Chemical mutagenesis of human interferon-beta: construction, expression in E. coli, and biological activity of sodium bisulfite-induced mutations

Quantitative liquid-phase chemiluminescence ELISA: detection of subtle epitope differences in HuIFN-beta

We have developed a new liquid-phase, chemiluminescence-enhanced, inhibition ELISA (LP-CEI-ELISA) to explore the binding sites recognized by two neutralizing monoclonal antibodies (mAb) against recombinant human IFN-(beta)ser (rHuIFN-(beta)ser). In this assay, the initial antigen-antibody reaction occurs in solution under more physiologic conditions than in a standard solid-phase ELISA. Subsequently, the reaction mixture is applied to a membrane that is exposed to a second, peroxidase-labeled mAb, chemiluminescent reagents are added, and the membrane is photographically recorded. Competitive inhibition of binding of a second, labeled mAb by the first mAb decreases the signal detected. Two well-characterized mAb A1 and A7, have been shown to recognize distinct epitopes on rHuIFN-(beta)ser and to neutralize its antiviral and antiproliferative activity (Proc. Natl. Acad. Sci. USA 88, 4040-4044, 1991). In conventional solid-phase ELISA, mAb A1 does not inhibit the binding of A7 to rHuIFN-(beta)ser, but we observed partial inhibition in the new liquid-phase assay. In contrast, A7 did not inhibit the binding of A1, consistent with the solid-phase ELISA results. This observation suggests that in the LP-CEI-ELISA, A1 and A7 may recognize epitopes differently than in solid-phase assays. Thus, the LP-CEI-ELISA, which is simple, sensitive, and quantifiable, appears also to be able to detect subtle, conformational differences of epitopes not evident in a standard solid-phase ELISA.

The crystal structure of human interferon beta at 2.2-A resolution

Type I interferons (IFNs) are helical cytokines that have diverse biological activities despite the fact that they appear to interact with the same receptor system. To achieve a better understanding of the structural basis for the different activities of alpha and beta IFNs, we have determined the crystal structure of glycosylated human IFN-beta at 2.2-A resolution by molecular replacement. The molecule adopts a fold similar to that of the previously determined structures of murine IFN-beta and human IFN-alpha2b but displays several distinct structural features. Like human IFN-alpha2b, human IFN-beta contains a zinc-binding site at the interface of the two molecules in the asymmetric unit, raising the question of functional relevance for IFN-beta dimers. However, unlike the human IFN-alpha2b dimer, in which homologous surfaces form the interface, human IFN-beta dimerizes with contact surfaces from opposite sides of the molecule. The relevance of the structure to the effects of point mutations in IFN-beta at specific exposed residues is discussed. A potential role of ligand-ligand interactions in the conformational assembly of IFN receptor components is discussed.

Neutralising and binding anti-interferon-beta-I b (IFN-beta-I b) antibodies during IFN-beta-I b treatment of multiple sclerosis

Interferon-beta-I b (IFN-beta-I b) is an immunomodulatory therapy of multiple sclerosis (MS), reducing the numbers and severity of exacerbations and the total lesion load measured by magnetic resonance imaging of the brain. The benefits of IFN-beta-I b could be hampered by the development of neutralising antibodies against the compound. Our results confirmed earlier studies, showing that 42% of MS patients treated with IFN-beta-I b for more than 3 months had developed neutralising antibodies. The occurrence of binding anti-IFN-beta-I b antibodies, presently not believed to impede the clinical efficacy of IFN-beta-I b, were demonstrated by an immunoassay in some patients after 1 month of treatment and in 78% after 3 months. The development of binding antibodies seemed to be an early phenomenon, preceding the appearance of neutralising antibodies. Antibodies crossreacting with IFN-beta-I a and natural IFN-beta were also found in a majority of IFN-beta-I b treated patients with high titres of binding antibodies. Employing a solid-phase enzyme-linked immunospot (ELISPOT) assay, 68% of MS patients treated with IFN-beta-I b for 1-23 months had elevated numbers of anti-IFN-beta-I b-antibody secreting cells in blood, compared to 18% of untreated MS patients and 20% among patients with other neurological diseases. Thus, our findings confirm that IFN-beta-I b is immunogenic in MS patients. High levels of anti-IFN-beta-I b antibody secreting cells were, however, also found in two untreated control patients with inflammatory diseases, suggesting that anti-IFN-beta-I b antibodies might also occur spontaneously.

Structure-activity of type I interferons

Type I IFNs constitute a family of proteins exhibiting high homology in primary, secondary, and tertiary structures. They interact with the same receptor and transmit signals to cellular nucleus through a similar mechanism, eliciting roughly homogeneous biological activity. Nevertheless, the members of that family, IFN alpha species, IFN beta and IFN omega, due to local differences in the structure sometime show distinct properties. From the reported data it results that even minute changes or differences in the primary sequences could be responsible for a significant variety of biological actions, thus inducing to the hypothesis that Type I IFNs, rather than to be the result of a redundant replication during the evolution play definite roles in the defense of living organisms to foreign agents.

Three-dimensional model of a human interferon-alpha consensus sequence

A computer-built, three-dimensional, atomic-level model for human interferon-alpha (IFN-alpha) was constructed. This model was prepared using the primary amino acid sequence of consensus IFN-alpha (IFN-alpha Con1) and the alpha-carbon Cartesian coordinates of murine IFN-beta as a homolog guide to the model building. In agreement with an earlier report from this laboratory, the two domains 29-35 and 123-140 are in close spatial proximity in this model, and may constitute a receptor recognition domain, whereas the region bounded by residues 78-95 is somewhat removed from this region on the molecule and may constitute an alternative active site. Extrapolating from the model, we propose that, of the stretch 123-140, the residues that are exposed are 123, 125, 126, 128-130, and 132-139; and of the stretch 29-35, all are accessible. Additionally, we propose that there may be sufficient complexity in the Type 1 IFN receptor to account for the differential sensitivities between IFN-alpha s and IFN-beta that may be associated with residue differences in the region 78-95, specifically at residues 84, 86, and 87. This model conforms with experimental data that identify specific amino acid residues in human IFN-alpha that either do, or do not, affect the active conformation and biological activities of the molecule.

Antibodies that neutralize human beta interferon biologic activity recognize a linear epitope: analysis by synthetic peptide mapping

The location of biologically relevant epitopes on recombinant human beta interferon in which Ser-17 replaces Cys-17 (rh[Ser17]IFN-beta) was evaluated by testing the immunoreactivity of antibodies against 159 sequential, overlapping octamer peptides. Three monoclonal antibodies (mAbs) that neutralize rh[Ser17]IFN-beta biologic activity, designated A1, A5, and A7, bound to peptides spanning only residues 39-48, whereas nonneutralizing mAb bound less specifically at multiple sites near the amino terminus. The immunoreactivity of peptides spanning residues 40-47 that contained a series of single amino acid substitutions suggested that residues 41-43 (Pro-Glu-Glu) and 46 (Gln) are important for the binding of neutralizing mAbs. The reactivity of mAbs to larger synthetic peptides containing rh[Ser17]IFN-beta sequences from residue 32 through residue 56 was evaluated. All mAbs except A7 reacted with synthetic peptides representing rh[Ser17]IFN-beta residues 32-47, 40-56, and 32-56, but only mAbs A1 and A5 bound to the core peptide composed of residues 40-47. Peptide 32-56 effectively blocked the binding of mAbs A1 and A5 to rh[Ser17]IFN-beta and markedly inhibited their neutralizing activity. Biologic activity of the peptides was undetectable. Rabbit antisera raised against peptides 32-47 and 40-56 recognized rh[Ser17]IFN-beta but did not neutralize its antiviral activity. Thus, structure-function analysis by peptide mapping has permitted the identification of a linear epitope recognized by neutralizing antibody on a biologically active cytokine. We conclude that the region spanning residues 32-56 is of major importance in the expression of the biologic activity of human IFN-beta.

Immunochemical characterization of antigenic domains on human interferon-beta: spatially distinct epitopes are associated with both antiviral and antiproliferative activities

The use of a panel of monoclonal antibodies (mAb) raised against recombinant (serine-17) human interferon-beta (rHuIFN-beta ser) has permitted the identification of three epitopes on HuIFN-beta, designated as sites I, II and III, based solely on functional differences, i.e., the neutralization of antiviral and antiproliferative activities of natural and recombinant HuIFN-beta (Redlich, P.N. and Grossberg, S. E., J. Immunol. 1989. 143: 1887). Site I- and II-directed mAb possessed neutralizing activity whereas none was noted by mAb recognizing site III. To characterize further these epitopes by immunochemical means, we studied their (a) spatial relationship by competitive binding assays, (b) antigenic structure by Western blotting, and (c) sensitivity to chemical modification by the measurement of mAb reactivity after radioiodination. Competitive antibody binding studies revealed site II to be spatially distinct from sites I and III. Furthermore, site I- and II-directed mAb could easily recognize rHuIFN-beta ser on a Western blot, suggesting that both these epitopes are primarily sequential in structure or denaturation resistant. Chemical modification by radioiodination, which did not alter the biologic activity of rHuIFN-beta ser, had likewise little effect on mAb reactivity to site I; however, reactivity to site II was diminished and reactivity to site III was minimal following the radioiodination process. Both site I- and II-directed mAb inhibited the binding of 125I-rHuIFN-beta ser to intact Daudi cells, suggesting that inhibition of receptor binding is their mechanism of neutralization. Thus, we conclude that epitopes I and II, which are associated with both antiviral and antiproliferative activities of rHuIFN-beta, are spatially and immunochemically distinct.

Structure-function studies of interferon-alpha based on random mutagenesis and expression in vitro

An efficient procedure for random chemical mutagenesis was used to create analogs of human interferon (IFN)-alpha 4. Unique restriction enzyme sites were introduced into the human IFN-alpha 4 gene to enable cassetting of the gene for localized random mutagenesis. Single-stranded IFN-alpha 4 DNA was treated with nitrous acid, followed by second-strand synthesis using reverse transcriptase. A 72 base pair cassette spanning the coding region for amino acid residues 120 to 136 (120-136 region) was isolated and cloned into a phagemid vector adjacent to a GC-rich sequence. A DNA segment comprising the IFN-alpha 4 cassette sequence and the GC clamp was excised and electrophoresed on a denaturing gradient gel, which allowed the separation from unmutated DNA of DNA fragments with single base pair changes. DNA fragments with mobility different from that of the unmutated fragment were pooled and cloned into an expression vector. Using this procedure, mutations were found in the DNA of 48% of the clones analyzed. However, mutations at two "hot spots" accounted for 89% of these clones. Four of the IFN-alpha 4 analogs with mutations in the 120-136 region were expressed in vitro. The antiproliferative activities on human Daudi cells of most of the analogs were less than 0.2% of the activity of unmodified IFN-alpha 4, suggesting that the integrity of the carboxy terminus is important for the antiproliferative activity of human IFN-alpha 4.