Membrane immunoglobulins of B lymphocytes: inability to detect certain characteristic IgM and IgD antigens

Monoclonal antibodies specific for murine IgM I. Characterization of antigenic determinants on the four constant domains of the mu heavy chain

Seventeen monoclonal rat antibodies with specificities for mouse mu heavy chain recognize seven distinguishable determinants that are located on the four constant region domains. All determinants are present on secreted, intracellular and membrane bound IgM, and all but two are expressed on isolated mu heavy chain. One antibody, specific for a site in the first constant region domain, recognizes a determinant that is present on IgM of AKR, C3H/HeJ, C57BL/6J, SJL, DBA/2, BALB/c, NZB and CBA/J mouse strains, but not on IgM of A.TH, A/J and A.CA strains of mice.

Coexpression of multiple immunoglobulin isotypes on human B-lymphocytes

Endogenous immunoglobulin (Ig) determinants on blood B-lymphocytes (B-cells) were investigated in 13 healthy individuals, 9 patients with thyrotoxic Graves disease, 5 patients with chronic sarcoidosis, and 4 patients with IgA deposition in renal glomeruli. Specificities of goat antisera to Ig determinants were confirmed by studying Ig isotypes on leukemic B-cells. Absence of nonspecific attachment of the goat antisera was ascertained by reacting cells with goat IgG. Lymphocytes were distinguished from monocytes by morphology and by reacting monocytes with rhodamine-conjugated immune complexes. The endogenous nature of the cell surface Ig was established by an antibody-prelabeling technique as follows: after the surface Ig had been labelled with fluorescent antibody, the cells were cultured for 3 days. Antibody-prelabelled surface Ig diminished by the third day of incubation because of shedding. Thus restaining of the cells at the end of the culture identified the membrane Ig determinants expressed during the incubation. Our results indicated that endogenous gamma and alpha chains were present on B-cells of all donors. In Graves disease, epsilon chain was also found. In all cases of Graves disease, 2 cases of sarcoidosis and 2 normal individuals, gamma, alpha, mu and delta chains were present on the majority of B-cells suggesting coexpression of these heavy chains on a single cell. I conclude that all 5 Ig isotypes may be coexpressed on B-cells under certain clinical conditions.

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.

Intracellular immunoglobulin in lymphocytes from patients with chronic lymphocytic leukemia: an immunoelectron microscopic study

The presence of ultrastructural distribution of intracellular immunoglobulin (Ig) in leukemic cells from patients with chronic lymphocytic leukemia (CLL) and in normal B cells were investigated by an immunoelectron microscopic technique. Most normal B cells had no intracellular Ig in spite of the presence of surface Ig. However, leukemic cells from 10 to 11 patients with CLL contained intracellular Ig and showed various staining patterns. In eight patients, Ig was present in the perinuclear space (PN), the endoplasmic reticulum (ER) and, if identified, the Golgi complexes. In four patients, most cells had diffuse staining of the cytoplasm. In five patients, Ig was detected in the ER-associated structures or the vesicles, in addition to the PN and ER. These findings suggest that CLL cells have a greater capacity to produce Ig than those of normal B cells and include various clones with distinct staining patterns of intracellular Ig.

Study of proteolytic removal of Fab delta and Fe delta determinants of lymphocyte membrane IgD using a direct rosette assay

Proteolysis of lymphocyte surface membrane immunoglobulin (SIg) by trypsin, papain and pronase was investigated. Cleavage in the hinge region resulting in removal of Fab delta was distinguished from removal of the whole SIgD molecule using antisera specific for c delta l and Fc delta. Using the sensitive direct rosette assay SIgD determinants were detected after digestion under conditions previously thought to remove IgD from the membrane. By increasing digestion times is was shown that whilst papain and pronase remove the whole IgD molecule, Fc delta but not Fab delta determinants survive trypsin. The implications of the results for structural differences between membrane and serum IgD and for models of insertion of SIgD into the plasma membrane are discussed.

Demonstration of immunoglobulin-related molecules on human peripheral blood T lymphocytes using chicken anti-F(ab')2 and anti-IgM antibodies

A mean of 98% of lymphocytes in T-enriched preparations of human peripheral blood bound purified chicken anti-human F(ab')2 antibodies to their surface membranes as demonstrated by the mixed antiglobulin rosetting reaction (MARR). By reverse passive hemagglutination these antibodies reacted strongly with kappa light chains and with Fd gamma but gel diffusion analyses, absorption on affinity columns and inhibition experiments established that the anti-F(ab')2 antibodies were not isotype specific and that there was extensive serological cross-reactivity between Fd gamma and kappa and probably also between Fd gamma and mu chains. The antigenic determinants recognized on the T-cells surfaces were not shared by Ig fractions from several other mammalian species. Trypsin treatment of lymphocytes removed all determinants recognized by anti-F(ab')2 antibodies from approximately two-thirds of the cells but these cells almost completely re-expressed these determinants during in vitro culture; this indicates that the T-cell determinants seen by anti-F(ab')2 antibodies are T cell products and do not represent adsorbed Ig. Complete inhibition of the MARR by human F(ab')2 excludes false positive rosette formation due to contaminating specificities directed against non-Ig molecules on T cells. Together the various findings are consistent with the conclusion that most human peripheral blood T cells express Ig or Ig-related molecules in their surface membranes. A mean of 13% of lymphocytes in T-enriched preparations were reactive with mu-chain specific chicken antibodies; it may be that a minority of T cells express mu chain determinants in addition to those recognized by anti-F(ab')2 antibodies.

The immunoglobulin mu chains of membrane-bound and secreted IgM molecules differ in their C-terminal segments

The B lymphocytes synthesizes two forms of IgM molecules during its development from a stem cell to a mature antibody-secreting plasma cell. The monomeric receptor IgM molecule is affixed to the plasma membrane and triggers the later stages of B cell differentiation, whereas the pentameric secreted IgM molecule is an effector of humoral immunity. The structural differences between membrane-bound and secreted IgM molecules are reflected in the differences between their heavy or mu chains. We have previously determined the complete amino acid sequence of a murine secreted mu (microsecond) chain. In this study, we have compared the structures of the secreted and membrane-bound mu (micron) heavy chains by peptide mapping, micro-sequence and carboxypeptidase analyses. These studies demonstrate that the micron and microsecond chains are very similar throughout their VH, C mu 1, C mu 2, C mu 3 and C mu 4 domains. The micron and microsecond chains differ in the amino acid sequence of their C-terminal segments. These studies in conjunction with those carried out on the micron and microsecond mRNAs and the C mu gene suggest that the micron and microsecond chains from a given B cell are identical except for their 41 and 20 residue C-terminal segments, respectively. The amino acid sequence of the 41 residue C membrane terminal segment predicted from the corresponding micron mRNA is in agreement with all the protein studies reported in this paper.

Lymphocyte membrane IgG and secreted IgG are structurally and allotypically distinct

We have demonstrated that there are structurally distinct membrane and secreted IgG2a immunoglobulin molecules. The membrane heavy chain is both larger and more acidic than the secreted molecule. This difference is not a result of different N-glycosidic-linked oligosaccharide chains. The membrane heavy chain also is antigenically different from its secreted homologue. This is based on the fact that secreted IgG2a molecules express an allotypic determinant absent on membrane molecules. We discussed the genetic control and gene organization of membrane and secreted immunoglobulin heavy chain sequences and suggest mechanisms controlling the expression of the simian virus 40 genome as models for alternate gene expression of membrane and secreted heavy chain polypeptide chains from the same DNA sequence. The possible biological significance of the membrane immunoglobulin acting as a recognition site for regulatory T cells also is discussed. The difference between membrane and secreted immunoglobulin is proposed as a possible explanation for the manner in which T cells interact with IgG on memory B cells in the presence of a large excess of IgG present in body fluids.

[In vitro studies on two cell lines from patients with Hodgkin's disease (author's transl)]

Two cell lines (Ma,Bi) were established in vitro from pleura effusions from two patients with Hodgkin's disease. After some months the monolayer line Ma spontaneously became nonadherent and proliferated in suspension. Immunological properties, enzyme activities, surface characteristics and chromosomal analyses suggested that the culture Ma is composed of lymphoid cells and macrophages. Line Bi propagated attached to the glass and had properties typical for macrophages only.

Studies in chronic lymphocytic leukaemia II. Lymphocyte markers, cellular and humoral immunity and the effect of treatment

Observations were made on 15 patients with chronic lymphocytic leukaemia, 3 with non-Hodgkin's lymphoma, and 18 healthy controls. These include characterization of lymphocytes, assessment of humoral and cell-mediated immunity and the effect of treatment. Those responding to therapy showed a disappearance of 'null' lymphocytes from the blood with improvement in clinical and haematological parameters. Their immune capacity, however, remained unchanged or continued to deteriorate.

Human cell lines containing Epstein-Barr virus but distinct from the common B cell lymphoblastoid lines

A group of very similar cell lines was established from peripheral blood or bone marrow of 12 patients with a variety of disorders. The cells in these cell lines were uniform and round in shape. They grew as single-cell suspensions or as aggregates of small numbers of cells in stationary culture. The most striking characteristic of these lines was the lack of cells with surface immunoglobulin or with demonstrable immunoglobulin synthesis. This lack of immunoglobulin synthesis and their special growth characteristics distinguished them from the lymphoblastoid B cell lines previously described. The cells of these unusual cell lines had strong Fc receptors and C3 receptors and expressed Ia antigens. They did not form rosettes with sheep erythrocytes and did not have detectable levels of terminal deoxynucleotidyltransferase. They did not secrete lysozyme and failed to stain for peroxidase. The presence of the Epstein-Barr virus nuclear antigen in the cells indicated the presence of Epstein-Barr viral genome. The possibility that these cells represent some type of precursor cell in the B cell lineage is discussed, but the exact cellular origin remains to be ascertained.

Density comparisons of heavy chains of membrane and secreted immunoglobulins of mouse

In order to explore structural differences between membrane and secreted immunoglobulins the buoyant densities of mouse immunoglobulin (Ig) heavy (H) chains were compared by isopycnic centrifugation in CsCl containing guanidine hydrochloride. The buoyant densities, under denaturing conditions, of mouse myeloma protein MOPC 21 IgG, MOPC 315 IgA and MOPC 104E IgM H chains were consistent with their carbohydrate contents. Mouse membrane IgM and MOPC 104E-secreted IgM H chains were of equal density. The buoyant densities of MOPC 104E-secreted IgM and spleen-cell-secreted IgM H chains were indistinguishable. The IgD-like membrane H chain was denser than membrane IgM H chain, and its carbohydrate content was calculated to be 15.5%. The resolution of the technique was sufficient to conclude that the apparent 1500 mol.wt. difference, as determined by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, between membrane and secreted IgM H chains was due to peptide rather than to carbohydrate. The results also imply that intact membrane IgM and IgD bind detergent and are thus integral membrane proteins.

Delineation of IgM-receptor bearing human T and B lymphocytes using a direct plaque forming cell (PFC) assay

Based on the observation that binding of IgM cytophilic antibodies to lymphocytes is temperature dependent, a direct plaque forming cell (PFC) assay was developed to detect IgM-receptor bearing human peripheral blood T and B lymphocytes. Lymphocytes were passively sensitized with IgM anti-SRBC molecules at 4 degrees C, added to SRBC monolayers then incubated at 37 degrees C with guinea pig complement to develop the plaques. The PFC assay has methodological advantages over rosetting methods which demonstrate IgM receptors, and under certain conditions is more sensitive than these rosette techniques. A mean of 17% of freshly isolated uncultured lymphocytes, enriched for B cells, formed direct plaques while a mean of 3% of T-enriched preparations formed direct plaques. However, if the lymphocytes were preincubated with vibrio cholerae neuraminidase (VCN) these figures increased to 46% and 35% respectively. The specificity of plaque formation by VCN-treated lymphocytes was established. SRBC sensitized with a F(ab')2 preparation of an IgG anti-SRBC reagent failed to bind to VCN-treated lymphocytes, inclusion of IgM, but not other Ig molecules in the test medium, inhibited plaque formation, and, most important, plaque formation by T and B cells was inhibited by F(c)5 mu but not by Fab mu fragments. These results indicate that T and B lymphocytes express IgM-class specific membrane receptors, that these receptors may be hidden on normal lymphocytes but are revealed by treatment with VCN and that the IgM receptor on VCN-treated lymphocytes is F(c)mu specific. These findings are discussed briefly with regard to other and partly contradictory data obtained after overnight in vitro lymphocyte culture. As demonstrated by direct PFC assay, the B cell IgM receptor is trypsin sensitive.

Lactoperoxidase-catalyzed iodination of human IgM. Differences between 7 S IgM and 19 S IgM and between cell surface 7 S IgM and serum 7 S IgM

We have studied the lactoperoxidase-catalyzed iodination of human IgM and have measured the ratio of radioactivity incorporated into the mu chain to that incorporated into the L chain (i.e. the mu/L ratio). Both 7 S and 19 S IgM were examined. The ratio of radioactivity was found to be larger for 7 S IgM than for 19 S IgM for all four of the monoclonal IgM proteins examined. The data suggest that some tyrosines of the mu chain which are buried and not available for iodination in 19 S IgM become exposed on conversion of 19 S IgM to 7 S IgM. The mu/L ratio for the IgM found on the cell surface of RPMI 8392 cells was significantly smaller than the ratios for all of the five 7 S IgM proteins studied in solution. It appears, therefore, that a portion of the mu chain of the cell surface IgM of the RPMI 8392 cells is buried in the membrane.

Phagocytic peripheral blood monocytes from rabbits and humans express membrane receptors specific for IgM molecules: evidence that incubation with neuraminidase exposes cryptic IgM (Fc) receptors

Phagocytic human and rabbit peripheral blood monocytes, identified by their ingestion of polystyrene particles, were investigated for the presence of surface membrane receptors for IgM molecules. After incubation of freshly isolated monocytes with IgM anti-sheep erythrocyte (SRBC) preparations, a mean of 0.7% of human monocytes and a mean of 16.2% of rabbit monocytes formed rosettes with SRBC. However, if the monocytes were pre-incubated with vibrio cholerae neuraminidase (VCN), these figures increased to 32.6% and 37.8% respectively. The specificity of rosette formation by VCN-treated monocytes was established in several experiments; SRBC sensitized with a F(ab')2 preparation of an IgG anti-SRBC reagent completely failed to rosette with VCN-treated monocytes, and inclusion of IgM, but not other Ig or non-Ig protein molecules in the test medium, inhibited rosette formation. Further, and most important, rosette formation by human monocytes was inhibited by F(c)5mu but not by Fabmi fragments. These findings indicate that both rabbit and human monocytes express IgM-class specific membrane receptors for IgM molecules, that these receptors may be cryptic or hidden but can be revealed by treatment with VCN and that the human monocyte IgM receptor is F(c) specific. Further, the rabbit monocyte IgM receptor was shown to be trypsin-resistant.

B lymphocytes contain three species of mu chains

The murine B cell line cloned from a single cell, 38C-13, synthesizes three species of mu chains, that of cell surface membrane IgM (m-mu), that of secreted IgM (s-mu) and that of intracellular IgM (i-mu). They differ in their mobility on sodium dodecyl sulfate polyacrylamide gel electrophoresis. Sequence analysis of the different mu chains suggests that they are identical in the N-terminal as well as in their C-terminal positions. The ratio between incorporated radioactive monosaccharides to radioactive amino acids into the three different mu chains was higher in s-mu than in m-mu, but nevertheless m-mu migrated more slowly than s-mu on sodium dodecyl sulfate polyacrylamide gel electrophoresis. However, since this ratio may also be influenced by the rate of synthesis, it may not represent a real molar ratio of carbohydrate to protein. Studies with normal spleen cells clearly indicated the presence of the same three types of mu chains.

Allelic exclusion of M1 (IgM) allotype on the surface of chicken B cells

The membrane expression of M1 (IgM) and G1 (IgG) allotype markers on peripheral blood lymphocytes was examined by immunofluorescence. In homozygous chickens 15% lymphocytes stained with either anti-M1 or with polyspecific rabbit anti-Ig serum, suggesting that M1 is expressed on the surface of probably all B lymphocytes. In heterozygous M1a/M1b individuals antisera against either allelic antigens reacted with 50% of the total surface Ig positive cells as evidence for the allelic exclusion of surface M1 expression. Donor allotype synthesis was determined from serum allotype levels in congenic cyclophosphamide-treated recipients of lymphoid cells. Treatment of cells with anti-M1 serum prior to transfer inhibited both M1 and G1 allotype synthesis. The treatment of heterozygous (M1a/M1b, G1a/G1e) cells with anti-M1b serum inhibited specifically the synthesis of M1b and G1e allotypes controlled by linked genes from the same parental chromosome. Thus, B cells which had been the targets for anti-M1 antibody mediated suppression are also subject to allelic exclusion.

Expression of immunoglobin G on blood lymphocytes in chronic lymphocytic leukaemia

A sensitive rosette test utilizing antibody-coated red cells has been applied to the study of the immunoglobulins on the surface of blood lymphocytes in chronic lymphocytic leukaemia (CLL). Contrary to other reports, IgG has been found to be a common surface membrane immunoglobulin (SmIg) on CLL cells. The reasons for this variation are discussed. Evidence is presented to show that (a) the anti-IgG-coated erythrocytes really are detecting IgG and not a cross-reacting substance, (b) the IgG is intrinsic to the cell and not cytophilically bound and (c) Fc-binding and other artefacts have been excluded. Expression of Ig by individual cells was studied by using mixtures of fluorescein- and rhodamine-labelled red cells coated with various anti-Ig. Fifty-one cases of untreated CLL were tested and the lymphocytes of thirty-eight of these cases bore Ig of a single light chain type. These cases could be classified on the basis of their lymphocyte SmIg as follows: eighteen expressing M and D and G, eleven expressing D and G but not M, six expressing M and D but not G, and three expressing G alone.

Allotypes of IgM and IgD receptors in the mouse: a probe for lymphocyte differentiation

Antibodies to IgD and IgM are present in many mouse alloantisera made against lymphocytes. Antibodies to IgM are also present in a small minority of conventional antiallotype sera made against pertussis/anti-pertussis complexes. These antibodies reflect different allelic forms of IgD and IgM in different mouse strains, and allowed the mapping of the delta- and mu-chain genes (Ig-5 and Ig-6, respectively) to the heavy-chain complex. The variable portions of IgM and IgD receptors on individual cells bearing both chains are similar or identical. Both receptors show allelic exclusion and come from the same chromosome on individual cells (haplotype exclusion). It is suggested that very early in B lymphocyte differentiation there is a commitment to a given chromosome, and translocation of one copy of a given variable region gene to each heavy-chain gene. Subsequent switches of immunoglobulin class then involve differential gene activation. Immature B cells possess IgM receptors only and mature into cells bearing both IgM and IgD receptors. After activation with antigen, IgD is probably lost. These findings are discussed within the framework of the clonal abortion theory of B-lymphocyte tolerance.

B- and T-cells and Ig-containing blast cells in healthy children

Studies on rosette-forming cells and IgG, IgA, IgM and IgE-bearing lymphocytes were carried out in 70 healthy children aged 5 to 15 years. Furthermore, in 20 of these children, the percentages of IgG, IgA, IgM and IgE-containing blast cells after pokeweed mitogen stimulation of peripheral lymphocytes were evaluated. No differences in the percentages of rosette-forming cells, or in Ig-bearing and Ig-containing cells were found when three different age-groups were compared.

Surface immunoglobulin on rabbit lymphoid cells. IV. Ultrastructural labelling of A and B group allotypic determinants

Equal numbers of rabbit peripheral blood lymphocytes (PBLs) label for either a- or b-group immunoglobulin allotypes using a modification of the mixed antiglobulin reaction of Coombs adapted for electron microscopy. This technique which employs immunoferritin markers is consistently more sensitive than other methods used for labelling surface Ig on lymphocytes. Although approximately 75 per cent of PBLs label for either a- or b-group allotypes, the number of immunoferritin grains on the cell surface after labelling for b group allotypes is nearly twice as great as that observed after labelling for a group allotypes. We conclude that essentially all Ig-bearing blood lymphocytes of the rabbit express both heavy chains (a allotypes) and light chains (b allotypes) on their surface membranes, but that light chain allotypic determinants are more accessible or exposed than the heavy chain (Fd) allotypic determinants.

Density differences between membrane and secreted immunoglobins of murine splenocytes

The buoyant densities of mouse immunoglobulins were determined by isopycnic centrifugation in phosphate-buffered cesium chloride using beta-galactosidase as marker. The buoyant densities of IgG, TEPC 15 IgA, secreted IgM, and MOPC 104E IgM were consistent with their carbohydrate contents both in the presence and the absence of the nonionic detergent, Nonidet P-40. Intracellular IgM from spleen cell lysates had a buoyant density corresponding to a carbohydrate content of 6%. Membrane IgM from detergent lysates of spleen cells was less dense than either intracellular or secreted IgM in the presence of detergent. The IgD-like membrane molecules were more dense than membrane IgM.

Establishment in continuous culture of a new type of lymphocyte from a "Burkitt like" malignant lymphoma (line D.G.-75)

The isolation and establishment in vitro of a hitherto undescribed type of lymphocyte designated D.G.-75 is reported. The original inoculum was derived from the pleural effusion of a child with a primary abdominal lymphoma, which clinically and histologically resembled Burkitt's lymphoma. In addition to the absence of the EBV genome and EBV receptors, this line possesses a number of other properties which distinguish it from previously described lymphoblastoid cell lines. It has different growth characteristics and morphology; does not form EAC or E rosettes (representative of B and T) cell surface markers, respectively); possesses IgM-kappa immunoglobulins on the cell surface (B lymphocyte), has an unusually high cap-forming ability and low agglutinability with fluorescent concanavalin A. One homologue of the No.14 chromosome pair possesses extra chromatin material as revealed on chromosome banding. This abnormal chromosome marker is similar to that described in biopsies and cultured tumor cells from patients with African Burkitt's lymphoma.

Lymphocyte surface immunoglobulins: detection, characterization, and occurrence in disease of the lymphoid system

Surface immunoglobulins (Igs) of lymphocytes are of considerable interest because these molecules probably function as receptors for antigen, and knowledge of their molecular properties should provide information on the mechanisms of immune differentiation. The density and types of surface Ig on a cell provide markers useful in indicating the class of a lymphocyte and its stage of maturity. Moreover, knowledge of the specificities of the surface Ig of neoplastic lymphocytes might suggest the nature of agents involved in the generation of the disease. Two broad classes of lymphocytes, bone marrow-derived lymphocytes (B cells) active in antibody secretion, and thymus-derived lymphocytes (T cells) which mediate cellular immune reactions, and their subpopulations must be considered with reference to the nature, origin, and function of their surface immunoglobulin. This article analyzes direct and indirect methods for the demonstration of surface Igs and describes certain physicochemical properties of isolated surface Ig molecules. Roles of these surface molecules in recognition of antigen, initiation of all differentiation, and cooperation among lymphocytes and accessory cells are discussed.

Human T cell receptor for IgM: specificity for the pentameric Fc fragment

The specificity of the IgM receptor expressed by human T cells cultured in IgM-free media has been investigated. IgM receptors have been detected using a rosette system with ox erythrocytes coated with rabbit antibody (EA(IgM)), and the inhibitory capacity of different IgM fragments in the rosette system has been tested. It was found that F(c)5mu but not F(ab')2mu, nor monomeric IgM (8 S IgM) inhibited EA (IgM) rosette-forming cells. This indicates that the receptor present on the surface of T cells has affinity for a structure located in the Fc portion of the pentameric IgM.

Studies on J chain and binding site for secretory component in circulating human B cells

Lymphocyte-enriched mononuclear cell fractions were obtained from the peripheral blood of four healthy young adults. Living B cells with membrane immunoglobulin (Ig) were studied by immunofluorescence to reveal exposed J-chain determinants or surface affinity for the secretory component (SC). Alcohol-fixed cell smears were similarly studied with and without prior denaturation in acid urea. It was concluded that J chain-containing polymeric Ig of endogenous origin is generally not present on the surface of circulating B cells. If occasional cells bear IgM or IgA polymers rather than monomers, their SC-binding site must be concealed to a degree that it is not functional. Affinity for SC is therefore unlikely to be involved in a selective homing of IgM- and IgA-immunocyte precursors from peripheral blood to glandular regions.

The variable pattern of circulating lymphocyte subpopulations in chronic lymphocytic leukemia

The percentages of T and B lymphocytes in 13 patients with chronic lymphocytic leukemia were determined at two-week intervals. One patient with "B-cell predominant" disease showed a decrease from 77 to 49 in the percentage of circulating B cells, which bind heat aggregated immunoglobulin and anti-human immunoglobulin. This patient had a comparable increase in T cells, which form rosettes with sheep erythrocytes. The lymphocytes of three other patients, who originally had equal percentages of cells that bound heat-aggregated immunoglobulin and anti-human immunoglobulin, lost lg determinants from 47 to 63 per cent of cells without changing the proportion of cells with receptors for heat-aggregated immunoglobulin or for sheep erythrocytes. The characterization of lymphocytes in chronic lymphocytic leukemia should include sequential determination of cell-surface markers for both T and B cells, since the disease does not represent a proliferation of B cells alone.

Evolutionary and developmental aspects of T-cell recognition

Studies relating to the nature of the antigen-specific T-cell receptor are reviewed in the light of present knowledge of phylogenetic and ontogenetic development. It is suggested that this evidence supports the concept that immunoglobulin (Ig) is the T-cell receptor, and that the following conclusions may be tentatively drawn.

Subunit structure, cell surface orientation, and partial amino-acid sequences of murine histocompatibility antigens

Detergent and papain solubilized murine histocompatibility (H-2) antigens have been compared by gel exclusion chromatography, ultracentrifugation, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and amino-acid sequence analysis. From these data, we propose a molecular model for the H-2 antigens that includes the size and arrangement of the subunits on the cell surface and in solution, and we provide evidence for the orientation of these molecules on the cell surface. Detergent solubilized H-2 antigens (molecular weight 116,000) consist of two disulfide-linked heavy chains (46,000 daltons) and two monocovalently associated light chains (12,000 daltons). Alkylation with iodoacetamide prior to extraction prevented the formation of a disulfide linkage between the two heavy chains. A water-soluble 51,000-dalton molecule (Fs) consisting of a 39,000-dalton fragment (FH) of the heavy chain and one intact light chain was obtained by papain digestion of cells or detergent extracts. Therefore, the disulfide linkage between the heavy chains is located in the remaining membrane-associated portion (Fm). Amino-acid sequence analysis of the FH fragment of H-2Kb by radiochemical techniques showed that it is identical to the detergent solubilized H-2Kb heavy chain in eight positions for the three amino acids tested. These data indicate that the fragment FH derives from the amino-terminus of the heavy chain and suggest that it projects outward from the cell surface, while the carboxyl-terminal region is associated with the plasma membrane. The described amino-terminal sequence data have been found constant in H-2Kb, H-2Kd, H-2Kk, H-2Db, and H-2Dd gene products. These data support the hypothesis that the K and D products of the major histocompatibility antigen complex have evolved by gene duplication.