The codon for the methionine at position 129 (M129) in the human prion protein provides an alternative initiation site for translation and renders individuals homozygous for M129 more susceptible to prion disease.

Single amino-acid substitutions in the prion protein have been found to lead to resistance or susceptibility to amyloid fibril formation. In humans, the presence of methionine at position 129 in the prion protein results in increased susceptibility to prion disease, while the presence of valine at that position appears to be protective. It is hypothesized that the codon for M129 is an alternative initiation site for translation, which results in a truncated molecule that is missing the first 128 amino acids, including the signal peptide. This N-terminal truncated form of the prion molecule will not be transported to the extracellular space and thus will accumulate in the cytosol where it is more susceptible to fibril formation and aggregation; this aggregation could hinder normal degradation processes and cause disease. The results of experimental studies on truncated prion molecules support this hypothesis. To test the hypothesis, a gene segment, which when transcribed would result in a prion molecule starting at methionine 129, could be introduced into a convenient experimental animal to see if there is increased incidence of prion disease. Or, fibrils from the brains of affected M129/M129 homozygous individuals could be isolated and the molecules in the fibrils analyzed to determine the identity of the N-terminal amino acid(s). We predict that those isolates will have a preponderance of molecules that start with the methionine at position 129 in the intact protein.

Why don't humans get scrapie from eating sheep? A possible explanation based on secondary structure predictions.

In an effort to find a structural explanation for the lack of direct transmission of scrapie from sheep to humans, secondary structure predictions are used to locate the segments of the prion sequence which may be involved in the transformation from the normal form of the prion protein, which has high helix content, to the pathogenic form, which has high beta-sheet content. The Chou-Fasman algorithm, which calculates propensities for both helix and sheet formation, was used to predict the secondary structures of the scrapie-resistant and the scrapie-susceptible variants of the ovine prion protein. The scrapie-susceptible variant, which has a glutamine at residue position 168 (human prion protein numbering), is predicted to have a propensity for sheet formation in that region of the molecule, while the scrapie-resistant variant, which has an arginine at position 168, does not. The valine at position 133, additionally present in the ovine variant which is the most susceptible to scrapie, is predicted to result in even more sheet formation. When the predicted secondary structure of the human prion protein is compared to those of the ovine prion protein variants, the human protein is found to be most similar to the scrapie-resistant variant. This result is proposed to provide a possible explanation for the observation that scrapie is not directly transmitted from sheep to humans.

Are humans getting 'mad-cow disease' from eating beef, or something else?

Bovine spongiform encephalopathy (BSE) or 'mad-cow disease' is believed to have been caused by the consumption of scrapie-infected sheep matter that had been added to cattle feed. BSE is then believed to have been transmitted to humans by the consumption of infected beef. We have compared the sequences of human and various animal prion proteins with regards to the fragments that could result from gastric digestion. We noted the close similarity of the sequences of human and rodent prion proteins in a peptic fragment that corresponds very closely to one that had been shown by others to be protease resistant and infective. Since rats and mice are known to be susceptible to prion disease, we propose that ingestion of infected rodent parts, possibly droppings, may be a possible mode of transmission of scrapie or BSE to humans.

A humanized anti-human CD154 monoclonal antibody blocks CD154-CD40 mediated human B cell activation.

Humanized anti-CD154 antibody, IDEC-131, had a slightly, but reproducibly, better binding affinity for CD154 (Kd = 5.6 nM), compared to the parent antibody 24-31 (Kd = 8.5 nM). Otherwise it was indistinguishable from the murine parent antibody in its ability to bind to CD154, block CD154 binding to CD40 and inhibit T cell-dependent B cell differentiation. The latter activity was independent of FcR binding as the Fab'1 fragment of IDEC-131 had an equivalent biological activity to that of the whole antibody. IDEC-131 blocked soluble CD154 from inducing proliferation of purified B cells, and blocked T cell dependent anti-tetanus toxoid specific antibody production by human B cells in vitro. IDEC-131, gamma1, kappa, had strong Fc gammaRI, Fc gammaRII and C1q binding, but was unable to induce complement dependent (CDC) or antibody dependent cell-cytotoxicity (ADCC) of activated peripheral blood T cells, which express relatively low levels of CD154. IDEC-131 antibody inhibited both primary and secondary antibody responses to ovalbumin in cynomolgus monkeys at a dose of 5 mg/kg. In non-immunized animals, treatment with IDEC-131 at 50 mg/kg weekly for 13 weeks induced no change in any of the measured lymphocyte subsets, including B cells, CD4+ and CD8+ T cells. Similarly, a safety study in chimpanzees showed no discernible safety related issues at 20 mg/kg, including B and T cell subsets. These results show that the humanized anti-CD154 antibody, IDEC-131, has retained the affinity and functional activity of its murine parent antibody, is unlikely to deplete CD154 positive lymphocytes in humans, and is safe and effective in blocking antibody production in monkeys. Based on its safety and efficacy profile, IDEC-131 is being developed for therapy of systemic lupus erythematosus.

Development of a minimally immunogenic variant of humanized anti-carcinoma monoclonal antibody CC49.

Monoclonal antibody (MAb) CC49 reacts with a pancarcinoma antigen, tumor associated glycoprotein (TAG)-72. To circumvent human anti-murine antibody (HAMA) responses in patients, we earlier developed a humanized CC49 (HuCC49) by grafting the complementarity-determining regions (CDRs) of MAb CC49 onto variable light (VL) and variable heavy (VH) frameworks of the human MAbs LEN and 21/28'CL, respectively. With the aim of minimizing its immunogenicity further, we have now generated a variant HuCC49 MAb by grafting the specificity-determining residues (SDRs) of MAb CC49 onto the frameworks of the human MAbs. Based on the evaluation of its binding affinity for TAG-72 and its reactivity with anti-idiotypic antibodies present in sera from patients who have been treated with murine CC49, this variant retains its antigen-binding activity and shows minimal reactivity with anti-idiotypic antibodies in patients' sera. Development of this variant, which is a potentially useful clinical reagent for diagnosis and therapy of human carcinomas, demonstrates that for humanization of a xenogeneic antibody grafting of the potential SDRs should be sufficient to retain its antigen-binding properties.

Structure-function analysis of a lupus anti-DNA autoantibody: central role of the heavy chain complementarity-determining region 3 Arg in binding of double- and single-stranded DNA.

To determine the contribution of the somatic point mutations and that of the complementarity-determining region (CDR)3 Arg to DNA binding, we engineered the germline V(H) and V(kappa) gene revertant and site-mutagenized the CDR3 Arg residues of the mutated and "antigen-selected" mAb 412.67. This anti-DNA autoantibody was derived from B-1 cells of a lupus patient and bore two H-CDR3 Arg, Arg105 and Arg107, encoded by N segment additions, and one kappa-CDR3 Arg, Arg97, resulting from a point mutation (Kasaian et al. 1994. J. Immunol. 152: 3137-3151; Kasaian et al. 1995. Ann. N.Y Acad. Sci. 764: 410-423). The germ-line revertant bound double-stranded (ds) DNA and single-stranded (ss) DNA as effectively as its wild-type counterpart (relative avidity: 6.4x10(-7) and 9.9x10(-9) vs. 6.7x10(-7) and 9.1 x10(-9) g/microl), raising the possibility that an antigen other than DNA was responsible for the selection of the mAb 412.67 V(H) and V(kappa) point mutations. H-CDR3 Arg105 and Arg107 were both required for dsDNA binding, but either Arg105 or Arg107 was sufficient for ssDNA binding. The central role of Arg105 and Arg107 in DNA binding reflected their solvent-exposed orientation at the apex of the H-CDR3 main loop. Consistent with its inward orientation afar from the antigen-binding surface, the kappa-CDR3 Arg97 played no role in either dsDNA or ssDNA binding.

Does mRNA translation starting from an alternative initiation site contribute to the pathology of Huntington's disease?

Huntington's disease is associated with an expanded and unstable trinucleotide repeat (CAG)(n). Various possibilities have been suggested to explain the significance of poly-(CAG) length in HD, including changes in the structure of the product (huntingtin) which result in the protein acquiring deleterious properties. We have looked at the nucleotide sequence coding for huntingtin and find that another possibility may exist for the correlation between the occurrence of HD and poly-CAG length. We have noted an alternative reading frame that includes the trinucleotide repeat, now read as (GCA)(n). Upon close examination of this alternative gene product, we observe features that suggest it can likewise have deleterious properties.

Structural correlates of an anticarcinoma antibody: identification of specificity-determining residues (SDRs) and development of a minimally immunogenic antibody variant by retention of SDRs only.

Clinical utility of murine mAbs is limited because many elicit Abs to murine Ig constant and variable regions in patients. An Ab humanized by the current procedure of grafting all the complementarity determining regions (CDRs) of a murine Ab onto the human Ab frameworks is likely to be less immunogenic, except that its murine CDRs could still evoke an anti-variable region response. Previous studies with anticarcinoma mAb CC49 showed that light chain LCDR1 and LCDR2 of humanized CC49 could be replaced with the corresponding CDRs of a human Ab with minimal loss of Ag-binding activity. The studies reported in this paper were undertaken to dissect the CC49 Ag-binding site to identify 1) specificity determining residues (SDRs), the residues of the hypervariable region that are most critical in Ag-Ab interaction, and 2) those residues that contribute to the idiotopes that are potential targets of patients' immune responses. A panel of variants generated by genetic manipulation of the murine CC49 hypervariable regions were evaluated for their relative Ag-binding affinity and reactivity to sera from several patients who had been immunized with murine CC49. One variant, designated HuCC49V10, retained only the SDRs of CC49 and does not react with the anti-variable region Abs of the sera from the murine CC49-treated patients. These studies thus demonstrate that the genetic manipulation of Ab variable regions can be accomplished by grafting only the SDRs of a xenogeneic Ab onto human Ab frameworks. This approach may reduce the immunogenicity of Abs to a minimum.

Generation and characterization of a single gene-encoded single-chain-tetravalent antitumor antibody.

Monoclonal antibody (mAb) CC49, a murine IgG1, reacts with the tumor-associated glycoprotein-72 expressed on a variety of carcinomas. In clinical trials, radiolabeled CC49 has shown excellent tumor localization to a variety of carcinomas. To minimize the immunogenicity of CC49 mAb in patients, a humanized CC49 (HuCC49) was generated by complementarity-determining region (CDR) grafting. The relative affinity of HuCC49 was 2-3-fold less than that of the murine mAb. With the aim of improving tumor targeting, attempts have been made to enhance the avidity of the HuCC49 mAb. Previous research has yielded a single gene-encoded immunoglobulin, SCIgcCC49deltaCH1, which is a dimer of a single chain consisting of CC49 single-chain Fv linked to the NH2 terminus of the human gamma1 Fc through the hinge region. This molecule is comparable to the mouse-human chimeric CC49 in terms of in vitro antigen binding properties, cytolytic activity, and rate of plasma clearance in athymic mice bearing human tumor xenografts. Recently, a single gene encoding a single-chain immunoglobulin consisting of a HuCC49 diabody attached to human gamma1 Fc via the hinge region was constructed. The diabody, a bivalent antigen-binding structure, is made up of variable heavy (V(H))/variable light (V(L)) domains and V(L)/V(H) domains. In each of the variable domain pairs, the V(H) and V(L) domains are linked through a short linker peptide. Meanwhile, the two pairs are linked via a 30-residue Gly-Ser linker peptide to yield two antigen-binding sites by lateral and noncovalent association of the V(L) of one pair with the V(H) of the other. Transfectomas expressing the single-gene immunoglobulin secrete a homodimer of about Mr 160,000 that reacts to tumor-associated glycoprotein-72. This tetravalent humanized antitumor immunoglobulin molecule may potentially be an efficacious therapeutic and diagnostic reagent against a wide range of human carcinomas.

Structure-function studies of an anti-asialo GM1 antibody obtained from a phage display library.

Although gangliosides elicit human autoantibodies, they are extremely weak immunogens in mice. We obtained a monoclonal antibody Fab fragment (clone 10) that is specific for asialo GM1 (GA1), from a phage display library. The Vkappa domain of clone 10 could be replaced by two different Vkappa domains without changing the specificity of the antibody. Mutagenesis of the third hypervariable regions of the heavy and light chains of clone 10 yielded three mutants that exhibited a 3 to 4 times increase in avidity for GA1. A molecular model of clone 10 indicated that the putative antigen-binding site contained a shallow surface pocket. These data illustrate the use of recombinant DNA techniques to obtain anti-ganglioside antibodies, and to explore the molecular basis of their antigen-binding activity.

Generation and characterization of a novel single-gene-encoded single-chain immunoglobulin molecule with antigen binding activity and effector functions.

Monoclonal antibody (MAb) CC49 is a murine IgG1 that reacts with tumor-associated glycoprotein (TAG)-72, a pancarcinoma antigen. Clinical trials using radiolabeled CC49 for diagnostic imaging have demonstrated specific localization of more than 90% of carcinomas. The feasibility of adopting in vivo gene inoculation methods for antibody-based immunotherapy requires introduction and expression of two genes, encoding immunoglobulin (Ig) heavy and light chains, in a single cell to generate a functional antibody. To circumvent the problems inherent in this approach, we have constructed a single-gene encoding a single-chain immunoglobulin (SCIg) that, unlike previously developed SCIgs, contains all IgG domains. To construct the novel SCIg, the carboxyl end of the constant region of the chimeric (c) CC49 kappa chain is joined, via a 30 residue Gly-Ser linker peptide, to the amino terminus of the CC49 heavy chain. To our knowledge, neither a linker peptide this long nor a linkage between the constant light (C(L)) and variable heavy domains has been reported previously. Transfectomas developed by introducing the expression construct of the amplifiable gene in dihydrofolate reductase-deficient Chinese hamster ovary (CHO dhfr-) cells secrete a 160 kDa homodimeric molecule, SCIgcCC49. The in vitro antigen binding properties of SCIgcCC49 are comparable to those of cCC49 and SCIgcCC49deltaC(H)1, a single-chain Ig deficient in constant heavy chain-1 (C(H)1) and C(L) domains. The antibody-dependent cellular cytotoxicity (ADCC) of SCIgcCC49 and cCC49 were also comparable. This single-gene approach for generating an immunoglobulin molecule may facilitate in vivo gene inoculation as well as ex vivo transfection of patients' cultured tumor-infiltrating lymphocytes for immunotherapy protocols for a variety of diseases, including cancer.

The X-ray crystal structure of a Valpha2.6Jalpha38 mouse T cell receptor domain at 2.5 A resolution: alternate modes of dimerization and crystal packing.

We describe here the structure of a murine T cell receptor (TCR) Valpha2.6Jalpha38 (TCRAV2S6J38) domain, derived from a T cell hybridoma with specificity for the H-2Ddmajor histocompatibility complex class I molecule bound to a decamer peptide, P18-I10, from the HIV envelope glycoprotein gp120, determined by X-ray crystallography at 2.5 A resolution. Unlike other TCR Valpha domains that have been studied in isolation, this one does not dimerize in solution at concentrations below 1 mM, and the crystal fails to show dimer contacts that are likely to be physiological. In comparison to other Valpha domains, this Valpha2.6 shows great similarity in the packing of its core residues, and exhibits the same immunoglobulin-like fold characteristic of other TCR Valpha domains. There is good electron density in all three complementarity-determining regions (CDRs), where the differences between this Valpha domain and others are most pronounced, in particular in CDR3. Examination of crystal contacts reveals an association of Valpha domains distinct from those previously seen. Comparison with other Valpha domain structures reveals variability in all loop regions, as well as in the first beta strand where placement and configuration of a proline residue at position 6, 7, 8, or 9 affects the backbone structure. The great variation in CDR3 conformations among TCR structures is consistent with an evolving view that CDR3 of TCR plays a plastic role in the interaction of the TCR with the MHC/peptide complex as well as with CDR3 of the paired TCR chain.

CDR substitutions of a humanized monoclonal antibody (CC49): contributions of individual CDRs to antigen binding and immunogenicity.

One of the major obstacles in the successful clinical application of monoclonal antibodies has been the development of host immune responses to murine Ig constant and variable regions. While the CDR grafting of MAbs may alleviate many of these problems, the potential remains that one or more murine CDRs on the human Ig backbone of a "humanized" MAb may still be immunogenic. Studies were undertaken employing a MAb of potential clinical utility, CC49, to define those CDRs that are essential for antigen binding and those that may be immunogenic in humans. We previously developed a humanized CC49 (HuCC49) by grafting the MAb CC49 hypervariable regions onto frameworks of human MAbs. To identify those CDRs essential for binding, a panel of variant HuCC49 MAbs was generated here by systematically replacing each of the murine CDRs with their human counterparts. The relative affinity constant of each variant was determined. Serum from a patient who received murine CC49 was used to determine the potential immunogenicity of each CDR in humans. The serum was shown to react with the anti-CC49 variable region. Results showed that patients' anti-idiotypic responses are directed mainly against LCDR3 and moderately against LCDR1 and HCDR2. These studies demonstrate for the first time that variants containing individual CDR substitutions of a humanized MAb can be constructed, and each CDR can be defined for the two most important properties for potential clinical utility: antigen binding and immunogenicity.

Protein and cell engineering of components of the human immunoglobulin E receptor/effector system: applications for therapy and diagnosis.

Adaptive immune responses characterised by the synthesis of antibodies of the immunoglobulin E (IgE) isotype play an important role in type I hypersensitivity disorders and parasitic infestations, diseases which have an significant socioeconomic impact world-wide. This paper considers potential applications of recent advances in our understanding of the origin of isotype specific immune responses which emerged as a result of cell and protein engineering studies on components of the human IgE/receptor/effector system. Furthermore, the identification of the receptor binding regions in IgE as a result of the development of a stable assay system has important applications for the design of rational therapeutic interventions in allergy and asthma, the treatment of mast cell tumours, and the establishment of procedures for the selective isolation of cells expressing the high-affinity receptor for IgE for functional studies.

Amino acid residues that influence Fc epsilon RI-mediated effector functions of human immunoglobulin E.

Immunoglobulin E (IgE) mediates its effector functions via the Fc region of the molecule. IgE binding to and subsequent aggregation of the high-affinity receptor (Fc epsilon RI) by allergen plays a pivotal role in type I hypersensitivity responses. Earlier studies implicated the C epsilon 2 and 3 interface and the A-B loop in C epsilon 3 in the IgE-Fc epsilon RI interaction. These regions and glycosylation sites in C epsilon 3 were now targeted by site-specific mutagenesis. IgE binding to Fc epsilon RI was compared with surface plasmon resonance (SPR) measurements, which assessed the binding of the soluble extracellular domain of Fc epsilon RI to IgE. Kinetic analysis based on a pseudo-first-order model agrees with previous determinations. A more refined SPR-based kinetic analysis suggests a biphasic interaction. A model-free empirical analysis, comparing the binding strength and kinetics of native and mutant forms of IgE, identified changes in the kinetics of IgE-Fc epsilon RI interaction. Conservative substitutions introduced into the A-B loop have a small effect on binding, suggesting that the overall conformation of the loop is important for the complementary interaction, but multiple sites across the C epsilon 3 domain may influence IgE-Fc epsilon RI interactions. Asn394 is essential for the generation of a functional IgE molecule in mammalian cells. A role of Pro333 in the maintenance of a constrained conformation at the interface between C epsilon 2-3 emerged by studying the functional consequences of replacing this residue by Ala and Gly. These substitutions cause a dramatic decrease in the ability of the ligand to mediate stimulus secretion coupling, although only small changes in the association and dissociation rates are observed. Understanding the molecular basis of this phenomenon may provide important information for the design of inhibitors of mast cell degranulation.

Binding of modified fragments of the Shigella dysenteriae type 1 O-specific polysaccharide to monoclonal IgM 3707 E9 and docking of the immunodeterminant to its modeled Fv.

The O-specific polysaccharide (O-SP) of Shigella dysenteriae type 1 has been shown by others to have the structure-->3)-alpha-L-Rhap-(1-->3)-alpha-L-Rhap-(1-->2)-alp ha-D- Galp-(1-->3)-alpha-D-GlcpNAc-(1-->. We have shown in the past that IgM 3707 E9, an anti S. dysenteriae type 1 O-SP monoclonal antibody, binds specifically to the -alpha-L-Rhap-(1-->2)-alpha-D-Galp-determinant of the polysaccharide. In this report we show that determinant to have hydrogen bonds, necessary for binding to the antibody, involving positions 3, 4 and 6 of the galactopyranosyl residue. The hydroxyl groups of the rhamnopyranosyl moiety of the immunodeterminant appear not to partake in hydrogen-bond interactions with the antibody. A model is presented of the Fv of IgM 3707 E9 based on our previously established cDNA-sequence and two known, highly homologous immunoglobulin crystal structures. The methyl glycoside of the immunodeterminant alpha-L-rhamnopyranosyl-(1-->2)-alpha-D-galactopyranose is docked to the combining area of the Fv.

Development of more efficacious antibodies for medical therapy and diagnosis.

Two procedures for improving the efficacy of medically important antibodies are described. The first procedure is designed to reduce the immunogenicity of nonhuman antibodies to the barest minimum--the "humanization" is accomplished by transplanting only the specificity-determining residues of the nonhuman antibody onto a human antibody template. The second procedure is designed to permit the easy production of multispecific/multivalent antibodies via heterodimer formation of electrostatically complementary Fc regions.

Does base composition help predispose the complementarity-determining regions of antibodies to hypermutation?

A survey of the base usage in genes coding for human antibodies reveals that more (A+T) and less (C+G) are found in the segments coding for the complementarity-determining regions, while the opposite is true for the segments coding for framework and constant regions. The possibility that this bias in base usage may contribute to hypermutation is explored.