bcl-2 protein in non-small-cell lung carcinoma

BACKGROUND

The proto-oncogene bcl-2 encodes a protein that inhibits programmed cell death (apoptosis). The protein is expressed in basal cells in normal human epithelium, but no data are available on the frequency or clinical importance of its expression in carcinoma. We studied bcl-2 expression in patients with non-small-cell lung carcinoma and correlated this phenomenon with survival.

METHODS

Immunochemical analysis with a monoclonal antibody specific for bcl-2 was used to detect the protein in tumor samples from 122 patients undergoing surgery for squamous-cell carcinoma (80 patients) or adenocarcinoma (42 patients). The possibility that bcl-2 expression correlated with survival was investigated with use of the log-rank test, hazard ratios, and their confidence intervals.

RESULTS

We detected bcl-2 protein in 25 percent of squamous-cell carcinomas (20 of 80) and 12 percent of adenocarcinomas (5 of 42). In adjacent normal respiratory epithelium, bcl-2 was expressed only in basal cells. Survival at five years was higher among patients with bcl-2-positive tumors, both in the group as a whole (P < 0.1) and in the group with squamous-cell carcinoma (P < 0.02). Patients 60 years of age or older who had bcl-2-positive tumors had the best prognoses, both in the group as a whole (P < 0.02) and in the group with squamous-cell carcinoma (P < 0.01).

CONCLUSIONS

The proto-oncogene bcl-2 is abnormally expressed in some lung carcinomas, and its expression may have prognostic importance.

A new macrophage differentiation antigen which is a member of the scavenger receptor superfamily

The antibodies Ki-M8, Ber-Mac3, GHI/61 and SM4 define a human macrophage-associated antigen with a relative molecular mass of 130,000 which we designate M130. The protein was purified by immunoaffinity chromatography and an N-terminal and three internal amino acid sequences were obtained. A cDNA fragment was initially obtained by polymerase chain reaction (PCR) using reverse-translated primers. Several variant cDNA clones, derived from alternative spliced messages, were obtained from a lipopolysaccharide-stimulated human monocyte library and were sequenced. The relative abundance of these variants was evaluated by a series of overlapping PCR reactions. The size of the most representative cDNA is 3.7 kb and closely agrees with the mRNA size of 3.8 kb determined by Northern blot analysis. The membrane protein encoded contains a leader peptide of 40 residues, a putative extracellular domain of 1003 residues, followed by a hydrophobic segment of 24 residues and a cytoplasmic domain of 49 residues. The extracellular domain was found to contain nine repeating elements, of about 110 residues, which are similar to those of the scavenger receptor superfamily.

ZB51: a monoclonal antibody reactive with human plasma cells

Several monoclonal antibodies have been raised in the past which react with human plasma cells but they have all shown additional reactions with other cell types. In this paper we describe a new monoclonal antibody, ZB51, which recognizes an intracellular antigen in normal plasma cells in cryostat tissue sections and cell smears, and which also reacts with neoplastic cells in most cases of myeloma and with plasma cell lines. The antibody shows minimal reactivity with a few eosinophils and myelocytes in bone marrow and stains a myeloid cell line. Whilst normal epithelium is not labelled, antibody ZB51 stains two carcinoma cell lines. Although it was not possible to characterize the target antigen in terms of molecular weight, the reactivity of antibody ZB51 with normal and neoplastic plasma cells makes it a useful new immunocytochemical reagent.

mb-1: a new marker for B-lineage lymphoblastic leukemia

The expression of the Ig-linked mb-1 polypeptide was analyzed by immunocytochemistry (alkaline phosphatase anti-alkaline phosphatase technique) using a specific monoclonal antibody in 165 cases of acute leukemia, with 88 being lymphoblastic (ALL) and 77 myeloid (AML). The purpose of the study was to investigate the specificity of this reagent for B-lineage cases and its reactivity on leukemias that coexpress myeloid and B-cell antigens (biphenotypic). The majority (89%) of 72 B-cell precursor ALL patients were positive. Of these, mb-1 was expressed in all 9 patients with early-B-ALL (CD10-, c mu-), in all 11 patients with pre-B-ALL (c mu+) and in the single case of B-ALL (smIgM+). Forty-three of 51 patients with common-ALL (CD10+, c mu+) were also positive. All 16 T-lineage ALL patients and 72 (93.5%) of the AML patients examined were mb-1 negative. Four of the 5 mb-1-positive AML patients were considered biphenotypic and expressed other B-cell antigens such as CD10, CD19, and/or cCD22 and all showed rearrangement of the Ig heavy chain genes. Within the AML cases, mb-1 and cCD22 were more useful than other B-cell antigens in detecting biphenotypic cases, and mb-1 showed the highest correlation with the clonal rearrangement of Ig heavy chain genes. These results indicate that mb-1 is a sensitive and specific reagent for B-lineage blasts that will aid in the classification of B-cell precursor ALL and in the identification of biphenotypic leukemia presenting as AML.

Detection of T and B cells in many animal species using cross-reactive anti-peptide antibodies

A wide range of lineage-specific Ag are detectable in the human lymphoid system using mAb, but only a few such markers are detectable in animal species. In this paper, we have investigated the interspecies reactivity of antibodies raised against intracytoplasmic peptide sequences from two T cell Ag (CD3 and CD5) and two B cell markers (the Ig-associated polypeptides encoded by the mb-1 and B29 genes). Immunocytochemical labeling of tissue sections showed that these antibodies cross-react widely between different species (including ungulates, rodents, and marsupials), staining B or T cell areas selectively in lymphoid tissue. The specificity of these antibodies for the animal homologues of the human T and B cell markers was confirmed for the rat by Western blotting analysis. The broad cross-reactivity of these antibodies appears to be due to the fact that they were raised against intracytoplasmic peptide sequences that are highly conserved between humans and rodents, i.e., 80% for mb-1, 85% for CD5, and 100% for CD3 and B29. This strategy should, in the future, widen the range of lineage-associated markers detectable in experimental animals.

Detection of T and B cells in many animal species using cross-reactive anti-peptide antibodies

A wide range of lineage-specific Ag are detectable in the human lymphoid system using mAb, but only a few such markers are detectable in animal species. In this paper, we have investigated the interspecies reactivity of antibodies raised against intracytoplasmic peptide sequences from two T cell Ag (CD3 and CD5) and two B cell markers (the Ig-associated polypeptides encoded by the mb-1 and B29 genes). Immunocytochemical labeling of tissue sections showed that these antibodies cross-react widely between different species (including ungulates, rodents, and marsupials), staining B or T cell areas selectively in lymphoid tissue. The specificity of these antibodies for the animal homologues of the human T and B cell markers was confirmed for the rat by Western blotting analysis. The broad cross-reactivity of these antibodies appears to be due to the fact that they were raised against intracytoplasmic peptide sequences that are highly conserved between humans and rodents, i.e., 80% for mb-1, 85% for CD5, and 100% for CD3 and B29. This strategy should, in the future, widen the range of lineage-associated markers detectable in experimental animals.

An immunocytochemical study of p53 and bcl-2 protein expression in Hodgkin's disease

In view of their reported reciprocal effects on apoptosis, the expression of p53 and bcl-2 proteins was studied in 46 cases of Hodgkin's disease by immunocytochemical labeling. We found p53 protein in Reed-Sternberg cells and their mononuclear variants in 16 of the 46 cases (34.7%) of Hodgkin's disease, mainly in a nuclear pattern. This restricted expression on Reed-Sternberg cells and variants supports their neoplastic nature. This overexpression of p53 protein in one third of Hodgkin's disease cases is similar to that seen in many other human malignancies. bcl-2 protein was present in mantle zone B cells and scattered T cells in all cases, and in 17 cases (37.7%) of Hodgkin's disease in Reed-Sternberg cells and their mononuclear variants. Six cases coexpressed both proteins, whereas in 18 cases neither was identified. There is no apparent relationship between p53 and bcl-2 protein expression, and on the basis of the present results there is no reason to suppose that they have any particular complementary effects on the neoplastic transformation in Hodgkin's disease.

The structure of the mu/pseudo light chain complex on human pre-B cells is consistent with a function in signal transduction

Prior to immunoglobulin (Ig) light (L) chain rearrangement, pre-B cells can express mu heavy (H) chains at the cell surface in association with pseudo (psi) L chains. This complex may be essential for B cell development. We have investigated the composition of the mu/psi L chain complex of a human pre-B cell line, in view of its potential role in transmembrane signal transduction. The mu/lambda receptor of a mature B cell line was analyzed in comparison. The mu/psi L chain complex is associated with disulfide-linked molecules that are homologous or identical to the mb-1 and B29 proteins, known to be integral components of membrane Ig receptors on mature B cells. Both receptors contain tyrosine (Tyr) kinase activity. In the mu/lambda receptor, the lyn and lck Tyr kinases could clearly be identified. The mb-1 and B29 proteins in both mu/lambda and mu/psi L chain receptors are substrates for in vitro phosphorylation on Tyr, but also on serine (Ser) and threonine (Thr) residues. The undefined mu-associated Ser/Thr kinase also phosphorylates the src-related kinases in the mu/lambda receptor and a 43-kDa mu-associated protein that is present in both complexes. The 43-kDa protein may be an integral part of both receptor types, or a transiently associated molecule instrumental in the signaling process. We conclude that the mu/psi L receptor on human pre-B cells fulfills the presently known criteria to function as a signal transduction unit.

CD68 reactivity of non-macrophage derived tumours in cytological specimens

AIMS

To investigate the presence of the macrophage associated antigen CD68 in non-haematopoietic tumours.

METHODS

Cytological specimens from non-macrophage derived tumours were stained using the alkaline phosphatase anti-alkaline phosphatase immunocytochemical method (APAAP) and three monoclonal anti-CD68 antibodies, Y1/82A, EBM11, and KP1.

RESULTS

Reactivity of malignant cells with one or more of the antibodies was seen in 11 out of 40 adenocarcinomas and in one of seven poorly differentiated carcinomas; other neoplasms, including 10 cases of squamous carcinoma, three of malignant melanoma, and four of oat cell carcinoma were negative. Monoclonal antibody KP1 gave the strongest staining and reacted with the highest proportion of neoplastic cells.

CONCLUSIONS

CD68 is expressed in a proportion of epithelial tumours although the labelling is usually less intense than in macrophages. Anti-CD68 antibodies should therefore be used as part of a panel in the diagnosis of poorly differentiated neoplasms in cytological material.

Heterogeneity of CD3 antigen expression in T-cell lymphoma

CD3 antigen expression was studied in a series of 98 T-cell lymphomas, using polyclonal antibodies which recognize this molecule in routinely processed, paraffin-embedded, tissue. We identified 40 cases in which CD3 was present on only a proportion of the neoplastic cells. This phenomenon of heterogeneous CD3 expression was commonest in pleomorphic T-cell lymphomas (22/42 cases) and in CD30 (Ki-1)-positive lymphomas (5/11 cases), and was less frequently observed in mycosis fungoides (4/18 cases) and not seen in T-cell lymphoblastic lymphoma (0/9 cases). CD3 expression was often related to cell morphology, with CD3 antigen being present on the smaller neoplastic cells but absent from the larger ones. The diagnostic significance of these observations is that, on occasion, it may be possible to diagnose a lymphoma as being of T-cell origin in paraffin sections by demonstrating a minor subpopulation of CD3-positive neoplastic cells.

Immunohistological patterns of myeloid antigens: tissue distribution of CD13, CD14, CD16, CD31, CD36, CD65, CD66 and CD67

A number of differentiation antigens on myeloid cells have been defined on the CD classification system by the four International Workshops on Human Leucocyte Differentiation Antigens. The distribution of eight of these antigens (CD13, CD14, CD16, CD31, CD36, CD65, CD66, CD67) have been studied in human tissues, with the aim of documenting their immunohistological patterns and their degree of myeloid restriction. CD13, the most widely distributed antigen, was found in skin, bile canaliculi, kidney and pancreas. CD14 was not restricted to monocytes and tissue macrophages, being also strongly expressed on dendritic reticulum cells. CD16 was expressed on granulocytes and tissue macrophages (alveolar and Kupffer cells) and in the red pulp of the spleen. CD31 and CD36 gave a characteristic staining of vascular endothelium, corresponding to their identification as the platelet glycoproteins gp IIa and gp IV. Antibodies against the most recently defined myeloid antigens (CD65, CD66 and CD67) appeared to be more specific for myeloid differentiation than previously described 'myeloid antigens'.

Expression of the Ig-associated heterodimer (mb-1 and B29) in Hodgkin's disease

Eighty-three cases of Hodgkin's disease were studied immunocytochemically for the presence of the Ig associated heterodimer (mb-1 and B29) which is believed to be a specific pan B-cell marker. These results were compared with those achieved using other B-cell markers against CD19, CD20 and CD22. Although a small number of cases of nodular sclerosis and mixed cellularity subtype showed positivity for CD19, CD20 or CD22, no case showed any reactivity with antibodies against mb-1 or B29. This contrasted markedly with the cases of lymphocyte predominance where all seven cases expressed one or more of the B-cell antigens, with six cases being positive for mb-1. These results confirm previous studies that have suggested lymphocyte-predominance Hodgkin's disease is of B-cell origin and different from the other subtypes. However, they do not provide support for the thesis that these other subtypes may also have a B-cell origin, albeit with a different phenotype to lymphocyte predominance.

Antibody By114 is selective for the 90 kD PI-linked component of the CD66 antigen: a new reagent for the study of paroxysmal nocturnal haemoglobinuria

This paper describes a monoclonal antibody which reacts with transfected cells carrying a gene (NCA-50/90) which has been shown to encode the human CD66 antigen. However, antibody By114 recognizes only a single 90 kD polypeptide from human neutrophils, whereas the antibodies which originally defined the CD66 antigen also recognize a larger 180-200 kD protein. We conclude that antibody By114 is selective for the smaller of the two CD66 gene products, which is a surface membrane phosphatidyl-inositol (PI)-linked molecule. The reactivity of antibody By114 on peripheral blood cells (positive on neutrophils, weak or negative on eosinophils, and negative on basophils, monocytes and lymphocytes) and myeloid precursor cells was identical to those of a reference CD66 antibody, as was the staining of leukaemic cells. However, the reactions of the two antibodies differed on kidney, liver and pancreas, and in cases of myeloma, Waldenström's macroglobulinaemia and lymphoma, indicating that By114 represents a new CD66 sub-specificity. Granulocytes from a case of paroxysmal nocturnal haemoglobinuria (PNH) were negative with antibody By114, indicating that it may be of value in detecting the defect in PI-linked surface molecules characteristic of this condition. Antibody By114 also stained formalin fixed paraffin embedded tissues and may therefore be of use in routine diagnostic histopathology.

Immunohistochemical detection of p53 and bcl-2 proteins in non-Hodgkin's lymphoma

We have investigated the immunohistochemical expression of p53 protein in 96 cases of non-Hodgkin's lymphoma using a panel of five antibodies. Positive neoplastic cells were found in 30 (31.2%) cases, which could be divided into two groups according to their patterns of reactivity with the different antibodies; i.e. those positive with both polyclonal and monoclonal antibodies, and those which were stained only by monoclonal antibodies PAb1801 and/or PAb240. Positivity was nuclear in all but six cases in which cytoplasmic staining was found. In view of the hypothesis recently raised that p53 protein induces apoptosis we have compared our results with parallel staining for bcl-2 protein since bcl-2 is believed to be important, at least in lymphomas, in suppression of apoptosis. Staining for bcl-2 protein was performed on 83 cases and it was shown that p53-positive cases accounted for 10 out of 17 (59%) of the bcl-2-negative lymphomas but only for 15 out of the 66 (23%) bcl-2-positive cases, suggesting a possible relationship between the expression of these two proteins. Thus, our data show that p53 protein is abnormally expressed in a substantial proportion of non-Hodgkin's lymphomas and bears a significant inverse relationship to bcl-2 protein expression. However the molecular basis of this expression remains to be elucidated.

Molecular cloning of ICAM-3, a third ligand for LFA-1, constitutively expressed on resting leukocytes

The co-ordinated function of effector and accessory cells in the immune system is assisted by adhesion molecules on the cell surface that stabilize interactions between different cell types. Leukocyte function-associated antigen 1 (LFA-1) is expressed on the surface of all white blood cells and is a receptor for intercellular adhesion molecules (ICAM) 1 and 2 (ref. 3) which are members of the immunoglobulin superfamily. The interaction of LFA-1 with ICAMs 1 and 2 provides essential accessory adhesion signals in many immune interactions, including those between T and B lymphocytes and cytotoxic T cells and their targets. In addition, both ICAMs are expressed at low levels on resting vascular endothelium; ICAM-1 is strongly upregulated by cytokine stimulation and plays a key role in the arrest of leukocytes in blood vessels at sites of inflammation and injury. Recent work has indicated that resting leukocytes express a third ligand, ICAM-3, for LFA-1 (refs 11, 12). ICAM-3 is potentially the most important ligand for LFA-1 in the initiation of the immune response because the expression of ICAM-1 on resting leukocytes is low. We report the expression cloning of a complementary DNA, pICAM-3, encoding a protein constitutively expressed on all leukocytes, which binds LFA-1. ICAM-3 is closely related to ICAM-1, consists of five immunoglobulin domains, and binds LFA-1 through its two N-terminal domains.

Immunohistological detection of human cytotoxic/suppressor T cells using antibodies to a CD8 peptide sequence

AIMS

To evaluate whether cytotoxic/suppressor T cells can be detected in paraffin wax embedded human tissue samples using antibodies to a synthetic CD8 peptide sequence.

METHODS

Polyclonal and monoclonal antibodies were raised against a 13 amino acid peptide sequence from the cytoplasmic portion of the alpha chain of the human CD8 molecule.

RESULTS

These antibodies specifically detected the native form of the CD8 polypeptide when tested by immunoprecipitation with radiolabelled T cells, and gave the expected staining pattern for cytotoxic/suppressor T cells in cryostat sections. Being raised in rabbits, the polyclonal antibodies were also useful for double labelling for CD8 in conjunction with monoclonal antibodies. CD8 positive cells could also be detected in paraffin wax embedded tissues. This was achieved without prior treatment of the sections if the tissue had been fixed in Bouin's fixative. When tissues had been exposed to conventional formalin fixation, preliminary microwave treatment was required.

CONCLUSIONS

These findings provide further evidence that antibodies against leucocyte associated antigens, capable of reacting on paraffin wax embedded tissue, can be produced by immunisation with synthetic peptide sequences.

Ultrastructural localization of bcl-2 protein

Previous cell subfractionation studies have indicated that bcl-2 is an inner mitochondrial membrane protein. We have sought to determine the ultrastructural localization of bcl-2 protein in lymphoma and breast carcinoma cell lines and biopsy material known to overexpress bcl-2 using immunoelectron microscopy. To avoid the possibility of processing artifacts, samples were prepared by three different methods: progressive lowering of temperature, cryosectioning, and freeze-substitution. In all instances the labeling of bcl-2 protein was relatively weak but the distribution the same. In both lymphoma and breast carcinoma tissues, bcl-2 protein was detected on the periphery of mitochondria: little labeling of either the mitochondrial matrix or cristae could be detected. Labeling was also detected on the perinuclear membrane and throughout the cytoplasm, as also indicated by confocal microscopy. These data therefore indicate that bcl-2 protein can be detected at several intracellular sites and that at the likely functional destination, the mitochondria, there appears to be, contrary to expectations, a preferential association with the outer membrane.

The B29 and mb-1 polypeptides are differentially expressed during human B cell differentiation

Surface immunoglobulin on mouse B cells is associated with a heterodimer comprising the products of the mb-1 and B29 genes. Here we report that antibodies raised against a peptide sequence from the intracytoplasmic C terminus of the B29 murine gene product detect the 37-kDa component of the human heterodimer, indicating that this component in man is also encoded by the B29 gene. The immunocytochemical reactivity of these anti-B29 antibodies was compared with those of antibodies to the mb-1 protein. Of 25 cases of acute lymphoblastic leukaemia (ALL), 24 were positive for mb-1 whereas B29 was expressed in only 13 cases. Most of these B29-positive ALL expressed immunoglobulin mu heavy chain in their cytoplasm (pre-B ALL). In lymphoid tissue sections, anti-B29-labeled B cell follicles in a similar fashion to anti-mb-1, with the striking exception that plasma cells were unreactive for B29, but positive for mb-1. These results suggest that the synthesis of B29 begins later in precursor B cells than that of mb-1, and ceases before the terminal plasmacyte phase.

Monoclonal antibody JC1: new reagent for studying cell proliferation

AIM

To characterise a newly developed mouse monoclonal antibody JC1 which recognises a nuclear antigen present in proliferating cells in normal tissues and neoplastic lesions, and which is absent in resting cells.

METHODS

The methodology was established using a representative range of frozen sections from normal tissues and from certain tumours which were immunostained with antibodies Ki67 and JC1. The molecular weight of the antigen recognised by JC1 was obtained by western blot analysis and this was compared with that of Ki67. IM-9 cell lysates containing Ki67 derived plasmids were also tested with JC1 antibody.

RESULTS

Biochemical investigation indicated that the antigen recognised by JC1 gives two molecular weight bands of 212 and 123 kilodaltons, which is distinct from the well characterised anti-proliferation monoclonal antibody Ki67 (395-345 kilodaltons). In addition recombinant Ki67 protein is not recognised by JC1. Immunohistological reactivity was seen in areas known to contain numerous proliferating cells such as lymphoid germinal centres, splenic white matter, cortical thymocytes and undifferentiated spermatogonia. In tumours many cells from adenocarcinomas, oat cell carcinomas, squamous cell carcinomas of lung, and seminomas were labelled by JC1 with a distribution and proportion similar to that seen with Ki67. In normal tissues the only apparent difference was in testis where JC1 stained a considerably greater number of cells than Ki67. In all cases studied the new antibody showed nuclear reactivity only. JC1 did not show any cytoplasmic crossreactivity with squamous cells as is frequently seen with Ki67.

CONCLUSION

Antibody JC1, which recognises a nuclear antigen present in proliferating cells, should provide a useful adjunct to Ki67 as a marker of proliferation especially in those cases such as squamous cell carcinomas where a Ki67 index cannot be determined.

Immunoglobulin M and D antigen receptors are both capable of mediating B lymphocyte activation, deletion, or anergy after interaction with specific antigen

A series of immunoglobulin (Ig)-transgenic mice were generated to study the functional capabilities of the IgM and IgD classes of B lymphocyte antigen receptor in regulating both cellular development and responses to specific antigen. B cells from Ig-transgenic mice expressing either hen-egg lysozyme (HEL)-specific IgM or IgD alone were compared with B cells from mice that coexpressed IgM and IgD of the same anti-HEL specificity. In all three types of Ig-transgenic mice, conventional B cells specific for HEL exhibited exclusion of endogenous Ig expression and matured to populate the usual microenvironments in peripheral lymphoid tissues. These peripheral B cells could be stimulated by HEL through either IgM or IgD antigen receptors to generate T cell dependent antibody production in vivo or to enhance T cell independent proliferative responses to lipopolysaccharide in vitro. Conversely, when HEL was encountered in vivo as a self-antigen, B cells expressing HEL-specific IgM or IgD alone were both rendered tolerant. In each case this occurred by clonal anergy in response to soluble autologous HEL, and clonal deletion when HEL was recognized as a membrane-bound self-antigen. Taken together, these findings indicate that IgM and IgD antigen receptors expressed alone on conventional B cells can support normal differentiation, antigen-dependent activation, and induction of self-tolerance, the only overt difference lying in a greater degree of receptor downregulation for IgM relative to IgD after induction of clonal anergy by soluble HEL.

Relative expression of surface IgM, IgD and the Ig-associating alpha(mb-1) and beta(B-29) polypeptide chains

Membrane immunoglobulins are associated with a transmembrane disulphide-linked heterodimer composed of an alpha-chain (mb-1) and a beta-chain (B-29). The relative surface expression of all of the polypeptide chains comprising the Ig-alpha beta complex has been investigated using surface labelling coprecipitation analysis and two-colour flow cytometric analysis. The main conclusions are that mb-1 and B-29 are B-cell surface markers on immature and mature B cells, and that all components of the surface Ig-alpha beta complex are expressed in stoichiometrically equivalent amounts. Thus the complex was quantitatively precipitated from digitonin lysates of 125I-surface-labelled cells with anti-B-29, anti-mb-1 or anti-Ig. Secondly, by two-colour FACS analysis there was a proportionality between the relative amounts of cell surface mb-1 or B-29 and surface IgM or IgD, but not other B-cell markers (class II, B220, FcR gamma, FcR epsilon). Finally there was an insignificant number of B cells expressing membrane Ig without alpha- and beta-chains, and vice versa. Thus there appears to be a closely controlled relative synthesis and surface expression of all components of the B-cell receptor complex.

Comparison of human B cell antigen receptor complexes: membrane-expressed forms of immunoglobulin (Ig)M, IgD, and IgG are associated with structurally related heterodimers

We have recently reported that on human B lymphocytes, membrane IgM (mIgM) associates with a heterodimer of 47- and 37-kD polypeptides, the 47-kD subunit being encoded by the mb-1 gene. We show here that expression of mb-1, both at the mRNA and the protein level, is not restricted to IgM+ B cells but can also be found in IgM- pre-B cells and mIgM-IgG+ B cells. Membrane forms of IgD and IgG, isolated from freshly isolated human B cells and B cell lines, are expressed together with heterodimeric protein structures biochemically similar to the mIgM-associated polypeptides, and these were shown to comprise the products of the mb-1 and B29 genes, or homologous genes. Finally, all three classes of antigen receptors are linked to protein kinases, capable of phosphorylating the Ig-associated heterodimers. Our findings provide insight in the structural organization of the different antigen receptors on human B cells and have implications for their function.