Molecular characterization of human monoclonal antibodies specific for several HIV proteins: analysis of the VH3 family expression

Plant-derived chimeric antibodies inhibit the invasion of human fibroblasts by Toxoplasma gondii

The parasite Toxoplasma gondii causes an opportunistic infection, that is, particularly severe in immunocompromised patients, infants, and neonates. Current antiparasitic drugs are teratogenic and cause hypersensitivity-based toxic side effects especially during prolonged treatment. Furthermore, the recent emergence of drug-resistant toxoplasmosis has reduced the therapeutic impact of such drugs. In an effort to develop recombinant antibodies as a therapeutic alternative, a panel of affinity-matured, T. gondii tachyzoite-specific single-chain variable fragment (scFv) antibodies was selected by phage display and bioinformatic analysis. Further affinity optimization was attempted by introducing point mutations at hotspots within light chain complementarity-determining region 2. This strategy yielded four mutated scFv sequences and a parental scFv that were used to produce five mouse-human chimeric IgGs in Nicotiana benthamiana plants, with yields of 33-72 mg/kg of plant tissue. Immunological analysis confirmed the specific binding of these plant-derived antibodies to T. gondii tachyzoites, and in vitro efficacy was demonstrated by their ability to inhibit the invasion of human fibroblasts and impair parasite infectivity. These novel recombinant antibodies could therefore be suitable for the development of plant-derived immunotherapeutic interventions against toxoplasmosis.

A novel strategy for efficient production of anti-V3 human scFvs against HIV-1 clade C

Background

Production of human monoclonal antibodies that exhibit broadly neutralizing activity is needed for preventing HIV-1 infection, however only a few such antibodies have been generated till date. Isolation of antibodies by the hybridoma technology is a cumbersome process with fewer yields. Further, the loss of unstable or slowly growing clones which may have unique binding specificities often occurs during cloning and propagation and the strongly positive clones are often lost. This has been avoided by the process described in this paper, wherein, by combining the strategy of EBV transformation and recombinant DNA technology, we constructed human single chain variable fragments (scFvs) against the third variable region (V3) of the clade C HIV-1 envelope.

Results

An antigen specific phage library of 7000 clones was constructed from the enriched V3- positive antibody secreting EBV transformed cells. By ligation of the digested scFv DNA into phagemid vector and bio panning against the HIV-1 consensus C and B V3 peptides followed by random selection of 40 clones, we identified 15 clones that showed V3 reactivity in phage ELISA. DNA fingerprinting analysis and sequencing showed that 13 out of the 15 clones were distinct. Expression of the positive clones was tested by SDS-PAGE and Western blot. All the 13 anti-V3 scFvs showed cross-reactivity against both the clade C and B V3 peptides and did not show any reactivity against other unrelated peptides in ELISA. Preliminary neutralization assays indicated varying degrees of neutralization of clade C and B viruses. EBV transformation, followed by antigen selection of lines to identify specific binders, enabled the selection of phage from un-cloned lines for scFv generation, thus avoiding the problems of hybridoma technology. Moreover, as the clones were pretested for antigen binding, a comparatively small library sufficed for the selection of a considerable number of unique antigen binding phage. After selection, the phage clones were propagated in a clonal manner.

Conclusions

This strategy can be efficiently used and is cost effective for the generation of diverse recombinant antibodies. This is the first study to generate anti-V3 scFvs against HIV-1 Clade C.

Different pattern of immunoglobulin gene usage by HIV-1 compared to non-HIV-1 antibodies derived from the same infected subject

A biased usage of immunoglobulin (Ig) genes is observed in human anti-HIV-1 monoclonal antibodies (mAbs) resulting probably from compensation to reduced usage of the VH3 family genes, while the other alternative suggests that this bias usage is due to antigen requirements. If the antigen structure is responsible for the preferential usage of particular Ig genes, it may have certain implications for HIV vaccine development by the targeting of particular Ig gene-encoded B cell receptors to induce neutralizing anti-HIV-1 antibodies. To address this issue, we have produced HIV-1 specific and non-HIV-1 mAbs from an infected individual and analyzed the Ig gene usage. Green-fluorescence labeled virus-like particles (VLP) expressing HIV-1 envelope (Env) proteins of JRFL and BaL and control VLPs (without Env) were used to select single B cells for the production of 68 recombinant mAbs. Ten of these mAbs were HIV-1 Env specific with neutralizing activity against V3 and the CD4 binding site, as well as non-neutralizing mAbs to gp41. The remaining 58 mAbs were non-HIV-1 Env mAbs with undefined specificities. Analysis revealed that biased usage of Ig genes was restricted only to anti-HIV-1 but not to non-HIV-1 mAbs. The VH1 family genes were dominantly used, followed by VH3, VH4, and VH5 among anti-HIV-1 mAbs, while non-HIV-1 specific mAbs preferentially used VH3 family genes, followed by VH4, VH1 and VH5 families in a pattern identical to Abs derived from healthy individuals. This observation suggests that the biased usage of Ig genes by anti-HIV-1 mAbs is driven by structural requirements of the virus antigens rather than by compensation to any depletion of VH3 B cells due to autoreactive mechanisms, according to the gp120 superantigen hypothesis.

Preferential use of the VH5-51 gene segment by the human immune response to code for antibodies against the V3 domain of HIV-1

Human anti-V3 monoclonal antibodies (mAbs) generated from HIV-1 infected individuals display diversity in the range of their cross-neutralization that may be related to their immunogenetic background. The study of the immunoglobulin (Ig) variable region gene usage of heavy chains have shown a preferential usage of the VH5-51 gene segment which was detected in 35% of 51 human anti-V3 mAbs. In contrast, human mAbs against other envelope regions of HIV-1 (anti-Env), including the CD4-binding domain, the CD4-induced epitope, and gp41 preferentially used the VH1-69 gene segment, and none of them used the VH5-51 gene. Furthermore, the usage of the VH4 family by anti-V3 mAbs was restricted to only one gene segment, VH4-59, while the VH3 gene family was used at a significantly lower frequency by all of the analyzed anti-HIV-1 mAbs. Multivariate analysis showed that usage of VH gene segments was significantly different between anti-V3 and anti-Env mAbs, and compared to antibodies from healthy subjects. In addition, the anti-V3 mAbs preferentially used the JH3 and D2-15 gene segments. The preferential usage of selected Ig gene segments and the characteristic pattern of Ig gene usage by anti-V3 mAbs can be related to the conserved structure of the V3 region.

Direct impact of inactivated HIV-1 virions on B lymphocyte subsets

Although there is no convincing evidence that HIV infects primary B cells, marked changes in B cell responses have been described in HIV-1-infected subjects, including B cell repertoire perturbations, depression of B cell memory and paucity of CD5(+) B cells. As it is hard to assess the consequences of these in vitro and ex vivo observations in patients, the pathogenic mechanisms responsible for the B cell deficit are unclear, and direct and indirect effects of HIV-1 remain possible. To gain further insight into the impact of HIV-1 on the B cell compartment in vivo, we used XenoMouse mice, mice genetically engineered to express human antibodies with an absence of mouse antibody expression. In these transgenic animals, B cells expressing a virtually full human Ig repertoire develop, which allows investigation of the in vivo consequences of confronting B cells expressing human immunoglobulins with HIV-1. We found that soluble gp120 induced an inversion in the B-1a/B-1b cell ratios, without impacting B-2 cells or affecting substantially the T cell compartment. Virion treatment specifically and dramatically depressed B-1a cells, which represent the majority of B-1 cells in normal mice. The observed B cell changes were associated with a functional alteration of the humoral response to tetanus toxoid. Thus, the results reveal a capacity of HIV-1 to specifically impact a highly specialized B cell subpopulation. Because there is evidence that human IgM memory B cells are functionally equivalent to murine B-1a cells, our findings suggest that gp120 may have a direct deleting activity on B cell memory.

Molecular determinants of the human antibody response to HIV-1: implications for disease control

Various aspects of the immune response to HIV-1 infection remain unclear. While seropositive subjects generally mount a strong humoral response, the antibodies produced are not effective in halting disease progression. Molecular characterization of the antibody repertoire specific for HIV-1 antigens represents an approach to further our understanding of the mechanisms involved in mounting a humoral immunity in this infection. Recently, the content, structure, and organization of the human immunoglobulin-variable gene loci have been elucidated and a number of laboratories have characterized the variable gene elements of human anti-HIV-1 antibodies derived from infected persons by cell fusion or by Epstein-Barr virus transformation. The results show evidence for extensive somatic mutations that lead to preferential amino acid substitutions in the hypervariable regions, an indication of an antigen-driven process. Multiple other molecular events also are engaged in generating antibody diversity, including various types of fusions of variable genes, usage of inverted diversity genes, and addition of extragenomic nucleotides. Most importantly, there is a paucity of antibodies expressing the major V(H)3 gene family, which could result from the capacity of gp120 to act as superantigen for human B cells. This V(H)3+ antibody deficit also has been observed in B cells isolated ex vivo from the patients. Since V(H)3+ antibodies play an essential role in immune defense against infections, the abnormalities observed in HIV-1 infection may predispose to opportunistic infections and further compromise the immune defense mechanisms of the subjects.

Secretory IgA specific for a conserved epitope on gp41 envelope glycoprotein inhibits epithelial transcytosis of HIV-1

As one of the initial mucosal transmission pathways of HIV (HIV-1), epithelial cells translocate HIV-1 from apical to basolateral surface by nondegradative transcytosis. Transcytosis is initiated when HIV-1 envelope glycoproteins bind to the epithelial cell membrane. Here we show that the transmembrane gp41 subunit of the viral envelope binds to the epithelial glycosphingolipid galactosyl ceramide (Gal Cer), an alternative receptor for HIV-1, at a site involving the conserved ELDKWA epitope. Disrupting the raft organization of the Gal Cer-containing microdomains at the apical surface inhibited HIV-1 transcytosis. Immunological studies confirmed the critical role of the conserved ELDKWA hexapeptide in HIV-1 transcytosis. Mucosal IgA, but not IgG, from seropositive subjects targeted the conserved peptide, neutralized gp41 binding to Gal Cer, and blocked HIV-1 transcytosis. These results underscore the important role of secretory IgA in designing strategies for mucosal protection against HIV-1 infection.

Design and intracellular activity of a human single-chain antibody to human immunodeficiency virus type 1 conserved gp41 epitope

A human lymphoid cell line (F172-D8) excreting a human immunodeficiency virus type 1 (HIV-1) anti-gp41 monoclonal antibody was used to construct a plasmid containing the cDNA of the single-chain variable fragment (scFvD8) corresponding to this antibody. A stable human osteosarcoma cell line was obtained which expressed the scFvD8 protein in the cytoplasm. Whereas a cell line transfected with a control construct (pCI-neo) was readily and productively infected with laboratory (Ba-L) or primary HIV-1 isolates, the scFvD8 cell line did not support productive infection. Binding of the virus, internalization, and reverse transcription were not altered by scFvD8 expression, but gp160 expression was dramatically reduced. These data suggest that cytoplasmic expression of this artificial single-chain antibody can interfere with gp160 expression, thereby reducing the production of mature viral envelope proteins.

Mapping the B cell superantigen binding site for HIV-1 gp120 on a V(H)3 Ig

The emerging class of B cell superantigens includes HIV-1 gp120, which binds to many members of the V(H)3 Ig gene family. The present study addresses the structural features of V(H)3 antibodies conferring gp120 binding activity using a panel of recombinant full-length and Fab Ig proteins. Binding activity was fully conferred by the Fab portion of the Ig molecule. The V(H) region was the major determinant of binding; diverse light chains were permissive for gp120 binding. A series of recombinant V(H)3-V(H)1 chimeric molecules was created to analyze the contribution of different subregions of V(H)3 to gp120 binding. Hypervariable loop 1 (H1) substitution alone caused a 10-fold reduction in binding activity. The framework subregions (FR1, FR2 and FR3) and H2 also influenced binding, since substitutions of various combinations of these subregions conferred 10- to 100-fold binding reductions. We conclude that gp120 binding occurs through a non-conventional interaction involving multiple discontinuously arrayed residues spanning the V(H), and including roles in gp120 contact and favorable conformation of the V(H).

Selective deficit in antibodies specific for the superantigen binding site of gp120 in HIV infection

HIV infection is characterized by accelerated apoptosis and progressive loss of B cells. To see whether these abnormalities are related to the property of gp120 to act as a superantigen for VH3(+) B cells, we probed the temporal development of VH3(+) antibodies in HIV-1-infected subjects over a 7-year period. We found that VH3(+) antibodies specific for the gp120 superantigen binding site are deficient. Since VH3(+) antibodies impart protective responses to infectious agents, we quantified VH3(+) antibodies in serum samples from HIV-seropositive slow progressors and from patients who progressed to AIDS-related manifestations. We found that paucity in VH3(+) antibodies is a marker of rapid clinical decline. Remarkably, anti-gp160 VH3(+) antibodies showed a gradual decrease in progressors and, with time, varied depending on the viral load. We conclude that disease aggravation is associated with a decrease of the magnitude of the humoral response, that VH3(+) antibodies play an important role in protection, and that their underexpression may accelerate disease progression. We propose that vaccine preparations able to trigger VH3(+) antibodies might confer a better protection against HIV infection. This work also represents a novel mechanism of humoral deficiency resulting from the capacity of a viral antigen to affect an important subset of the B cell repertoire and to induce B cell death by apoptosis.

Molecular characterization of five neutralizing anti-HIV type 1 antibodies: identification of nonconventional D segments in the human monoclonal antibodies 2G12 and 2F5

We have stabilized a panel of 33 hybridomas producing human monoclonal antibodies (MAbs) against HIV-1 gp160 and p24. Five of these antibodies were able to neutralize different HIV-1 isolates, and two of them (2F5 and 2G12) revealed remarkable potential to neutralize primary virus isolates of different clades in several in vitro tests. To determine whether a structural basis for neutralization could be identified, we analyzed the antibodies at the molecular level. This study reports the primary nucleotide and deduced amino acid sequences of the rearranged heavy and light chain V segments (VH, Vkappa) of the neutralizing MAbs (1B1, 1F7, 2F5, 2G12, and 3D5) and the nonneutralizing anti-gp41 MAb 3D6. Aligning the V segments with the nearest related germline genes illustrated the occurrence of somatic mutations. The neutralizing MAbs show mutational rates comparable to those of antibodies that appear in patients in whom the immune system is under constant antigenic pressure over a long period of time. In contrast, 3D6, which recognizes the immunodominant region on gp41, displays homologies as high as 97 and 98% compared with its VH and Vkappa germline genes. The diversity segments [D(H)] of 1B1, 1F7, 3D5, and 3D6 were assigned to single D(H) segments on the chromosomal D(H) locus. 2F5 presents a D(H) segment 52 nucleotides in length, which could be explained by fusion of two segments on the immunoglobulin heavy chain locus that have not yet been described as rearranged regions. 2G12 D(H) shows best homologies to a D(H) segment between D3-22 and D4-23. This D(H) segment could be the reason for the rare occurrence of antibodies competing with 2G12. Since this nearest related chromosomal region on the D(H) locus does not display recombination signals at the flanking regions, this segment is normally not taken into consideration as a site for immunoglobulin heavy chain rearrangement.

Characteristics of human antibody repertoires following active immune responses in vivo

Possibilities to develop human monoclonal antibody specificities have recently been much facilitated by improvements of human hybridoma technology but even more so by the emerging phage-display technique. However, until recently very little has been known about the characteristics at the molecular level of the induced, T cell-dependent human antibody response, frequently targeted by these techniques. Rather, the major part of available sequence information has been related to tumor-derived or autoreactive antibodies. We have now investigated high affinity, monospecific, human antibody repertoires as developed by hybridoma technology. The VH region gene usage among such in vivo-induced repertoires is in only some respects similar to that found in the total B cell population. A limited number of heavy-chain variable segment loci account for the majority of all induced antibodies. A particular VH gene locus (4-34) frequently employed by peripheral B cells and associated with autoreactive antibodies was rarely used by the induced repertoire. Furthermore, in particular antigen systems, V region usage differs from the total available repertoire, and heavy-chain CDR3 is generally longer among antibodies induced against foreign protein antigens than in the average B cell population. Light-chain gene usage is often restricted to just a few dominant genes frequently found among B cells in general. In comparison, variable regions derived by phage-display technology in some antigen systems display even longer heavy-chain CDR3 than hybridoma-derived antibodies. This technique also appears to select a different set of germline genes preferentially (both with respect to VH and JH) as compared to hybridoma technology. In summary, the T cell-dependent antibody response against foreign antigens appears to differ from the average circulating B cell in several ways, and thus does not seem to represent a random selection of the available repertoire.