Surface immunoglobulins of B and T lymphocytes: molecular properties, association with the cell membrane, and a unified model of antigen recognition

Changes in the expression of Ig-associated proteins on B lymphocytes activated by anti-IgM antibodies

Binding of antigen to receptor complexes on B cells elicits a cascade of intracellular signalling events leading to proliferation and, together with T-cell help, Ig secretion. Components of the antigen receptor (AgR) complex have been demonstrated to be either covalently bound or associated with surface Ig (sIg) molecules. The function of these proteins is still unknown. In order to address this question, we have stimulated B cells with anti-mu antibodies and have studied possible changes in the expression of AgR complexes. After anti-mu stimulation, the IgM molecules disappeared rapidly from the cell surface together with the covalently bound proteins. The IgM molecules were internalized and probably degraded. The IgM-associated heterodimer Ig-alpha/Ig-beta was also removed from the cells, leaving the IgD-associated heterodimer unaffected. Two proteins showed an enhanced association with sIg after 15 min and then were gradually removed from the cell surface. Two other proteins became increasingly attached to sIg. This association remained stable for the rest of the culture period (up to 4 h). Further studies are underway to characterize these proteins more closely and to examine possible interactions with downstream members of the signalling cascade.

A 30-kDa protein is disulfide linked to IgM on normal and neoplastic murine B lymphocytes

Four cloned continuously cultured mouse B lymphoma cells of the lines WEHI-279, BCL1 and 38C-13 were used to study the surface presentation of IgM. Cells of all lines expose, in addition to mu 2L2, various, disulfide-linked subunits as already shown for normal B lymphocytes (Koch, N. and Haustein, D., Mol. Immunol. 1982. 19:477). In contrast to normal B cells, B lymphoma cells also carry IgM structures of higher molecular weight than mu 2L2 on their surface. Furthermore, B lymphoma cells of all investigated lines expose a mu 2L2 structure to which a 30-kDa protein is disulfide-linked. This 30-kDa protein could also be identified on normal mouse B lymphocytes when more drastic conditions were employed. mu 2L2 disulfide linked to this 30-kDa protein is the main IgM structure on normal and neoplastic B cells whereas only small amounts of mu 2L2 (unlinked to this protein) can be detected.

Lymphocyte surface proteins recognized by an anti-thymocyte-globulin

Rat lymphocyte surface proteins were labelled by lactoperoxidase-catalysed iodination and immunoprecipitated by an anti-thymocyte globulin (ATG). ATG-Reactive proteins from cortical and medullary thymocytes, peripheral T cells and B cells were examined by gel electrophoresis. Several surface molecules were identified which showed differences in their distribution in the four cell populations. A 105 000-Da component was found in relatively high density on thymocytes and peripheral T cells only. Two high-molecular mass components of 205 000 Da and 190 000 Da were observed which were highly specific for peripheral T cells. These molecules appeared to share antigenic determinants with two other proteins found on the surface of both peripheral T cells and thymocytes. Limited proteolysis with Staphylococcus aureus protease showed these four proteins to have a similar peptide composition indicating the existence of a family of related surface molecules with varying expression on different cells. One of these T cell-specific proteins (P 190) was present at a comparatively high density, making up about 4% of the membrane protein. The main protein recognised by ATG was a component with a molecular mass of 175 000 Da found on all lymphocytes which made up about 7% of membrane protein. The other ATG-reactive proteins were all minor components of the cell surface, constituting less than 1% of membrane protein.

T lymphocytes specific for immunoglobulin allotype. II. Cloned Igh-1b-specific cytotoxic T cells

This study describes long-term-cultured lines and clones of cytotoxic T cells (Tc) with specificity for determinants of the Igh-1(b) immunoglobulin allotype. These Tc clones were initiated by repeated stimulation of immune spleen cells from BALB/c mice with an Igh-1(b)-producing myeloma, and then they were maintained in medium supplemented with mitogen-induced growth factors in the absence offurther antigenic stimulation . The lytic potency of these clones was 30-100-fold greater than the primary cultures from which they were derived, and specificity studies showed them to be lytic for Igh-1(b) targets and not for targets expressing Igh-1(a) or Igh-4(b), nor the lipopolysaccharide blasts . Finally, soluble preparations of Ig were tested for their ability to block lysis of labeled Igh-1(b)-expressing targets. The results showed that Igh-1(b) and not other immunoglobulin allotypes or isotypes could block lysis, and that the mechanism of lytic inhibition is due to Igh-1(b)-induced autolysis of the killer cells.

Expression of an antigen receptor on T cells does not require recombination at the immunoglobulin JH-C mu locus

Considerable evidence has accumulated suggesting that the antigen receptor(s) on T cells is coded for by genes for the variable (V) region of the immunoglobulin heavy (H) chains. In B cells, a complete gene for the immunoglobulin VH region is formed by somatic recombination of VH and joining region heavy chain (JH) gene segments [through an intermediate diversity(D) region gene segment]. In an attempt to determine whether a complete immunoglobulin VH region is expressed on T cells that bear an antigen receptor, we analyzed the restriction map of the JH-C mu locus in genomic DNA from two cloned murine cytotoxic T-lymphocyte (CTL) lines specific for the x-ray-induced leukemia RL male 1. We found no rearrangement of the JH C mu locus in the CTL lines, indicating that the T-cell antigen receptor(s) in these CTLs is not coded for by a complete immunoglobulin VH gene formed by joining of VH, (DH), and JH genes. In addition, we determined that C mu genes on both chromosomes were present and that there was no rearrangement of the C alpha, C kappa, or lambda chain genes in these CTL cells.

Spatial structure of immunoglobulin molecules

Immunoglobulin molecules of the class G (antibody molecules) consist of two heavy chains (50,000 dalton molecular weight) and two light chains (25,000 dalton). The overall shape is a Y with the arms formed by the light chains and the N-terminal half of the heavy chains in tight association. The stem is formed by the C-terminal halves of the heavy chains. The heavy and the light chains fold into globular domains of molecular weights of 12,000 dalton. There are four domains of the heavy chain and two of the light chain. All these domains show a similar fold, consisting of two B-sheets but display considerable differences in detail. The N-terminal variable domains of heavy and light chains and specifically the hypervariable polypeptide segments of the domains, located at the tips of the Y, constitute the antigen and hapten binding site. The nature of the amino acid residues of the hypervariable loops determines the shape and the specificity of the antibody. All domains pair tightly laterally, except the CH2 domains of the heavy chain. This domain has carbohydrate bound which prevents lateral association. Longitudinal interaction between the domains is loose and allows flexibility in the arrangement. Flexibility is probably of significance for antibody function. Arm (Fab) and stem (Fc) parts are linked by the hinge peptide which contains a segment with a unique conformation of two parallel poly-proline helices. Antigen binding triggers effector functions of antibodies. Antigen binding is at the tips of the Y-shaped antibody, but effector functions are displayed by the stem part. It is an open question whether conformational changes of the antibody molecule play a significant role in the trigger mechanism.

The immunoglobulin mu constant region gene is expressed in mouse thymocytes

It has been a matter of controversy whether the functional capacity of T cells to discriminate between antigens is mediated via immunoglobulin, an immunoglobulin-like molecule, or by the product(s) of unrelated genes. The progenitors of immunoglobulin-secreting cells, B cells, express membrane-bound immunoglobulin as the antigen-specific receptor on their surface. For T cells, although products of immunoglobulin heavy chain variable region genes are implicated as receptor components, there has been no compelling immunochemical evidence for participation of either immunoglobulin light chains or heavy chain constant regions (see refs 2-6 for the disparate views). Recently, using cloned immunoglobulin DNA sequences as hybridization probes, we have demonstrated that the immunoglobulin Cmu gene, but not the Cmu gene, is expressed as polyadenylated RNA in some T cell tumour (T lymphoma) cell lines. Individual T lymphoma lines yielded up to three discrete mu RNA species of different sizes (1.9, 2.2 and 3.0 kilobases), each species being different in size from the major mu RNA species present in B lymphoma cells (2.4 and 2.7 kilobases). We show here that cells from the normal mouse thymus contain mu RNA species, indistinguishable in size from those in T lymphoma cells, but contain little if any kappa RNA.

Expression of the immunoglobulin C mu gene in mouse T and B lymphoid and myeloid cell lines

We have used cloned nucleotide sequences as probes to search for immunoglobulin RNA in T and B lymphoid and nonlymphoid tumor cell lines. Polyadenylated RNA from the cells was fractionated electrophoretically by size and fixed to diazobenzyloxymethyl-paper, and then immunoglobulin RNA species were revealed by hybridization with 32P-labeled cloned sequences for mu heavy chain and kappa light chain. kappa mRNA was detected only in B lymphoid lines known to synthesize kappa chains. No mu RNA was detectable in erythroleukemia, mastocytoma, or sarcoma cells. RNA bearing mu constant region (C mu) sequences, however, was detected in four of nine T lymphoma and four of five myeloid tumor cell lines tested, as well as in B lymphoma and "pre-B" Abelson lymphoma cells. In contrast to the single mu RNA species of 2.6 kilobases (kb) in a plasmacytoma, individual T lymphoma and myeloid lines yielded up to three discrete mu RNA species of apparent size 1.9, 2.2, and 3.0 kb, each different from the two mu RNA species in a B lymphoma line (2.4 and 2.7 kb) or the single species in "pre-B" lymphoma cells (2.9 kb). Both chromosomal complements of the C mu gene in STRij-4 T cells were found to be rearranged from their embryonic (germ line) context. The results suggest that the C mu gene functions not only in T cells, wherein the ability to recognize antigen is long established, but also in myeloid cells.

Radio-iodinated surface proteins of electrophoretically separated rat lymphocytes

Rat thymocytes and lymph node cells were separated into three T and one B subpopulation by means of free flow electrophoresis. The surface proteins of the separated cells were labeled by lactoperoxidase catalysed radioiodination. Most of the label was demonstrated to be at the cell surface. The labelled cells were either lysed in sodium dodecyl sulphate or treated with Nonidet P-40 to extract cell proteins. The radioiodinated proteins were analysed on sodium dodecyl sulphate polyacrylamide gel electrophoresis followed by autoradiography. In contrast to sodium dodecyl sulphate Nonidet P-40 did not extract cell proteins completely. Furthermore the degree of extraction varied considerably in the different cell populations. 58% of protein bound radioactivity was extracted from thymocytes, 67% from peripheral T cells and 81% from B cells. Gel electrophoresis revealed that four proteins were not extracted at all, whereas five components were better soluble in Nonidet P-40 than all other proteins. One protein was extracted from B cells only although it was present in all cells. Although the surface protein patterns of the four lymphocyte subpopulations were rather similar, distinctive differences could be found. B cells had six labelled proteins which seemed to be absent in the other cells, In the T cell group, three protein bands were identified, each with specificity for peripheral T cells, thymocytes and all T cells respectively. Four other proteins were found which showed quantitative differences between the four cell groups.

Molecular properties of T lymphoma immunoglobulin. I. Serological and general physicochemical properties

Immunoglobulin (Ig) released into the medium by monoclonal continuously cultured murine T lymphoma cells of the lines WEHI-22 and WEHI-7 was isolated by serological precipitation or solid-phase immunoadsorption techniques. The intact immunoglobulin had an electrophoretic mobility on sodium dodecyl-sulphate (SDS) containing polyacrylamide gels comparable to that of IgG (mass 150,000). This mobility was significantly faster than that of '7S' IgM of murine B lymphocyte surfaces. The T lymphoma immunoglobulin consisted of a pair of heavy chains linked by disulphide bonds and light chains non-covalently bound to the heavy chains. The isolated heavy chains migrated slightly faster than the mu chains of MOPC 104E IgM. Some, but not all, antisera directed against mu chains of normal mouse serum IgM bound T lymphoma immunoglobulin apparently via a cross-reaction localized to the Fd fragment. These data indicate that immunoglobulin of T lymphoma cells and, presumably normal T lymphocytes, represents an immunoglobulin isotype which is distinct from those immunoglobulins found on the B cell surface.