD after internalization is strikingly different in the B cell lines studied. These findings support the hypothesis that the role of the two Ig classes in the antigen-presenting function may be different.     

IgM and IgG but not cytokine secretion is restricted to the CD27+ B lymphocyte subset.

In a recent study we reported that CD27 is expressed on a subpopulation of human B lymphocytes and presented circumstantial phenotypic evidence that CD27 expression may be acquired late during B cell differentiation. Here we present functional data showing that, after in vitro stimulation, CD27+ but not CD27- B cells secrete large amounts of both IgM and IgG. Using double immunofluorescence staining of CD27 and IgD, three functionally different B cell subsets representing distinct stages of B cell differentiation can be isolated: 1) the CD27- IgD+ B cells, which do not secrete appreciable Ig; 2) the CD27+IgD+ B cells, which exclusively secrete IgM; and 3) the CD27+IgD- B cells, which comprise the IgG-producing cells. Furthermore, costimulation of CD27- B cells with low m.w. B cell growth factor, in the presence or in the absence of a CD40 mAb, does not induce these cells to become Ig-secreting cells. Although CD27- B cells hardly secrete Ig of any isotype in response to Staphylococcus aureus+IL-2, these cells proliferate vigorously and express the IL-2R alpha chain (CD25) under these stimulatory conditions. Furthermore, both CD27- and CD27+ B cells are capable of producing similar amounts of IL-6 and TNF-alpha. Taken together, these findings indicate that CD27 is a unique non-Ig surface marker discriminating naive from primed B lymphocytes. Furthermore, the capacity to proliferate and to secrete the B cell differentiation factors IL-6 and TNF-alpha already exists at an early B cell differentiation stage at which the cells lack CD27 expression and are not induced to produce Ig.     

Differences in glycoprotein complexes associated with IgM and IgD on normal murine B cells potentially enable transduction of different signals.

Studies presented here demonstrate that IgM and IgD molecules on normal murine B lymphocytes exist in different, noncovalently associated molecular complexes containing distinct but potentially related glycoproteins. The glycoproteins in these complexes, particularly those associated with IgD, show striking differences in various lymphoid organs and in X-linked immunodeficient (Xid) mice. These differences are due in part to post-translational processing. They apparently reflect the differential expression of the Ig-associated glycoproteins in the various B cell subpopulations and lineages and the differential distribution of the subpopulations and lineages in the various lymphoid organs. In addition, they reflect structural differences in the IgM and IgD complexes which, we suggest, permit differential signal transduction by IgM and IgD on the same B cell.     

The class of surface immunoglobulin on virgin and memory B lymphocytes.

The class of surface immunoglobulin receptors for antigen on B cell precursors of different classes of antibody-forming cells was determined by utilizing a technique of class-specific antigen suicide. Spleen cells are first treated with a class-specific antiserum under conditions that result in the stripping of that class from the cell surface. The cells are then permitted to bind a highly radioactive trinitrophenyl (TNP)-conjugated protein, which leads to lethal irradiation of all TNP-specific B cells except those whose TNP receptors had been removed by the class-specific stripping of surface immunoglobulin. In this way, the class of antibody-forming cells resulting from TNP stimulation of B cells with different classes of surface immunoglobulin can be examined. It was found that the virgin B cell precursors of IgM-producing cells are two types: cells bearing IgM receptors only and those bearing both IgM and IgD receptors. All virgin B cells that gave rise to IgG1 antibody-forming cells had both IgM and IgD on their surfaces, demonstrating that an antigen-dependent switch from IgM and IgD to IgG1 production is a common feature of B cell maturation. In contrast, memory B cell precursors of IgG1 antibody-forming cells had predominantly IgG1 as their surface antigen receptor. The implications of these findings on current models of B cell maturation are analyzed.     

Two spontaneous BALB/c lymphomas synthesize IgM: monomers and half molecules are isolated and characterized whereas another molecule resembles IgD.

Spontaneous lymphomas of BALB/c mice, both in vivo tumors and cell lines established in long term tissue cultures, were investigated for their ability to synthesize IgM by using radiolabeled amino acid precursors. Immunoglobulins manufactured by lymphomas K46 and L10A had the m.w. of monomeric IgM and IgM half molecule. Both of these molecules could be immunoprecipitated with class-specific anti-IgM but not anti-IgA or anti-IgG. When precipitated with polyvalent anti-Ig L10A synthesized monomeric immunoglobulins that migrated as two peaks in contrast to their single counterpart precipitated with anti-IgM. The second peak migrated in the region expected for IgD. Monomer and half molecules were composed of similar ratios of mu-chains to light chains linked by disulfide bonds. The mu2L2 monomer of these B cell lines migrated slightly slower in SDS PAGE than a mu2L2 secreted by a myeloma. Thus, these lymphomas synthesize immunoglobulins with the chemical and antigenic characteristics typical of monomeric membrane-attached IgM and IgM half molecules, plus a molecule resembling IgD on L10A only. Lymphoma assembly of monomeric IgM may follow the same initial biosynthetic sequence as myeloma assembly.     

Electrophoretic mobility of lymphocytes in chronic lymphatic leukemia (author's transl)]

Lymphocytes and other blood cells can be separated by means of free flow cell electrophoresis. Immunofluorescence of the separated lymphocytes of four healthy volunteers with antiimmunoglobulins IgD and IgM produced different distribution profiles for each immunoglobulin class, the IgD positive cells migrating faster than the IgM positive ones. Amongst five patients with chronic lymphocytic leukemia four with IgD positive lymphocytes (greater than 80%) showed an identical electrophoretic distribution. The IgM positive lymphocytes (greater than 80%) of the fifth patient migrated much more slowly. The weighted mean of each distribution profile of either the IgD or IgM positive lymphocytes in CLL is similar to that of normal subjects.     

The expression of the sIgD isotype in wild-derived mice

IgD and IgM are concomitantly expressed on the surface of most mouse B lymphocytes and both molecules serve as receptor for antigen. In this communication we report that in contrast to IgM, which is expressed in a constant manner on the surface of spleen B lymphocytes of different laboratory and wild-derived mice, IgD expression is variable among the spleen cells of wild-derived mice. SPE, SEI, and SFM mice belonging to the Mus 3 subgroup show a fluorescence profile characterized by a marked diminution in the population of B lymphocytes expressing the IgD isotype; in addition, these cells have a low sIgD density on their membranes. These findings were confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the iodinated membrane proteins. Polyclonal in vitro activation with lipopolysaccharide increases the frequency of surface IgD (sIgD)-bearing spleen cells and sIgD density in the SPE strain but decreases both the frequency and the density of IgD bearing cells in the BALB/c strain. This result suggests that delta gene expression is regulated differently in SPE and BALB/c mice. In addition, genetic analysis of sIgD expression in (BALB/c X SPE)F1 hybrids suggests that the proportion of sIgD-bearing cells and sIgD density are independently regulated.     

B cell abnormalities induced by a mu Ig transgene extend to L chain isotype usage

We have analyzed the phenotype of B cell populations from mice transgenic for a rearranged Ig mu H chain gene. We find a decrease in the number of B cells in the spleens of these mice. Transgenic B cells have decreased surface levels of both IgM and IgD. The circulating IgM in these mice is 3- to 10-fold enriched in lambda L chains, compared with that in non-transgenic mice. Analysis of IgM-producing hybridomas, from transgenic mice that express the transgene at high levels, demonstrates that this higher lambda frequency is observed in transgene-nonexpressing as well as transgene-expressing hybridomas. A partial loss of L chain isotype exclusion is also noted in these hybridomas, and a significant proportion of primary B cells expressing both kappa and lambda L chains on their surface can be demonstrated. These findings suggest an ability of the transgenic Ig H chain to affect events in B cell ontogeny beyond the H chain locus. Our results support a quantitative model of exclusion for both the H chain alleles and the L chain isotypes.     

Terminal differentiation of PWM-stimulated human B lymphocytes.

Patterns of surface and cytoplasmic immunoglobulins were simultaneously studied on human B blast cells induced by pokeweed stimulation of peripheral blood lymphocytes. A double-staining immunofluorescent technique was used. After 4 and 7 days of culture, a gradual loss of surface IgD was observed on blast cells whereas numbers of plasmablasts with cytoplasmic immunoglobulin showed a marked increase. After 7 days, 92% of surface IgA positive blasts had passed terminal differentiation to cytoplasmic IgA-producing plasma-blasts. At the same time, 73% of surface IgM positive blasts were found to contain cytoplasmic IgM, and 30% of surface IgG positive cells had cytoplasmic IgG. Only a small fraction of blast cells with surface IgD was able to mature to IgD producing plasmablasts. In general, the class of surface and cytoplasmic immunoglobulin coincided in single blast cells, with the exception of surface IgD which was present on 10% of the cytoplasmic IgM-containing blasts.     

Human B cell development. II. Subpopulations in the human fetus

In man, during fetal development the B cell populations show distinct phenotypes at different tissue sites. The pre-B and B lymphocytes of the fetal liver and bone marrow express IgM and B cell markers, B1 (CD20) and BA-1 (CD24). These "early" cells are negative with a number of other reagents, anti-IgD, RFB4 (CD22), RFB6 (CD21), and RFA-2, which on the other hand recognize peripheral B cells. These peripheral B lymphocytes in the developing fetus are heterogeneous. The diffusely distributed B cells in the earliest lymph node samples, 16 to 17 wk of gestational age, and from 16 to 21 wk in the spleen, are strongly IgM+ (IgD+,RFB4+,RFB6+, and RFA-2+) but lack T cell-associated markers such as T1 (CD5, p 67,000 dalton equivalent of murine Ly-1) and Tü-33. In fetal lymph nodes, primary nodules develop around the follicular dendritic (FD) cells from 17 wk onward, and contain a virtually pure population of B cells; B1+,BA1+,RFB4+,RFB6+,RFA-2+, which simultaneously express IgM,IgD together with T1 (CD5), a T cell-associated antigen. A sizeable subpopulation of these IgM+,T1+ cells are also positive for Tü-33, another T cell-associated marker. In the spleen, the B cells of the IgM+,IgD+,T1+ type appear in smaller numbers and only relatively late around wk 22. These cells are diffusely distributed at first, and start accumulating around the small FD cell clusters as soon as these emerge about the 23rd gestational wk. At that time, the IgM+,T1+B cells can also be washed out from the peritoneal and pleural cavities. The T1+,IgM+B cells may represent the normal equivalent cells of B chronic lymphoid leukemia and centrocytic lymphoma, and appear to be the counterpart of Ly-1+,IgM+B cells in the mouse.     

In vitro and in vivo B lymphocyte-activating properties of monoclonal anti-delta antibodies. I. Determinants of B lymphocyte-activating properties

To appreciate better the mechanisms by which B lymphocytes are activated by anti-Ig antibodies, we characterized seven monoclonal mouse allo-antibodies to IgD of the a allotype for their isotypes, fine specificities, IgD-cross-linking abilities, avidities, and abilities to activate B cells in vitro and in vivo. Three of the monoclonal antibodies tested bound to the Fc fragment of IgD with relatively high avidity and were effective at cross-linking IgD, since they precipitated soluble IgD and rapidly capped B cell membrane IgD. These were the only antibodies tested that induced B cell DNA synthesis in vitro and were the most effective antibodies at inducing in vivo increases in B cell size and DNA synthesis and in vitro and in vivo increases in B cell surface Ia expression. Two antibodies bound to the Fd fragment of IgD with relatively high avidity but could not rapidly cap cell membrane IgD or precipitate soluble IgD even in the presence of 2% polyethylene glycol. These high-avidity, poorly cross-linking antibodies were unable to stimulate B cell DNA synthesis in vitro and were much less effective than the first group of anti-delta antibodies at stimulating in vivo increases in B cell DNA synthesis, size, or surface Ia expression or in vitro increases in surface Ia expression. One antibody, which bound to the Fc fragment of IgD with an intermediate avidity, was unable to rapidly cap B cell membrane IgD or precipitate soluble IgD in saline, but could precipitate soluble IgD in the presence of 2% polyethylene glycol. This antibody failed to induce B cell DNA synthesis in vitro and was as effective as the higher-avidity, poorly cross-linking antibodies at stimulating increases in B cell size, surface Ia expression, and DNA synthesis in vivo, and surface Ia expression in vitro. One antibody, which bound to the Fd fragment of IgD with low avidity and was unable to precipitate soluble IgD or to cap cell membrane IgD, had little ability to activate B cells by any of the parameters studied. Each of the monoclonal anti-delta antibodies, regardless of isotype or fine specificity, when bound to agarose to increase its ability to cross-link IgD, was mitogenic for B cells in vitro. None of the monoclonal antibodies to IgD of the a allotype stimulated B cells from b allotype mice to increase their size, surface Ia expression, or synthesis of DNA in vitro or in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)