Studies on the chemical basis of the antigenicity of proteins. 5. Synthesis, characterization and immunogenicity of some multichain and linear polypeptides containing tyrosine

Relationship of epitope glycosylation and other properties of blood group proteins to the immunogenicity of blood group antigens

BACKGROUND

The intrinsic properties of polypeptide blood group antigens that determine their relative immunogenicities are unknown. Because size, composition, charge, dose, and epitope glycosylation affect the immunogenicity of other polypeptides, we examined whether similar properties were related to the immunogenicity of blood group antigens.

STUDY DESIGN AND METHODS

Amino acid (AA) sequences of antithetical blood group antigens were searched for N- and O-glycosylation sites. Regression analysis was carried out to determine whether blood group protein properties, including total and ectodomain size, red blood cell (RBC) antigen site density, number of mismatched AAs between an antigen and its closest homolog, and differences in mass, charge, and hydrophobicity of the mismatched AAs, were related to immunogenicity.

RESULTS

The immunogenicities of non-RhD polypeptide antigens were directly related to the total and ectodomain sizes of their carrier proteins. A negative power relationship existed between RBC antigen site density and immunogenicity, such that the most immunogenic antigens had the lowest site density. The strong immunogenicity of RhD was related to the number of AA mismatches between RhD and RhCE, to their cumulative hydrophobicity and electrostatic mismatch scores, and the cumulative AA mass difference. No N- or O-glycosylation differences were predicted for antithetical or homologous antigens, other than a previously known N-glycosylation difference between K and k.

CONCLUSION

Epitope glycosylation appeared not to be a determinant of immunogenicity for blood group antigens, except possibly for K. The immunogenicity of blood group antigens was positively related to total and ectodomain sizes of blood group proteins and negatively related to antigen site density. Such findings should be considered hypothesis generating for future, more definitive studies.

Structural and functional impacts of amino acid substitutions that create blood group antigens: implications for immunogenicity

BACKGROUND

The immunogenicities of polypeptide blood group antigens vary widely. One possible determinant of immunogenicity is antigenic foreignness. The goal was to employ alternative ways of assessing foreignness and determine whether foreignness was related to immunogenicity.

STUDY DESIGN AND METHODS

Foreignness was assessed as the extent of protein functional disruption caused by the exofacial amino acid (AA) substitutions that create blood group antigens, using AA substitution prediction algorithms such as Meta-SNP and according to whether those substitutions were radical or conservative.

RESULTS

AA substitutions that create the most immunogenic antigens had the highest Meta-SNP scores, predictive of greater protein structure and function changes. Four of the 11 exofacial AAs that distinguish the most immunogenic antigen, RhD, from RhCE, and substitutions creating four of the five next most immunogenic antigens had the highest Meta-SNP scores (0.293-0.649). Excluding the outlier Jk^a , the mean Meta-SNP score of the four most immunogenic non-RhD antigens (K, Lu^a , E, c) was 3.7-fold higher than the mean of the four least immunogenic (M, Fy^a , C, S), 0.459 versus 0.123 (p = 0.0026). Regression analysis revealed a relationship between immunogenicity and Meta-SNP score (R²  = 0.953). Actual protein functional disruption was predicted for the AA substitution creating the E antigen. An AA cluster at Positions 350, 353, and 354 of RhD was unique, containing radical substitutions according to two classification schemes and relatively high Meta-SNP scores (0.351-0.432).

CONCLUSION

The immunogenicity of blood group antigens was related to the functional disruption caused by the AA substitutions that create the antigens, as measured by Meta-SNP score.

Experimental Autoimmune Myasthenia Gravis (EAMG): from immunochemical characterization to therapeutic approaches

Myasthenia Gravis (MG) is an organ-specific autoimmune disease. In high percentage of patients there are autoantibodies to the nicotinic acetylcholine receptor (AChR) that attack AChR on muscle cells at the neuromuscular junction, resulting in muscle weakness. Experimental Autoimmune Myasthenia Gravis (EAMG) is an experimental model disease for MG. EAMG is induced in several animal species by immunization with acetylcholine receptor (AChR), usually isolated from the electric organ of electric fish, which is a rich source for this antigen. Our lab has been involved for several decades in research of AChR and of EAMG. The availability of an experimental autoimmune disease that mimics in many aspects the human disease, provides an excellent model system for elucidating the immunological nature and origin of MG, for studying various existing treatment modalities and for attempting the development of novel treatment approaches. In this review in honor of Michael Sela and Ruth Arnon, we report first on our early pioneering contributions to research on EAMG. These include the induction of EAMG in several animal species, early attempts for antigen-specific treatment for EAMG, elicitation and characterization of monoclonal antibodies and anti-idiotypic antibodies, measuring humoral and cellular AChR-specific immune responses in MG patient and more. In the second part of the review we discuss more recent studies from our lab towards developing and testing novel treatment approaches for myasthenia. These include antigen-dependent treatments aimed at specifically abrogating the humoral and cellular anti-AChR responses, as well as immunomodulatory approaches that could be used either alone, or in conjunction with antigen-specific treatments, or alternatively, serve as steroid-sparing agents.

Differentiation of Gelatins Using Polyclonal Antibodies Raised Against Tyrosylated Bovine and Porcine Gelatins

Gelatin is obtained from bones and hides/skin, mainly from cows and pigs using alkaline or acidic processes. The use of bovine gelatin in feed, food, and pharmaceutical products has been restricted by regulatory authorities as a consequence of the outbreak of bovine spongiform encephalopathy (BSE). On the other hand, some religions ban the porcine gelatin for human consumption. Thus, there is a need for methods able to control the species origin of gelatins. The large similarity in structure of gelatins from different origins has made unsuccessful their differentiation by physicochemical methods. Moreover, the development of immunochemical methods has been hampered by the poor immunogenicity of gelatins. We obtained high titers antibodies upon immunization of rabbits with tyrosylated bovine and porcine gelatins. Using indirect and competitive indirect ELISAs we observed large differences in titers and specificity among rabbits and during the course of immunization. Some of the antisera were not sensitive to the species origin of raw material or to the process used for gelatin production and could be used for gelatin quantitation in food. Other antisera detected the porcine acidic gelatins with 10- to 30-fold higher sensitivity than their bovine counterparts and could be used for the differentiation of the species origin of gelatins. Lastly, other antisera were highly sensitive to subtle changes in conformation of gelatins obtained by alkaline or acidic processes such as a 1,000-fold higher reactivity of bovine acid hide gelatin compared to that of its limed counterpart or a 30,000-fold higher reactivity of porcine acid bone compared to that of its limed counterpart; such antisera could be used to monitor the process induced structural changes of collagen during its transformation to gelatin.

Highly branched poly(L-lysine)

This paper describes the synthesis of several novel water-soluble highly branched polypeptides. The synthesis starts with the ring-opening polymerization of epsilon-benzyloxycarbonyl-l-lysine N-carboxyanhydride (Z-Lys NCA) or epsilon-trifluoroacetyl-l-lysine N-carboxyanhydride (TFA-Lys NCA), followed by end functionalization of the peptide chain with N(alpha),N(epsilon)-di(9-fluorenylmethoxycarbonyl)-l-lysine (N(alpha),N(epsilon)-diFmoc Lys). Deprotection of the N(alpha),N(epsilon)-diFmoc Lys end group affords two new primary amine groups that can initiate the polymerization of a second generation of branches. Repetition of this ring-opening polymerization-end functionalization sequence affords highly branched poly(epsilon-benzyloxycarbonyl-l-lysine) (poly(Z-Lys)) and poly(epsilon-trifluoroacetyl-l-lysine) (poly(TFA-Lys)) in a small number of straightforward synthetic steps. Removal of the side-chain protective groups yields water-soluble and highly branched poly(l-lysine)s, which may be of potential interest for a variety of medical applications.

Discovering the role of the major histocompatibility complex in the immune response

The discovery that genes in the major histocompatibility complex (MHC) play an important role in the immune response depended on the chance interaction of several unrelated events. The first, and most important, was the decision by Michael Sela to synthesize a series of branched, multichain, synthetic polypeptides based on a backbone of poly-l-lysine. The prototype compound, (T,G)-A-L, was tipped with short random sequences of tyrosine and glutamic acid. This resulted in a restricted range of antigenic determinants composed of only two or three amino acids with a variable length-ideal for binding to the peptide binding groove of MHC class II molecules. The second was the decision by John Humphrey to immunize various strains of rabbits with this synthetic polypeptide. Two of these rabbit strains showed very large quantitative differences in antibody response to (T, G)-A-L. In transferring this system to inbred mouse strains, the third bit of good fortune was the availability at the National Institute of Medical Research, in Mill Hill (London), of the CBA (H2(k)) and C57 (H2(b)) strains. The H2(b) haplotype is the only one mediating a uniform high antibody response to (T,G)-A-L. The fourth critical ingredient was the availability of numerous congenic and H2 recombinant inbred strains of mice produced earlier by Snell, Stimpfling, Shreffler, and Klein. A search for congenic pairs of mice expressing the responder and nonresponder H2 haplotypes on the same background revealed that these strains responded as a function of their H2 haplotype, not of their inbred background. Extensive studies in a variety of inbred strains carrying recombinant H2 haplotypes, as well as a four-point linkage cross, mapped immune response to (T,G)A-L within the murine MHC, between the K and Ss loci. The demonstration that stimulation in the mixed lymphocyte reaction (MLR) mapped to the same region quickly led to attempts to produce antisera in congenic H2 recombinant strain combinations. These antisera identified I-region associated (Ia) antigens. Immunoprecipitation and blocking studies showed that the gene products controlling specific immune responses, the mixed lymphocyte reaction, and the structure of Ia antigens were one and the same-now designated as the I-A MHC class II molecules. These antisera and inbred strains enabled Unanue to demonstrate the peptide binding function of class II MHC molecules.

Detrimental effect of bovine leukemia virus (BLV) on the immunological state of cattle

Bovine leukemia virus (BLV) is a retrovirus which seems to affect both the humoral and the cellular immune response. Cows affected by enzootic bovine leukemia (EBL) showed a reduction of IgM-producing cells in the spleen and lymph nodes. Experimentally infected calves had lower levels of secretory IgM and a decrease in T lymphocytes in the peripheral blood. The reduction in the amount of T cells was noticed mainly in cells bearing the CD4 markers. BLV-infected animals showed diminished responsiveness to newly encountered antigens. Cows naturally infected by BLV produced Igs with impaired structural or biological reactivity. The primary immune response was shown to be deficient in BLV-infected cows following vaccination with synthetic antigen. A marked shift in the proportion of PBL, especially of the CD5+ subset, was noticed. Peripheral blood mononuclear cells from BLV-infected cows secrete elevated levels of certain cytokines and contain increased levels of cytokine mRNA. High levels of cytokines are also found in the sera of BLV-infected cows compared to non-infected animals. A correlation was found between BLV infection and lack of spontaneous recovery from Trichophyton verrucosum infection. Moreover, some studies ascertained a significant association between the herd BLV infection status and disease incidence. The culling rate was higher and milk production lower in BLV-infected vs. BLV-free herds. It seems that BLV infection affects the immune system of a cow to such an extent that it ceases to be productive enough to be kept and, in most cases, the animal is culled before any symptoms of illness associated with persistent immunodeficiency become apparent.

Dichotomy between the T and the B cell epitopes of the synthetic polypeptide (T,G)-A--L

Studies with the well-characterized, synthetic, random-multichain polypeptide poly(LTyr,LGlu)-poly(DLAla)-poly(LLys) (T,G)-A-L) led to the discovery of determinant-specific genetic control of the immune response, as well as to other immunological phenomena. Moreover, the tetrapeptide TyrTyrGluGlu built on the same backbone ("(T-T-G-G)-A--L") was found to represent its major B cell epitope. We have recently shown that for interaction with major histocompatibility complex class II molecules and stimulation of T cells, (T,G)-A--L requires proteolytic processing and the resulting T cell epitopes are close to the N termini of the branched polymer's side chains. Thus, we were interested to elucidate the major T cell epitope of (T,G)-A--L, by using the ordered polypeptides (T-T-G-G)-A--L and (T-G-T-G)-A--L, in which only the two internal amino acids of the tetrapeptide attached to the side chains are switched. We established T cell lines to these antigens, and found that the ordered analog (T-T-G-G-)-A--L, which was defined as the B cell epitope of (T,G)-A--L, did not represent its T cell epitope, whereas (T-G-T-G)-A--L, to which only a minor anti-(T,G)-A--L Ab response was directed, was found to be its major T cell epitope. In addition, there was no cross-reaction between (T-G-T-G)-A--L and (T-T-G-G)-A--L at the T cell level, similar to the lack of cross-reaction of their antibodies. Analysis of the repertoire of the T cell receptors used by these lines revealed that the (T,G)-A--L and the (T-T-G-G)-A--L specific T cell lines were not restricted in their V alpha and V beta TCR usage, whereas the (T-G-T-G)-A--L-specific line was restricted by both V alpha and V beta T cell receptor gene products. This difference might be due to the thymus-independent characteristics previously described for the latter antigen.

Binding of glycoprotein 120 and peptides from the HIV-1 envelope by autoantibodies in mice with experimentally induced systemic lupus erythematosus and in patients with the disease

Systemic lupus erythematosus (SLE) and infection with the human immunodeficiency virus type 1 (HIV) are diseases that are characterized by immune dysregulation and autoantibody production. In this article we identify and characterize IgG antibodies from mice with SLE and SLE patients that bind HIV gp120 and HIV envelope-derived peptides. SLE can be induced in susceptible mouse strains by immunization with a human monoclonal anti-DNA antibody that bears a common idiotype designated 16/6 Id. We tested sera from various strains of mice in which experimental SLE was induced by this method, as well as from 93 patients with SLE and 31 controls (17 healthy controls, 14 patients with other autoimmune diseases) for the presence of antibodies reactive to gp120 by an ELISA. Antibodies reactive with gp120 were produced by BALB/c, C3H.SW, AKR, and DBA/2 mice, all of which were 16/6 Id immunized and had experimental SLE. C57BL/6 mice, which are resistant to induction of SLE by this method, did not produce antibodies reactive with gp120 despite 16/6 immunization. Forty-three percent of SLE patients made antibodies that bound to gp120 at titers greater than 1:40, whereas 12% of healthy control sera (p < or = 0.02) and 14% of patients with other autoimmune diseases contained such antibodies (p < or = 0.05). We delineated the specificity of this antibody activity by testing for reactivity to six HIV envelope peptides. In both mice and SLE patients, sera reactive with gp120 recognized the same three envelope peptides. Removal of the anti-DNA antibodies from the sera by DNA-agarose affinity purification did not change anti-gp120 specificity.

Influence of the conformation of a macromolecule on the generation of T-cell proliferative response. A study with model polypeptides

To study the influence of the conformation of polypeptidic macromolecules on the generation of T-cell epitopes, sequential polypeptides with an octamer repeat unit were designed and synthesized. They adopt mainly unordered and alpha-helical conformations. Among these polypeptides, those containing proline are fully or partly unordered, and are more effective at inducing T-cell proliferation than a proline-free very stable alpha-helical polypeptide. This extremely stable alpha-helical conformation, probably stabilized by aggregation, would enhance its stability against proteolytic processing.

Utilization of modified surfactant-associated protein B for delivery of DNA to airway cells in culture

Pulmonary surfactant lines the airway epithelium and creates a potential barrier to successful transfection of the epithelium in vivo. Based on the functional properties of pulmonary surfactant protein B (SP-B) and the fact that this protein is neither toxic nor immunogenic in the airway, we hypothesized that SP-B could be modified to deliver DNA to airway cells. We have modified native bovine SP-B by the covalent linkage of poly(lysine) (average molecular mass of 3.3 or 10 kDa) to the N terminus of SP-B and formed complexes between a test plasmid and the modified SP-B. Transfection efficiency was determined by transfection of pulmonary adenocarcinoma cells (H441) in culture with the test plasmid pCPA-RSV followed by measurement of activity of the reporter gene encoding chloramphenicol acetyltransferase (CAT). Transfections were performed with DNA.protein complexes using poly(lysine)10kDa-SP-B ([Lys]10kDa-SP-B) or poly(lysine)3.3kDa-SP-B ([Lys]3.3kDa-SP-B), and results were compared with transfections using unmodified poly(lysine).DNA, unmodified SP-B.DNA, or DNA only. For [Lys]10kDa-SP-B.pCPA-RSV preparations, CAT activity was readily detectable above the background of [Lys]3.3kDa-SP-B or unmodified SP-B. The SP-B-poly(lysine) conjugates were effective over a broad range of protein-to-DNA molar ratios, although they were optimal at approximately 500:1-1000:1. Transfection efficiency varied with the tested cell line but was not specific to airway cells. Addition of replication-defective adenovirus to the [Lys]10kDa-SP-B.pCPA-RSV complex enhanced CAT activity about 30-fold with respect to that produced by the [Lys]10kDa-SP-B.pCPA-RSV complex alone. This increase suggests routing of the adenoviral.[Lys]10kDa-SP-B.pCPA-RSV complex through an endosomal pathway. Effects of covalent modification on the secondary structure of SP-B were examined by Fourier transform infrared spectrometry (FTIR). Results of FTIR indicated that the conformation of [Lys]10kDa-SP-B was comprised primarily of alpha-helical structure compared with a predominantly aggregated structure of unmodified poly(lysine). We conclude that poly(lysine) conjugates of SP-B effectively deliver DNA in vitro and may have utility as DNA delivery vehicles to the airway in vivo.

Ia-antigen-T-cell interactions for a thymus-independent antigen composed of D amino acids

Synthetic polypeptide antigens of L amino acids, although bearing repeating sequences, are thymus-dependent (L-TD), whereas the same polymers composed of D amino acids are thymus-independent (D-TI), probably due to a slower rate of metabolism. Yet we found that lymph-node cells of BALB/c mice immunized with D-TI proliferate in response to it in vitro. To follow T-cell activation by D-TI, we established T-cell hybridomas to D-TI and to its analog composed of L isomers, L-TD, for comparison. The T-cell hybridomas express membrane alpha/beta T-cell receptors and secrete interleukin 2 upon stimulation with the respective antigen. In addition, D-TI-specific hybridomas are stimulated, to a lesser extent, by the L-TD antigen, whereas only some L-TD-specific hybridomas recognize D-TI. Moreover, biotinylated analogs of D-TI and L-TD bind to splenic antigen-presenting cells (APCs) from BALB/c mice. Binding is inhibited by an excess of nonbiotinylated L-TD, and by an excess of a peptide comprising residues 259-271 of the human acetylcholine receptor alpha subunit, which binds to I-Ad and I-Ed molecules without prior processing. Analysis of APC lysates following incubation of the APCs with biotinylated D-TI and L-TD reveals that the biotinylated antigen moiety is associated with Ia molecules. D-TI and L-TD bind to Ia molecules on intact APCs with similar KD values, 5 x 10(-8) M and 3 x 10(-8) M, respectively. However, D-TI has faster kinetics of binding than L-TD, probably due to different processing requirements. Hence, we have demonstrated a major histocompatibility complex class II-mediated T-cell response to a thymus-independent antigen.

Structural origin of the immunological diversity of two closely related tetrapeptides: CIDNP study of TyrTyrGluGlu and TyrGluTyrGlu epitopes

Photochemically induced dynamic nuclear polarization (photoCIDNP) measurements, specific for exposed tyrosine residues, have been applied to elucidate conformational differences responsible for the immunological diversity of the synthetic multichain copolymers, Tyr1Tyr2Glu3Glu4-poly-DL-Ala-poly-Lys and Tyr1Glu2Tyr3Glu4-poly-DL-Ala-poly-LS. These two copolymers are essentially identical in their molecular weight, size, shape and composition, and differ only in the order of the two internal amino acid residues within the sequence of the tetrapeptide epitopes. Nonetheless, previous studies have shown that the two macromolecules behave differently, as evidenced by their immunological and immunogenic properties. As immunogens they act under different genetic control mechanisms, and differ in their interactions with antigen presenting cells, T cells and B cells. Antibodies elicited against these two antigens do not cross react. The photoCIDNP measurements of these two polymers, intended to elucidate discrete structural differences controlling immune recognition, showed that in the TyrTyrGluGlu polymer, Tyr1 and Tyr2 rings are free, non-interacting and undergo fast internal rotation. Computed minimum energy conformations confirm these conclusions and indicate that Tyr1 and Tyr2 point to different regions in space. In TyrGluTyrGlu, however, CIDNP measurements give rise to one broad tyrosine 3,5 proton signal, the result of a strong Tyr1-Tyr3 hydrophobic interaction. These two tyrosine residues are thus close in space, and undergo slow internal rotation. These results are in agreement with the computed minimum energy conformations.

Synthetic copolymer 1 inhibits human T-cell lines specific for myelin basic protein

Copolymer 1 (Cop 1) is a synthetic basic random copolymer of amino acids that has been shown to be effective in suppression of experimental allergic encephalomyelitis and has been proposed as a candidate drug for multiple sclerosis. Cop 1 is immunologically cross reactive with myelin basic protein (BP) and was shown to inhibit murine BP-specific T-cell lines of various H-2 restrictions. In the present study these findings were extended to include human T-cell lines. Cop 1 competitively inhibited the proliferative responses and interleukin 2 secretion of six BP-specific T-cell lines and 13 clones with several DR restrictions and epitope specificities. Conversely, BP inhibited--albeit to a lesser extent--the response of all the Cop 1-specific T-cell lines and clones, irrespective of their DR restrictions. Another random copolymer of tyrosine, glutamic acid, and alanine, denoted TGA, had no effect on these lines. Neither Cop 1 nor BP inhibited the response of lines and clones specific for purified protein derivative. Cop 1 and BP exerted their cross-inhibitory effects only in the presence of antigen-presenting cells. These results suggest that Cop 1 can compete with BP for the binding to human major histocompatibility complex molecules. In view of recent studies implicating BP reactivity in multiple sclerosis, these findings suggest a possible mechanism for the beneficial effect of Cop 1 in this disease.

Direct binding of a synthetic multichain polypeptide to class II major histocompatibility complex molecules on antigen-presenting cells and stimulation of a specific T-cell line require processing of the polypeptide

T-cell activation involves the recognition of foreign antigens as a complex with self-major histocompatibility complex (MHC) proteins on the surface of antigen-presenting cells (APC). Protein antigens usually require uptake by the APC and processing that results in the generation of peptide fragments. The branched synthetic polypeptide (Tyr, Glu)-Ala--Lys was chosen as a model antigen to follow the processing requirements, leading to T-cell activation. It has been demonstrated, by using fixed APC and various inhibitors of proteases, that (Tyr, Glu)-Ala--Lys has to be processed to stimulate a (Tyr, Glu)-Ala--Lys-specific T-cell line of C3H.SW (H-2b) origin to proliferate. To determine whether processing of (Tyr,Glu)-Ala--Lys is required to allow its association with the MHC class II molecules, biotin was covalently attached to it. Binding of the biotinylated (Tyr,Glu)-Ala--Lys to MHC class II gene products on the surface of intact normal APC was directly detected by phycoerythrin-streptavidin. The specificity of the binding was confirmed by its inhibition with anti-I-Ab antibodies as well as with excess of nonlabeled (Tyr,Glu)-Ala--Lys. Furthermore, introducing several inhibitors of proteases to the binding assay, we could substantiate that the proteolysis of (Tyr,Glu)-Ala--Lys is required to allow association of the resulting peptidyl T-cell epitopes with the MHC class II molecules themselves. The presence of the biotin moiety in the resulting peptides suggests that the T-cell epitopes of (Tyr,Glu)-Ala--Lys contain the N-terminal portion of the side chains of the branched polypeptide. An apparent Kd of 8.05 x 10(-8) M was determined, and optimal binding was detected after 10 hr of incubation with the antigen. The latter phenomenon is not due to slow uptake, since uptake of (Tyr,Glu)-Ala--Lys occurs mainly during the first 30 min of incubation, but rather reflects the events of processing that precede MHC interaction.

Specific proliferative responses following the induction of experimental SLE in mice

We have previously reported the induction of experimental systematic lupus erythematosus (SLE) in mice by immunization with a human monoclonal antibody that expresses a common anti-DNA idiotype (16/6 Id). Following immunization, antibodies directed against various nuclear autoantigens could be detected in the sera of the mice. In the present study, we investigated the proliferative responses of lymph node cells to one particular autoantigen (DNA) following the induction of experimental SLE. Cells reactive with ssDNA could be detected following immunization of BALB/c mice with the 16/6 Id. The appearance of these DNA-reactive cells succeeded the appearance of 16/6 Id-specific cells. The activation of this subset of autoreactive cells could be achieved only by the immunization of the mice with the 16/6 Id, but not by their immunization with DNA, thus suggesting that the induction of experimental SLE is associated with the alteration of the low responsive potential of the mice to DNA.

The influence of the side-chain sequence on the structure-activity correlations of immunomodulatory branched polypeptides. Synthesis and conformational analysis of new model polypeptides

New branched polypeptides were synthesized for a detailed study of the influence of the side-chain structure on the conformation and biological properties. The first subset of polypeptides were prepared by coupling of tetrapeptides to poly[L-Lys]. These polymers contain either DL-Ala3-X [poly[Lys-(X-DL-Ala3)n]] or X-DL-Ala3 [poly[Lys-(DL-Ala3-X)n] (n less than or equal to 1)] tetrapeptide side chains. Another group of branched polymers comprise a mixture of DL-Alam and of DL-Alam-X oligomeric branches in a random distribution [poly[Lys-(DL-Alam-Xi)] (i less than 1, m approximately 3)]. In each subset the X = Leu or Phe derivatives were prepared. The N-protected tetrapeptides were synthesized by conventional liquid phase methods and were coupled as active esters. The degree of racemization was found relatively high both for active esters and coupled derivatives, when optically active amino acids were in the C-terminal position of the tetrapeptides. In the case of the poly[Lys-(Leu-DL-Ala3)n] derivative, comparative experiments were carried out using various methodical alterations. The highest stereochemical homogeniety could be achieved when the tetrapeptide active ester was synthesized by the "backing off" method. CD spectra of poly[Lys-(Xi-DL-Alam)] (i less than 1, m approximately 3) and of poly[Lys-(X-DL-Ala3)n] were analyzed and compared to those of poly[Lys-(DL-Alam-Xi)] and of poly[Lys-(DL-Ala3-X)n]. All measurements were performed in water solutions of varying pH values and ionic strengths. The data obtained suggest that branched polypeptides containing a mixture of two different types of oligomeric side chains (DL-Alam and DL-Alam-Xi or Xi-DL-Alam) distributed randomly adopt an almost identical conformation to those that comprise only the respective tetrapeptide (DL-Ala3-X or X-DL-Ala3) branches. The results also indicate that the tendency to form an ordered structure is determined by the identity and the position of the chiral amino acid X (Phe or Leu) in the side chain.

The insulin receptor: structure and function

Promising progress in understanding the molecular basis of insulin action has been achieved by demonstrating that the insulin receptor is an insulin-sensitive tyrosine kinase. Here we discuss the structure of this receptor kinase and compare it with receptors for related growth factors. We review the known modes to regulate the receptor kinase activity, either through its autophosphorylation (on tyrosine residues) or through its phosphorylation by other kinases (on serine and threonine residues). We discuss the role of the receptor kinase activity in hormone signal transduction in light of results indicating a reduced kinase activity in insulin-resistant states. Finally, studies to identify natural substrates for the insulin receptor kinase are presented. The possible physiological role of these phosphorylated substrates in mediating insulin action is evaluated.

Specific inhibition of the T-cell response to myelin basic protein by the synthetic copolymer Cop 1

Cop 1 is a synthetic basic random copolymer of L-alanine, L-glutamic acid, L-lysine, and L-tyrosine in a residue molar ratio of 6.0:1.9:4.7:1.0 and with a molecular weight of 21,000 which proved to be effective in specific suppression of experimental allergic encephalomyelitis and has been proposed as a candidate drug against multiple sclerosis. In the present study we further investigated the mechanism of Cop 1 suppressive activity and tested whether Cop 1 could inhibit the specific T-cell response to myelin basic protein (BP). Eight BP-specific T-cell lines and clones with various H-2 restrictions and antigenic specificities were used. The responses of all these lines and clones to BP, as followed by both cell proliferation and interleukin 2 secretion assays, were affected by Cop 1. For one line, a direct cross proliferation with Cop 1 was observed, whereas in the other seven lines and clones, Cop 1 specifically inhibited the responses to BP in a competitive dose-dependent manner. The inhibition of the response to BP is specific to Cop 1, as D-Cop 1 and another random acidic polymer, poly(Tyr,Glu,Ala) (TGA), both of which were previously demonstrated to be ineffective in suppression of experimental allergic encephalomyelitis, did not inhibit the response to BP. Furthermore, Cop 1 specifically inhibited only the response of the T-cell lines and clones to BP. It did not inhibit their response to the mitogen Con A, nor did it inhibit the responses of the purified protein derivative-specific T-cell line and clone. These results suggest that Cop 1 may be effective in suppression of experimental allergic encephalomyelitis, not only because of the selective stimulation of suppressor T cells, as we have previously demonstrated, but also by specific inhibition of BP-specific effector T cells.

Induction of a systemic lupus erythematosus-like disease in mice by a common human anti-DNA idiotype

Systemic lupus erythematosus (SLE) is considered to be the quintessential autoimmune disease. It has not been possible to induce SLE in animal models by DNA immunization or by challenge with anti-DNA antibodies. We herewith report a murine model of SLE-like disease induced by immunization of C3H.SW female mice with a common human monoclonal anti-DNA idiotype (16/6 idiotype). Following a booster injection with the 16/6 idiotype, high levels of murine anti-16/6 and anti-anti-16/6 antibodies (associated with anti-DNA activity) were detected in the sera of the immunized mice. Elevated titers of autoantibodies reacting with DNA, poly(I), poly(dT), ribonucleoprotein, autoantigens [Sm, SS-A (Ro), and SS-B (La)], and cardiolipin were noted. The serological findings were associated with increased erythrocyte sedimentation rate, leukopenia, proteinuria, immune complex deposition in the glomerular mesangium, and sclerosis of the glomeruli. The immune complexes in the kidneys were shown to contain the 16/6 idiotype. This experimental SLE-like model may be used to elucidate the mechanisms underlying SLE.

Phosphorylation of synthetic random polypeptides by protein kinase P and other protein-serine (threonine) kinases and stimulation or inhibition of kinase activities by microbial toxins

A synthetic random polymer of threonine and glutamate (1:4.4) is readily phosphorylated by protein kinase P but not by five other protein-serine (threonine) kinases. A synthetic random polymer of serine and arginine (1:3) is readily phosphorylated by protein kinase A and protein kinase C but not by protein kinase P. Although the amino acid sequences surrounding the phosphorylated serine (threonine) residue have been demonstrated in studies with small synthetic polypeptides to be decisive factors in the rate at which they are phosphorylated, the findings with the large synthetic polypeptides suggest that in the case of proteins the size, the tertiary structure, and particularly the electrostatic interactions are equally or more important contributing factors. Syringomycin, a toxin from Pseudomonas syringae, and polymyxin B, from Bacillus polymyxa, stimulate protein kinase P, strongly inhibit protein kinase C, and have no effect on protein kinase A. Basic polypeptides with high lysine content are phosphorylated by ATP nonenzymatically.

Analysis of the antigen specific helper T cell function and HLA-DR of Israeli patients with rheumatoid arthritis (RA)

Forty-three patients with rheumatoid arthritis (RA) were studied for their ability to respond to the synthetic polypeptide antigen (T, G)-A-L as measured by the production of a T cell helper factor by their antigen activated T cells. Sixteen patients (37%) responded to (T, G)-A-L by the production of an antigen specific helper T cell factor, a percentage not significantly different from healthy donors. The production of antigen specific T cell helper factors was affected, although not significantly, by immune modulating drugs and by the presence of rheumatoid factor in sera of patients. The high incidence of HLA-DR 4 reported for RA patients was not observed in this group of RA patients.

Mode of action of antigen-specific T-cell helper factors secreted by a T-cell line and clones

Antigen-specific T-cell helper factors were secreted from a (T,G)-A--L specific T-cell line and clones. The factors were released upon antigenic stimulation and could be induced by a low or a high dose of antigen. The factors secreted upon low-dose stimulation possessed the antigenic specificity of the secreting cells, while the high dose-induced factors had a broader antigenic specificity and could react with the closely related polypeptide (Phe,G)-A--L, even when the cells were restricted to (T,G)-A--L. Both the low dose- as well as the high dose-induced factors could not trigger antibody production in the presence of a non-relevant antigen, and did not collaborate with B cells immunized with a non-related antigen for the production of antibodies. The helper factors, like their secreting cells, were H-2-restricted in the collaboration with B cells. In contrast to the helper cells, however, they did not require accessory cells for triggering the B cells in the process of antibody production. Some preparations of helper factors were found to be inactive. The helper activity could be restored by IL-2. Thus, IL-2 is an additional essential factor required for the antigen-specific collaboration of B cells and T-cell helper factors.

Humoral immune response of asymptomatic cats naturally infected with feline leukemia virus

The humoral immune response of cats that were naturally infected with the feline leukemia virus (FeLV) was examined after antigenic stimulation with the synthetic antigen poly(L-Tyr, L-Glu)-poly(DL-Ala)-poly(L-Lys). The primary humoral antibody response in FeLV-infected cats was both delayed and greatly reduced, compared with that seen in uninfected control cats. A similar discordance was observed after secondary stimulation with the antigen, in the FeLV-infected cats had both a delayed response and a reduced response, compared with uninfected cats. The levels of total immunoglobulins of the immunoglobulin G and immunoglobulin M classes in the sera of FeLV-infected cats were significantly higher (two- and threefold, respectively) than were those of the uninfected control animals. The presence of an impaired humoral immune response to newly presented antigens in the presence of elevated immunoglobulin levels has been thoroughly documented in the case of people with the acquired immunodeficiency syndrome. This further emphasizes the potential value of FeLV-infected cats as a model for human acquired immunodeficiency syndrome.

Analysis of antigen specific T cell helper function in first degree relatives of patients with systemic lupus erythematosus (SLE)

Fourteen families with first degree relatives of patients with systemic lupus erythematosus (SLE) were studied for the ability of their members to respond to the synthetic polypeptide antigen (T,G)-A-L. The family members were also tested for their HLA determinants. All SLE patients tested responded to (T,G)-A-L as measured by the production of (T,G)-A-L specific T cell helper factors by their antigen activated T cells, confirming our previous findings that 100% of SLE donors responded to (T,G)-A-L in contrast to 50% responders in a control population of healthy donors. The general defect in the regulation of immune responses in SLE patients was further indicated by the demonstration that an SLE patient who is a daughter of non-responder parents to (T,G)-A-L, responded to this genetically regulated antigen. In contrast to our observations with SLE patients, the genetic regulation of the ability to respond to (T,G)-A-L was shown not to be impaired in healthy first degree family members of SLE patients and the segregation of the immune response potential in these families was as expected from an inherited dominant trait.

Analysis of the biological functions and fine specificity of (T,G)-A--L specific T cell clones

Two T cell lines, TPB1 and TPB2, specific for the synthetic polypeptide antigen (T,G)-A--L, were established from (T,G)-A--L primed lymph node cells of C3H.SW(H-2b) mice. Both lines proliferated in the presence of (T,G)-A--L, helped in antibody production in vitro, and secreted IL2 upon stimulation with antigen. The lines differed in the fine specificity of their responses to antigenic stimulation. The line with the broader specificity TPB2 was cloned by limiting dilution, and its derived clones were analyzed. No efficient manifestation of both proliferative activity and helper function could be detected in a single clone. Most of the clones were highly specific to (T,G)-A--L, although 2 of them cross-reacted with the closely related polypeptide (Phe,G)-A--L. Individual clones could trigger B cells for the production of antibodies of the IgM and IgG classes. All helper clones secreted (T,G)-A--L specific helper factors. No correlation was found between efficient secretion of IL2 by the clones and their other biological functions.

Antigen-presenting cells in human decidual tissue

The responses of peripheral blood human T lymphocytes supported by decidual antigen-presenting cells (DAPCs) to a variety of immunogenic stimuli were studied and compared to those of T cells supported by peripheral blood antigen-presenting cells (PAPCs). Antigen-presenting cells were isolated from early normal decidual tissue or peripheral blood by elution with ethylenediamine tetraacetic acid of cells that after Ficoll-Paque separation bear receptors for all have bound to fibronectin. DAPCs pulsed with soluble or particulate antigens induced proliferation of T cells with an efficiency equivalent to PAPCs. Decidual tissue APCs also showed the ability to stimulate auto- and alloreactivity. Treatment with anti-human lymphocyte antigen (HLA) class II antibody and ultraviolet radiation resulted in substantial inhibition of the accessory cell function of DAPCs as well as of PAPCs. Bromodeoxyuridine and light treatment of alloreactive T cells generated in vitro was used to demonstrate that DAPCs primed with a synthetic polypeptide antigen (T,G)-A-L can stimulate only HLA class II-compatible T lymphocytes.

Possible association of antibody responses to human serum albumin and (T,G)-A--L with the bovine major histocompatibility complex (BoLA)

Antibody responses to human serum albumin (HSA) and (T,G)-A--L were determined in 130 young bulls in Norway and the BoLA types of the bulls were defined. Significant associations of some BoLA antigens with immune responsiveness were shown, indicating the likely existence of an immune response (Ir) region linked to the BoLA class I antigens. High response to HSA seems to be a dominant trait. BoLA w2 showed an association with low response to HSA. This may reflect the effect of a specific MHC-associated immune suppressor gene.

Isotype distribution and fine specificity of the antibody response of inbred mouse strains to four compounds belonging to a new group of synthetic branched polypeptides

A new group of synthetic branched polypeptides was developed to initiate a systematic study of the relationships between the chemical structure (charge, size, primary structure, configuration and conformation), the carrier potential and the antigenic properties of these biodegradable and biocompatible macromolecules. This model system has two main advantages over the previously used ones: (i) the side chains grafted to the poly(L-lysine) backbone are composed of about three DL-Ala and a single chain-terminating amino acid with different absolute configuration and/or identity, and (ii) the conformation of these polypeptides is characterized in solution. The size, charge and inside area of the four molecules selected for this study were identical; however, the identity, the absolute configuration of the chain-terminating amino acids (D-Leu, Leu, Phe or D-Phe) and, in consequence, the conformation of the macromolecules were different. The qualitative and quantitative features of the antibody response induced by the four polypeptides were characterized in inbred mouse strains by IgM and IgG type antibody levels, as well as by isotype distribution and fine specificity of antibodies produced during the primary and memory response. The intensity of the memory response and the characteristics of subclass distribution were dependent on the conformation of the branched polypeptides. These molecules carry at least two types of antigenic determinants. One is ordered to the tetrapeptide side chain, the expression of which proved to be inversely correlated with the backbone-originated helix content of the molecules. The other antigenic determinant corresponds to the common inside area of the polypeptides which is less conformation-dependent and therefore common to all four polypeptides.

HLA-linked immune responsiveness to (T,G)-A-L: a family study

The heredity of the immune response potential to the synthetic polypeptide poly(LTyr,LGlu)-poly(DLAla)-poly(LLys) [(T,G)-A-L] and its possible linkage to the major histocompatibility complex of man were studied in 24 families. Peripheral blood lymphocytes (PBL) obtained from 174 donors belonging to 24 unrelated families were educated to (T,G)-A-L on autologous antigen-pulsed adherent cells. The supernatants obtained from these activated PBL were tested for their antigen-specific helper activity in an in vitro antibody production system. All donors were typed for their HLA haplotypes. The results obtained indicated that the ability to respond to (T,G)-A-L by production of an antigen-specific T cell helper factor is inherited as an autosomal dominant trait linked to the responder HLA haplotype.

Recognition of peptide orientation: studies with angiotensin II in the guinea-pig

Antipeptide sera of defined subspecificities were obtained when angiotensin II was unidirectionally conjugated to its carrier protein. Antisera with predominantly amino terminal specificity were produced when guinea-pigs were immunized with angiotensin II conjugated via its carboxy terminus to thyroglobulin. When angiotensin II was coupled via its amino terminus, carboxy terminal-specific antisera were obtained. The same conjugates failed to provide antisera with the corresponding specificities when rabbits were used instead of guinea-pigs. This work demonstrates that the peptide-protein coupling strategy may affect the specificity of the resulting antiserum.

(T,G)-A-L specific immune response potential and HLA typing of Israeli patients with systemic lupus erythematosus

Thirty-three Israeli patients with systemic lupus erythematosus (SLE) were studied for their ability to respond to the synthetic polypeptide poly (Tyr,Glu)-poly (DLAla)-poly(Lys) [( T,G]-A-L) as measured by the production of a T cell helper factor by their antigen activated T cells. Twenty-seven of the patients were typed for their HLA phenotypes. Nineteen patients were with more active disease and 14 with a milder non-active disease. All the patients of the two groups responded to (T,G)-A-L by the production of an antigen specific helper T cell factor, in contrast to only 50% responders among healthy donors. Thus, lack of normal regulation of T cell helper function was observed among all patients with SLE, independently of their disease activity and/or treatment. A higher frequency of DR5 (75%) was observed in patients with a milder non-active disease (vs 46.6% in normal healthy control individuals) while 53.3% of patients with active disease possessed DR7 (21.8% in controls). These findings may suggest a possible association between the severity of the disease and a specific DR determinant.

Use of tyrosine-containing polymers to characterize the substrate specificity of insulin and other hormone-stimulated tyrosine kinases

Synthetic copolymers containing tyrosine residues were used to characterize the substrate specificity of the insulin receptor kinase and compare it to tyrosine kinases stimulated by epidermal growth factor, insulin-like growth factor-1 and phorbol ester. In partially purified receptor preparations from eight different tissues insulin best stimulated (highest V) phosphorylation of a random copolymer composed of glutamic and tyrosine residues at a 4:1 ratio (Glu/Tyr, 4:1). The insulin-stimulated phosphorylation of this polymer was highly significant also in receptor preparations from fresh human monocytes, where insulin binding and autophosphorylation were difficult to detect. Other tyrosine-containing polymers Ala/Glu/Lys/Tyr (6:2:5:1) and Glu/Ala/Tyr (6:3:1) were also phosphorylated by the insulin-stimulated kinase but to a lower extent. A tyrosine kinase stimulated by insulin-like growth factor-1, and one stimulated by phorbol ester also best phosphorylated the polymer Glu/Tyr (4:1). The three kinases differed only in their capability to phosphorylate Glu/Ala/Tyr (6:3:1) or Ala/Glu/Lys/Tyr (6:2:5:1). Glu/Tyr (4:1) was a poor substrate for the epidermal growth factor receptor kinase which best phosphorylated the polymer Glu/Ala/Tyr (6:3:1). Three additional polymers: Glu/Tyr (1:1), Glu/Ala/Tyr (1:1:1), and Lys/Tyr (1:1) failed to serve as substrates for all four tyrosine kinases tested. Taken together these findings suggest that. Hormone-sensitive tyrosine kinases have similar yet distinct substrate specificity and are likely to phosphorylate their native substrates on tyrosines adjacent to acidic (glutamic) residues. Tyrosine-containing polymer substrates are highly sensitive and convenient tools to study (hormone-sensitive) tyrosine kinases whose native substrates are unknown or present at low concentrations.