Virus-binding receptors: similarities to immune receptors as determined by anti-idiotypic antibodies

Development of biologically active peptides based on antibody structure

Antibody molecules are composed of several functional domains, including a variable domain that contacts antigen and a constant domain. The hypervariable regions of antibody molecules play an integral role in determining their specificity. However, the delineation of specific residues most critical in binding is difficult. We have been studying a monoclonal antibody (87.92.6) that binds to the reovirus type 3 receptor on a number of cell types, down-modulates the receptor, and inhibits DNA synthesis in the cells. We have shown that a peptide derived from the second complementarity-determining region of the monoclonal antibody 87.92.6 light-chain variable region can reproduce both of these effects. We were also able to demonstrate specific amino acid residues and structural features involved in producing these effects. The study of antibody structure, coupled with molecular synthetic techniques, can lead to the development of biologically active substances with potential clinical use.

Reovirus type 3 and [125I]-iodocyanopindolol bind to distinct domains on the beta-adrenergic like receptor

Antireceptor antibodies have been developed as a probe to study the cellular receptor for reovirus type 3. Using this probe, a glycoprotein with a molecular weight of 65-67 kilodaltons and a pI of 5.8-6.0 was isolated and identified as the reovirus receptor. This protein was also structurally similar to the affinity-purified beta-adrenergic receptor from calf lung. In this report, we employ [125I]-iodocyanopindolol, a high affinity beta-adrenergic antagonist, to further characterize this protein. We show that R1.1, a murine thymoma cell line, possesses about 2,000 receptors per cell with high affinity for ICYP (kD = 3.3 X 10(-11) M). Competitive inhibition studies suggest that the receptor is of the beta-2 subtype. Solubilized receptor proteins from R1.1 cells bound to the antireceptor antibody were further purified by SDS-PAGE and electroelution from the gel. Five percent of the proteins thus obtained could bind ICYP with high affinity (kD = 1.6 X 10(-10) M). This suggests that the purification procedure produced a collection of forms of this 65- to 67-kilodalton protein, some of which retained the conformation for binding the beta ligands. We also demonstrate that the isolated receptor protein was able to bind ICYP even when the virus binding site was occupied by the anti-idiotype, suggesting that reovirus type 3 and the beta ligands bind to distinct domains on the receptor protein.

Nucleic acid sequence of an internal image-bearing monoclonal anti-idiotype and its comparison to the sequence of the external antigen

The monoclonal anti-idiotypic antibody (mAb2) 87.92.6 directed against the 9B.G5 antibody specific for the virus neutralizing epitope on the mammalian reovirus type 3 hemagglutinin was previously demonstrated to express an internal image of the receptor binding epitope of the reovirus type 3. Furthermore, this mAb2 has autoimmune reactivity to the cell surface receptor of the reovirus. The nucleotide and deduced amino acid sequences of the 87.92.6 mAb2 heavy and light chains are described in this report. The sequence analysis reveals that the same heavy chain variable and joining (VH and JH) gene segments are used by the 87.92.6 anti-idiotypic mAb2 and by the dominant idiotypes of the BALB/c anti-GAT (cGAT) and anti-NP (NPa) responses. [GAT; random polymer that is 60% glutamic acid, 30% alanine, and 10% tyrosine. NP; (4-hydroxy-3-nitrophenyl)-acetyl.] Despite extensive homology at the level of the heavy chain variable regions, the NPa positive BALB/c anti-NP monoclonal antibody 17.2.25 binds neither 9B.G5 nor the cellular receptor for the hemagglutinin. Amino acid sequence comparison between the viral hemagglutinin and the 87.92.6 mAb2 light chain "internal image," reveals an area of significant homology indicating that antigen mimicry by antibodies may be achieved by sharing primary structure.

Site-specific alteration of murine hepatitis virus type 4 peplomer glycoprotein E2 results in reduced neurovirulence

Strains of the murine coronavirus mouse hepatitis virus type 4 (MHV-4) which contained a mutation in the E2 peplomer glycoprotein were obtained by selection for resistance to neutralization by monoclonal antibodies. Characterization of six variants representing two independent epitopes on E2, E2B and E2C, by in vitro neutralization and antibody-binding assays demonstrated that selection for an alteration in epitope E2B also resulted in changes in epitope E2C and vice versa. We observed a mutation frequency of approximately 10(-4.3) to 10(-4.6), which is consistent with the expected occurrence of single point mutations. The variant virus strains were attenuated with respect to neurovirulence when compared with wild-type MHV-4. Mice normally develop encephalomyelitis and die after wild-type MHV-4 infection. Mice receiving 2- to 3-log-higher doses of the variant strains survived and developed demyelinating disease. As the disease progressed, evidence of remyelination and ongoing demyelination was observed up to 65 days after infection. Virus reisolated 15 days after infection retained the variant phenotype. The data indicate that the E2 glycoprotein plays a central role in determining the cellular tropism and virulence of MHV-4 in the mouse.

Structural similarities between the mammalian beta-adrenergic and reovirus type 3 receptors

The mechanism by which viruses bind to and infect specific tissues to cause disease has only recently begun to be understood. The mammalian reoviruses provide an especially attractive model for studying the details of cell surface recognition. The cell and tissue tropism of reovirus is determined by a portion of the viral hemagglutinin termed the neutralization domain. We have reported previously on the generation of both monoclonal and polyclonal anti-idiotypic antibodies that mimic the viral hemagglutinin in the specificity of binding to the reovirus receptor. By using these anti-idiotypic antibodies as specific probes, we have successfully isolated the mammalian reovirus receptor from neuronal and lymphoid cells. In the present study, we report that the reovirus receptor is structurally similar to the mammalian beta-adrenergic receptor. This conclusion is based on the following observations: (i) purified beta-adrenergic receptor is immunoprecipitable by anti-reovirus receptor antibody; (ii) purified reovirus receptor obtained from murine thymoma cells and beta-adrenergic receptor obtained from calf lung exhibit identical molecular masses and isoelectric points; (iii) trypsin digests of purified reovirus and beta-adrenergic receptors display indistinguishable fragment patterns; (iv) purified reovirus receptor binds the beta-antagonist [125I]iodohydroxybenzylpindolol and this binding is blocked by the beta-agonist isoproterenol.

Isolation and biochemical characterization of the mammalian reovirus type 3 cell-surface receptor

A cell-surface receptor for the mammalian reovirus type 3 hemagglutinin was isolated by using antiidiotypic anti-receptor antibodies. The receptor is a glycoprotein with a molecular mass of 67,000 daltons and a pI of 5.9. Evidence that the isolated structure represents the reovirus receptor was obtained by electrophoretic immunoblot studies, which demonstrated that the 67,000-dalton glycoprotein is the only cell-surface structure recognized by both reovirus type 3 and the anti-receptor immunoglobulin. Comparison of the reovirus receptor on murine thymoma (R1.1) and rat neuroblastoma (B104) cells indicated that similar structures on the cell surface are recognized by the reovirus type 3 and the anti-receptor antibodies as previously suggested from cellular and binding studies. This receptor was found on mouse, rat, monkey, and human cells. Furthermore, diverse tissue types, including lymphoid and neuronal cells, express the receptor structure. The receptor structure is discussed in terms of its role in mediating viral tropism and as an essential cell-surface protein.

Sendai virus-specific T-cell clones: induction of cytolytic T cells by an anti-idiotypic antibody directed against a helper T-cell clone

We used a monoclonal anti-idiotypic antibody that is directed against a Sendai virus-specific helper T-cell clone to induce an anti-viral immune response in vivo. Splenocytes primed with the anti-idiotypic antibody mediated an antigen-specific cytolytic response. Preimmunization of mice with the anti-idiotypic antibody resulted in protection against a subsequent lethal infection with Sendai virus.

Identification of idiotypic receptors on reovirus-specific cytolytic T cells

Cytolytic T lymphocytes (Tc) specific for cells infected with reovirus type 3 were shown to lyse an uninfected B-cell hybridoma line (designated 87.92.6). This hybridoma expresses and secretes an anti-idiotypic antibody that reacts with a monoclonal antibody (termed G-5). G-5 recognizes a domain on the hemagglutinin of the reovirus that is relevant to virus tropism. The Tc cell response was H-2 restricted and could be inhibited by G-5. As shown by limiting dilution analysis, identical clones lysed reovirus-infected and anti-idiotype-bearing target cells. Tc cells induced by a variant of wild-type reovirus type 3 (immunologically selected by resistance to neutralization by G-5) were unable to recognize the anti-idiotype-bearing cells although they lysed reovirus type 3-infected tumor cells. We conclude that Tc cells must bear determinants that bind anti-idiotype molecules and, furthermore, that B cells and cytolytic T cells can share these idiotypic determinants.