Structural characterization of a cross-reactive idiotype shared by monoclonal antibodies specific for the human CD4 molecule

Cyanovirin-N, a potent human immunodeficiency virus-inactivating protein, blocks both CD4-dependent and CD4-independent binding of soluble gp120 (sgp120) to target cells, inhibits sCD4-induced binding of sgp120 to cell-associated CXCR4, and dissociates bound sgp120 from target cells

Cyanovirin-N (CV-N), an 11-kDa protein originally isolated from the cyanobacterium Nostoc ellipsosporum, potently inactivates diverse strains of human immunodeficiency virus type 1 (HIV-1), HIV-2, simian immunodeficiency virus, and feline immunodeficiency virus. It has been well established that the HIV surface envelope glycoprotein gp120 is a molecular target of CV-N. We recently reported that CV-N impaired the binding of virion-associated gp120 to cell-associated CD4 and that CV-N preferentially inhibited binding of the glycosylation-dependent neutralizing monoclonal antibody 2G12 to gp120. However, CV-N did not interfere with the interactions of soluble CD4 (sCD4) with either soluble gp120 (sgp120) or virion-associated gp120. In the present study, we have evaluated the effects of CV-N on the binding of sgp120 to cell-associated CD4 to clarify the experimental basis of the previous binding results, and we further address the detailed mechanism of action of CV-N. Here we present evidence that (i) CV-N impairs both CD4-dependent and CD4-independent binding of sgp120 to the target cells, (ii) CV-N blocks the sCD4-induced binding of sgp120 with cell-associated coreceptor CXCR4, and (iii) CV-N dissociates bound sgp120 from target cells. The results illustrate that the measured effects of CV-N on gp120-CD4 binding interactions depend upon the type of CD4 (soluble or cell associated), but not upon the type of gp120 (soluble or virion associated), employed in the experimental protocol. In addition, this study reinforces that CV-N acts uniquely to prevent essential interactions between the envelope glycoprotein and target cell receptors and further supports the potential broad utility of CV-N as a microbicide to prevent the transmission of HIV and AIDS.

Idiotype cascades associated with the CD4-HIV glycoprotein 120 interaction: immunization with anti-idiotypic antibodies induces anti-anti-idiotypic responses with anti-CD4 specificity and in vitro neutralizing activity

Anti-idiotypic antibodies (Ab-2), which were generated in baboons against a mouse monoclonal antibody specific for the CD4 molecule expressed on human T cells, were used to produce anti-anti-idiotypic antibodies (Ab-3) in mice. This response induced by Ab-2 immunization of BALB/c mice was classified as anti-anti-idiotype (Ab-3) on the basis of the ability of the mouse Ab-3 to (1) specifically bind the baboon Ab-2 preparation, but not irrelevant baboon IgG preparations, (2) inhibit the binding of the anti-CD4 Ab-1 preparation to the baboon Ab-2, and (3) recognize a second baboon Ab-2, along with a rabbit Ab-2 specific for the monoclonal anti-CD4 Ab-1 preparation. The murine Ab-3 response also recognizes the CD4 molecule expressed on a human CD4+ T cell line, as determined by flow cytometry; recognizes the same epitopes on the CD4 molecule as the Ab-1; inhibits HIV-1 syncytium formation; and neutralizes HIV-1 primary isolates in vitro. These studies suggest that Ab-3 responses can be induced by anti-Id immunization, which serologically mimicks the antigen and Id specificities of the monoclonal anti-CD4 preparation used to generate the anti-Id. Thus, the Ab-3 response exhibits the characteristics of a population that represents the internal image of the Ab-1.

Cyanovirin-N binds to gp120 to interfere with CD4-dependent human immunodeficiency virus type 1 virion binding, fusion, and infectivity but does not affect the CD4 binding site on gp120 or soluble CD4-induced conformational changes in gp120

Cyanovirin-N (CV-N), an 11-kDa protein isolated from the cyanobacterium Nostoc ellipsosporum, potently inactivates diverse strains of human immunodeficiency virus type 1 (HIV-1), HIV-2, and simian immunodeficiency virus. While it has been well established that the viral surface envelope glycoprotein gp120 is a molecular target of CV-N, the detailed mechanism of action is of further interest. We compared matched native and CV-N-treated virus preparations in a panel of assays that measure viral replication, assessing successive stages of the viral life cycle. CV-N-treated virions failed to infect cells as detected by p24 production and quantitative PCR for HIV-1 reverse transcription products, whereas treatment of the target cells did not block infection, confirming that CV-N acts at the level of the virus, not the target cell, to abort the initial infection process. Compared to native HIV-1 preparations, CV-N-treated HIV-1 virions showed impaired CD4-dependent binding to CD4(+) T cells and did not mediate "fusion from without" of CD4(+) target cells. CV-N also blocked HIV envelope glycoprotein Env-induced, CD4-dependent cell-cell fusion. Mapping studies with monoclonal antibodies (MAbs) to defined epitopes on the HIV-1 envelope glycoprotein indicated that CV-N binds to gp120 in a manner that does not occlude or alter the CD4 binding site or V3 loop or other domains on gp120 recognized by defined MAbs and does not interfere with soluble CD4-induced conformational changes in gp120. Binding of CV-N to soluble gp120 or virions inhibited subsequent binding of the unique neutralizing MAb 2G12, which recognizes a glycosylation-dependent epitope. However, prior binding of 2G12 MAb to gp120 did not block subsequent binding by CV-N. These results help clarify the mechanism of action of CV-N and suggest that the compound may act in part by preventing essential interactions between the envelope glycoprotein and target cell receptors. This proposed mechanism is consistent with the extensive activity profile of CV-N against numerous isolates of HIV-1 and other lentiviruses and supports the potential broad utility of this protein as a microbicide to prevent the sexual transmission of HIV.

Inactivation of human immunodeficiency virus type 1 infectivity with preservation of conformational and functional integrity of virion surface proteins

Whole inactivated viral particles have been successfully used as vaccines for some viruses, but procedures historically used for inactivation can denature virion proteins. Results have been inconsistent, with enhancement of disease rather than protection seen in some notable instances following vaccination. We used the compound 2,2'-dithiodipyridine (aldrithiol-2; AT-2) to covalently modify the essential zinc fingers in the nucleocapsid (NC) protein of human immunodeficiency virus type 1 (HIV-1) or simian immunodeficiency virus (SIV) virions, thereby inactivating infectivity. The inactivated virus was not detectably infectious in vitro (up to 5 log units of inactivation). However, in contrast to virions inactivated by conventional methods such as heat or formalin treatment, viral and host cell-derived proteins on virion surfaces retained conformational and functional integrity. Thus, immunoprecipitation of AT-2-treated virions was comparable to precipitation of matched untreated virus, even when using antibodies to conformational determinants on gp120. AT-2 inactivated virions bound to CD4(+) target cells and mediated virus-induced, CD4-dependent "fusion from without" comparably to native virions. However, viral entry assays demonstrated that the viral life cycle of AT-2-treated virions was arrested before initiation of reverse transcription. The major histocompatibility complex (MHC) class II molecules on the surface of AT-2-treated virions produced from MHC class II-expressing cells retained the ability to support class II-dependent, superantigen-triggered proliferative responses by resting T lymphocytes. These findings indicate that inactivation via this method results in elimination of infectivity with preservation of conformational and functional integrity of virion surface proteins, including both virally encoded determinants and proteins derived from the host cells in which the virus was produced. Such inactivated virions should provide a promising candidate vaccine antigen and a useful reagent for experimentally probing the postulated involvement of virion surface proteins in indirect mechanisms of HIV-1 pathogenesis.

Dependence of the murine antibody response to an anti-CDR2 VH peptide on immunogen formulation

A peptide corresponding to the second complementarity determining region of the heavy chain (CDR2 VH) from a murine anti-CD4 monoclonal antibody, designated L202, was synthesized by solid phase methodology in a number of different antigenic forms, for the purpose of comparing the effectiveness of different adjuvant-carrier systems in the induction of a murine antibody response against the immunizing peptide and parent antibody molecule. Two of the synthetic constructs contained the palmitoyl and N-palmitoyl-cysteinyl-S-(2,3-palmitoyloxy)-propanediol (PAM3Cys) moieties, respectively, attached to the peptide amino terminus with the immunogen comprising liposomal formulations of each. A third immunogen consisted of the CDR2 VH peptide admixed with the PAM3Cys non-covalently and incorporated into liposomes (PAM3Cys + CDR2 VH). A fourth composition comprised the CDR2 VH peptide conjugated to KLH via the sulfhydryl of an added N terminal cysteine (KLH-CDR2 VH) and injected with Complete Freund's adjuvant (CFA). A fifth immunogen consisted of the CDR2 VH peptide synthesized on an octameric, branched polylysine core as a multiple antigenic peptide (MAP-CDR2 VH) injected in the presence of Freund's adjuvant. Groups of five mice were injected intramuscularly with each of these immunogens and bled at two week intervals. The highest anti-peptide gamma-immunoglobulin (IgG) responses (against uncoupled peptide by ELISA) after 56 days were obtained with mice receiving the PAM3Cys-CDR2 VH peptide. However, when screened against the CDR2 V(H) peptide present as the MAP derivative by ELISA, IgG raised against the cognate MAP-CDR2 peptide was much more reactive than IgG raised against the liposomal PAM3Cys-CDR2 VH immunogen. In either case, IgG raised against the KLH-CDR2VH conjugate was poorly reactive. These differences in reactivity to the two forms of the CDR2 VH peptide by ELISA did not correspond to major differences in reactivities to the intact L202 Ab by ELISA. Although the IgG against the MAP immunogen was slightly more reactive than the other antisera against the l202 Ab, all titers were less than 1:100. These data illustrate some limitations of using anti-peptide responses as indicators of potential reactivity against the native protein, but suggest that alternate formulations including lipoidal peptides are more effective than corresponding KLH-peptide conjugates in eliciting Ab responses against poorly immunogenic epitopes.

Serologic crossreactions among primate immunoglobulins

We have generated and characterized 50 murine monoclonal antibodies (mAb) specific for baboon IgG. We examined crossreactivity of these mAb to baboon IgM and immunoglobulin (Ig) of various other primates including human, chimpanzee, rhesus monkey, cynomolgus monkey, and African green monkey. Those mAB that crossreacted with human IgG were further examined using myeloma proteins for specificity to human Ig subclasses. One mAB crossreacted with all four human IgG subclasses and with human IgM. We further analyzed this reactivity utilizing Bence Jones proteins representative of various light (L) chain germline gene family products. This mAB reacted with Bence Jones proteins indicating the recognition of a kappa (k) L chain specificity associated with the kappa I, kappa III, and kappa IV subgroups, but not with kappa II. Based on the differences between kappa II germ line gene encoded L chains and the other kappa L chain subgroups, we ascribe this reactivity to six amino acids that define a discontinuous epitope.

Idiotypic restriction of murine monoclonal antibodies to a defined antigenic region of human thyroglobulin

In previous studies, we demonstrated that anti-human thyroglobulin (hTg) autoantibodies in patients with thyroid disorders exhibit a restricted epitopic specificity towards antigenic region II defined by its reactivity with four murine monoclonal antibodies (mAb 3, 6, 10, 15). To analyze the relationships between epitopic specificity and idiotypic expression of these mAb, two polyclonal anti-idiotypic sera were generated in rabbits by immunization with F(ab')2 fragments of mAb 3 and mAb 10. These anti-idiotypic preparations (AI 3 and AI 10) were tested against a panel of hTg-mAb produced in different strains of mice (HR BIOZZI and BALB/c). The idiotypic analysis showed that AI 3 and AI 10 specifically recognized framework-associated idiotopes as well as paratope-associated idiotopes shared by region II mAb. These results demonstrate that specificity for region II was strongly associated with a restricted idiotype suggesting a high sequence homology between V regions. In addition, naïve BALB/c mice immunized with AI 3 or AI 10 produced anti-hTg (Ab3) antibodies that recognize region II epitopes. These latter findings reveal that anti-Id contain a population of Ab2 beta carrying the internal image of region II epitopes.

Humoral immune response in a marsupial Monodelphis domestica: anti-isotypic and anti-idiotypic responses detected by species-specific monoclonal anti-immunoglobulin reagents

We examined the humoral immune response of the laboratory opossum (Monodelphis domestica) as a model marsupial species. To evaluate antibody responses, IgM and IgG preparations were purified from the sera of naïve Monodelphis. These two immunoglobulin (Ig) preparations were used to generate specific murine monoclonal antibodies for use in ELISA-based serology. Individual Monodelphis were then immunized with a multideterminant protein antigen, a murine monoclonal antibody (Mab) IgG preparation designated Pab 405. In contrast with the primary IgM response of eutherian mammals, the primary response of Monodelphis to mouse IgG involved both IgG and IgM. The specificity of this anti-mouse IgG response appeared isotypic in nature, specifically, the immune sera recognized Ig determinants common to both Pab 405 and a control IgG Mab. Further, to evaluate the antibody responses to mouse IgG, immune sera were adsorbed against a control IgG Mab to remove the anti-isotypic reactivity. The adsorbed Monodelphis sera recognized idiotope specificities expressed on Pab 405. Based on an inhibition ELISA, the anti-idiotype (anti-Id) response recognized an idiotope on Pab 405 associated with its antigen combining site. These results demonstrate that Monodelphis respond to a multideterminant protein antigen such as murine IgG, similarly but not identically to eutherian mammals, and can serve as a useful marsupial model for additional comparative immunological studies.

Variable region gene selection of immunoglobulin G-expressing B cells with specificity for a defined epitope on type II collagen

Immunization with type II collagen (CII) induces collagen-induced arthritis (CIA) in animals, and B cells reactive with CII are involved in the induction and manifestation of the disease. In this study, B cell hybridomas producing IgG antibodies specific for a major epitope on mouse CII (the "C1" epitope, amino acid 316-333), were isolated 11 days after immunization from draining lymph nodes in DBA/1 mice. Injection into neonatal mice of purified and biotinylated monoclonal antibodies binding the C1 epitope led to a specific binding to joint cartilage, demonstrating that the antibodies interact with native antigen in vivo. cDNA sequencing of the B cell clones revealed that they all expressed the same combination of a variable heavy chain (VH J558 family) and light chain (V kappa 21 family) germ-line gene, apparently lacking somatic mutations. The presence of isotype-switched B cells expressing a certain combination of V genes encoding antibodies that bind epitopes in vivo, indicates that this B cell population has been peripherally selected.

Anti-peptide reagent identifies a primary-structure-dependent, cross-reactive idiotype expressed on heavy and light chains from a murine monoclonal anti-CD4

A synthetic peptide corresponding to the second complementarity determining region (CDR2) of the immunoglobulin (Ig) variable (V) region heavy (H) chain (CDR2VH) of anti-Leu3a, a murine monoclonal antibody specific for the human CD4 molecule, was used to elicit the production of specific rabbit anti-peptide antibodies. The rabbit anti-peptide antiserum was tested for reactivity against the immunizing peptide, anti-Leu3a, and a panel of mouse monoclonal anti-CD4. Only the immunizing peptide and anti-Leu3a were recognized by ELISA, whereas the H chains of anti-Leu3a and five other monoclonal anti-CD4 preparations were recognized by Western blot analysis. These data suggest that linear structures corresponding to the CDR2VH are not normally exposed on the surface of these monoclonal antibodies and become accessible only upon unfolding of the Ig molecule. In addition, Western blot analysis demonstrated that the anti-CDR2VH peptide antiserum was able to recognize the Ig light (L) chain of anti-Leu3a. This reactivity to both H and L chains from anti-Leu3a was ascribed to a homologous five amino acid sequence region shared by the two chains. The region of homology was associated with the third framework (FR3) of the L chain and was included as a portion of the sequence in the CDR2VH synthetic peptide. This observation was confirmed by the ability of the CDR2VH anti-peptide antiserum to bind the L chains of three mouse myeloma proteins that exhibited the five amino acid sequence region of homology within their respective FR3. Together, these data provide information on the structural basis of idiotypes shared by the H and L chains from the same antibody molecule and indicate that five amino acids might be sufficient to define a minimal continuous idiotypic determinant.