Immunoglobulin isotype expression

Epstein-Barr virus-induced lymphoblastoid cell lines derived from the peripheral blood of patients with X-linked agammaglobulinemia can secrete IgM

We have established lymphoblastoid cell lines (LCL) from the peripheral blood of three individuals with X-linked agammaglobulinemia as well as three of their immunodeficient first-degree relatives. Lines could be induced with Epstein-Barr virus only when T lymphocytes were depleted from total mononuclear leukocytes. The LCLs derived from XLA patients expressed characteristics of IgM-secreting plasmacytes, including intense cytoplasmic fluorescence after staining with anti-mu, easily detectable amounts of IgM in culture supernatants, and radiolabeled IgM with both heavy and light chains in culture media and cell lysates. The cell lines induced from blood of the first-degree relatives of these patients were more varied. They often exhibited multiple isotypes for both heavy and light chains in different cells or expressed a B-lymphocyte phenotype (easily detectable surface Ig but no Ig secretion). These studies suggest that B-cell precursors are present in peripheral blood of individuals with classical XLA . Differentiation of such cells to mature cells of the B lineage can be accomplished using Epstein-Barr virus after removal of T cells.

Hyper IgM immunodeficiency. A primary dysfunction of B lymphocyte isotype switching

Immunological evaluations (lymphocyte markers, B cell differentiation, T cell function) were performed on peripheral blood mononuclear cells from four individuals with hyper IgM immunodeficiency. Number, proportion, and proliferation of T lymphocytes and T lymphocyte subpopulations were relatively normal in affected individuals. The percentage and number of B cells expressing surface IgM and IgD were either normal or elevated in both blood and lymph nodes. However, surface IgG- and IgA-bearing B lymphocytes were completely absent. In vitro stimulation of blood lymphocytes with both T cell-dependent and T-cell independent polyclonal B cell activators resulted in normal numbers of IgM plasma cells and IgM secretion in cultures, but failed to induce any IgG- or IgA-producing cells. This failure of isotype switching was intrinsic to the B cell population and did not involve aberrant T cell help or suppression. Therefore, individuals with this disorder possess an intrinsic B cell dysfunction that is not related to abnormal T cell regulation.

Biosynthesis and structure of membrane and secretory immunoglobulins

Almost all of the body's extracellular immunoglobulin (Ig) is derived from Ig-secreting plasma cells of lymphoid tissues. The secreted material is a heterogeneous mixture of different classes and specificities. Lymphoid tissues also contain a large number of essentially non-secretory cells--B lymphocytes--which bear Ig firmly associated with their plasma membranes. Ig molecules thus exist in two functionally different forms, as membrane-bound antigen receptors on the surface of B lymphocytes on the one hand, and as humoral secreted Ig antibodies on the other. On B cells, membrane-bound heavy chains have an apparent mol. wt. slightly larger than that of secreted heavy chains from plasma cells. Membrane-bound but not secreted heavy chains bind detergents, thus suggesting the presence of a hydrophobic region in membrane-bound heavy chains, which is absent in secreted heavy chains. Most investigations have dealt with immunoglobulin M. The two types of IgM heavy chains differ at their carboxy termini. Recent investigations at the nucleic acid level demonstrate that membrane-associated mu chains contain a 41-residue hydrophobic tail adjacent to the last constant domain, whereas secretory mu chains contain a 20-residue hydrophilic tail. At the present time, evidence is accumulating that all membrane-bound Ig heavy chain classes may contain similar hydrophobic structures necessary for anchorage of the molecules into the lipid bilayer.

Immunoglobulin isotype expression. II. Frequency analysis in mitogen-reactive B cells

The frequency of lipopolysaccharide (LPS)-reactive B cells developing into clones that secrete the various immunoglobulin (Ig) classes has been determined in vitro, in cells from BALB/c mice, under culture conditions which detect all growth-inducible cells. Secretion of the different Ig classes was assessed in the protein A plaque assay for Ig-secreting, plaque-forming cells by using developing antisera specific for either IgM, IgG1, IgG2a, IgG2b, IgG3 or IgA. In all lymphoid organs tested (spleen, bone marrow, mesenteric lymph nodes and thoracic duct), a considerable proportion of all B cells (5-20%) was induced by LPS to yield a clone of IgM-secreting cells. Frequency determinations of LPS-reactive cells giving rise to descendants secreting other Ig isotypes revealed that, on an average, and irrespective of the origin of the cells, 7% of all IgM-secreting clones switched to the synthesis of IgG1, 39% to IgG2, 41% to IgG3 and 1% to IgA. Roughly the same frequencies of B cells switching CH gene expression were found among spleen cells of athymic nude mice. No correlation was found between the clonal frequencies of CH gene expression in polyclonally activated B cells and the in vivo "background" Ig-secreting cells suggesting that the CH gene expression in B cells is influenced by the quality of stimulation and other regulating influences.

Expression of IgD may use both DNA rearrangement and RNA splicing mechanisms

From a library of mouse sperm DNA, we have isolated two overlapping clones which contain the C(delta) gene. One of these clones also contains the C(mu) gene. The C(delta) gene is separated from the C(mu) membrane exons by approximately 2 kilobases (kb) of DAN. The C(delta) gene was identified by (a) hybridization to poly(A)(+)RNA prepared from the IgD-producing rat plasma cell tumor IR731, and (b) homology of a translated nucleotide sequence to the amino acid sequence of the human delta chain. The C(delta) gene spans 8 kb of DNA in the germ line. Plasmid subclones of the C(delta) gene were used as probes in Southern and RNA blot experiments. RNA blot analysis of cytoplasmic poly(A)(+)RNA from IR731 and a mu(+)delta(+) B-cell hybridoma revealed 1.6- and 2.7-kb delta mRNA species with different 3' ends, which presumably encode the secreted and membrane-bound forms, respectively, of the delta chain. Southern blot analysis of DNA from two mu(+)delta(+) lymphomas revealed that the C(delta) gene is in the germ-line configuration in each case. Restriction map analysis of C(mu) and C(delta) genomic clones isolated from a library of normal mu(+)delta(+) B-cell DNA also gave no evidence for DNA rearrangement in the region between the C(mu) and C(delta) genes. Taken together, these data suggest that IgD expression in mu(+)delta(+) B cells does not involve a V(H)-to-C(delta) DNA switch rearrangement. We propose that simultaneous expression of C(delta) and C(delta) with a single V(H) gene is mediated by two alternative routes of RNA processing of a primary nuclear transcript which contains the V(H), C(mu), and C(delta) genes. In contrast, analogous experiments with myeloma IR731 DNA revealed that the C(mu) gene has been deleted from the myeloma DNA and that the C(delta) gene has undergone DNA rearrangement, presumably including a switch recombination of the V(H) gene from the C(mu) to the C(delta) gene. These results indicate that two alternative mechanisms may be used in the expression of IgD molecules-RNA splicing in B cells and DNA rearrangement in plasma cells.

Surface Ig on rabbit lymphocytes. Rabbit B and T cells are distinct populations

Rabbit peripheral blood lymphocytes (PBL) were analyzed by immunofluorescence using anti-T cell conjugates and anti-Fab, anti-a1 allotype, anti-IgM and anti-IgA conjugates. In addition, T cells were demonstrated by rosetting with papain-treated homologous erythrocytes. Control experiments, using acid treatment and incubation at 37 degrees C for 18 h after or without pronase treatment, revealed the endogenous origin of all surface determinants tested. A good correlation was found between results obtained with the two anti-T cell conjugates used and the T rosette test on PBL and on lymphoid cells isolated from various organs. In lymphocytes isolated from peripheral blood and from various lymphoid organs, the percentages of T and B cells were respectively 45 and 38 for PBL, 10 and 46 for bone marrow, 27 and 31 for appendix, 40 and 45 for spleen, 42 and 46 for Peyer's patches, 96 and 0.3 for thymus and 70 and 16 for peripheral lymph nodes. The percentage of "null" cells in lymphocyte populations derived from bone marrow and appendix is rather high. The final percentages of T and B cells in rabbit PBL depend to a significant extent on the method of isolation, especially isolation by Ficoll-Hypaque centrifugation results in a depletion of T cells. Moreover, a rather impure lymphoid cell suspension is obtained. In double incubation experiments, T cells (as defined by T cell antigen(s) or rosette formation) and B cells (Fab-bearing cells) were entirely different subpopulations. Allotypes of the a locus could not be detected on the surface of T cells. The results are discussed with respect to genetic coding of antigen receptors on B and T cells.

The ontogeny of B lymphocytes. V. Lipopolysaccharide-induced changes of IgD expression on murine B lymphocytes

Murine splenocytes contain two minor subpopulations of B cells, one inducible by lipopolysaccharide to convert within 2 h from IgD- to IgD+ and the other to change from IgD+ TO IgD-. These two subpopulations can be separated by density centrifugation. Their relative proportions show a marked age dependency: IgD- leads to IgD+ cells are more frequent in suckling mice, while IgD+ leads to IgD- inducible cells become predominant in mice older than 3 weeks. The age dependency observed in the relative proportions between the two cell types suggest that they are ontogenetically related as progenitor-successor. This hypothesis is corroborated by phenotype analysis of the two subsets, revealing IgD- leads to IgD+ cells as IgM+, Ia+, complement receptor- (CR-) and IgD+ leads to IgD- cells as IgM+, Ia+, CR+. Our data show that IgD and CR are expressed concomitantly during B cell differentiation. On further differentiation, induced by lipopolysaccharide, both markers are lost from the cell surface at different rates: IgD decreases significantly in a very short period (less than 2.5 h) while induction of a decline in CR requires longer culture periods (greater than 8 h). Th: loss of IgD may thus herald an early differentiation event toward antibody-producing cells.