Rearrangements and deletions of immunoglobulin heavy chain genes in the double-producing B cell lymphoma I.29

The control of membrane and secreted heavy chain biosynthesis varies in different immunoglobulin isotypes produced by a monoclonal B cell lymphoma

The control of production of the membrane (m) vs secreted (s) forms of immunoglobulin heavy chains was investigated in a panel of cell lines expressing different heavy chain classes but identical light chains (lambda) and variable regions. These cell lines could be induced towards Ig secretion by mitogen treatment. During this process a shift from m to s heavy chain production takes place. Here we show that, similarly to IgA- and IgE-producing B cells, in IgG2a-producing I.29 cells the gamma m-gamma s shift was accompanied by a shift in the corresponding mRNAs, with a decrease of gamma m mRNA and an increase of the gamma s mRNA in LPS-stimulated cells. By contrast, the micron mRNA was increased in LPS-stimulated IgM-producing cells, albeit these cells synthesized reduced amounts of micron polypeptides. The utilization of the translational level in the early steps of B lymphocyte maturation thus distinguishes the mode of regulation of mu chains from those of the other isotypes. In addition, in B cells a post-translational event blocks the secretion of IgM but not of IgG or IgE.

Differentiation in the murine B cell lymphoma I.29: individual mu + clones may be induced by lipopolysaccharide to both IgM secretion and isotype switching

Cells from the monoclonal B cell lymphoma I.29 expressing surface IgM (mu +) are capable of differentiating in vitro to IgM secretion and of switching to IgA or IgE production in response to lipopolysaccharide (LPS) stimulation. To determine whether a single mu + B cell is capable of undertaking both differentiative pathways (isotype switch and plasma cell differentiation) I.29 mu + cells were cloned by limiting dilution and a panel of clones were analyzed by immunofluorescence, endogenous labeling and Northern blotting. While 100% of the clones could differentiate toward IgM secretion, only a proportion of them (greater than 70%) also switched to IgA and/or IgE production. Certain clones switched preferentially to a specific isotype. Taken together with the observation that C gamma genes were never the target of switching in our experiments, these data suggest that individual mu + clones from the I.29 lymphoma are "precommitted" as for their switching potentials. The subclones that showed a high frequency of switching to IgA transcribed the germ line C alpha gene(s), suggesting a role for chromatin structure in determining the isotype switch specificity. Switch variant clones expressing either IgA or IgE on the cell surface were isolated and found capable of further differentiating toward Ig secretion in response to LPS. On the contrary, we could not induce switch to IgA in IgE-producing cells. Unlike mu + and alpha + cells, all the switch variant clones expressing IgE tested by endogenous labeling constitutively secreted large amounts of IgE in the supernatants even in the absence of LPS stimulation.

Components of immune function studied with lymphoid tumors

Lymphoid tumors have proven to be productive models for investigating genetic, biochemical, physiological and developmental mechanisms of lymphoid cells. Lymphoid tumors of a given lineage exhibit a spectrum of phenotypes from clones whose features overlap extensively with their normal counterparts to clones whose features are not obviously represented in normal lymphoid populations. In addition to the phenotypic continuum represented in a library of related tumors, many clones exhibit phenotypic diversity spontaneously or in response to various stimuli. As laboratory technology has advanced many interesting biological features of lymphoid tumors have become amenable to precise analysis. Progress made in understanding the mechanisms that underly the phenotypic diversity and plasticity of lymphoid tumor cells has the dual impact of improving our knowledge of malignant and normal lymphoid cells.

Surface immunoglobulin light chain expression in pre-B cell leukemias

Cytofluorographic analysis of surface immunoglobulin (sIg) light chain clonal excess (CE), defined as (%kappa+ - %lambda+)/(%kappa+ + %lambda+) cells per discrete level of fluorescence intensity, was carried out on mononuclear cells of 32 leukemic patients. Eight demonstrated sIg light chain CE, including four blastic chronic myeloid leukemias (BL-CML), three "null" acute lymphoblastic leukemias (ALL), and one leukemic lymphoblastic lymphoma. Six of the leukemias demonstrated a kappa CE and two had a lambda CE. Sorted kappa+ PB cells from a BL-CML patient were shown to have a diploid DNA stem line and to bear the "common" ALL antigen. To provide further support for our finding of the expression of sIg light chains in ALL, we studied the REH cell line, derived from a "common" ALL patient and found cytoplasmic mu heavy chain and surface Ig lambda CE. Nucleic acid blotting experiments on REH revealed that both kappa genes had been deleted and that lambda genes had been rearranged, as expected in B cells expressing lambda light chains. Moreover, REH cells contained mu and lambda RNA. When REH cells were treated with TPA the amount of mu chain RNA increased by approximately fivefold and the amount of lambda chain RNA increased by approximately twofold. The finding of sIg light chain in pre-B cell leukemias and in the REH cell line, suggests that these leukemic cells are further differentiated along the B-cell lineage than was previously believed.

The commitment of secretory cells to the selective expression of immunoglobulin CH genes is determined by the available concentrations of the triggering ligand

The property of lipopolysaccharide to induce B cells to both proliferate and differentiate to IgM, IgG3 and IgG2b expression can be ascribed either to a precommitted sequence of molecular events in the activated B cells or, alternatively, to separate activities which independently modulate the two events. To discriminate between these two possibilities we have investigated the relationship between the doses of the polyclonal stimulus and the commitment of the activated cells to proliferate and to produce various isotypes. Low doses of ligand supported proliferation as well as IgM but not IgG2b secretion. On the contrary, high doses of the same ligand were less efficient in supporting proliferation but strongly induced heavy chain class switch. The effect of lipopolysaccharide concentrations on CH genes expression decayed with the distance from mu to the respective C gamma gene. Although we could define different B cell subsets on the basis of their proliferative response to various doses of the ligand, all these B subpopulations were found to be multipotential in terms of their switching capacity. Taken together our data show that in lipopolysaccharide cultures B cell proliferation and heavy chain switch are two events completely dissociable on the basis of their inducing requirements.

Biosynthesis of membrane and secreted epsilon-chains during lipopolysaccharide-induced differentiation of an IgE+ murine B-lymphoma

A switch variant of the I.29 murine B-cell lymphoma expressing membrane IgE and inducible by lipopolysaccharide (LPS) to increase the rate of IgE secretion was characterized. The cells (I.29 epsilon +2) express membrane-bound IgE, and also secrete considerable amounts of IgE when grown in regular culture medium. Membrane and secreted IgE contain structurally different heavy chains. The former is constituted by a 93-kd molecule (epsilon m), while secretory chains (epsilon s) have an apparent mol. wt of 86,000. Both epsilon m and epsilon s are heavily glycosylated: in the presence of tunicamycin their apparent mol. wt is reduced by approx. 35% (61 kd for epsilon m and 56 kd for epsilon s). Glycosylation is necessary for membrane expression and for secretion of IgE molecules. Stimulation with LPS leads to the disappearance of IgE molecules from the cell surface (determined by radioiodination) although epsilon m-chains are still synthesized, suggesting a defective transport of membrane IgE in LPS-treated cells. The epsilon m:epsilon s ratio decreases upon LPS stimulation. A similar change can be observed in the messenger RNAs specific for epsilon m and epsilon s, possibly suggesting a major pretranslational control for epsilon m and epsilon s biosynthesis.

Surface immunoglobulin light-chain expression by the "common" all cell line REH

To provide further support of the B-cell lineage of "common" acute lymphoblastic leukemia (ALL), we studied the REH cell line and found not only cytoplasmic heavy chain but also surface lambda light-chain expression by flow cytometry. Nucleic acid blotting experiments on REH revealed that both kappa genes had been deleted and that lambda genes had been rearranged, as expected in B cells expressing lambda light chain. Moreover, REH cells contained mu and lambda RNA. When REH cells were treated with TPA the amount of mu chain RNA increased by approximately fivefold and the amount of lambda chain RNA increased by approximately twofold. The finding of surface immunoglobulin lambda light chain in the REH cell line, suggests that these leukemic cells are further differentiated along the B-cell lineage than was previously believed.

Differentiation in the murine B cell lymphoma I.29: inductive capacities of lipopolysaccharide and Mycoplasma fermentans products

Cells from the murine B lymphoma I.29, expressing IgM or IgA of identical idiotype, were found inducible by lipopolysaccharide to differentiate into plasma cells. Within 3 days, differentiating cells lost membrane-bound immunoglobulin (Ig) and accumulated large quantities of intracytoplasmic Ig. At day 6 of culture, IgA secretion increased 50-100-fold, as determined by enzyme-linked immunoassay. Proliferation increased for the first days of culture but decreased thereafter; by day 10 very few viable cells were present in lipopolysaccharide-stimulated cultures. Similar results were obtained by culturing I.29 cells in the presence of supernatants of certain B cell lines (e.g. BFO.3). The finding of a strict correlation between the inductive activity and presence of contaminating Mycoplasma fermentans suggested that factor(s) released by mycoplasma were responsible for the mitogenic activities. This was further indicated by the findings that: the supernatants of BFO.3 that were rendered free of mycoplasma were not inductive, and a nonactive cell line could be made active by infection with supernatants of BFO.3 cells containing viable microorganisms. Thus, supernatants of mycoplasma-infected cell lines may act as potent polyclonal activators on both normal and malignant B lymphocytes. The ability to induce membrane Ig on 70Z/3 cells indicates that mycoplasma-related mitogens are also active on pre-B cells. The possibility of mycoplasma contamination should thus be carefully excluded when presumptive factors of cloned cell lines are being evaluated.

The role of glycosylation in secretion and membrane expression of immunoglobulins M and A

The role of glycosylation in membrane expression and secretion of IgM and IgA was investigated in murine lymphoma and hybridoma cell lines, derived from I.29 tumor, which synthesize IgM or IgA with identical variable regions. Tunicamycin, a selective inhibitor of N-linked glycosylation, prevented the membrane expression of both isotypes, as demonstrated by immunofluorescence, radioiodination and endogenous labeling experiments. Selective immunoprecipitation and immunochemical analysis of membrane, intracellular and secreted molecules permitted us to determine the amount of membrane heavy chain externalized in the presence or absence of tunicamycin. Id 150 and Id 43, two I.29-derived hybridomas secreting IgA and IgM respectively, were differently affected by tunicamycin. While secretion of IgM was inhibited to greater than 95%, no inhibition of secretion of non-glycosylated IgA could be detected in Id 150 cells. These results indicate that different requirements for glycosylation exist in the biosynthetic pathways of immunoglobulin isotypes, and suggest that distinct intracellular transport systems may operate for membrane and secreted alpha-chains.