Molecular structure and granulocyte/macrophage colony-stimulating factor activity

Chimeric HIV-1 envelope glycoproteins with potent intrinsic granulocyte-macrophage colony-stimulating factor (GM-CSF) activity

HIV-1 acquisition can be prevented by broadly neutralizing antibodies (BrNAbs) that target the envelope glycoprotein complex (Env). An ideal vaccine should therefore be able to induce BrNAbs that can provide immunity over a prolonged period of time, but the low intrinsic immunogenicity of HIV-1 Env makes the elicitation of such BrNAbs challenging. Co-stimulatory molecules can increase the immunogenicity of Env and we have engineered a soluble chimeric Env trimer with an embedded granulocyte-macrophage colony-stimulating factor (GM-CSF) domain. This chimeric molecule induced enhanced B and helper T cell responses in mice compared to Env without GM-CSF. We studied whether we could optimize the activity of the embedded GM-CSF as well as the antigenic structure of the Env component of the chimeric molecule. We assessed the effect of truncating GM-CSF, removing glycosylation-sites in GM-CSF, and adjusting the linker length between GM-CSF and Env. One of our designed Env(GM-CSF) chimeras improved GM-CSF-dependent cell proliferation by 6-fold, reaching the same activity as soluble recombinant GM-CSF. In addition, we incorporated GM-CSF into a cleavable Env trimer and found that insertion of GM-CSF did not compromise Env cleavage, while Env cleavage did not compromise GM-CSF activity. Importantly, these optimized Env(GM-CSF) proteins were able to differentiate human monocytes into cells with a macrophage-like phenotype. Chimeric Env(GM-CSF) should be useful for improving humoral immunity against HIV-1 and these studies should inform the design of other chimeric proteins.

An immunodominant epitope in a functional domain near the N-terminus of human granulocyte-macrophage colony-stimulating factor identified by cross-reaction of synthetic peptides with neutralizing anti-protein and anti-peptide antibodies

We produced polyclonal and monoclonal antibodies (MAbs) against recombinant human (rh) granulocyte-macrophage colony-stimulating factor (GM-CSF) and performed studies of epitope mapping by ELISA, using five synthetic peptides corresponding to sequences along this molecule. Additionally, anti-peptide MAbs were generated. The antibody ability to inhibit rhGM-CSF activity was determined using as bioassay the MO7e cell line, which is dependent on hGM-CSF for growth in vitro. An immunodominant epitope able to induce the highest neutralization antibody titers was identified near the N terminus of hGM-CSF. A synthetic peptide 14-24, homologous to a sequence including part of the first alpha-helix of the molecule, was recognized by neutralizing anti-protein antibodies. Similarly, MAbs anti- 14-24 cross-reacted with rhGM-CSF and specifically blocked its function. Replacement of Val16 or Asn17 with alanine greatly reduced the antibody-binding capacity to peptide 14-24, whereas substitution of Gln20 or Glu21 was less critical. Monoclonal antibodies generated against residues 30-41 (corresponding to an intrahelical loop) and 79-91 (homologous to a sequence including part of the third alpha-helix) or its analog [Ala88](79-91)beta Ala-Cys, were conformation dependent and nonneutralizing: they failed to react or bound poorly to rhGM-CSF in ELISA, but readily recognized the homologous sequence in the denatured protein, by Western blotting.

Isolated VH4 heavy chain variable regions bind DNA characterization of a recombinant antibody heavy chain library derived from patient(s) with active SLE

In many autoimmune diseases autoantibodies are intimately involved in disease manifestations. Molecular characterization of these autoantibodies should provide insights into the pathogenesis of these diseases, as well as suggest novel avenues for development of therapeutics. While some prior studies suggest that DNA binding may be a characteristic of individual heavy chain variable regions, the ability of these V regions to bind DNA in isolation has not been investigated. We have utilized a bacterial vector for cloning and expressing isolated antibody heavy chain variable regions. RNA was extracted from peripheral blood mononuclear cells of patients with active SLE, cDNA synthesized and heavy chain V regions amplified with VH specific oligonucleotide primers. The VH fragments were cloned into a bacterial expression plasmid including the pelB leader peptide to direct appropriate expression. Recombinant antibodies were screened for binding to 32P-labeled double-stranded plasmid DNA and later also characterized for binding to single-stranded DNA. Binding was confirmed by standard ELISA methodology. Sequence analysis of seven DNA binding VH fragments revealed that they utilized the VH gene family previously described to be associated with autoimmune responses, with a JH6 segment. On VH sequence analysis only one residue substitution in the consensus sequence is needed to form a VH4 germline gene. Potential contact residues with DNA were delineated by three-dimensional structure analysis. We concluded that the DNA binding characteristics of VH regions can be examined in the absence of light chain. DNA binding specificity appears to be a property of the germline VH4 gene. Analysis of such V regions can aid in the identification of hypervariable region contact residues important for DNA binding.

Molecular cloning of a soluble form of the granulocyte-macrophage colony-stimulating factor receptor alpha chain from a myelomonocytic cell line. Expression, biologic activity, and preliminary analysis of transcript distribution

OBJECTIVE

To analyze the molecular and functional characteristics of a soluble form of the granulocyte-macrophage colony-stimulating factor receptor alpha chain (sGM-CSFR alpha), and analyze transcript expression in immune cells and the cellular constituents of rheumatoid arthritis synovial tissue.

METHODS

We amplified, cloned, and expressed the sGM-CSFR alpha and transmembrane form of the receptor (tmGM-CSFR alpha) from complementary DNA derived from a human myelomonocytic cell line. Competitive polymerase chain reaction assays were developed to determine the absolute and relative amounts of tmGM-CSFR alpha versus sGM-CSFR alpha message synthesized by various cell lines and tissues.

RESULTS

sGM-CSFR alpha transcripts were detected in bone marrow, monocyte/macrophages (cultured in GM-CSF), rheumatoid synovial tissue, and rheumatoid synovial tissue T cell lines, and represented the predominant transcript in synovial fibroblasts and osteoarthritis synovial tissue. Levels of expression in monocyte/macrophages and some synovial fibroblast and T cell lines approached those seen in transfected cell lines producing functional sGM-CSFR alpha.

CONCLUSION

sGM-CSFR alpha represents a functional antagonist of GM-CSF activity in vitro. Expression of sGM-CSFR alpha in bone marrow, rheumatoid synovial tissue T cells, and synovial fibroblasts suggests an important role in vivo, both in regulating myelopoiesis and in modulating the immune response.

Granulocyte-macrophage colony-stimulating factor mimicry and receptor interactions

Development of small molecular mimics of larger polypeptide ligands is an important approach to pharmacophore design. One strategy for the development of such mimics is analysis of alternative ligands that bind to the same site as the native ligand. These allow examination of the structural and chemical constraints for binding within the setting of diverse backbone geometries. The use of antireceptor antibodies as alternative ligands has allowed the development of biologically active peptides in several ligand-receptor systems. This technology has been applied to the study of interactions between human granulocyte-macrophage colony-stimulating factor (GM-CSF) and its receptor (GM-CSFR). GM-CSF is one of a family of signal-transducing cytokines and growth factors characterized by a four-helix bundle core structure. The GM-CSFR is comprised of an alpha-chain (GM-CSFR alpha) specific for GM-CSF, and a beta-chain (beta c) shared with the interleukin-3 and interleukin-5 receptors. At least two sites on GM-CSF have been implicated in the GM-CSF-GM-CSFR alpha/beta c ternary complex. In studies summarized here, synthetic peptide analogs of GM-CSF sequences were designed and used to map neutralizing epitopes. One neutralizing epitope corresponded to the A helix of GM-CSF, and a synthetic analog displayed biological activity as a GM-CSF antagonist in vitro, suggesting interaction with the GM-CSFR alpha/beta c complex. A second peptide comprising the B and C helices was recognized by monoclonal neutralizing antibodies and similarly displayed antagonist activity. Recombinant antibody (rAb) technology was also employed. An expression library of rAbs from mice immunized with neutralizing anti-GM-CSF antibodies was developed and screened with a neutralizing anti-GM-CSF monoclonal antibody. One clone which displayed receptor binding activity exhibited structural similarity with epitopes on GM-CSF previously implicated as interaction sites with the neutralizing monoclonal antibody. A synthetic peptide analog of the rAb inhibited GM-CSF bioactivity. Critical contact residues were predicted on the basis of structural similarity of the rAb peptide and GM-CSF. These studies indicate the feasibility of using rAbs in bioactive peptide design, providing lead compounds and information regarding contact residues for drug design.