A V lambda x-bearing monoclonal antibody with similar specificity and sequence to encephalitogenic T cell receptors

The fine specificity of mAb F28C4 to myelin basic protein (MBP), acetyl residues 1-9, has been compared with the previously described specificity of an encephalitogenic T cell clone, PJR-25. F28C4 has been found to express a cross-reactive idiotope (CRI) that is shared with MBP acetyl peptide 1-9-specific TCR. The CRI seems to be located at or near the Ag-combining site of F28C4 and the TCR and, thus, might possibly result from overlapping epitope specificity. We tested the fine epitope specificity of F28C4 by using alanine-substituted peptide analogues and found that residues critical for TCR recognition, Cln3 and Pro6, are also necessary for F28C4 recognition. By using nuclear magnetic resonance, we found that the MBP acetyl peptide 1-9 binds F28C4 in an extended conformation and that the central residues are more tightly bound than the terminal residues, much like the MBP-TCR interaction. Furthermore, sequence homology (75% overall) was found between the regions that contained CDR3 of F28C4 VL and VH and the VDJ junction of the TCR V beta. This homology is not shared by other Ig CDR3 regions and arises, in part, because F28C4 uses an unusual V lambda light chain, V lambda x. Thus, F28C4 shares a CRI with the TCRs, possibly as a result of having similar fine epitope specificity and sequence homology. The anti-CRI mAb can down-modulate experimental allergic encephalomyelitis; thus, it is possible that Abs that are similar to F28C4 may play an important immunoregulatory role in experimental allergic encephalomyelitis in vivo.

A V lambda x-bearing monoclonal antibody with similar specificity and sequence to encephalitogenic T cell receptors

The fine specificity of mAb F28C4 to myelin basic protein (MBP), acetyl residues 1-9, has been compared with the previously described specificity of an encephalitogenic T cell clone, PJR-25. F28C4 has been found to express a cross-reactive idiotope (CRI) that is shared with MBP acetyl peptide 1-9-specific TCR. The CRI seems to be located at or near the Ag-combining site of F28C4 and the TCR and, thus, might possibly result from overlapping epitope specificity. We tested the fine epitope specificity of F28C4 by using alanine-substituted peptide analogues and found that residues critical for TCR recognition, Cln3 and Pro6, are also necessary for F28C4 recognition. By using nuclear magnetic resonance, we found that the MBP acetyl peptide 1-9 binds F28C4 in an extended conformation and that the central residues are more tightly bound than the terminal residues, much like the MBP-TCR interaction. Furthermore, sequence homology (75% overall) was found between the regions that contained CDR3 of F28C4 VL and VH and the VDJ junction of the TCR V beta. This homology is not shared by other Ig CDR3 regions and arises, in part, because F28C4 uses an unusual V lambda light chain, V lambda x. Thus, F28C4 shares a CRI with the TCRs, possibly as a result of having similar fine epitope specificity and sequence homology. The anti-CRI mAb can down-modulate experimental allergic encephalomyelitis; thus, it is possible that Abs that are similar to F28C4 may play an important immunoregulatory role in experimental allergic encephalomyelitis in vivo.

Predicted structural motif of IFN tau

Ovine interferon tau (IFN tau) is a type I interferon that was originally identified as ovine trophoblast protein and is associated with the maternal recognition of pregnancy in sheep. Additionally, IFN tau possesses potent antiviral and antiproliferative activity without the corresponding toxicity found in known IFN alpha s. Structure-function studies with synthetic peptides have identified three discontinuous functional sites on the protein that are involved in receptor interaction and biological activity. However, the structural relationship of these regions is unknown. Therefore, a model relationship of these regions is unknown. Therefore, a model of the 3-D structure of IFN tau would be useful in interpretation of existing data and the design of future structure-function studies. Combining information from circular dichroism (CD) of both the full length recombinant IFN tau and synthetic peptides representing regions of the IFN tau molecule, with sequence homology of IFN tau to IFN beta, a protein of known 3-D structure, we have constructed a model of IFN tau using distance geometry and energy minimization methods. The most striking feature of this model is that functionally active domains of IFN tau, discontinuous in the primary structure, are localized to one side of the molecule and found to be spatially contiguous. This observation is consistent with multiple binding sites on IFN tau interacting simultaneously with the IFN tau receptor.

Binding site on the murine IFN-gamma receptor for IFN-gamma has been identified using the synthetic peptide approach

We have studied the structural parameters involved in the binding of murine IFN-gamma (MuIFN-gamma) to its receptor. Ten synthetic overlapping peptides corresponding to the extracellular domain of the MuIFN-gamma receptor (MuIFN-gamma R) were synthesized. In direct binding studies, biotinylated MuIFN-gamma bound specifically to receptor peptide (95-120). Further, the NH2-terminal IFN-gamma peptide, MuIFN-gamma (1-39), also specifically bound to receptor peptide (95-120). Binding of both labeled MuIFN-gamma and MuIFN-gamma (1-39) to MuIFN-gamma R peptide (95-120) was inhibited by either unlabeled molecule. The COOH-terminal receptor binding peptide, MuIFN-gamma (95-133), neither bound to any receptor peptides nor blocked the binding of intact MuIFN-gamma or MuIFN-gamma (1-39) to receptor peptide (95-120). Polyclonal antibodies to each of the peptides were then produced. Each of the anti-peptide antisera recognized its corresponding peptide and bound denatured cloned soluble receptor by Western blotting. Furthermore, the antisera to peptides representing the inner region of the extracellular domain of the receptor bound to nondenatured soluble MuIFN-gamma R. Specifically, antisera to the receptor peptides (73-97), (95-120), (118-143), (142-163), and (161-182) bound to soluble MuIFN-gamma R, whereas antisera to peptides (1-21), (20-49), (46-74), (178-203), and (202-227) did not bind. Most important, antisera to peptides (95-120) and (118-143) competed with [125I]MuIFN-gamma for binding to soluble receptor. These results show that the region of the MuIFN-gamma R encompassing amino acid residues (95-120) is a binding site on the receptor for the NH2-terminal of MuIFN-gamma by direct binding, and that the larger region (95-143) on the receptor may play a role in binding of intact MuIFN-gamma based on blocking of binding by site-specific antibodies.

Binding site on the murine IFN-gamma receptor for IFN-gamma has been identified using the synthetic peptide approach

We have studied the structural parameters involved in the binding of murine IFN-gamma (MuIFN-gamma) to its receptor. Ten synthetic overlapping peptides corresponding to the extracellular domain of the MuIFN-gamma receptor (MuIFN-gamma R) were synthesized. In direct binding studies, biotinylated MuIFN-gamma bound specifically to receptor peptide (95-120). Further, the NH2-terminal IFN-gamma peptide, MuIFN-gamma (1-39), also specifically bound to receptor peptide (95-120). Binding of both labeled MuIFN-gamma and MuIFN-gamma (1-39) to MuIFN-gamma R peptide (95-120) was inhibited by either unlabeled molecule. The COOH-terminal receptor binding peptide, MuIFN-gamma (95-133), neither bound to any receptor peptides nor blocked the binding of intact MuIFN-gamma or MuIFN-gamma (1-39) to receptor peptide (95-120). Polyclonal antibodies to each of the peptides were then produced. Each of the anti-peptide antisera recognized its corresponding peptide and bound denatured cloned soluble receptor by Western blotting. Furthermore, the antisera to peptides representing the inner region of the extracellular domain of the receptor bound to nondenatured soluble MuIFN-gamma R. Specifically, antisera to the receptor peptides (73-97), (95-120), (118-143), (142-163), and (161-182) bound to soluble MuIFN-gamma R, whereas antisera to peptides (1-21), (20-49), (46-74), (178-203), and (202-227) did not bind. Most important, antisera to peptides (95-120) and (118-143) competed with [125I]MuIFN-gamma for binding to soluble receptor. These results show that the region of the MuIFN-gamma R encompassing amino acid residues (95-120) is a binding site on the receptor for the NH2-terminal of MuIFN-gamma by direct binding, and that the larger region (95-143) on the receptor may play a role in binding of intact MuIFN-gamma based on blocking of binding by site-specific antibodies.

The structure of a myelin basic protein-associated idiotope

A cross-reactive idiotope (CRI) has been previously described on monoclonal antibodies (mAbs) specific for encephalitogenic peptides from myelin basic protein (MBP). The anti-CRI mAb, F25F7, binds an idiotope (Id) localized to the light chains of an anti-MBP peptide 1-9 mAb, denoted F23C6, and an anti-MBP peptide 80-89 mAb, denoted 845D3. It is the purpose of this study to further delineate the CRI being recognized by F25F7. To this end, we have found a structural correlation between the CRI and the antigen, a small synthetic peptide, denoted PBM 9-1, used to elicit the anti-Id mAb. Sequence comparison between the light chain of F23C6 and PBM 9-1 reveals a region of homology in CDR 2/FWK 3. The configuration of this site in the VL, as determined by comparison with a mAb, HyHEL-10, whose structure has been determined and is 97% homologous to the light chain of F23C6, conforms to the rules used to define antigenic determinants or Ids. A synthetic peptide having the F23C6 VL CDR 2/FWK 3 sequence inhibited the binding of F25F7 to F23C6 and 845D3. Taken together, these data suggest the Id recognized by F25F7 is defined, in part, by the PBM 9-1-like sequence of F23C6.

Solution structure of trypsin modulating oostatic factor is a left-handed helix

The solution structure of trypsin modulating oostatic factor (TMOF), a decapeptide (H-YDPAPPPPPP-OH) hormone that signals the termination of trypsin-like biosynthesis in mosquito midgut epithelial cells, was determined by 2-D 1H nuclear magnetic resonance spectroscopy and molecular modeling. The peptide forms a rod-shaped left-handed helix about 30 A long. No evidence was found to support a poly-L-proline beta-turn model. Hydrophobic contacts between the rings of tyrosine 1 and proline 3 may enhance the stability of the N-terminal segment. This peptide provides an interesting exception to the normal chemical shift index (csi) rules. Our results suggest that a sequence of positive csi indices, normally expected for a beta-strand structure, could also describe a left-handed poly-L-proline-like helix.

Proton NMR sequence-specific assignments and secondary structure of a receptor binding domain of mouse gamma-interferon

Previous studies using synthetic peptides and monoclonal antibodies have implicated the N-terminal 39-residue segment as a receptor binding region of mouse gamma-interferon (MuIFN gamma). In this work, we report the solution structure of this fragment (dissolved in water with 40% trifluoroethanol) as determined by proton NMR spectroscopy. The proton sequence-specific assignments were determined from TOCSY and NOESY spectra using established procedures. The secondary structure is characterized by two well-defined alpha-helical regions composed of residues 5-16 and 22-37. These two helices are joined by a loop. No NOESY contacts between the two helical regions were detected. Molecular models consistent with the NMR data were generated for MuIFN gamma (1-39) using distance geometry and restrained molecular dynamics/energy minimization calculations. Comparison with similar N-terminal domains in the published NMR and crystallographic studies on the dimeric human and rabbit IFN gamma suggests some similarities in the structures except that the helical regions in the fragment are longer, and considerable variation may exist in the relative orientation of the two helices in the solution phase. The presence of stronger alpha N sequential NOE's suggests that this peptide is flexible. The absence of NOESY contacts involving the N-terminal tripeptide suggests that this region undergoes rapid segmental motion. The data presented here on MuIFN gamma (1-39), combined with the studies on human and rabbit IFN gamma, suggest that the N-terminal receptor binding domain of the protein can undergo structural changes, the understanding of which may provide insight into the basis for receptor interaction by this lymphokine.

Stable conformation of an interferon-gamma receptor binding peptide in aqueous solution is required for interferon-gamma antagonist activity

A peptide antagonist for mouse interferon-gamma (IFN-gamma) activity corresponding to the amino-terminal 39 amino acids of mouse IFN-gamma [MuIFN-gamma (1-39)] has been identified previously. In an analogous manner, we have assessed the ability of the corresponding peptide of human IFN-gamma [HuIFN-gamma (1-39)] to antagonize human IFN-gamma. HuIFN-gamma (1-39) has the ability to block the antiviral activity of human IFN-gamma in a functional assay. In receptor competition, the peptide can also block human IFN-gamma receptor binding. Surprisingly, the murine analog, MuIFN-gamma (1-39), possesses a 10-fold greater ability to block human IFN-gamma antiviral activity and receptor binding than HuIFN-gamma (1-39). Both peptides showed alpha-helical structure in water by circular dichroism, however MuIFN-gamma (1-39) possessed a greater amount of alpha-helix compared to HuIFN-gamma (1-39), suggesting a requirement for a stable secondary structure for optimal antagonist activity. In trifluoroethanol, a helix-stabilizing agent, both peptides showed relatively equal alpha-helices, suggesting that both sequences have an equal potential for helical structure. As IFN-gamma is species specific, the observation that MuIFN-gamma (1-39) can antagonize human IFN-gamma raises questions about the role of this region in species specificity. These studies provide insight into the structural requirements, both primary and secondary, for IFN-gamma receptor binding. In the future, this information, along with more detailed three-dimensional structural analyses of the peptides, could prove useful in the rational design of IFN-gamma agonists and antagonists.

Identification of binding domains on the superantigen, toxic shock syndrome-1, for class II MHC molecules

Toxic shock syndrome toxin-1 (TSST-1) is a member of the staphylococcal enterotoxin superantigen family. In order to determine the regions on the TSST-1 molecule involved in binding to class II MHC, seven overlapping peptides of the entire TSST-1 molecule were synthesized and tested for their ability to compete with 125I-TSST-1 for binding to class II MHC on murine A20 cells and HLA on Raji cells. Peptides corresponding to N-terminal amino acid residues 39 through 68 and C-terminal residues 155 through 194 competed with 125I-TSST-1 for binding to class II MHC. Also, binding studies with class II MHC beta-chain peptides indicate that regions encompassed by I-A beta b(30-60) and I-A beta b(60-90) are binding regions for TSST-1. Thus, we have identified binding domains on the TSST-1 molecule for class II MHC molecule receptors on antigen presenting cells.

The N-terminus and C-terminus of IFN-gamma are binding domains for cloned soluble IFN-gamma receptor

The mechanism of binding of murine IFN-gamma to its receptor has not been determined. We have studied this mechanism by examining the binding of overlapping synthetic peptides of IFN-gamma to cloned soluble murine IFN-gamma R. IFN-gamma (1-39) and IFN-gamma (95-133) were able to compete with [125I]IFN-gamma for binding to cloned soluble receptor. Peptides corresponding to the inner region of IFN-gamma--IFN-gamma (36-60), IFN-gamma (54-91), and IFN-gamma (78-107)--showed a markedly reduced ability to compete with [125I]IFN-gamma for receptor binding relative to the N-terminal and C-terminal peptides. In direct binding studies, the binding of [125I]-IFN-gamma (1-39) to soluble receptor could only be competed by IFN-gamma (1-39) and IFN-gamma and not by any of the other peptides including IFN-gamma (95-133). This suggests that the N- and C-termini of IFN-gamma bind to different regions of the receptor. These data in conjunction with previous structure/function studies and x-ray crystallographic data have allowed us to formulate a "velcro-key" model of IFN-gamma binding to receptor that involves both the N- and C-terminal domains. The N-terminus binds in the classical "lock-and-key" manner characterized by specific ligand-receptor binding. The hydrophilic C-terminus binds to a region of the receptor distinct from the N-terminus likely through the polycationic region, which is conserved across species barriers. Binding of this type would exhibit high affinity and low specificity similar to a piece of velcro. This interaction becomes specific when the C-terminus is in the context of the whole IFN-gamma molecule and may act to increase the affinity of receptor binding and/or facilitate signal transduction.

The N-terminus and C-terminus of IFN-gamma are binding domains for cloned soluble IFN-gamma receptor

The mechanism of binding of murine IFN-gamma to its receptor has not been determined. We have studied this mechanism by examining the binding of overlapping synthetic peptides of IFN-gamma to cloned soluble murine IFN-gamma R. IFN-gamma (1-39) and IFN-gamma (95-133) were able to compete with [125I]IFN-gamma for binding to cloned soluble receptor. Peptides corresponding to the inner region of IFN-gamma--IFN-gamma (36-60), IFN-gamma (54-91), and IFN-gamma (78-107)--showed a markedly reduced ability to compete with [125I]IFN-gamma for receptor binding relative to the N-terminal and C-terminal peptides. In direct binding studies, the binding of [125I]-IFN-gamma (1-39) to soluble receptor could only be competed by IFN-gamma (1-39) and IFN-gamma and not by any of the other peptides including IFN-gamma (95-133). This suggests that the N- and C-termini of IFN-gamma bind to different regions of the receptor. These data in conjunction with previous structure/function studies and x-ray crystallographic data have allowed us to formulate a "velcro-key" model of IFN-gamma binding to receptor that involves both the N- and C-terminal domains. The N-terminus binds in the classical "lock-and-key" manner characterized by specific ligand-receptor binding. The hydrophilic C-terminus binds to a region of the receptor distinct from the N-terminus likely through the polycationic region, which is conserved across species barriers. Binding of this type would exhibit high affinity and low specificity similar to a piece of velcro. This interaction becomes specific when the C-terminus is in the context of the whole IFN-gamma molecule and may act to increase the affinity of receptor binding and/or facilitate signal transduction.

Mapping of multiple binding domains of the superantigen staphylococcal enterotoxin A for HLA

Multiple binding sites on the staphylococcal enterotoxin A (SEA) molecule which interact with class II MHC Ag have been suggested by previous studies comparing SEA binding with that of another superantigen, toxic shock syndrome toxin-1. Using the synthetic peptide approach we have identified multiple regions of the SEA molecule which are responsible for binding to HLA Ag on Raji cells. Overlapping peptides were synthesized corresponding to the complete amino acid sequence of SEA: SEA(1-45), SEA(39-66), SEA(62-86), SEA(83-104), SEA(102-124), SEA(121-149), SEA(146-173), SEA(166-193), SEA(187-217), and SEA(211-233). Like the native SEA molecule, all of the peptides exhibited relatively high beta-sheet and low alpha-helical structure as determined by circular dichroism spectroscopy. A direct competition assay was employed with peptide blockage of 125I-SEA binding to MHC Ag. SEA(1-45), SEA(39-66), SEA(62-86), and SEA(121-149) but none of the other peptides blocked binding to Raji cells. The relative potency of the peptides in blocking SEA binding was determined with SEA(39-66) much greater than SEA(1-45) = SEA(62-86) = SEA(121-149). Peptide competition was seen at concentrations as low as 55 microM. Further, antibodies were produced to all of the peptides and tested for their ability to bind to SEA and inhibit SEA binding to HLA. Consistent with the direct inhibition of binding, antisera to SEA(1-45), SEA(39-66), and SEA(62-86) reduced the ability of SEA to bind Raji cells, whereas, antisera to the remaining peptides failed to block binding. The data suggest that the binding of the superantigen SEA to MHC molecules involves several N-terminal regions on SEA as well as an additional internal domain. This allows for the presence of multiple binding sites in an extended N-terminal region of the SEA molecule or a discontinuous binding epitope.

Mapping of multiple binding domains of the superantigen staphylococcal enterotoxin A for HLA

Multiple binding sites on the staphylococcal enterotoxin A (SEA) molecule which interact with class II MHC Ag have been suggested by previous studies comparing SEA binding with that of another superantigen, toxic shock syndrome toxin-1. Using the synthetic peptide approach we have identified multiple regions of the SEA molecule which are responsible for binding to HLA Ag on Raji cells. Overlapping peptides were synthesized corresponding to the complete amino acid sequence of SEA: SEA(1-45), SEA(39-66), SEA(62-86), SEA(83-104), SEA(102-124), SEA(121-149), SEA(146-173), SEA(166-193), SEA(187-217), and SEA(211-233). Like the native SEA molecule, all of the peptides exhibited relatively high beta-sheet and low alpha-helical structure as determined by circular dichroism spectroscopy. A direct competition assay was employed with peptide blockage of 125I-SEA binding to MHC Ag. SEA(1-45), SEA(39-66), SEA(62-86), and SEA(121-149) but none of the other peptides blocked binding to Raji cells. The relative potency of the peptides in blocking SEA binding was determined with SEA(39-66) much greater than SEA(1-45) = SEA(62-86) = SEA(121-149). Peptide competition was seen at concentrations as low as 55 microM. Further, antibodies were produced to all of the peptides and tested for their ability to bind to SEA and inhibit SEA binding to HLA. Consistent with the direct inhibition of binding, antisera to SEA(1-45), SEA(39-66), and SEA(62-86) reduced the ability of SEA to bind Raji cells, whereas, antisera to the remaining peptides failed to block binding. The data suggest that the binding of the superantigen SEA to MHC molecules involves several N-terminal regions on SEA as well as an additional internal domain. This allows for the presence of multiple binding sites in an extended N-terminal region of the SEA molecule or a discontinuous binding epitope.

Evidence for the alpha-helicity of class II MHC molecular binding sites for the superantigen, staphylococcal enterotoxin A

Circular dichroism (CD) spectra of class II MHC peptides revealed the alpha-helical conformation of superantigen-binding peptides I-A beta b(60-90), I-A beta b(65-85), and I-A alpha b(51-80), but not the nonbinding peptide I-A beta b(80-100). These CD spectra provide biophysical evidence for the alpha-helicity of class II MHC molecular binding sites for the superantigen, staphylococcal enterotoxin A (SEA). Alanine-substituted analogs of the SEA binding-site peptide, I-A beta b(65-85), were used to implicate beta-chain residues 72 and 80 in class II MHC-SEA binding. The data support SEA binding away from the class II antigen binding cleft along the faces of the alpha-helices.

Topology of receptor binding domains of mouse IFN-gamma

IFN-gamma is an essential immunoregulatory lymphokine for a variety of immunologic functions including upregulation of MHC Ag. The elucidation of the structure, particularly the receptor binding domains, should further enhance our understanding of its mechanism of action, and provide a rational basis for modulation of its activity by alteration of its structure. A predicted model of murine IFN-gamma structure has been constructed based on data derived from our synthetic peptide studies, circular dichroism spectra, and predictive algorithms for secondary structure, surface accessibility, and tertiary structure, as well as predicted structural similarities to IL-2. Direct synthetic peptide competition studies using five overlapping peptides that encompassed the entire IFN-gamma sequence of 133 amino acids showed that only the N-terminus of IFN-gamma, IFN-gamma(1-39), blocked binding to receptor, suggesting that the N-terminus is directly involved in receptor binding. Rabbit antibodies to the N-terminal (IFN-gamma(1-39)) and C-terminal (IFN-gamma(95-133)) peptides neutralized IFN-gamma activity, whereas antibodies to the three peptides that spanned the internal region, sequence 36-107, were without effect. Thus, the antibody data support the involvement of the N-terminus as a receptor binding domain and also suggest that the C-terminus of the molecule is also a binding domain. Predictive algorithms assign six alpha-helices and five turns to the molecule and circular dichroism spectra of both intact human and murine IFN-gamma and synthetic peptides of murine IFN-gamma showed that the molecule is mainly alpha-helical in structure. Drawing mainly on the four alpha-helix bundle model, a common motif in globular proteins such as IFN-gamma and IL-2 (whose crystalline structure is known), we constructed a simple model of the tertiary structure of IFN-gamma that fits well with our synthetic peptide and circular dichroism data. The model consists of the four-alpha-helical bundle along with N- and C-terminal helices that are predicted to be closely associated and together form the receptor binding domains. The model presented should contribute to further understanding the molecular basis of IFN-gamma action and allow us to begin modulating the function of IFN-gamma through the design of agonists and antagonists.

Topology of receptor binding domains of mouse IFN-gamma

IFN-gamma is an essential immunoregulatory lymphokine for a variety of immunologic functions including upregulation of MHC Ag. The elucidation of the structure, particularly the receptor binding domains, should further enhance our understanding of its mechanism of action, and provide a rational basis for modulation of its activity by alteration of its structure. A predicted model of murine IFN-gamma structure has been constructed based on data derived from our synthetic peptide studies, circular dichroism spectra, and predictive algorithms for secondary structure, surface accessibility, and tertiary structure, as well as predicted structural similarities to IL-2. Direct synthetic peptide competition studies using five overlapping peptides that encompassed the entire IFN-gamma sequence of 133 amino acids showed that only the N-terminus of IFN-gamma, IFN-gamma(1-39), blocked binding to receptor, suggesting that the N-terminus is directly involved in receptor binding. Rabbit antibodies to the N-terminal (IFN-gamma(1-39)) and C-terminal (IFN-gamma(95-133)) peptides neutralized IFN-gamma activity, whereas antibodies to the three peptides that spanned the internal region, sequence 36-107, were without effect. Thus, the antibody data support the involvement of the N-terminus as a receptor binding domain and also suggest that the C-terminus of the molecule is also a binding domain. Predictive algorithms assign six alpha-helices and five turns to the molecule and circular dichroism spectra of both intact human and murine IFN-gamma and synthetic peptides of murine IFN-gamma showed that the molecule is mainly alpha-helical in structure. Drawing mainly on the four alpha-helix bundle model, a common motif in globular proteins such as IFN-gamma and IL-2 (whose crystalline structure is known), we constructed a simple model of the tertiary structure of IFN-gamma that fits well with our synthetic peptide and circular dichroism data. The model consists of the four-alpha-helical bundle along with N- and C-terminal helices that are predicted to be closely associated and together form the receptor binding domains. The model presented should contribute to further understanding the molecular basis of IFN-gamma action and allow us to begin modulating the function of IFN-gamma through the design of agonists and antagonists.

Structure of an epitope in an immunodominant region of the interferon-gamma molecule that is involved in receptor interaction

An amino-terminal immunodominant region of murine interferon-gamma (IFN-gamma) that may be involved in function is characterized using a neutralizing amino-terminal-specific monoclonal antibody (mAb) and synthetic peptides. Competition studies with peptide truncations determined that residues 3, 4, and 5 of IFN-gamma were required for binding to the mAb. These residues are predicted to participate in an amphipathic alpha-helix spanning residues 3-11 of IFN-gamma. Conservative peptide analogs that maintain the amphipathicity of the alpha-helix of IFN-gamma (1-20) retained the ability to block IFN-gamma binding to mAb. Peptide analogs with substitutions that disrupted the amphipathicity of the alpha-helix lost their ability to block binding. The tyrosine at position 14 was required as its removal in carboxy-terminal truncations caused the loss of blocking ability. We conclude that the IFN-gamma epitope for the neutralizing mAb involves residues 3-14, spanning 12 residues, and it appears that residues 3, 4, 5, and 14 are an important part of the epitope. Based on these characteristics and the observation that most continuous epitopes are reported to involve shorter regions of up to 8 residues, this epitope may be described as a linear discontinuous epitope that may require the amphipathic helix. The data presented here provide further insight to the structure of a site that is involved in receptor interaction.

Site of nonrestrictive binding of SEA to class II MHC antigens

We have used the synthetic peptide approach to show that the N-terminal 45-amino acids of staphylococcal enterotoxin A (SEA), SEA(1-45), constitute an important part of its binding site on class II major histocompatibility complex (MHC) molecules. SEA(1-45) and to a lesser extent SEA(1-27) were able to displace SEA from HLA-DR on Raji cells as assessed by flow cytometry and to compete with radiolabeled SEA for interaction with HLA-DR in a direct binding assay. Specific binding of SEA to Ia on murine A-20 cells could be inhibited by the same peptides [i.e. SEA(1-45) greater than SEA(1-27)] that blocked binding to HLA-DR. Therefore, different class II MHC molecules associate with the same functional site on SEA. Further, an ELISA system was used to demonstrate that SEA(1-45) is able to directly bind to a mouse synthetic I-A beta b peptide, I-A beta b (65-85), which contains a binding site of the class II MHC molecule involved in SEA presentation to T cells. Thus, we have localized a site on SEA that is involved in selective surface association with class II MHC antigens and identified the region on the class II MHC antigen to which that site binds.

Causal association of interferon-gamma with tumor regression

Mouse interferon-gamma (MuIFN-gamma) can cause the rejection of malignant cells in vivo. The evidence presented here in support of this claim includes, first, that spontaneous regression of MSC sarcomas was associated with high intratumoral concentrations of endogenously-produced MuIFN-gamma. By contrast, progressively growing, lethal neoplasms of the same kind invariably contained very little IFN-gamma. Second, spontaneously regressing MSC sarcomas were converted into progressively growing, lethal neoplasms by injecting mice with a monoclonal antibody that neutralized the biological effects of endogenous IFN-gamma. Another monoclonal antibody that bound to, but did not neutralize, mouse IFN-gamma had no effect on the course of tumor regression. Together, these data causally relate MuIFN-gamma to the successful rejection of malignant cells in vivo. They also suggest that findings of poor therapeutic efficacy for IFN-gamma are probably attributable to problems other than an intrinsic lack in the biological activity of the lymphokine.

Macrophage activation-associated proteins. Characterization of stimuli and conditions needed for expression of proteins 47b, 71/73, and 120

The expression of cellular proteins was analyzed by two-dimensional gel electrophoresis during and after exposure of mouse macrophages to either mouse rIFN-gamma or natural MuIFN-beta sufficient to prime macrophages for tumor cell killing. The reversible inhibitor of protein synthesis, cycloheximide (CY), was included in some experiments during exposure to IFN. While it was present, CY suppressed protein synthesis by greater than 90%, but did not affect priming for tumor cell killing that was induced by either kind of IFN, as measured in cytotoxicity assays. Further analysis showed that, after CY and IFN were removed, protein synthesis recovered fully within 1 h. p47b, a protein that has been associated closely with the induction of the primed state in mouse macrophages, was then substantially expressed despite no new stimulation by IFN. Thus, macrophages in which protein synthesis had been reversibly inhibited delayed full processing of a signal delivered by IFN, until after protein synthesis had resumed. Such a delay in processing may explain how macrophages subsequently became activated, despite treatment with CY. The expression of the protein doublet, p71/73, was induced, regardless of which of three dissimilar agents (LPS, heat killed Listeria monocytogenes, poly I:C) was used to trigger the expression of cytolytic activity by primed macrophages. Therefore, the likelihood was increased that p71/73, expressed with p47b, is a valid phenotypic marker for fully activated, cytolytic macrophages. By contrast, p120, another protein that has been proposed as a marker of full activation in peritoneal macrophages, was expressed by bone marrow culture-derived macrophages regardless of whether or not they were cytolytic for tumor cells. It cannot be regarded as a reliable marker of macrophage activation in all circumstances, therefore.

Macrophage activation-associated proteins. Characterization of stimuli and conditions needed for expression of proteins 47b, 71/73, and 120

The expression of cellular proteins was analyzed by two-dimensional gel electrophoresis during and after exposure of mouse macrophages to either mouse rIFN-gamma or natural MuIFN-beta sufficient to prime macrophages for tumor cell killing. The reversible inhibitor of protein synthesis, cycloheximide (CY), was included in some experiments during exposure to IFN. While it was present, CY suppressed protein synthesis by greater than 90%, but did not affect priming for tumor cell killing that was induced by either kind of IFN, as measured in cytotoxicity assays. Further analysis showed that, after CY and IFN were removed, protein synthesis recovered fully within 1 h. p47b, a protein that has been associated closely with the induction of the primed state in mouse macrophages, was then substantially expressed despite no new stimulation by IFN. Thus, macrophages in which protein synthesis had been reversibly inhibited delayed full processing of a signal delivered by IFN, until after protein synthesis had resumed. Such a delay in processing may explain how macrophages subsequently became activated, despite treatment with CY. The expression of the protein doublet, p71/73, was induced, regardless of which of three dissimilar agents (LPS, heat killed Listeria monocytogenes, poly I:C) was used to trigger the expression of cytolytic activity by primed macrophages. Therefore, the likelihood was increased that p71/73, expressed with p47b, is a valid phenotypic marker for fully activated, cytolytic macrophages. By contrast, p120, another protein that has been proposed as a marker of full activation in peritoneal macrophages, was expressed by bone marrow culture-derived macrophages regardless of whether or not they were cytolytic for tumor cells. It cannot be regarded as a reliable marker of macrophage activation in all circumstances, therefore.

Migration of neutrophils from lung to tracheobronchial lymph node

Translocation of particulate antigen deposited in the lung to tracheobronchial lymph nodes (TBLN) is important in the induction of pulmonary immune responses. We have previously shown that alveolar macrophages can contribute to particle translocation to TBLN, but whether neutrophils can also contribute to this process is not known. To determine if neutrophils can carry particles to the TBLN, dog neutrophils were elicited by instillation of red or green fluorescent microspheres into individual lung lobes. Autologous neutrophils that had phagocytized fluorescent microspheres were then instilled into an unexposed lobe of the same dog's lung. After 24 hr, the TBLN of instilled dogs had numerous neutrophils, 99% of which contained either red or green fluorescent microspheres but not both. Use of the two different colored microspheres as labels precluded the possibility that neutrophils had phagocytized the microspheres in the TBLN. In a second experiment, dogs were depleted of peripheral blood neutrophils by injections of hydroxyurea. Hydroxyurea-treated and normal dogs were instilled with fluorescent microspheres and killed after 40 h. Hydroxyurea treatment reduced neutrophil accumulation in the lung by 79% and reduced particle translocation to the TBLN by 80%. Results of these experiments indicate that neutrophils are similar to pulmonary alveolar macrophages (PAM) in their ability to phagocytize particles in the lung and then migrate to the TBLN.

Modification of the antitumor action of Corynebacterium parvum by stress

Social grouping and isolation of mice, in the presence of an acute stressor, were found to differentially affect the antitumor action of the immunological adjuvant Corynebacterium parvum. Socially grouped DBA/2j mice were injected intradermally with P815 mastocytoma ascites cells. Half the mice had a threshold dose of C. parvum admixed with the P815 cells. Half the mice in each of those conditions were given acute, inescapable electric footshock. In a second experiment, the stressed mice were socially isolated prior to the acute stress. Tumor growth itself was not affected by the stress procedures. C. parvum inhibited tumor growth in non-stressed and socially isolated, stressed mice. However, social grouping selectively negated the C. parvum effect resulting in tumor growth and mortality equivalent to mice not given the adjuvant. Psychological factors may be important to the development of concomitant immunity and the efficacy of immunotherapies.