Structural studies of human autoantibodies. Crystal structure of a thyroid peroxidase autoantibody Fab

Structural Studies of Thyroid Peroxidase Show the Monomer Interacting With Autoantibodies in Thyroid Autoimmune Disease

Thyroid peroxidase (TPO) is a critical membrane-bound enzyme involved in the biosynthesis of multiple thyroid hormones, and is a major autoantigen in autoimmune thyroid diseases such as destructive (Hashimoto) thyroiditis. Here we report the biophysical and structural characterization of a novel TPO construct containing only the ectodomain of TPO and lacking the propeptide. The construct was enzymatically active and able to bind the patient-derived TR1.9 autoantibody. Analytical ultracentrifugation data suggest that TPO can exist as both a monomer and a dimer. Combined with negative stain electron microscopy and molecular dynamics simulations, these data show that the TR1.9 autoantibody preferentially binds the TPO monomer, revealing conformational changes that bring together previously disparate residues into a continuous epitope. In addition to providing plausible structural models of a TPO-autoantibody complex, this study provides validated TPO constructs that will facilitate further characterization, and advances our understanding of the structural, functional, and antigenic characteristics of TPO, an autoantigen implicated in some of the most common autoimmune diseases.

Modelling of Thyroid Peroxidase Reveals Insights into Its Enzyme Function and Autoantigenicity

Thyroid peroxidase (TPO) catalyses the biosynthesis of thyroid hormones and is a major autoantigen in Hashimoto's disease--the most common organ-specific autoimmune disease. Epitope mapping studies have shown that the autoimmune response to TPO is directed mainly at two surface regions on the molecule: immunodominant regions A and B (IDR-A, and IDR-B). TPO has been a major target for structural studies for over 20 years; however, to date, the structure of TPO remains to be determined. We have used a molecular modelling approach to investigate plausible modes of TPO structure and dimer organisation. Sequence features of the C-terminus are consistent with a coiled-coil dimerization motif that most likely anchors the TPO dimer in the apical membrane of thyroid follicular cells. Two contrasting models of TPO were produced, differing in the orientation and exposure of their active sites relative to the membrane. Both models are equally plausible based upon the known enzymatic function of TPO. The "trans" model places IDR-B on the membrane-facing side of the myeloperoxidase (MPO)-like domain, potentially hindering access of autoantibodies, necessitating considerable conformational change, and perhaps even dissociation of the dimer into monomers. IDR-A spans MPO- and CCP-like domains and is relatively fragmented compared to IDR-B, therefore most likely requiring domain rearrangements in order to coalesce into one compact epitope. Less epitope fragmentation and higher solvent accessibility of the "cis" model favours it slightly over the "trans" model. Here, IDR-B clusters towards the surface of the MPO-like domain facing the thyroid follicular lumen preventing steric hindrance of autoantibodies. However, conformational rearrangements may still be necessary to allow full engagement with autoantibodies, with IDR-B on both models being close to the dimer interface. Taken together, the modelling highlights the need to consider the oligomeric state of TPO, its conformational properties, and its proximity to the membrane, when interpreting epitope-mapping data.

A residue-specific shift in stability and amyloidogenicity of antibody variable domains

Variable (V) domains of antibodies are essential for antigen recognition by our adaptive immune system. However, some variants of the light chain V domains (VL) form pathogenic amyloid fibrils in patients. It is so far unclear which residues play a key role in governing these processes. Here, we show that the conserved residue 2 of VL domains is crucial for controlling its thermodynamic stability and fibril formation. Hydrophobic side chains at position 2 stabilize the domain, whereas charged residues destabilize and lead to amyloid fibril formation. NMR experiments identified several segments within the core of the VL domain to be affected by changes in residue 2. Furthermore, molecular dynamic simulations showed that hydrophobic side chains at position 2 remain buried in a hydrophobic pocket, and charged side chains show a high flexibility. This results in a predicted difference in the dissociation free energy of ∼10 kJ mol(-1), which is in excellent agreement with our experimental values. Interestingly, this switch point is found only in VL domains of the κ family and not in VLλ or in VH domains, despite a highly similar domain architecture. Our results reveal novel insight into the architecture of variable domains and the prerequisites for formation of amyloid fibrils. This might also contribute to the rational design of stable variable antibody domains.

Probing structural variability at the N terminus of the TSH receptor with a murine monoclonal antibody that distinguishes between two receptor conformational forms

Despite elucidation of the crystal structure of M22, a human thyroid-stimulating autoantibody (TSAb) bound to the TSH receptor (TSHR) leucine-rich repeat domain (LRD), the mechanism by which TSAs activate the TSHR and cause Graves' disease remains unknown. A nonstimulatory murine monoclonal antibody, 3BD10, and TSAb interact with the LRD N-terminal cysteine cluster and reciprocally distinguish between two different LRD conformational forms. To study this remarkable phenomenon, we investigated properties of 3BD10, which has a linear epitopic component. By synthetic peptide ELISA, we identified 3BD10 binding to TSHR amino acids E34, E35, and D36 within TSHR cysteine-bonded loop 2 (C31-C41), which includes R38, the most N-terminal contact residue of TSAb M22. On flow cytometry, despite not contributing to the 3BD10 and M22 epitopes, chimeric substitution (but not deletion) of TSHR cysteine-bonded loop 1 (C24-C29) eliminated 3BD10 binding to the TSHR ectodomain (ECD) expressed on the cell surface, as found previously for TSAb including M22. Furthermore, 3BD10 did not recognize all cell surface TSHR ECDs, consistent with recognition of only one conformational receptor form. Reversion to wild-type of small components of the loop 1 chimeric substitution partially restored 3BD10 binding to the TSHR-ECD but not to synthetic peptides tested by ELISA. Molecular modeling supports the concept that modification of TSHR C-bonded loop 1 influences loop 2 conformation as well as LRD residues further downstream. In conclusion, the present study with mouse monoclonal antibody 3BD10 confirms TSHR conformational heterogeneity and suggests that the N-terminal cysteine cluster may contribute to this structural variability.

Recognition of highly restricted regions in the β-propeller domain of αIIb by platelet-associated anti-αIIbβ3 autoantibodies in primary immune thrombocytopenia

Platelet-associated (PA) IgG autoantibodies play an essential role in primary immune thrombocytopenia (ITP). However, little is known about the epitopes of these Abs. This study aimed to identify critical binding regions for PA anti-αIIbβ3 Abs. Because PA anti-αIIbβ3 Abs bound poorly to mouse αIIbβ3, we created human-mouse chimera constructs. We first examined 76 platelet eluates obtained from patients with primary ITP. Of these, 26 harbored PA anti-αIIbβ3 Abs (34%). Further analysis of 15 patients who provided sufficient materials showed that the epitopes of these Abs were mainly localized in the N-terminal half of the β-propeller domain in αIIb (L1-W235). We could identify 3 main recognition sites in the region; 2 eluates recognized a conformation formed by the W1:1-2 and W2:3-4 loops, 5 recognized W1:2-3, and 4 recognized W3:4-1. The remaining 4 eluates could not be defined by the binding sites. Within these regions, we identified residues critical for binding, including S29 and R32 in W1:1-2; G44 and P45 in W1:2-3; and P135, E136, and R139 in W2:3-4. Of 11 eluates whose recognition sites were identified, 5 clearly showed restricted κ/λ-chain usage. These results suggested that PA anti-αIIbβ3 Abs in primary ITP tended to recognize highly restricted regions of αIIb with clonality.

Structural basis for the dual recognition of IL-12 and IL-23 by ustekinumab

Interleukin (IL)-12 and IL-23 are heterodimeric proinflammatory cytokines that share a common p40 subunit, paired with p35 and p19 subunits, respectively. They represent an attractive class of therapeutic targets for the treatment of psoriasis and other immune-mediated diseases. Ustekinumab is a fully human monoclonal antibody (mAb) that binds specifically to IL-12/IL-23p40 and neutralizes human IL-12 and IL-23 bioactivity. The crystal structure of ustekinumab Fab (antigen binding fragment of mAb), in complex with human IL-12, has been determined by X-ray crystallography at 3.0 Å resolution. Ustekinumab Fab binds the D1 domain of the p40 subunit in a 1:1 ratio in the crystal, consistent with a 2 cytokines:1 mAb stoichiometry, as measured by isothermal titration calorimetry. The structure indicates that ustekinumab binds to the same epitope on p40 in both IL-12 and IL-23 with identical interactions. Mutational analyses confirm that several residues identified in the IL-12/IL-23p40 epitope provide important molecular binding interactions with ustekinumab. The electrostatic complementarity between the mAb antigen binding site and the p40 D1 domain epitope appears to play a key role in antibody/antigen recognition specificity. Interestingly, this structure also reveals significant structural differences in the p35 subunit and p35/p40 interface, compared with the published crystal structure of human IL-12, suggesting unusual and potentially functionally relevant structural flexibility of p35, as well as p40/p35 recognition. Collectively, these data describe unique observations about IL-12p35 and ustekinumab interactions with p40 that account for its dual binding and neutralization of IL-12 and IL-23.

A possible role for metallic ions in the carbohydrate cluster recognition displayed by a Lewis Y specific antibody

Background

Lewis Y (Le^y) is a blood group-related carbohydrate that is expressed at high surface densities on the majority of epithelial carcinomas and is a promising target for antibody-based immunotherapy. A humanized Le^y-specific antibody (hu3S193) has shown encouraging safety, pharmacokinetic and tumor-targeting properties in recently completed Phase I clinical trials.

Methodology/Principal Findings

We report the three-dimensional structures for both the free (unliganded) and bound (Le^y tetrasaccharide) hu3S193 Fab from the same crystal grown in the presence of divalent zinc ions. There is no evidence of significant conformational changes occurring in either the Le^y carbohydrate antigen or the hu3S193 binding site, which suggests a rigid fit binding mechanism. In the crystal, the hu3S193 Fab molecules are coordinated at their protein-protein interface by two zinc ions and in solution aggregation of Fab can be initiated by zinc, but not magnesium ions. Dynamic light scattering revealed that zinc ions could initiate a sharp transition from hu3S193 Fab monomers to large multimeric aggregates in solution.

Conclusions/Significance

Zinc ions can mediate interactions between hu3S193 Fab in crystals and in solution. Whether metallic ion mediated aggregation of antibody occurs in vivo is not known, but the present results suggest that similar clustering mechanisms could occur when hu3S193 binds to Le^y on cells, particularly given the high surface densities of antigen on the target tumor cells.

Purification, Properties and Extended Solution Structure of the Complex Formed between Human Immunoglobulin A1 and Human Serum Albumin by Scattering and Ultracentrifugation

Immunoglobulin A (IgA) is unique amongst antibodies in being able to form polymeric structures that may possess important functions in the pathology of specific diseases. IgA also forms complexes with other plasma proteins, the IgA1-human serum albumin (HSA) complex (IgA1-HSA) being typical. We have purified this complex using a novel two-step purification based on thiophilic chromatography and gel filtration, and characterised this. HSA is linked covalently to the tailpiece of IgA1 by a disulphide bond between Cys471 in IgA1 and Cys34 in HSA. IgA1-HSA binds to IgA receptors on neutrophils and monocytes, and elicits a respiratory burst that is comparable in magnitude to that of monomeric IgA1. The solution arrangement of IgA1-HSA was identified by X-ray scattering and ultracentrifugation. The radius of gyration R(G) of 7.5(+/-0.3) nm showed that IgA1-HSA is more extended in solution than IgA1 (R(G) of 6.1-6.2 nm). Its distance distribution function P(r) showed two peaks that indicated a well-separated solution structure similar to that for IgA1, and a maximum dimension of 25 nm, which is greater than that of 21 nm for IgA1. Sedimentation equilibrium showed that the IgA1:HSA stoichiometry is 1:1. Sedimentation velocity resulted in a sedimentation coefficient of 6.4S and a frictional ratio of 1.87, which is greater than that of 1.56 for IgA1. The constrained modelling of the IgA1-HSA structure using known structures for IgA1 and HSA generated 2432 conformationally randomised models of which 52 gave good scattering fits. The HSA structure was located at the base of the Fc fragment in IgA1 in an extended arrangement. Such a structure accounts for the functional activity of IgA1-HSA, and supports our previous modelling analysis of the IgA1 solution structure. The IgA1-HSA complex may suggest the potential for creating a new class of targeted therapeutic reagents based on the coupling of IgA1 to carrier proteins.

A novel and conserved pocket of human kappa-Fab fragments: design, synthesis, and verification of directed affinity ligands

Antibodies of type IgG may be divided into two classes, called lambda or kappa, depending on the type of light chain. We have identified a conserved pocket between the two domains CH1 and CL of human IgG kappa-Fab, which is not present in the lambda type. This pocket was used as a target docking site with the purpose of exploring the possibilities of designing affinity ligands that could function as such even after immobilization to gel. The idea of the design arose mainly from the results of the saturated transfer difference (STD-NMR) screening of 46 compounds identified by means of virtual docking of 60 K diverse compounds from the Available Chemicals Directory (ACD). Surface plasmon resonance (SPR) was used as an alternative method to monitor binding in solution. A total of 24 compounds belonging to a directed library were designed, synthesized, and screened in solution. They consist essentially of an amino acid condensed to a N,N'-methylated phenyl urea. STD-NMR results suggest that a small hydrophobic side chain in the condensed amino acid promotes binding, whereas a hydroxyl-group-containing side chain implies absence of STD-NMR signals. Three compounds of the directed library were immobilized and evaluated as chromatographic probes. In one case, using D-Pro as the condensed amino acid, columns packed with ligand-coupled Sepharose (Amersham Biosciences) retained two different monoclonal samples of kappa-Fab fragments with different variable regions, whereas a sample of monoclonal lambda-Fab fragments was not retained under similar chromatographic conditions.

Solution Structure Determination of Monomeric Human IgA2 by X-ray and Neutron Scattering, Analytical Ultracentrifugation and Constrained Modelling: A Comparison with Monomeric Human IgA1

Immunoglobulin A (IgA), the most abundant human immunoglobulin, mediates immune protection at mucosal surfaces as well as in plasma. It exists as two subclasses IgA1 and IgA2, and IgA2 is found in at least two allotypic forms, IgA2m(1) or IgA2m(2). Compared to IgA1, IgA2 has a much shorter hinge region, which joins the two Fab and one Fc fragments. In order to assess its solution structure, monomeric recombinant IgA2m(1) was studied by X-ray and neutron scattering. Its Guinier X-ray radius of gyration R(G) is 5.18 nm and its neutron R(G) is 5.03 nm, both of which are significantly smaller than those for monomeric IgA1 at 6.1-6.2 nm. The distance distribution function P(r)for IgA2m(1) showed a broad peak with a subpeak and gave a maximum dimension of 17 nm, in contrast to the P(r) curve for IgA1, which showed two distinct peaks and a maximum dimension of 21 nm. The sedimentation coefficients of IgA1 and IgA2m(1) were 6.2S and 6.4S, respectively. These data show that the solution structure of IgA2m(1) is significantly more compact than IgA1. The complete monomeric IgA2m(1) structure was modelled using molecular dynamics to generate random IgA2 hinge structures, to which homology models for the Fab and Fc fragments were connected to generate 10,000 full models. A total of 104 compact best-fit IgA2m(1) models gave good curve fits. These best-fit models were modified by linking the two Fab light chains with a disulphide bridge that is found in IgA2m(1), and subjecting these to energy refinement to optimise this linkage. The averaged solution structure of the arrangement of the Fab and Fc fragments in IgA2m(1) was found to be predominantly T-shaped and flexible, but also included Y-shaped structures. The IgA2 models show full steric access to the two FcalphaRI-binding sites at the Calpha2-Calpha3 interdomain region in the Fc fragment. Since previous scattering modelling had shown that IgA1 also possessed a flexible T-shaped solution structure, such a T-shape may be common to both IgA1 and IgA2. The final models suggest that the combination of the more compact IgA2m(1) and the more extended IgA1 structures will enable human IgA to access a broader range of antigens than either acting alone. The hinges of both IgA subclasses appear to show reduced flexibility when compared to their equivalents in IgG, and this may be important for maintaining an extended IgA structure.

Monoclonal antibodies to thyroid specific autoantigens

Monoclonal antibody (MAbs) is a powerful and essential tool to perform studies concerning antigens and antibodies at molecular level. MAbs to major thyroid specific autoantigens, thyroglobulin (Tg), thyroid peroxidase (TPO) and TSH receptor (TSHR), have been prepared and applied for a variety of investigations including the structure of antigens and antibodies, the expression of antigens, the epitopes of antibodies, the functional regions of antigens, mutated antigens in congenital diseases, and clinical applications to diagnosis of various thyroid diseases. Recently, sodium iodide symporter (NIS) was identified and became a potential thyroid autoantigen related to autoimmune thyroid disease, although few MAbs to NIS have been prepared. In this manuscript, I primarily focus on studies concerning MAbs to three major thyroid specific autoantigens, Tg, TPO and TSHR, and summarize studies using the mAbs.

Humanization of the mouse anti-Fas antibody HFE7A and crystal structure of the humanized HFE7A Fab fragment

Binding of Fas ligand to Fas induces apoptosis. The Fas-Fas ligand system plays important roles in many biological processes, including the elimination of autoreactive lymphoid cells. We have previously obtained the mouse anti-Fas antibody HFE7A (m-HFE7A), which specifically induces apoptosis in inflammatory cells. In order to apply m-HFE7A for human therapy, we performed antibody humanization of m-HFE7A by grafting the mouse complementarity-determining regions (CDRs) to a human antibody. Five versions of humanized HFE7A (h-HFE7A) demonstrated the same antigen-binding affinity and same competition-binding activity against Fas as the chimeric HFE7A. Furthermore, these h-HFE7As induced the same degree of apoptosis in WR19L12a cells that express human Fas on their surface as chimeric HFE7A does. To further probe the structural basis for antibody humanization, we determined the three-dimensional structure of the h-HFE7A antigen-binding fragment (Fab) by X-ray crystallography and compared it with the crystal structure of the parent m-HFE7A Fab previously determined. The main-chain conformation in each h-HFE7A CDR is almost identical to that in m-HFE7A with root mean square (rms) deviations of 0.14-0.77 A. However, a significant segmental shift was observed in the CDR-L1 loop. Together with the high temperature factors of the CDR-L1 residues, both the loops are flexible, suggesting that the CDR-L1 loop would undergo conformational change upon binding to the antigen. Our results indicate that the humanization of m-HFE7A succeeded in maintaining the main-chain conformation as well as the flexibility of the CDR loop.

An engineered IN-1 F(ab) fragment with improved affinity for the Nogo-A axonal growth inhibitor permits immunochemical detection and shows enhanced neutralizing activity

The myelin axonal growth inhibitor NI-220/250 (Nogo-A) has attracted considerable attention in elucidating the mechanisms that account for the lack of plasticity in the adult central nervous system. The cognate monoclonal antibody IN-1, which was obtained prior to the molecular characterization of its Nogo-A antigen, has played a crucial role in this respect. However, this murine IgM/kappa antibody does not only provide an inappropriate format for in vivo studies, its low antigen affinity has also hampered the thorough structure-function analysis of its neutralizing effect toward the Nogo-A inhibitor on a molecular basis. We describe here the affinity maturation of a bacterially produced functional IN-1 F(ab) fragment via protein engineering. A soluble fragment of Nogo-A derived from the central exon 3 of its gene, which was prepared by secretion into the periplasm of Escherichia coli, served as a target in these experiments. After repeated cycles of site-directed random mutagenesis and screening, the mutant II.1.8 of the IN-1 F(ab) fragment was obtained, carrying five side chain substitutions within CDR-L3. Its dissociation constant for the complex with the recombinant Nogo-A fragment was determined in surface plasmon resonance measurements as approximately 1 microM. The affinity of the unmutated IN-1 F(ab) fragment was 8-fold lower. The engineered F(ab) fragment appeared to be well suited for the specific detection of Nogo-A in immunochemical assays and for the histochemical staining of myelin-rich tissue sections. Most importantly, its concentration-dependent neutralizing effect on the Nogo-A inhibitory activity was significantly enhanced in cell culture. This study confirms Nogo-A to be the antigen of the IN-1 antibody and it demonstrates increased potential of the engineered F(ab) fragment as a reagent for promoting axonal regeneration in vivo.

Crystal structures of human antibodies: a detailed and unfinished tapestry of immunoglobulin gene products

Sequencing of all human immunoglobulin (Ig) germline gene segments has recently been completed. However, our first glimpses of the recombined products of this combinatorial gene system were in the 1970s, in landmark publications, reporting the crystal structures of two human myeloma proteins, the Mcg lambda light chain dimer and the New IgG1(lambda) Fab. Although numerous crystal structures of murine and human antibodies have now been determined, only a relatively small proportion of the human germline genes have had their corresponding protein three-dimensional structures resolved. Therefore, further structural investigations are required before the inherent diversity of the antibody repertoire can be fully appreciated. We discuss the detailed structural information available for human antibodies with regard to their immune functions. Also discussed, is how the structural information is finding application in the 'humanization' of murine antibodies as part of their development as 'biopharmaceuticals' for the treatment of human disease.

Molecular study of an IgG1kappa cryoglobulin yielding organized microtubular deposits and glomerulonephritis in the course of chronic lymphocytic leukaemia

Glomerulonephritis with organized microtubular monoclonal immunoglobulin deposits (GOMMID) and glomerulonephritis related to type I cryoglobulin are well-known but rare complications of B cell derived chronic lymphocytic leukaemia. In these disorders, monoclonal Ig have never been studied at the molecular level. We conducted a pathological and molecular analysis in a patient with chronic lymphocytic leukaemia, glomerulonephritis and a single circulating monoclonal Ig. Unusual IgG1kappa kidney deposits were observed. The heavy and light chain variable region sequences of that cryoprecipitating monoclonal Ig were characterized. Light microscopy revealed glomerulonephritis typical of cryoglobulinaemia, with neutrophil and macrophage infiltration, endocapillary hyperplasia and few protein thrombi. Electron microscopic study clearly evidenced numerous subepithelial mixed granular and organized deposits with a unique microtubular organization, reminiscent of the GOMMID. The Ig molecule sequence revealed alterations of charge and hydrophobicity potentially promoting a crystal-like aggregation and the aggregation of microtubules. This description suggests that common mechanisms are involved in various forms of precipitation and/or deposition of complete Ig molecules, with a variable extent of organization and with a possible overlap between pathological patterns of either glomerulonephritis with microtubular deposits or type I cryoglobulinic glomerulonephritis.

Incorporation of long CDR3s into V domains: implications for the structural evolution of the antibody-combining site

Available data suggest that 'primitive' antibody-combining sites often include longer than average HCDR3s. Long HCDR3 sequences have been reported in diverse vertebrates, including humans, cattle, camels and sharks. These long HCDR3 segments contain unusual sequence features such as stretches of Gly or Pro residues and multiple Cys residues. We examined how longer than average HCDR3s were accommodated in the V domains of human, murine and camel antibodies with known three-dimensional structures. The main conclusions were that (1) HCDR3s longer than 12 residues should protrude outward from the V domains; (2) descending HCDR3 polypeptides may utilize VL (including LCDR3) constituents as a platform, supporting the protruding segments; (3) intra- and inter-HCDR disulfides are frequently formed to rigidify the structure of HCDR3 or the combining site, and (4) V and C domains were possibly more similar in primordial antibodies than they are in their present day counterparts.

Serine 132 is the C3 covalent attachment point on the CH1 domain of human IgG1

The covalent binding of C3 (complement component C3) to antigen-antibody complexes (Ag.Ab; immune complexes (ICs)) is a key event in the uptake, transport, presentation, and elimination of Ag in the form of Ag.Ab.C3b (IC.C3b). Upon interaction of C3 with IgG.IC, C3b.C3b.IgG covalent complexes are formed that are detected on SDS-polyacrylamide gel electrophoresis by two bands corresponding to C3b.C3b (band A) and C3b.IgG (band B) covalent complexes. This allows one to evaluate the covalent binding of C3b to IgG antibodies. It has been described that C3b can attach to both the Fab (on the CH1 domain) and the Fc regions of IgG. Here the covalent interaction of C3b to the CH1 domain, a region previously described spanning residues 125-147, has been studied. This region of the CH1 domain is exposed to solvent and contains a cluster of six potential acceptor sites for ester bond formation with C3b (four Ser and two Thr). A set of 10 mutant Abs were generated with the putative acceptor residues substituted by Ala, and we studied their covalent interaction with C3b. Single (Ser-131, Ser-132, Ser-134, Thr-135, Ser-136, and Thr-139), double (positions 131-132), and multiple (positions 134-135-136, 131-132-134-135-136, and 131-132-134-135-136-139) mutants were produced. None of the mutants (single, double, or multiple) abolished completely the ability of IgG to bind C3b, indicating the presence of C3b binding regions other than in the CH1 domain. However, all mutant Abs, in which serine at position 132 was replaced by Ala, showed a significant decrease in the ability to form C3b.IgG covalent complexes, whereas the remaining mutants had normal activity. In addition we examined ICs using the F(ab')2 fragment of the mutant Abs, and only those containing Ala at position 132 (instead of Ser) failed to bind C3b. Thus Ser-132 is the binding site for C3b on the CH1 domain of the heavy chain, in the Fab region of human IgG.

Mutation of a single conserved residue in VH complementarity-determining region 2 results in a severe Ig secretion defect

During an immune response, somatic mutations are introduced into the VH and VL regions of Ig chains. The consequences of somatic mutation in highly conserved residues are poorly understood. Ile51 is present in 91% of murine VH complementarity-determining region 2 sequences, and we demonstrate that single Ile51-->Arg or Lys substitutions in the PCG1-1 Ab are sufficient to severely reduce Ig secretion (1-3% of wild-type (WT) levels). Mutant H chains, expressed in the presence of excess L chain, associate with Ig binding protein (BiP) and GRP94 and fail to form HL and H2L assembly intermediates efficiently. The mutations do not irreversibly alter the VH domain as the small amount of mutant H chain, which assembles with L chain as H2L2, is secreted. The secreted mutant Ab binds phosphocholine-protein with avidity identical with that of WT Ab, suggesting that the combining site adopts a WT conformation. A computer-generated model of the PCG1-1 variable region fragment of Ig (Fv) indicates that Ile51 is buried between complementarity-determining region 2 and framework 3 and does not directly contact the L chain. Thus, the Ile51-->Arg or Ile51-->Lys mutations impair association with the PCG1-1 L chain via indirect interactions. These interactions are in part dependent on the nature of the L chain as the PCG1-1 VH single Ile51-->Arg or Ile51-->Lys mutants were partially rescued when expressed with the J558L lambda1 L chain. These results represent the first demonstration that single somatic mutations in V(H) residues can impair Ig secretion and suggest one reason for the conservation of Ile51 in so many Ig VH.

Evidence for the involvement of phosphatidylinositol 3-kinase in fMLP-stimulated neutrophil adhesion to ICAM-1-transfected cells

Phosphatidylinositol 3-kinase (PI-3K) controls important intracellular steps involved in inflammation, immunity, and cell growth. PI-3K also modulates leukocyte integrin adhesiveness. In this study we evaluated the role of PI-3K on neutrophil adhesion to intercellular adhesion molecule-1 (ICAM-1)-transfected cells. N-formyl-methionyl-leucyl-phenylalanine (fMLP)-stimulated neutrophil adhesion was inhibited by wortmannin and LY294002, two unrelated PI-3K inhibitors, whereas phorbol myristate acetate (PMA)-induced neutrophil adhesion was not inhibited by them. After fMLP stimulation, a rapid activation of AKT and ERK was observed. However, only activation of AKT was reversed by the PI-3K inhibitors. Neutrophil expression of the beta2-integrins Mac-1, lymphocyte function-associated antigen-1(LFA-1), and gp150.95 was not affected by wortmannin, nor was expression of the activation epitope recognized by MAB24. We conclude that (a) PI-3K is involved in fMLP-activated neutrophil adhesion to ICAM-1-transfected cells, (b) the mechanism involved is not mediated by the modulation of beta2-integrin expression or activation, and (c) another mechanism seems to involve the adhesion to ICAM-1 when a cellular system of adhesion is used.

Autoimmune response to the thyroid in humans: thyroid peroxidase--the common autoantigenic denominator

Autoimmunity to thyroid peroxidase (TPO), manifest as high affinity IgG class autoantibodies, is the common denominator of human thyroid autoimmunity, encompassing patients with overt hyper- or hypothyroidism as well as euthyroid individuals with subclinical disease. The identification and cloning of TPO (the "thyroid microsomal antigen") provided the critical tool for analyzing B and T cell reactivity to this major thyroid autoantigen. In particular, the availability of immunoreactive TPO permitted the isolation of essentially the entire repertoire of human monoclonal antibodies, a feat unparalled in an organ-specific autoimmune disease. These recombinant autoantibodies (expressed as Fab) provide insight into the genes encoding their H and L chains as well as the conformational epitopes on TPO with which serum autoantibodies interact. Analyses of TPO autoantibody epitopic "fingerprints" indicate a lack of epitope spreading as well as a genetic basis for their inheritance. Limited data are available for the responses and cytokine profiles of T cells to endogenously processed TPO. Moreover, the role of thyroid cells in initiating the autoimmune response to TPO, and of B cells in expanding and/or modulating the response of sensitized T cells, has yet to be established. Finally, because autoantibody (and likely T cell) responses to TPO parallel those to TSH receptor and thyroglobulin, manipulation of T and B cell responses to TPO may provide the basis for the development of immunospecific therapy for autoimmune thyroid disease in general.

Search for the autoantibody immunodominant region on thyroid peroxidase: epitopic footprinting with a human monoclonal autoantibody locates a facet on the native antigen containing a highly conformational epitope

Autoantibodies to thyroid peroxidase (TPO) are the hallmark of the humoral autoimmune response in human autoimmune thyroiditis (Hashimoto's thyroiditis). The majority of TPO autoantibodies in individual patients' sera interact with a restricted immunodominant region on TPO. Although this region can be mapped, previous studies have failed to localize its position on the TPO molecule. We, therefore, used a footprinting approach that can localize a highly conformational, discontinuous epitope on a very large molecule. Extensive biotinylation ( approximately 15 biotins/molecule protein) of lysine residues on the surface of purified, native TPO resulted in loss of multiple tryptic cleavage sites, as determined by analysis of tryptic polypeptide fragments on reverse-phase HPLC. TPO was then complexed with a monoclonal human autoantibody Fab (TR1.9) before biotinylation. After dissociation from TR1.9, TPO was recovered by gel filtration. A trypsin site, previously observed to be lost after TPO biotinylation, was restored when biotinylation was performed on the TPO-TR1.9 complex. The epitope-protected lysine (K) was present in a 30-aa TPO fragment that, by N-terminal sequencing, was found to be K713. Altered recognition by TR1.9 of a TPO-myeloperoxidase chimeric molecule involving this region supported the epitope protection data. In conclusion, we provide the first identification of an amino acid residue (K713) comprising part of an epitope within the TPO immunodominant region. This focal residue localizes the facet on the large, highly complex TPO molecule that contains the immunodominant region and provides the basis for rational guided mutagenesis studies to more fully characterize this region.

Covariance analysis of protein families: the case of the variable domains of antibodies

A nonrestrictive method for identifying covariance in protein families is described and applied to human and mouse germline Vkappa and VH sequence alignments. Amino acids that occur at each position in a sequence alignment are divided into two sets, called a word, by generating all possible combinations of alternative amino acids. Each word is associated with a pattern of changes. Words with identical patterns identify covariant positions. In antibody variable domains, the number of words generated ranged between 1103 and 2195 depending on the alignment, of which 4 to 12 % occurred in covariant pairs. Despite the nonrestrictive character of pattern generation, covariant residues did not reflect a random selection with respect to the nature of amino acid changes and/or their spatial proximity in a reference crystallographic structure. This approach allowed the identification of a covariance signal for positions with high variability, mostly located in the outer part of the common structural framework of antibody variable domains. Covariance in these regions may reflect the existence of alternative and mutually exclusive atomic arrangements that are compatible with antibody function. The method may be of general applicability to rationalize residue variability in protein families.

The Fab and Fc fragments of IgA1 exhibit a different arrangement from that in IgG: a study by X-ray and neutron solution scattering and homology modelling

Human immunoglobulin A (IgA) is an abundant antibody that mediates immune protection at mucosal surfaces as well as in plasma. The IgA1 isotype contains two four-domain Fab fragments and a four-domain Fc fragment analogous to that in immunoglobulin G (IgG), linked by a glycosylated hinge region made up of 23 amino acid residues from each of the heavy chains. IgA1 also has two 18 residue tailpieces at the C terminus of each heavy chain in the Fc fragment. X-ray scattering using H2O buffers and neutron scattering using 100 % 2H2O buffers were performed on monomeric IgA1 and a recombinant IgA1 that lacks the tailpiece (PTerm455). The radii of gyration RG from Guinier analyses were similar at 6.11-6.20 nm for IgA1 and 5.84-6.16 nm for PTerm455, and their cross-sectional radii of gyration RXS were also similar. The similarity of the RG and RXS values suggests that the tailpiece of IgA1 is not extended outwards in solution. The IgA1 RG values are higher than those for IgG, and the distance distribution function P(r) showed two distinct peaks, whereas a single peak was observed for IgG. Both results show that the hinge of IgA1 results in an extended Fab and Fc arrangement that is different from that in IgG. Automated curve-fit searches constrained by homology models for the Fab and Fc fragments were used to model the experimental IgA1 scattering curves. A translational search to optimise the relative arrangement of the Fab and Fc fragments held in a fixed orientation resembling that in IgG was not successful in fitting the scattering data. A new molecular dynamics curve-fit search method generated IgA1 hinge structures to which the Fab and Fc fragments could be connected in any orientation. A search based on these identified a limited family of IgA1 structures that gave good curve fits to the experimental data. These contained extended hinges of length about 7 nm that positioned the Fab-to-Fab centre-to-centre separation 17 nm apart while keeping the corresponding Fab-to-Fc separation at 9 nm. The resulting extended T-shaped IgA1 structures are distinct from IgG structures previously determined by scattering and crystallography which have Fab-to-Fab and Fab-to-Fc centre-to-centre separations of 7-9 nm and 6-8 nm, respectively. It was concluded that the IgA1 hinge is structurally distinct from that in IgG, and this results in a markedly different antibody structure that may account for a unique immune role of monomeric IgA1 in plasma and mucosa.

Structural bases of Fc gamma R functions

This review describes structures which determine the biological activities triggered by Fc gamma R and account for the cell-mediated functions of IgG antibodies in physiology and pathology. The binding specificity and affinity of Fc gamma R depend primarily on IgG-binding structures, in their immunoglobulin-like extracellular domains. Binding is however also influenced by subunits that associate to multichain Fc gamma R. Effector and regulatory intracytoplasmic sequences that are unique to molecules of the Fc gamma RIIB family determine the internalization properties of these receptors. Immunoreceptor Tyrosine-based Activation Motifs (ITAMs) are intracytoplasmic effector sequences shared by Fc gamma R and other receptors involved in the recognition of antigen, which trigger cell activation and internalization. Immunoreceptor Tyrosine-based Inhibition Motifs (ITIMs) are intracytoplasmic sequences, shared by Fc gamma RIIB and a growing number of negative coreceptors which negatively regulate cell activation via ITAM-bearing receptors. Altogether, these structures enable IgG antibodies to exert a variety of finely tuned biological effects during the immune response.

Influence of tyrosine phosphorylation on protein interaction with FcgammaRIIa

The cytoplasmic tail of Fc(gamma)RIIa present on human neutrophils shares with other antigen receptors a common amino acid sequence called ITAM (Immunoreceptor Tyrosine-based Activation Motif). After receptor ligation, the tyrosine residues within this motif become phosphorylated. We prepared a recombinant fusion protein of the cytoplasmic tail of Fc(gamma)RIIa (containing the ITAM) with glutathione-S-Transferase (GST-CT) to characterize the phosphorylation of Fc(gamma)RIIa and its ability to interact with other proteins involved in signal transduction. The GST-CT became phosphorylated in the presence of Lyn, Hck and Syk (immunoprecipitated from human neutrophils), but not in the presence of Fgr. Of the active kinases, only Lyn (mainly present in the membrane fraction) was found to associate with the GST-CT in the absence of ATP. This association was also observed in immunoprecipitates of Fc(gamma)RIIa from resting neutrophils, suggesting that Lyn might be the kinase responsible for the initial Fc(gamma)RIIa phosphorylation. Moreover, we observed specific association of Syk and the p85 subunit of PI 3-kinase after incubation of the GST-CT with neutrophil cytosol. This interaction was dependent on tyrosine phosphorylation of the GST-CT. Substitution of 269Tyr by Phe almost completely abolished tyrosine phosphorylation of the fusion protein. Substitution of either 253Tyr or 269Tyr eliminated Syk binding, but only 253Tyr appeared to be essential for p85 binding. We hypothesize that, upon activation, the membrane-associated Lyn is responsible for the initial tyrosine phosphorylation of Fc(gamma)RIIa, thus creating a docking site for Syk and PI 3-kinase.