Some biochemical properties of thymus leukemia antigens solubilized from cell membranes by papain digestion

Protein-bound carbohydrates on cell-surface as targets of recognition: an odyssey in understanding them

Multidisciplinary approaches by a number of investigators have established that cell-surface carbohydrates are integral components of recognition systems regulating survival, migration, adhesion, growth and differentiation of various cells. Our own experience and contributions to this exciting field are described. We discovered Endo D as the first endoglycosidase acting on glycoproteins, found complementary specificity of two endoglycosidases (Endo D and Endo H), and applied these enzymes for glycoprotein research. Endo-beta-galactosidase C, which hydrolyzes Galalpha1-3Galbeta1-4GlcNAc xenoantigenic determinant, was later found and molecularly cloned. We also found highly branched poly-N-acetyllactosamines in early embryonic cells, and demonstrated developmentally regulated carbohydrate changes during early mammalian development. The binding site for Dolichos biflorus agglutinin was introduced as a new differentiation marker. Basigin and embigin, two related members of the immunoglobulin superfamily, a sialomucin MGC-24 and other glycoproteins were discovered as carriers of developmentally regulated carbohydrate markers. We proposed enhancement of integrin action as a function of sugar chains with Lewis X epitope, and observed a relationship between the expression of carbohydrate markers and invasive properties of human carcinoma. Midkine, a heparin-binding growth factor, was discovered more recently and its interaction with heparin and oversulfated chondroitin sulfate was elucidated. N-Acetylglucosamine-6-sulfotransferase was cloned and used to reconstitute L-selectin ligands. Gene knockout was applied to reveal in vivo function of basigin, syndecan-4 and chondroitin 6-sulfate. Throughout my research on all these subjects, I have been fortunate in obtaining unexpected observations and enjoying fruitful collaborations.

Separation and comparison of human TL-like antigens and HLA(A, B, C) antigens expressed on cultured T cells

Beta 2-microglobulin-bound T-cell membrane components containing both human TL-like antigens and HLA(A, B, C) antigens were partially purified from Renex 30-solubilized membrane material of cells of a human T-cell-type leukemia cell line, HPB-ALL. The radioiodinated preparation was subjected to limited papain digestion; the HLA(A, B, C) antigens split, whereas a large portion of the human TL-like antigens remained intact. The antigen molecules were recovered by lentil-lectin affinity chromatography and separated by gel filtration on the basis of the induced difference in molecular size. The human TL-like-antigen preparation thus obtained was essentially free of HLA(A, B, C) antigens. The human TL-like antigens were immunospecifically precipitated and the component polypeptide, heavy and light, chains were separated by acid dissociation followed by gel filtration. The component chains were compared with the corresponding chains of HLA(A, B, C) antigens obtained similarly from the same HPB-ALL cells with respect to their fragmentation patterns on chemical or enzymatic cleavage. The results provided convincing evidence for the identity of the light chains of human TL-like antigens and HLA(A, B, C) antigens, and also evidence suggesting the presence of substantial differences in the fundamental structure of the heavy chains of human TL-like antigens and HLA(A, B, C) antigens.

Biochemical evidence for a separate, MHC-linked locus encoding H-2.28 antigens

In comparing the tryptic peptide maps of the H-2L and H-2D glycoprotein antigens isolated from NP-40 lysates of RADA1 (H-2 alpha) leukemic cells, no more than 37% of the observed arginine-containing tryptic peptides are found to be homologous. Thus, the primary amino-acid sequences of these two antigens are probably less than 90% homologous. This constitutes the strongest evidence to date that the MHC-linked H-2L region encodes H-2L antigens separately from the H-2D region, even though H-2L antigens bear D-end-associated antigenic determinants of the H-2.28 family. The anti-H-2.28 alloantiserum (k X r anti h2) used to precipitate H-2L antigens in this investigation was the NIH contract antiserum D28b. As the tryptic peptide maps also suprisingly revealed, D28b precipitates H-2D antigens as well and, thus, anti-H-2.4 immunoadsorbants were employed to isolate H-2L free of H-2D antigens. In light of the dual specificity of D28b, its reactivity with BALB/c-H-2dm2 mutant cells was re-examined. Even though mutant lymphocytes, which lack H-2L but not H-2D antigens, are not cytotoxically lysed by D28b (as are parental H-2d cells), D28b appears to precipitate H-2D antigens from NP-40 extracts of mutant splenocytes.

Synthesis and processing of molecules bearing thymus leukemia antigen

Thymus-leukemia (TL) antigens are expressed in murine lymphocytes under strict developmental regulation. To elucidate the molecular basis of TL expression, we have identified the molecular species that react with TL antiserum. At least three species can be resolved by metabolic radiolabeling of thymocytes and ASL1 leukemia cells, lysis, immune precipitation, and sodium dodecyl sulfate-polyacrylamide. After a brief incubation with [35S]methionine, the only radioactive molecule recognized by TL antiserum is a homogeneous species with an apparent Mr of 45,000 daltons. This molecule, 45K TL, includes high-mannose-type carbohydrate attached to a 45,000 dalton glycosidase-resistant backbone. In this form, 45K, it is never exposed on the cell surface. If pulse-labeled cells are further incubated with nonradioactive methionine before lysis, however, radioactivity disappears from the 45K TL species and appears in the slower migrating species 46K and 48K TL. Thus, 46K and 48K appear to represent products generated from the 45K TL precursor by posttranslational modification. These TL forms are displayed on the cell surface; they lack high-mannose carbohydrate but evidently include acidic complex-type carbohydrate. Normal thymocytes from Qa:Tla-negative mice lack not only the surface forms of TL but also the intracellular 45K TL form. Peripheral lymphoid cells of Qa:Tla-positive mice synthesize none of these TL species. But the TL antiserum, which contains Qa antibody, recognizes a distinct gene product in spleen and thymus of Qa-Tla-positive mice. In its pulse-labeled form, this molecule, which may represent Qa-1, has an apparent Mr of 44,000 daltons, and consists of a glycosidase-resistant polypeptide core of only 35,000 daltons linked to more high mannose carbohydrate than 45K TL.

Some structural properties of thymus leukemia antigen (TL) solubilized with detergent

Plasma membranes were isolated from the leukemia cell line ASL1w and extracted with detergent (DOC). DOC solubilized more TL activity than could be detected on isolated membranes. However, extraction of membranes with LDS or EDTA solubilized only 17% and 4%, respectively, of the activity. This indicated that TL was not loosely associated with the membrane but rather was integrated into the lipid bilayer. At low concentrations of DOC (0.05%), TL was found to be largely aggregated and was also prone to autolysis. Neither aggregation nor autolysis was observed at a higher DOC concentration (0.5%). The apparent molecular weight of TL in 0.5% DOC was determined by Sephadex G-200 chromatography to be about 65,000-70,000. Digestion of a 0.5% DOC extract of TL with either papain or trypsin produced a fragment of TL of about 35,000 molecular weight. These fragments were similar in size to a fragment produced by autolysis. These data suggested that a region of the TL molecule was very prone to proteolytic attack. The 35,000 molecular weight proteolytic fragments bound specifically to lentil lectin affinity columns, which indicated that they retained at least part of the carbohydrate present on the native molecule.

Macrophage-dependent, NK-cell-independent "natural" surveillance of tumors in syngeneic mice

The present study, which was designed to further characterize the "natural" T-independent rejection of syngenetic tumours (Greenberg and Greene, 1976), has revealed the following points: (1) no detectable DBA/2 NK cell activity was demonstrated against the syngeneic tumour lines studied, and these tumours were indensitive to NK cells from high-activity strains; (2) in addition the tumour frequencies in old and young mice receiving small tumour inocula were identical, in contrast with the reported decline in NK cell activity with age, suggesting that the surveillance of small inocula of these tumours was NK-cell-independent; (3) injection of silica intraperitoneally enhanced the frequency of tumours in normal and immunodeficient AT x BM mice, suggesting that the rejection mechanism was macrophage-dependent; (4) the effects of silica injection were maximal if administered 3 days prior to tumour injection, indicating that the period of time in which the rejection mechanism must act was very limited; (5) silica markedly decreased the survival of AKR mice dying of spontaneous tumours, providing evidence that the effect of this agent was not limited to model systems but would influence the appearance of spontaneous tumours; (6) reticuloendothelial stimulants such as mycobacterium butyricum and proteose peptone decreased the tumour frequency of small tumour inocula, indicating that the effector mechanism can be stimulated; and (7) soluble tumour antigen enhanced the tumour frequency in normal and immunodeficient mice, suggesting that the specific receptor molecule of the surveillance mechanism was not thymus-dependent.

Murine natural anti-tumor antibodies. I. Rapid in vivo binding of natural antibody by tumor cells in syngeneic mice

A competitive radioimmunoassay (RIA) for the detection of cell-bound antibody was used to study the in vivo acquisition of immunoglobulin (Ig) by tumor cells. Several tumor lines acquired Ig rapidly between 3 and 18 h after intraperitoneal implantation into normal syngeneic mice and this Ig was recovered by elution with basic or acid buffers. The Ig eluted from the L5178Y lymphoma showed higher binding to the L5178Y than to thymocytes, bone-marrow cells, 1509a sarcoma and P-815-X2 mastocytoma. In addition, binding of the eluates to the L5178Y was specifically inhibited by L5178Y cells or by solubilized membrane antigens of the L5178Y. The in vivo acquisition of Ig by the L5178Y could also be blocked by the IV and IP injections of tumor antigen although both L5178Y and 1509a solubilized membrane antigens were effective. Some of the Ig acquired by the tumor cells was found to be complement-fixing antibody since normal rabbit complement lysed 80% of L5178Y cells obtained from the peritoneal cavity of syngeneic mice 18 h after implantation, but did not lyse in vitro L5178Y cells. The in vivo binding of the complement-fixing antibodies was also inhibited by tumor antigens in the same way as the acquisition of Ig detected by RIA. It was shown that the acquisition of Ig during the first 18h of IP growth was a T-independent phenomenon because tumor cells acquire as much Ig in AT X BM mice as in sham-thymectomized controls. In a study with 11 different clones derived from the L5178Y lymphoma, a high correlation (r = 0.75, p less than 0.005) was found between the amount of Ig acquired after in vivo implantation and the amount of Ig bound to the cells after in vitro incubation with normal syngeneic serum. It is suggested that the rapid in vivo acquisition of Ig was due to the in vivo binding of natural antibodies to tumor cells.

Surface membrane changes of T cells induced by syngeneic tumour cells. I. Formation and uptake of complexes of Ig and tumour antigens by T cells

A large dose of tumour cells (10(7)) from two different tumours produced a significant increase of Ig-bearing spleen cells 6 h after intraperitoneal administration in syngeneic mice, like conventional antigens previously tested. Incubation of normal spleen cells in serum taken 6 h after administration of tumour cells reproduced in vitro the changes observed in the spleen cell population of the serum donors, demonstrating the presence of a cytophilic Ig, which was shown to be taken up by T cells. Serum collected 6 h after tumour cell inoculation contains also a 4S factor which can generate in vitro cytophilic Ig for T cells in the presence of a foreign soluble protein or tumour antigen. Extracts obtained from tumours cells, which were shown to contain tumour antigens, were labelled with 125I and mixed with the 4S factor and normal 7S Ig. Upon fractionation of the mixture, high molecular weight material containing radioactivity and 7S Ig were eluted in the void volume, well ahead of the position of both the tumour cell preparation and the 7S Ig. This material contained Ig cytophilic for splenic T cells and it is likely that it represents cytophilic complexes of Ig and tumour antigens. It is postulated that the cytophilic Ig detected in the serum of animals 6 h after injection of a large number of tumour cells represents cytophilic complexes of Ig and tumour antigens formed through the mediation of a soluble factor.

Differentiation antigens of the lymphocyte cell surface

Many alloantigens and xenoantigens of lymphocytes are not found generally on other tissues, and this suggests that much of the lymphocyte cell surface is differentiated in comparison with other cell membranes. These differentiation antigens are probably molecules that mediate lymphocyte-specific functions, and are also of interest in that they provide markers for different lymphoid cell types and may be important as target antigens for immunosuppressive anti-(lymphocyte) sera. The purification of differentiation antigens will be important in allowing their molecular properties to be discovered, and will also lead to the production of strong, specific antisera that can be used in functional studies. Radioimmunoassays have been developed for the analysis of anti-(lymphocyte) sera, and these assays provide advantages in purification studies over other techniques. The features of these assays are discussed, and studies of differentiation antigens of rat lymphocytes are described. These include the purification and characterization of the rat Thy-1 antigen, and preliminary studies on two other rat lymphocyte differentiation antigens.

Quantitative isolation of mouse thymus leukemia antigen, TL

This report describes the purification of TL from papain digests of a tumor line (ASL1) and of an established cell line (L251A). Through the use of gel filtration and ion-exchange chromatography, the TL was purified approximately 100-fold with respect to the original digest. It was noted that the TL isolated from ASL1 had a specific activity 1.4 times higher than that isolated from L251A. The reason for this anomaly is unexplained. However, this work indicates that classical methods of protein chemistry can be used in the purification of these membrane components which are present in only small amounts on the cell surface.

Distribution of transplantation antigens on cell surfaces

Significant advances have been made over the past few years in elucidating the genetics, the chemical composition, and, more recently, the in situ relation of the major histocompatibility antigens of the mouse and man. Attempts to map the arrangement of individual antigens on the surface of cells have revealed that some antigens specified by a given subregion of both the H-2 and HL-A systems are in close proximity on the cell membrane and that attachment of antibody to one site to a certain degree blocks or inhibits the binding of antibody to the adjacent site. Allelic antigens in the H-2 system tend to inhibit binding. H-2D and H-2K antigens show either inhibition or noninteraction, possibly reflecting a cis-trans effect. Unlike with the H-2, inhibition of binding occurs only between HL-A antigens specified by homologous chromosomes. Also, a number of instances have been noted where inhibition of binding is unidirectional, possibly reflecting a polymeric nature of antigen or stratification of moieties at cell surface. Inhibition of antibody attachment between several alloantigenic systems on thymocytes in mice and also a variation in the mobility of the histocompatibility antigens suggest that the moieties bearing histocompatibility antigens are comprised of several gene products. Further work is needed to establish the validity of this assumption and to fully define the composition of these units. Ample data have been obtained from both biological and biophysical experiments to support the suggestion that single or multiple complexes of glycoproteins can move in the plane of the membrane. Although the composition of these cell membrane components remains a question, direct visualization by fluorescence and electron microscopy indicates that these moieties are small and, under natural conditions, distributed uniformly over the cell surface. Direct and indirect labeling techniques have shown that the complexes have no fixed position in the cell membrane and can be displaced laterally in the plane of the membrane without affecting the distribution of other surface molecules, such as sIg and species-specific antigens. Additional evidence suggests that H-2D and H-2K antigen complexes, as well as their gene products specified by different parent chromosomes, may be displaced separately. These observations are especially interesting and must be reconciled and data obtained by proximity analysis which indicate an association of some allelic products and possibly certain combinations of D and K antigens. Whether the differences noted in reactivity of the various surface antigens following attachment of antibodies are attributable to difference in size or to differences in the manner of their intercalation in the cell membrane remains to be elucidated. The rapid advances in elucidation of the molecular structure of biological membranes suggest that experimental work should be done on the biophysics of the structure of the antigenic sites and the mechanism of migration...

The subunit structure of thymus leukemia antigens

EDTA-containing buffer solubilizes thymus leukemia antigens (TLa) from crude thymocyte membrane fractions. The TL antigens consist mainly of molecules of a size similar to immunoglobulin G when gel chromatography analyses were performed under physiological conditions. A single component of TLa was apparent on sucrose density gradient ultracentrifugation of solubilized thymocyte membrane macromolecules as monitored by indirect immunoprecipitation. The sedimentation constant for the TL antigens (5.8 S) was considerably less than that for immunoglobulin G. The gel chromatography and ultracentrifugation data suggest an apparent molecular weight for TLa of about 120000. TLa isolated by indirect immunoprecipitation is composed of two types of polypeptide chains. The smaller subunit was identified as beta2-microglobulin. The larger polypeptide chain carried the alloantigenic determinants and displayed a molecular weight of about 50000 after reduction and alkylation. TLa subjected to molecular weight determination under denaturing conditions was composed of two components. The smaller component was beta2-microglobulin which evidently is linked to the larger polypeptide chain by noncovalent interactions only. The larger component had a size greater than reduced and alkylated immunoglobulin G heavy chains. Upon reduction and alkylation of the latter component its size was reduced and it appeared to have a molecular weight of about 50000. Consequently, TLa is composed of two disulfide linked heavy polypeptide chains and two beta2-microglobulin molecules. TLa solubilized by papain digestion comprises two polypeptide chains, one of which is beta2-microglobulin. The larger 37000-dalton subunit is a fragment of the heavy polypeptide chain. This was demonstrated by digesting solubilized 120000-dalton TLa with papain. The proteolytic fragments obtained were indistinguishable from those directly released from the cell surface by proteolysis. The papain-derived TLa fragment exhibited most if not all the alloantigenic determinants.

Adaptation to tissue culture of the murine leukemia ASL1, a high producer of TL (thymus leukemia) antigen

ASL1, a murine thymus derived leukemia, was adapted to tissue culture and its suitability as a source of TL antigens was examined. Tissue culture medium was developed in which the cell grew with a mean doubling time of 12.5 h and 95% viability. The quantity of antigen expressed by the cells was determined by inhibition of lysis of 51Cr labelled leukemic cells meadiated by anti-TL 1,2,3 serum. Significant quantities of the antigens were retained by the cells during exponential growth in vitro.

Structural similarities between a product of the T/t-locus isolated from sperm and teratoma cells, and H-2 antigens isolated from splenocytes

Molecules specified by the H-2-linked T/t locus in the mouse are expressed on H-2-negative cells such as early embryos, sperm, and teratoma cells. By means of enzymatic radioiodination of cells and immunoprecipitation of lysates with congenic antiserum, one of these molecules, known as F9, has been obtained from sperm and teratoma cells and compared to H-2 isolated from murine splenocytes. Our studies indicate that both H-2 and F9 have identical molecular weights and subunit structure, including the presence of a B2-microglobulin-like subunit. These findings, taken together with previous studies of TL alloantigens, suggest that a number of gene products of the 9th linkage group show structural homology with each other and, in addition, with immunoglobulin. The genes in question may therefore have arisen from a primitive gene concerned with cellular recognition.

Surface carbohydrate composition of different types of chicken lymphocytes

A simple and quick procedure was used to analyse the carbohydrate composition of surface glycoproteins from chicken lymphocytes. The procedure included papain digestion of the cells, a two-step purification of the supernatant material and a sugar analysis by gaschromatography. The method made it possible to analyse samples of about 10(9) lymphocytes. The surface glycoproteins from different chicken lymphocyte preparations were found to differ significantly in their sugar composition. Lymphocytes from thymus, bursa, spleen or blood were characterized by typical relative amounts of glucose, galactosamine, fucose and sialic acid. The values for mannose, glucosamine and galactose, however, were approximately 1:1:1 for all four lymphocyte preparations. Bursectomy or thymectomy in combination with whole body irradiation altered the carbohydrate composition significantly. The results suggest the possibility that the carbohydrate composition can be used as a marker for different lymphocyte populations. The results are also discussed in respect to the hypothesis that carbohydrate determinants on the cell surface determine the migration of lymphocytes.

Differences in the surface proteins of mouse B and T cells

We have selectively labeled the surface of mouse thymus-dependent (T) and thymus-independent (B) lymphocytes by means of lactoperoxidase-catalyzed iodination. Examination of the labeled proteins by electrophoresis in the presence of sodium dodecylsulfate followed by autoradiography revealed differences in the cell surface proteins of B and T cells. Proteins from labeled thymocytes, T cell lymphomas, and both normal and activated peripheral T cells give a broad band of radioactivity corresponding to at least two protein components with apparent molecular weights of 170,000 to 190,000. This band is absent from autoradiographs of iodinated B cell proteins, which instead have another band corresponding to a protein with an apparent molecular weight of about 220,000. We have shown that these T and B cell marker proteins are synthesized by the cell and are not serum components selectively bound to the cell surface. We have also established that these proteins are the major iodinated species precipitated from Nonidet P-40 cell lysates by rabbit anti-mouse lymphocyte serum, suggesting that they are the major antigens recognized by rabbit antibodies to mouse lymphocytes.