The autoimmune response elicited by mouse hepatitis virus (MHV-A59) infection is modulated by liver tryptophan-2,3-dioxygenase (TDO)

In a previous work we demonstrated that inhibition of mouse indoleamine 2,3-dioxygenase (IDO) by methyltryptophan (MT) exacerbated the pathological actions of mouse hepatitis virus (MHV-A59) infection, suggesting that tryptophan (TRP) catabolism was involved in viral effects. Since there is a second enzyme that dioxygenates TRP, tryptophan-2, 3-dioxygenase (TDO), which is mainly located in liver, we decided to study its role in our model of MHV-infection. Results showed that in vivo TDO inhibition by LM10, a derivative of 3-(2-(pyridyl) ethenyl) indole, resulted in a decrease of anti- MHV Ab titers induced by the virus infection. Besides, a reduction of some alarmin release, i.e, uric acid and high-mobility group box1 protein (HMGB1), was observed. Accordingly, since alarmin liberation was related to the expression of autoantibodies (autoAb) to fumarylacetoacetate hydrolase (FAH), these autoAb also diminished. Moreover, PCR results indicated that TDO inhibition did not abolish viral replication. Furthermore, histological liver examination did not reveal strong pathologies, whereas mouse survival was hundred percent in control as well as in MHV-infected mice treated with LM10. Data presented in this work indicate that in spite of the various TDO actions already described, specific TDO blockage could also restrain some MHV actions, mainly suppressing autoimmune reactions. Such results should prompt further experiments with various viruses to confirm the possible use of a TDO inhibitor such as LM-10 to treat either viral infections or even autoimmune diseases triggered by a viral infection.

Changes in antibody specificities and cytokine release after infection with lactate dehydrogenase-elevating virus

Lactate dehydrogenase-elevating virus (LDV) is an apparently innocuous and persistent virus that can modify mouse immune reactions. We have shown that LDV-infected mice immunized with human growth hormone (hGH) showed a deep modification of the specificity of the anti-hGH antibodies (Ab) in CBA/Ht mice but not BALB/c animals. The aim of this work was to extend the previous observations to another mouse strain, C57BL/6, as well as to an antigen unrelated to hGH, ovalbumin (OVA), and to explore at the same time the production of various cytokines at serum and cellular levels. The amount of Ab directed to hGH or OVA native antigenic determinants versus the concentration of Ab to cryptic epitopes was evaluated by ELISA competition experiments. Results indicated that LDV infection affected Ab specificity solely in CBA/Ht mice. In CBA/Ht the virus infection was associated with a reduction of the Ab titers to hGH native epitopes and with a decrease of IL-13 and IL-17 serum levels, but Ab to native OVA epitopes were increased with a simultaneous increase of IL-17. Accordingly, only lymph node cells from infected CBA/Ht mice immunized with OVA were found to produce INF-γ, IL-13 and IL-17. Thus, a correlation of cytokine production with a change in Ab specificity after a viral infection was found, although this phenomenon was restricted to a given antigen and to the genetic background of immunized animals. These observations suggest that an apparent harmless virus can affect some immunological mechanisms, which could lead, for example, to inflammatory or autoimmune disorders.

Uric acid and HMGB1 are involved in the induction of autoantibodies elicited in mice infected with mouse hepatitis virus A59

We have shown that mice infected with mouse hepatitis virus A59 develop autoantibodies (autoAb) to liver and kidney fumarylacetoacetate hydrolase (FAH). Because it has been proposed that the immune system is stimulated by alarm signals called damage-associated molecular patterns or alarmins, we investigated the participation of uric acid and high-mobility group box protein 1 (HMGB1) in the autoimmune response elicited by mouse hepatitis virus (MHV). Mice subjected to MHV infection had increased plasmatic uric acid concentration that significantly decreased after 20 days of daily treatment with allopurinol and, simultaneously, autoAb to FAH were undetected. Furthermore, this autoAb disappeared after 30 days of treatment with ethyl pyruvate, along with a substantial reduction in serum HMGB1 concentration. Both results indicated a remarkable relationship between the autoimmune process induced by the virus and uric acid and HMGB1 liberation. Unexpectedly, it was found that allopurinol and ethyl pyruvate inhibited the release of both uric acid and HMGB1. Because HMGB1 is activated through binding to interleukin 1β, and that this cytokine is produced by the NLRP3 inflammasome that could be stimulated by uric acid, we propose that both alarmins could be acting in concert with the induction of the autoAb to FAH in MHV-infected mice.

Autoimmune hepatitis-like disease in C57BL/6 mice infected with mouse hepatitis virus A59

Mouse hepatitis virus A59 (MHV A59) induces autoantibodies (autoAb) to fumarylacetoacetate hydrolase (FAH), a soluble cytosolic enzyme present in the liver and kidneys, in various mouse strains. The aim of this work was to amplify and diversify the autoimmune response restricted to FAH through the use of the exogenous adjuvant called PADRE. Accordingly, C57BL/6 mice were chosen, because these animals respond to PADRE better than other mouse strains. Results presented herein indicate that, surprisingly, C57BL/6 mice developed signs of autoimmune hepatitis-like disease (AIH), including transient hypergammaglobulinemia, elevated transaminases, autoAb directed against different liver proteins and hepatic cellular infiltrates, indicating that a new model of experimental AIH could be generated by a viral inoculation. Furthermore, PADRE administration amplified the MHV effect, extending the duration of hypergammaglobulinemia and increasing the binding of autoAb as well as the degree of hepatic infiltrates. However, the adjuvant did not expand the time of the symptoms. Additionally, since plasmatic uric acid and high-mobility group box protein 1 (HGMB1) concentrations augmented in MHV- and/or PADRE-treated mice, it is suggested that both alarmins were probably involved in the spreading of the immune response induced by the viral infection and the adjuvant administration.

Fine specificity of autoantibodies induced by mouse hepatitis virus A59

We have shown that mice infected with mouse hepatitis virus A59 (MHV-A59) develop autoantibodies (autoAb) to liver and kidney fumarylacetoacetate hydrolase (FAH). The autoAb recognized conformational as well as linear antigenic determinants in the enzyme, and the autoimmune response was not entirely restricted to molecular mimicry and/or epitope spreading. Since the N- and C-terminal portions of the enzyme were the most reactive with autoAb, the fine specificity of these Ab was investigated. Immobilized 15-mer linear peptides (overlapping by 14 amino acids) spanning the N-terminal FAH sequence 1-49 were recognized by Ab from MHV-infected mice. The pattern of reactivity indicated the existence of two major epitope cores (i.e., sequences 9-23 and 30-44), and sequence comparison permitted the identification of two minimal epitopes, DSDFPIQ (amino acids 9-15) and IGDQILD (amino acids 36-42). Mutational analysis of sequences 9-23 and 30-44 indicated that residues 9-12 (DSDF) from the first major N-terminal epitope, and residue 36 (I) from the second, were the key amino acids energetically important for Ab contact. Interestingly, those residues were inside the two minimal epitopes previously predicted. The C-terminal portion of the enzyme (sequence 390-419) presented only one major epitope, located between residues 390 and 409. In this case, the minimal epitope had nine amino acids, CQGDGYRVG, corresponding to the FAH sequence 396-404 that outlines a loop specific for the enzyme. Data indicated that neither the FAH minimal epitopes nor the key residues important for binding to Ab from MHV-infected mice have their counterparts in the viral proteins. However, location of the energetically important residues in the tertiary structure of the enzyme originates a virtual conformational epitope. Such hypothetical B-cell epitopes could be present in any viral protein, originating a cross-reaction leading to the autoimmune response induced by MHV.

The yeast endocytic protein Epsin 2 functions in a cell-division signaling pathway

The epsins are a family of adaptors involved in recruiting other endocytic proteins, binding of ubiquitylated cargo and induction of membrane curvature. These molecules bear a characteristic epsin N-terminal homology (ENTH) domain and multiple peptide motifs that mediate protein-protein interactions. We have previously demonstrated that the ENTH domain of epsin is involved in Cdc42 signaling regulation. Here, we present evidence that yeast epsin 2 (Ent2) plays a signaling role during cell division. We observed that overexpression of the ENTH domain of Ent2 (ENTH2), but not Ent1, promoted the formation of chains of cells and aberrant septa. This dominant-negative effect resulted from ENTH2-mediated interference with septin assembly pathways. We mapped the ENTH2 determinants responsible for induction of the phenotype and found them to be important for efficient binding to the septin regulatory protein, Bem3. Supporting a physiological role for epsin 2 in cell division, the protein localized to sites of polarized growth and cytokinesis and rescued a defect in cell division induced by Bem3 misregulation. Collectively, our findings provide a potential molecular mechanism linking endocytosis (via epsin 2) with signaling pathways regulating cell division.

The autoimmune response induced by mouse hepatitis virus A59 is expanded by a hepatotoxic agent

Mouse hepatitis virus strain A59 (MHV-A59) triggers various pathologies in several mouse strains, including hypergammaglobulinaemia, hepatitis and thymus involution. We reported previously the presence of autoantibodies (autoAb) to liver and kidney fumarylacetoacetate hydrolase (FAH) in sera from mice infected with MHV-A59.

Long-term MHV-infected mice represented a good model of non-pathogenic autoimmune response since the animals were apparently healthy in spite of the presence of autoAb. The aim of this work was to see whether a severe liver injury, which releases endogenous adjuvants, i.e. danger signals, could elicit a broader spectrum of autoAb and perhaps signs of autoimmune hepatitis. Carbon tetrachloride (CCl₄) was injected into mice 30 days after MHV infection, and serum was assayed for autoAb and total IgG 20 days later. The association of MHV infection with the toxic effects of CCl₄ resulted in hypergammaglobulinaemia and the production of autoAb to various liver and kidney proteins. Histological examination of liver samples showed tissue damages but without significant differences between the animals submitted to MHV + CCl₄ and controls, which were either infected by MHV without CCl₄, or poisoned by CCl₄ in the absence of MHV infection. Those results show that liver injury after viral infection may lead to the spreading of the immune response and to an increase of serum IgG, suggesting that the procedure used herein could simulate the onset of autoimmune hepatitis.

Identification, synthesis and properties of a consensus peptide recognized by a monoclonal antibody directed to various type I cytokine receptors

Previous works demonstrated that the monoclonal antibody (MAb) called R7B4 is directed to an epitope shared by receptors for lactogenic and somatogenic hormones as well as interleukins 2 and 6 (IL-2 and IL-6). The MAb inhibited the biological effects of those hormones and cytokines by impairing their binding to receptors. It is known that the receptors for growth hormones (GH), prolactins (PRL), IL-2, and IL-6 pertain to the type I cytokine receptor family, sharing the common motif WSXWS or the homologous F(Y)GEFS. Thus, a set of 34 decapeptides corresponding to diverse receptors containing those sequences were synthesized by the PEPSCAN method and their reactions with MAb R7B4 were measured by ELISA. The MAb significantly recognized 21 peptides, allowing us to establish the consensus sequence HGYWSEWSPE as a portion of the R7B4 epitope. The consensus peptide was synthesized and purified by conventional methods, and its capacity to bind to MAb R7B4 paratope confirmed. Moreover, polyclonal Ab to the peptide elicited in mice were able to inhibit the hGH binding to lactogenic, somatogenic and human specific liver receptors. This fact suggests that the consensus peptide could be used as an immunogen to produce anti-hGH receptor Ab behaving as hormone or cytokine antagonists in certain pathological conditions.

Properties of cryptic epitopes and their corresponding antibodies as indicated by the study of human and ovine growth hormones

Antibodies (Ab) directed to hidden antigenic determinants (cryptotopes) are undesirable because they are not neutralizing. Additionally, we have previously demonstrated a close association between the extent of Ab to cryptic determinants and the expression of autoantibodies (autoAb) under some experimental conditions. Thus, the first objective of this work was to establish the physicochemical characteristics of Ab to cryptotopes and the second one was to examine the structural features of cryptic epitopes themselves. Using human and ovine growth hormones (hGH and oGH) as antigenic models and competition ELISA under different conditions of temperature, pH or ionic strength, we did not find any difference between the binding properties of anti-cryptic epitope antibodies (Ab) and anti-native epitope Ab. Then, using synthetic peptides and tryptic digests and direct and competition ELISAs we studied the structures of cryptic hGH and oGH epitopes. Isolated peptides either in solution or adsorbed on microplates failed to react. Partially digested hGH was recognized only when insolubilized on microplates, and anti-oGH Ab only reacted with a large fragment of the hormone either in solution or insolubilized. These results indicate that, at least in the case of hGH and oGH, cryptic epitopes are not simple linear sequences, as commonly referred without any evidence, but new exposed conformational structures different from those found in the native antigen.

Peltophorum dubium and soybean Kunitz-type trypsin inhibitors induce human Jurkat cell apoptosis

Plants constitute an important source of compounds which can induce apoptosis in a variety of cells. Previously, we reported the isolation of a trypsin inhibitor from Peltophorum dubium seeds (PDTI). This inhibitor, as well as soybean trypsin inhibitor (SBTI), both belonging to the Kunitz family, have lectin-like properties and trigger rat lymphoma cell apoptosis. In the present study, we demonstrate for the first time that PDTI and SBTI induce human leukemia Jurkat cell death. Induction of apoptosis was confirmed by flow cytometry after propidium iodide labeling of apoptotic nuclei, showing a considerable increase of the sub G(0)/G(1) fraction, with no cell cycle arrest. With the purpose of gaining insight into the signaling pathways involved, we investigated the activation of caspases and the effect of caspase inhibitors, and showed caspases-3 and -8-like activation by PDTI or SBTI-treatment. Consistent with these results, pan caspase inhibitor and caspase-8 inhibitor protected Jurkat cells from apoptosis. However, there was no caspase-9 activation, confirmed by the failure of caspase-9 inhibitor to prevent cell death. No significant release of cytochrome c from mitochondria was detected suggesting that the intrinsic mitochondrial pathway is not predominant in the apoptotic process. On the other hand, recruitment of Fas-associated death domain (FADD) to the cell membrane indicates the involvement of this adaptor protein in PDTI- and SBTI-induced apoptosis in Jurkat cells. Furthermore, human peripheral lymphocytes, either stimulated with phytohemagglutinin or not, are also susceptible to viability decrease induced by these inhibitors.

The peptide specificities of the autoantibodies elicited by mouse hepatitis virus A59

Synthetic decapeptides (N = 206) covering the entire sequence of mouse liver fumarylacetoacetate hydrolase (FAH) were used to analyze the specificities of the autoantibodies (autoAb) elicited towards this enzyme in mice infected with mouse hepatitis virus (MHV). These autoAb bound mainly to N- and C-terminal FAH peptides, the most reactive sequences being 1–50 and 390–420, respectively. Surprisingly, although FAH sequence 1–50 shares a high degree of homology with various MHV proteins, the C-terminal portion does not. Moreover, whereas the autoAb reacted with homologous peptides surrounding residues 70, 160 and 360, non-similar sequences around residues 130, 210, 240, 250, and 300 were also recognized, indicating that autoAb were not restricted to epitopes with sequence homologies. There was also a lack of correlation between the amount of anti-MHV or anti-FAH antibodies produced and the reactivity towards the peptides. Moreover, the spectrum of peptides recognized by the autoAb of a given mouse did not change significantly with time, which suggests that the MHV-elicited autoimmune response does not induce an epitope recognition spreading. Finally, anti-FAH Ab produced after immunization with rat liver FAH recognized essentially the same mouse FAH regions than autoAb from MHV-infected mice. Results indicated that the induction of the autoAb is not only related to molecular or structural mimicry, but rather supports the Danger model, in which any aggression, in this case the MHV infection, is susceptible to trigger the production of autoAb.

Sequence similarity and structural homologies are involved in the autoimmune response elicited by mouse hepatitis virus A59

The features of autoantibodies (autoAb) to liver fumarylacetoacetate hydrolase (FAH) elicited in mice infected with mouse hepatitis virus (MHV) were studied by ELISA and western-blot competition assays. All sera tested contained Ab to cryptic FAH epitopes according with results from western-blot tests, whereas ELISA data indicated that some of these same sera did recognize native epitopes of the autoantigen (autoAg). Such differences were detected in individual sera from various mouse strains, and were ascribed to the fact that proteins insolubilized on solid supports expose a variety of conformational and cryptic antigenic determinants.

On the other hand, whereas results from both experimental protocols showed that anti-MHV Ab did not cross-react with the soluble autoAg, the opposite situation did not show analogous results. Thus, binding of autoAb to insolubilized FAH could be inhibited by MHV depending on the mouse serum or the experimental protocol used.

Additionally, a set of synthetic homologous peptides from mouse FAH and various viral proteins was employed to analyze the Ab repertoire of MHV-infected mice. Results indicated that two homologous peptides were recognized by most Ab: the N-terminal sequences (1–10) from FAH and the nucleocapside, both sharing 50% of identity, and sequence 2317–2326 of the RNA polymerase, a peptide showing 30% of identity with FAH 11–20.

Results indicated that MHV-infection triggers at least three distinct Ab populations: anti-MHV, anti-FAH and cross-reacting Ab. This cross-reaction implies either sequential or conformational epitopes from both the viral proteins and the autoAg and may differ between individuals.

22- and 20 kDa-human growth hormones bind to different sites within certain cellular receptors

Human growth hormone 20 kDa (20 kDa-hGH) is a natural variant of the main hGH isoform (22 kDa-hGH). Since some 20- and 22 kDa-hGH biological activities are not identical, we decided to map the prolactin (PRLR) and growth hormone receptor (GHR) binding sites for both isoforms. Monoclonal antibody (MAb) R7B4, directed to both receptors, was employed to estimate the relative proximity between 20- and 22 kDa-hGH receptors binding sites. Results indicated that although both hGH isoforms share the same PRLR present in Nb2-cells and rat liver, MAb R7B4 differently affected hormone binding, suggesting that their receptor binding sites would be close in Nb2-cells and separate in rat liver membranes. Since labelled 20 kDa-hGH did not bind significantly to hGHR, we added to the incubating medium an allosteric MAb anti-hGH that improved 20 kDa-hGH affinity for receptors. Under these experimental conditions MAb R7B4 inhibited 20 kDa-hGH binding to human liver but not to placenta, whereas the Ab impaired 22 kDa-hGH binding to both receptors. Data thus suggested that both hGH isoforms share the same hGHR binding site in liver tissue but bind to different overlapped regions in placenta. Consequently, results presented in this paper indicate that PRLR and GHR binding sites for 22- and 20 kDa-hGH should not be always identical, a fact that could explain some of the isoforms different activities.

Monoclonal Antibodies Inducing Conformational Changes on the Antigen Molecule

Monoclonal antibodies (MoAbs) are extensively used as biological tools because of their invariable specificity. However, the interpretation of results can be misled by the behaviour of MoAb displaying allosteric effects, i.e. long-range conformational changes on the antigen (Ag). It has been shown that some MoAbs are able to modify the spatial structure of the corresponding protein Ag, affecting in this way its biological activity as well as its binding to a second MoAb. Thus, a researcher using a MoAb as a tool to investigate some features of an antigenic molecule must be aware of the possible positive or negative allosteric properties of the antibody.

Effects of various adjuvants and a viral infection on the antibody specificity toward native or cryptic epitopes of a protein antigen

An immunization protocol that induces antibodies (Abs) directed to cryptic epitopes of a protein antigen (Ag) reduces the efficacy of vaccines that ideally should induce Abs against native epitopes. We have shown earlier that viral infections concomitant with immunization against a protein tend to shift the Ab specificity toward cryptic epitopes and tend to induce the production of autoantibodies (autoAbs). Here, we show the effects of three adjuvants on the Ab specificity in the absence or presence of a viral infection (lactate dehydrogenase-elevating virus or LDV), with human growth hormone (hGH) being, as before, the protein Ag. Pathogen-free CBA/Ht and BALB/c mice were immunized with hGH in the presence of complete Freund's adjuvant (CFA), monophosphoryl lipid A (MPL) or alum, with the animals being either infected with LDV or not infected with LDV. Conventional and competition enzyme-linked immunosorbent assays (ELISAs) indicated that in noninfected mice, CFA induced higher titres of anti-hGH Ab than did MPL or alum, with the Ab being almost totally directed to cryptic hGH epitopes. Strikingly, CFA plus LDV infection in CBA/Ht mice shifted the specificity of the anti-hGH Ab toward native epitopes, whereas the virus decreased the Ab titre when MPL or alum was used. Our Western blot results showed that 70% of mice immunized with hGH in the presence of any adjuvant produced autoAbs against a variety of tissue Ags. The amount of autoAb and the concentration of Ab to hGH cryptic epitopes did correlate, suggesting a relationship between both kinds of Ab. Significant differences were observed in the various effects of adjuvants and the viral infection between the two mouse strains used in this work.

Detection of mouse hepatitis virus infection by assay of anti-liver autoantibodies

The observation that mice infected with mouse hepatitis virus (MHV) develop autoantibodies directed mainly to liver fumarylacetoacetate hydrolase (FAH) enabled the development of an ELISA applicable to the detection of MHV-infection. The method, based on the titration of antibodies to semipurified FAH from rat liver, is easy, economical, and does not require the isolation of viral proteins from large MHV stocks. Furthermore, since sera from mice immunized with a purified fraction of the rat liver enzyme do react with its homologous protein, this antiserum can be used as a positive control avoiding the manipulation of samples from MHV-infected animals.

Relative localization of the prolactin receptor binding sites for lactogenic hormones

A monoclonal antibody termed MAb R7B4, directed to an epitope present in prolactin receptors (PRLRs), was used as a tool to map the receptor binding sites for human growth hormone (hGH), ovine prolactin (oPRL) and human placental lactogen (hPL). Although the three hormones completely inhibited the binding of each other to Nb2 cells or rat liver receptors, MAb R7B4 behaviour was different depending on the hormone tested and the receptor source. According to the MAb effects, PRLR from Nb2 cells would locate both hGH and oPRL close to R7B4 epitope, whereas hPL would bind far from the MAb binding site. On the other hand, PRLR from rat liver should bind hGH close to the R7B4 epitope but oPRL and hPL would be recognized by a separate region of the same receptor. Thus, results presented in this paper suggest that PRLR binding sites for hGH, oPRL and hPL do not exactly overlap in spite of full competition between ligands.

Autoantibodies to cryptic epitopes elicited by infection with lactate dehydrogenase-elevating virus

Lactate dehydrogenase-elevating virus (LDV) produces a permanent infection in mice with a B-lymphocyte polyclonal activation leading to hypergammaglobulinaemia. Since LDV specifically suppressed antibodies to native epitopes in CBA/Ht, but not BALB/c, mice immunized against a protein antigen, we explored the relationship between such a change in antibody specificity and the expression of autoantibodies under the influence of LDV. Again in CBA/Ht, but not BALB/c, mice we observed another effect of LDV: the sera from infected CBA/Ht mice were found by enzyme-linked immunosorbant assay to contain antibodies to various mouse tissue extracts. Immunoblots revealed a large spectrum of autoantigens that differed markedly between animals. Western-blot competition experiments showed that the protein autoantigens had to be denatured to react with most of the autoantibodies. Despite the presence of these autoantibodies directed to cryptic epitopes, no specific tissue lesions could be ascribed to the autoimmune response elicited by LDV infection, since both mouse strains showed mild inflammatory reactions in liver and kidney.

Positive cooperative effects between receptors induced by an anti-human growth hormone allosteric monoclonal antibody

Monoclonal antibodies (MAb) anti-human growth hormone (hGH) termed MAb AE5, AC8 and F11 recognize a cluster of epitopes left exposed after hormone binding to receptors. Since these MAb were able to produce either positive (MAb AE5) or negative (MAb AC8 and F11) allosteric effects on hGH binding, the purpose of this work was to further characterize MAb behavior. Results indicated a straight correlation between MAb allosteric effects and affinity constant values for binding of different hGH:MAb complexes to lactogenic receptors from rat liver. Affinity of hGH:MAb AE5 as well as hGH:Fab AE5 complexes enhanced proportionally to the fraction of occupied receptors and Hill coefficients higher than 1 were obtained, suggesting the induction of positive cooperative effects between membrane-bound receptors. On the other hand, hGH:MAb AC8 and hGH:MAb F11 complexes binding affinity to lactogenic sites could not be related to receptor occupancy degree. It is proposed that binding of hGH:MAb AE5 complexes to receptors would elicit a conformational change on adjacent receptor molecules leading to an increase of their affinity to bind subsequent hGH:MAb AE5 complexes.

Ovine placental lactogen and human growth hormone bind to different regions of the same receptors

Anti-human growth hormone (hGH) polyclonal and monoclonal antibodies (MAb) failed to recognize ovine placental lactogen (oPL), indicating that the antigenic topographies of both hormones are different. Binding assays showed that oPL completely inhibited hGH binding to lactogenic receptors from Nb2-cells and to somatogenic receptors from rabbit or sheep liver; in contrast, oPL only bound to a subpopulation of rat liver receptors. Zinc ion increased hGH and oPL binding to Nb2-cell receptors and slightly inhibited both hormones' recognition by somatogenic receptors. However, ZnCl(2) increased hGH binding to rat liver microsomes but prevented that of oPL. Furthermore, MAb R7B4, recognizing lactogenic as well as somatogenic receptors, entirely blocked hGH binding to the various receptor systems but not affected oPL binding. Therefore, results presented in this paper suggest that oPL and hGH bind to different regions of the same receptors.

A monoclonal antibody recognizing an epitope shared by receptors for growth hormone, prolactin, interleukin 2 and interleukin 6

Monoclonal antibody (MAb) termed R7B4 was generated throughout the idiotypic-anti-idiotypic network from mice immunized with human and bovine growth hormones (GH). The Ab was selected on the basis that it did not recognize human GH (hGH) neither insolubilized nor in solution but inhibited 125I-hGH binding to receptors from rat and rabbit liver and from Nb2-cell membranes. Since it inhibited Nb2-cell mitogenesis stimulated by hGH, prolactins or placental lactogens, MAb R7B4 behaved as an antagonist of lactogenic hormones. Furthermore, the Ab impaired proliferative activity of interleukin 2 (IL-2) on Nb2 cells as well as growth of 7TD1 cells, an interleukin 6 (IL-6) dependent hybridoma not expressing GH receptors. Biotin-labeled MAb R7B4 specifically bound to rat liver microsomes, and the Ab was able to recognize Nb2 and 7TD1-cell membranes as shown by flow cytometry experiments. However, MAb binding was not hampered by hGH, indicating that the Ab did not mimic GH binding site to receptors. Immunoblot assays indicated that rat and rabbit liver as well as Nb2-cells membrane antigens recognized by MAb R7B4 were similar to those revealed by a MAb directed to prolactin receptors. In addition, MAb R7B4 was able to detect two bands probably corresponding to the somatogenic receptor in rabbit liver microsomes as well as three different proteins in 7TD1-cells showing molecular weights similar to those of the IL-6 receptor complex. Results suggest that MAb R7B4 is directed to an epitope shared by receptors for lactogenic and somatogenic hormones, IL-2 and IL-6. To our knowledge, these data are the first experimental evidence of the existence of structural similarity between some of the receptors grouped in the cytokine receptor superfamily.

Detection of negative allosteric effects between monoclonal antibodies by using an antigenic model-builder computer program

The ability of monoclonal antibodies (MAb) to bind or not simultaneously to the antigen (Ag) is used to establish antigenic maps considering that two different MAb do not bind to the Ag when the corresponding epitopes are overlapped (steric effect). Nevertheless, MAb inducing negative allosteric effect on the Ag could prevent the binding of the second MAb even if it is directed to a separate epitope. We report here that a knowledge-based expert module included in our previously described antigenic model-builder program (MAPAG) was able to differentiate between steric and negative allosteric effects between some MAb.

Changes in the specificity of antibodies in mice infected with lactate dehydrogenase-elevating virus

Infection with lactate dehydrogenase-elevating virus (LDV) modifies the isotypic distribution of antibodies (Ab) directed to several antigenic proteins with a preferential production of IgG2a. Because it was not known whether the virus could also affect the Ab specificity, the authors addressed this point using human growth hormone (hGH) as a model antigen. Anti-hGH monoclonal antibodies (MoAb) were used as probes to study the occurrence of Ab to three native hGH epitopes (3C11, F11 and 10D6) in sera from LDV-infected CBA/Ht and BALB/c mice immunized with hGH. Competition ELISA was used to determine the extent of Ab directed to cryptic hGH epitopes, i.e. antigenic determinants hidden in the native hormone. Results indicated that in LDV-infected CBA/Ht mice the titres of anti-hGH Ab were lower than in controls, although a consistent isotypic shift to IgG2a subclass was observed. Concurrently, the presence of Ab to epitopes 3C11, F11 and/or 10D6 were markedly reduced in infected animals and most anti-hGH Ab were directed to hGH cryptic epitopes. By contrast, LDV infection increased the amount of anti-KLH Ab elicited by CBA/Ht mice and did not affect Ab specificity, whilst control and LDV-infected BALB/c mice showed similar concentrations of anti-hGH Ab. Furthermore, the proportion of Ab to cryptic hGH epitopes did not change in infected animals even though an important shift to IgG2a was detected. Thus, data presented herein suggest that LDV infection modifies Ab specificity depending on the mice genetic background and on the antigenic characteristics of the immunogen.

Monoclonal antibodies to human growth hormone modulate its biological properties

Previous results indicated that monoclonal antibodies (mAbs) termed mAb AE5, mAb AC8 and mAb F11, recognizing the human growth hormone (hGH) region left exposed after binding to lactogenic, somatogenic and hGH-specific receptors, produce allosteric changes in the hormone which modify its binding properties. To study whether these mAbs could also influence hGH biological activity, experiments were carried out with Nb2 cells, a rat lymphoma cell line which proliferates in the presence of lactogenic hormones. Experiments involving previous binding of the hormone to receptors before adding 125I-mAbs indicated that the hGH domain defined by overlapped epitopes AE5, AC8 and F11 is uncovered in hGH when it is bound to the cell membranes. To reveal any alteration in the hGH molecule induced by the mAbs, preformed 125I-mAb:hGH complexes were added to the cell membranes. Data showed that 125I-mAb AE5:hGH complexes bound better to the receptors than free hormone. On the contrary, hGH previously bound to 125I-mAb AC8 or 125I-mAb F11 was poorly recognized by Nb2 receptors. Furthermore, both mAbs AC8 and F11 strongly inhibited 125I-hGH binding to Nb2 cell membranes and hGH-induced Nb2 cell proliferation whereas mAb AE5 enhanced both hormone binding and hGH mitogenic effect. Additionally, since mAb AC8 is directed towards an epitope shared by hGH and human placental lactogen (hPL), it was also shown that this mAb could impair hPL biological activity even though it recognizes the hPL region left exposed in hPL:Nb2 cell receptor complexes. Data presented in this work suggest that mAbs directed to the hGH or hPL regions unmasked after binding to Nb2 cell receptors produce allosteric alterations in the binding properties of these hormones leading to either enhancement or decrease of their biological activities.

MAPAG: a computer program to construct 2- and 3-dimensional antigenic maps

The contact area between an antibody (Ab) and the antigen (Ag) is called antigenic determinant or epitope. The first step in the characterization of an Ag by using monoclonal antibodies (MAb) is to map the relative distribution of the corresponding epitopes on the Ag surface. The computer program MAPAG has been devised to automatically construct antigenic maps. MAPAG is fed with a binary matrix of experimental data indicating the ability of paired MAb to bind or not simultaneously to the Ag. The program is interactive menu-driven and allows the user an easy data handling. MAPAG utilizes iterative processes to construct and to adjust the final map, which is graphically shown as a 2- or a 3-dimensional model. Additionally, the antigenic map obtained can be optionally modified by the user or readjusted by the program. The suitability of MAPAG was illustrated by running experimental data from literature and comparing antigenic maps constructed by the program with those elaborated by the investigators without the assistance of a computer. Furthermore, since some MAb could present negative allosteric effects leading to misinterpretation of data, MAPAG has been provided with an approximate reasoning module to solve such anomalous situations. Results indicated that the program can be successfully employed as a simple, fast and reliable antigenic model-builder.

Allosteric effects of monoclonal antibodies on human growth hormone

We have previously shown that a monoclonal antibody (MAb) recognizing the human growth hormone (hGH) antigenic domain left exposed after binding to lactogenic receptors enhanced hGH binding probably through allosteric effects on the hormone binding site. Since receptors displaying different specificities would not recognize exactly the same hGH region, we explored whether some of our MAb could affect hGH binding to somatogenic receptors from rabbit liver and to human liver hGH-specific receptors. The effect of MAbAE5,AC8 and F11 on hGH binding was measured by determining the formation of 125I-MAb:hGH:receptor complexes using two different experimental approaches. Results from procedure A, which involved the previous binding of the hormone to microsomes before adding 125I-MAb, indicated that the hGH domain defined by epitopes AE5, AC8 and F11 is uncovered in the various hormone:receptor complexes. Procedure B was devised to reveal any alteration in the hGH molecule induced by the MAb. In this case performed 125I-MAb:hGH complexes were added to microsomes. Data showed that 125I-MAb AE5:hGH complexes bound better to the various receptors than 125I-MAb AE5 to hGH:receptor complexes. On the contrary, hGH previously bound to 125I-MAb AC8 or 125I-MAb F11 was less recognized by the receptors than the free hormone. Furthermore, binding of MAb AE5 or MAb F11 to hGH 20 K (a natural hGH variant lacking residues 32-46) also enhanced its affinity to the various receptors whereas MAb AC8 did not inhibit hGH 20 K binding. Results indicated that MAb recognizing the hGH antigenic area that remains unmasked after binding to different membrane-bound receptors are able to affect hormone binding site.(ABSTRACT TRUNCATED AT 250 WORDS)

Synergistic monoclonal antibodies' interactions and their use for determination of antibody specificities

Three monoclonal antibodies (MAb 3C11, F11 and 10D6) to human growth hormone (hGH) recognize independent epitopes and show mutually enhancing properties. Thus, 125I-hGH binding to each of these MAb augmented significantly in the presence of each one of the other two MAb. Moreover, preincubation of the hormone with paired MAb gave rise to ternary complexes (Ag:Ab1:Ab2) which bound better than the free tracer to the third MAb previously captured on a solid-phase. Highly stable quaternary complexes (Ag:Ab1:Ab2:Ab3) were thus formed. Since Fab fragments from the three MAb displayed the same behavior as the whole Ab molecule, neither the formation of multimolecular cyclic complexes nor the occurrence of interactions through Fc fragments could explain the reciprocal MAb binding enhancement. Therefore, the results obtained suggest that MAb 3C11, F11 and 10D6 produce some modification in the Ag, each one improving the binding of the two other MAb. Additionally, the inhibition of the formation of quaternary complexes between the MAb and hGH was used to evaluate specific Ab populations in polyclonal antisera, avoiding the masking effect of enhancing Ab. The results obtained indicate that Ab directed to the hGH antigenic domains defined by MAb 3C11, F11 and 10D6 could be detected in spite of the presence of enhancing Ab to all three MAb.

Relationship between the antigenic topography and the structure of human growth hormone

Monoclonal antibodies (MAb) to human GH (hGH) were used to correlate the antigenic topography of the hormone with its structure. Competition experiments performed in a solid phase RIA system allowed us to measure the reactivity toward the MAb of the following hGH derivatives: hGH 20K (a natural variant lacking residues 32-46), hGH selectively modified in His or Met residues, hGH with the C and/or N-terminal disulfide bond reduced and carbamidomethylated, and hGH cleaved between residues 142-143. Results indicated that fragment 32-46 participates in the structure of epitopes EB1/EB3 and that the C-terminal bridge is located in epitope 10D6, whereas opening of both disulfide bridges alters the entire hGH antigenic surface. His-151 and Met-170 were placed in epitopes NA71 and AC8, respectively, whereas His-18 and Met-14 would be involved in the hGH antigenic domain formed by overlapping epitopes 3C11, 10C1, and HG3. MAb AE5, AE12, and AC3 define a flexible hGH region related to sequence 134-150; the respective epitopes show high conformational mobility induced by modifications in other regions of the molecule. Binding of the different hGH derivatives to lactogenic receptors from female rat liver gave some insights on the localization of the hormone-binding site. Epitopes EB1/EB3 and 10D6 were discarded because there was not a direct correlation between their drastic immunological alterations and the binding properties of the respective hGH derivatives. In the same way, epitopes AE5, AE12, and AC3 were excluded from the hGH-binding domain because a disruption in those sites did not affect the hGH interaction with receptors. We conclude that the hGH structure defined by epitopes 3C11, 10C1, and HG3 is probably related to the binding properties of the hormone.

Monoclonal antibodies as probes to study the human growth hormone-binding domain to lactogenic rat liver receptors

A set of monoclonal antibodies (MAb) to human GH (hGH) was used to study the hormone binding orientation to its receptors (R) from female rat liver. The hGH antigenic region left exposed after its binding to liver microsomes was detected by measuring the ability of various [125I]MAb to bind to the preformed hGH-R complexes. Results indicated that a cluster of epitopes defined by the MAb, termed AE5, AC8, and AE12, remains accessible in the hGH-R complex whereas overlapping epitopes 3C11 and HG3 would define a hGH region involved in the binding site. Supporting these findings, solubilization and HPLC gel filtration of [125I]MAb-hGH-R complexes showed a radioactive peak of about 450,000 mol wt for MAb AE5 or AC8, but not for MAb 3C11 or HG3. [125I]MAb AE12 behaved differently, suggesting that epitope AE12 may be masked or altered in hGH-R-solubilized complexes. MAb directed to the putative hGH-binding site (MAb 3C11, HG3, and the closely related MAb 10C1 and NA71) failed to inhibit binding of the preformed [125I]MAb AE5-hGH complex to the receptors, suggesting a hormone modification after MAb AE5 binding. Accordingly competition experiments indicated an increase in the affinity of hGH for its receptors induced by this MAb. A higher hGH concentration was required to obtain 50% [125I]hGH binding to liver microsomes in the presence of MAb AE5 than in its absence. As the MAb used define epitopes that were previously correlated with the hGH structure, we concluded that a high flexible region (sequences 134-150) is exposed in the hGH-R complex. Furthermore, some MAb directed to this region enhance the hormone affinity for its rat liver receptors, probably through an induced conformational change.

Equine growth hormone. Detection of immunoreactive sequences using poly- and monoclonal antibodies

The immunochemical behavior of several fragments of equine growth hormone (eGH) was examined using competitive binding assays with antibodies (Abs) to eGH obtained from different sources. Antigenicity was detected within the sequences 5-72 and 73-123 by rabbit Abs to eGH and by three mouse monoclonal antibodies (MAbs) produced by using bovine growth hormone as immunogen, but showing heteroclitic properties towards eGH. The polyclonal Abs to eGH also recognized as immunoreactive two smaller peptides corresponding to the amino acid residues 52-72 and 110-123. By contrast, the heteroclitic Abs to eGH developed by hypopituitary patients therapeutically injected with human growth hormone failed to react with any eGH-derived fragment. The rabbit polyclonal Abs and the mouse MAbs scarely discriminated between native and S-carbamidomethylated eGH, while the heteroclitic human Abs detected a clear difference between the native and the modified hormone.

Monoclonal antibodies to human growth hormone induce an allosteric conformational change in the antigen

We re-investigated the properties of a monoclonal antibody (mAb), 4D11, to human growth hormone (hGH) that showed a very weak affinity, recognizing hGH only when the hormone was solubilized on a solid surface. MAb4D11 did not significantly bind 125I-hGH. It was found that three mAb directed to different hGH epitopes (mAb 3C11, 10C1 and NA71) were able to induce the binding of the soluble antigen to mAb 4D11. The co-operative effect could be demonstrated by the formation of binary complexes (Ag:Ab, 1:2) detected by high-performance liquid chromatography (HPLC) and by the increase of radioactivity found when the synergistic mAb were added to 125I-hGH incubated with mAb 4D11 immobilized on polyvinyl microplates. Other possible explanations, such as the formation of cyclic complexes or the generation of a new epitope in the Fc fragment of the first antibody (Ab), were dismissed because the Fab fragment of one of the enhancing mAb (3C11) gave the same effect as the intact Ab. The data suggest that the hGH molecule undergoes a localized conformational change after binding to mAb 3C11, NA71 or 10C1 and that mAb 4D11 binds with high affinity to the modified region of the hormone. The formation or not of ternary complexes (Ag:Ab, 1:3) was used to localize the 4D11 epitope on the surface of the Ag. It is suggested that mAb 4D11 recognizes a conformational change produced in the region defined by the AE5/AC8 epitopes, which is close to the hGH antigenic domain only expressed when the protein is immobilized on plastic surfaces.

The antigenic topography of human growth hormone

Murine monoclonal antibodies (MAb) have been used as tools to probe the antigenic topography of human growth hormone (hGH). Mapping experiments were carried out by testing the ability of paired MAb to bind simultaneously or separately to 125I-hGH. A putative three-dimensional model of the relative distribution of 20 hGH epitopes indicated that they covered the entire molecular surface, showing the following essential characteristics. A domain of unique hGH specificity representing approximately 20% of the whole area was detected, as well as the presence of a discontinuous band of immunological identity between hGH and human placental lactogen (hPL) occupying 30% of the molecular surface. The rest of the surface (about 50%) displayed only partial cross-reactivity with hPL. Three restricted antigenic areas were also recognized. One of them appeared to correlate with a conformational change induced by the adsorption of the protein to plastic surfaces and the other two showed cross-reactivity with human prolactin and heterologous GH, respectively.

Stochastic humoral expression of human growth hormone epitopes

Competition experiments between insolubilized monoclonal antibodies (mAb) and polyclonal antisera has led to the description of the humoral expression of human growth hormone (hGH) epitopes. This study was carried out with sera from mice and hamsters submitted to different immunization schedules: chronic administration of the antigen, secondary response and conventional hyperimmunization. The results indicated the absence of a unique immunodominant epitope in hGH; a significant individual variation of antibody (Ab) population titres with time; changes with time in the relative proportion of one Ab population with respect to the others; and the occurrence of Ab enhancing the 125I-hGH binding to five mAb depending upon the individuals and the time of immunization. Heterocliticity towards non-human GH was also detected. Although most of the animals showed cross-reacting Ab, two out of 12 mice, chronically injected, developed heteroclitic Ab. The data suggest that the humoral response to different epitopes of a protein antigen during the maturation of the immune response is a stochastic process leading to transient humoral immunodominance, enhancing Ab populations and heterocliticity, depending upon individual characteristics, either in outbred or inbred populations.

Relative distribution of various antigenic determinants on the human growth hormone surface

The relative distribution of 12 antigenic determinants on the surface of the human growth hormone (hGH) molecule has been established. The necessary information was obtained by testing the ability of paired monoclonal antibodies (MAb) to bind simultaneously or not, to 125I-hGH which leads to the formation of 1:2 or 1:1, Ag-Ab complexes, respectively. The results obtained indicate that the epitopes occupy a large percentage of the total hGH molecular surface and revealed the existence of; an antigenic region specific for hGH; at least two independent domains of immunological identity between hGH and human placental lactogen (hPL), one of them also shared by heterologous GH; and other independent areas of partial cross reactivity with hPL. MAb competition experiments in a solid-phase RIA showed the unreliability of this technique for mapping purposes. The distribution of the hGH epitopes suggested in this work is in accord with present views on protein antigenicity and also explains data existing in the literature concerning the behavior of some of the MAb tested here.

Heteroclitic behaviour of some monoclonal antibodies against bovine growth hormone

Five monoclonal antibodies (MAbs) prepared against bovine growth hormone (bGH) were found to be directed against an immunodominant antigenic domain in bGH. MAbs D8 and H3 reacted equally well with bGH and ovine growth hormone and to a lesser extent with equine (eGH) or porcine (pGH) growth hormones while MAbs H1, H2 and C12 behaved as heteroclitic antibodies, i.e. they bound better a cross-reacting antigen (eGH on pGH) than the immunogen. The reactivity of bGH with the heteroclitic MAbs increased proportionally to the time that the native protein was kept frozen. Deamidation of bGH by treatment with alkali also increased its reactivity. Circular dichroism measurements indicated the occurrence of changes in the conformation of the bGH molecule by these treatments which presumably uncover normally buried or non-accesible epitopes. bGH shares epitopes with eGH and pGH which are immunologically expressed by bGH only when its native conformation is modified.

Comparison of human lactoferrins from milk and neutrophilic leucocytes. Relative molecular mass, isoelectric point, iron-binding properties and uptake by the liver

Human lactoferrins isolated from neutrophilic leucocytes and milk by CM-Sephadex chromatography were similar in Mr (76000) and pI (8.7). Upon acidification, both proteins released their two Fe3+ ions/molecule in a similar biphasic way. Both proteins intravenously injected into mice were cleared from plasma at the same rate. The maximal uptakes by the liver, which occurred 5 min after injection, were inhibited to the same extent by milk lactoferrin used as a competitor.

Specificities of antibodies to human growth hormone (hGH) in patients treated with hGH: longitudinal study and comparison with the specificities of animal antisera

The specificities of human and animal antibodies (Abs) against human GH (hGH) were analyzed using competition experiments with five monoclonal antibodies to hGH (MAbs). The results indicate that 1) the Abs produced by patients receiving long term hGH therapy as well as Abs of goat, rabbit, and mouse origin recognized the various hGH epitopes defined by the MAbs; 2) the proportion of each Ab population, with a given specificity, differed markedly in different patients and also with time in the same patient; 3) the titer of certain Ab populations was very low in some patients, and 4) polyclonal mouse Abs and some human Abs enhanced the binding of [125I] hGH to insolubilized MAbs.

Particle-counting immunoassay of human somatotropin

Human somatotropin was assayed by a novel automated nonradioisotopic technique, "particle-counting immunoassay," that requires a 45-min incubation and only 60 microL of 10-fold diluted sample. The principle of the assay is agglutination of antibody-coated latex particle by somatotropin, the reaction being measured by the (instrumented) counting of residual non-agglutinated particles. The dynamic range in serum extends from 0.2 to 40 micrograms/L. The between-assay CV was 12% for a concentration of 3.3 micrograms/L and 8.5% for 31.9 micrograms/L. The coefficient of correlation (r) with radioimmunoassay was 0.97. Curves for various dilutions of the macromolecular and monomeric forms of the hormone were not parallel.

Particle-counting immunoassay of human somatotropin

Human somatotropin was assayed by a novel automated nonradioisotopic technique, "particle-counting immunoassay," that requires a 45-min incubation and only 60 microL of 10-fold diluted sample. The principle of the assay is agglutination of antibody-coated latex particle by somatotropin, the reaction being measured by the (instrumented) counting of residual non-agglutinated particles. The dynamic range in serum extends from 0.2 to 40 micrograms/L. The between-assay CV was 12% for a concentration of 3.3 micrograms/L and 8.5% for 31.9 micrograms/L. The coefficient of correlation (r) with radioimmunoassay was 0.97. Curves for various dilutions of the macromolecular and monomeric forms of the hormone were not parallel.

Uptake of lactoferrin by the liver. III. Critical role of the protein moiety

Three possible modes of recognition of human lactoferrin (Lf), which is avidly taken up by the mouse liver, were examined. First, Lf has terminal galactosyl residues for which a receptor exists on hepatocytes. However, when large amounts of Lf were injected with 125I-asialoorosomucoid, no inhibition of asialoorosomucoid uptake by the liver was observed. Second, Lf that exposes fucosyl residues could be recognized by the sugar-polyspecific receptor of liver sinusoidal cells. Digestion of Lf by fucosidase did not affect considerably the uptake of Lf. In addition, bovine Lf, which lacks fucosyl residues, was also avidly taken up by the liver, and this uptake was inhibited by human Lf. Mannan and horseradish peroxidase, which are recognized by the sugar-polyspecific receptor, did not inhibit Lf uptake. These data demonstrate that galactosyl and fucosyl residues are not essential for Lf recognition. Third, Lf could be recognized by its protein moiety. To investigate this possibility, we used two Lf derivatives with intact carbohydrate side chains, carbamylated Lf and the C-terminal half molecule of Lf. Carbamylation reduced the uptake of Lf and its competitive activity toward the uptake of 125I-Lf. The C-terminal fragment, like carbamylated Lf, had a much weaker competitive activity than intact Lf. Therefore, the integrity of the protein moiety of Lf was required for its effective uptake by the liver. As Lf is a cationic protein, competition experiments were also done with lysozyme, another cationic protein which in the form of dimer is taken up by the liver reticuloendothelial system. The strong inhibition by dimerized lysozyme suggests that the liver reticuloendothelial system has common binding sites for certain cationic proteins, as recently shown for isolated macrophages.

Uptake of lactoferrin by the liver. I. Role of the reticuloendothelial system as indicated by blockade experiments

When 125I-labeled human lactoferrin was injected intravenously into mice it was rapidly taken up by the liver, where 75% of the dose was recovered after 15 minutes. The inhibition curve with unlabeled lactoferrin showed that the fates of labeled and unlabeled protein were similar and that the uptake capacity was saturable by milligram quantities of protein per gram of liver, suggesting the existence of numerous specific binding sites. The presence of these sites on the reticuloendothelial system was indicated by the blocking effect of dextran sulfate and latex particles. Fucoidin (fucan sulfate), which is used in the study of receptors specific for fucose, was found to inhibit the uptake of goat red blood cells as well as lactoferrin. Therefore, the inhibition exerted by fucoidin on the uptake of lactoferrin could be mediated by blockade of the reticuloendothelial system and not necessarily by competition for fucose receptors. These data indicate that the report by others that lactoferrin was taken up by a fucosyl receptor on hepatocytes is incorrect. The competition curve obtained when mouse lactoferrin was injected with its human homologue indicated that both proteins reacted with the same binding sites. However, a significant part of mouse lactoferrin was found to be taken up by the kidneys. Hepatic and renal uptakes were both reduced by prior injection of dextran sulfate.

Uptake of lactoferrin by the liver. II. Endocytosis by sinusoidal cells

Because of the controversy on the cell type involved in lactoferrin (Lf) uptake by the liver we localized injected Lf by immunofluorescence, light microscopic autoradiography, and light and electron microscope cytochemistry with Lf-peroxidase conjugates. These conjugates were essentially taken up by their Lf moiety as shown by competition experiments. By any procedure, Lf was almost exclusively found in the sinusoidal cells as well as in the wall of the central veins. These data directly demonstrate that both endothelial and Kupffer cells are responsible for Lf uptake by the liver.

The same region of human growth hormone is involved in its binding to various receptors

The Fab fragments of three monoclonal antihuman GH (anti-hGH) antibodies, among five tested, inhibited the binding of the hormone to the receptors of the human lymphoid cell line IM-9 and liver membranes of the pregnant rabbit. The results were similar for the receptors of human lymphocytes and rabbit liver, suggesting that both receptors reacted with the same region of the hormone. The Fab fragments of the most inhibitory antibody also inhibited the down-regulation by the hormone of the receptors on human lymphocytes. The fragments of this antibody completely blocked the binding of the hormone to the receptors of the rabbit liver, despite the fact that this carries two or more classes of receptors. Therefore, all of these various receptors apparently interact with the same region of the hormone. Three synthetic peptides extending from residues 19-128, 73-128, and 98-128 failed to inhibit the binding of hGH to its lymphocyte or liver receptors however, these peptides reacted significantly with the monoclonal antibody, which was the strongest inhibitor of the interaction of hGH with the cellular receptor, confirming that the receptor-binding site of the hormone is in the amino-terminal part of the molecule and suggesting indirectly that the short sequences 98-128 participates in the constitution of the receptor-binding site of the hormone.

The recognition by monoclonal antibodies of various portions of a major antigenic site of human growth hormone

The reactivities of five mouse monoclonal antibodies against human growth hormone (hGH) were defined by either a competitive radioimmunoassay with insolubilized antibodies or by an agglutination-inhibition method with hGH-coated polystyrene particles. The five antibodies reacted significantly but to various degrees with human placental lactogen and at least three antibodies reacted with human prolactin and three synthetic peptides extending from residues 19 to 128, 73 to 128 and 98 and 128 of hGH. Four tested monoclonal antibodies failed to react with bovine growth hormone and with hGH oxidized by performic acid. The antibodies were further distinguished by their different reactions with hGH modified by reduction and alkylation or by adsorption on a polystyrene surface. The unique specificity of each antibody was confirmed for most of them by an agglutination method in which the agglutinating activity of hGH was tested on latex particles coated with various paired combinations of the monoclonal antibodies. The lack of agglutination with certain combinations suggested that the specificities of such a pair of antibodies overlapped each other. These results suggest that the sequences corresponding to the synthetic peptides participate in the structure of a major antigenic site of which various portions are recognized by the monoclonal antibodies.