Common structural features among monoclonal antibodies binding the same antigenic region of cytochrome c

Recasting Human Vδ1 Lymphocytes in an Adaptive Role

γδ T cells are unconventional lymphocytes commonly described as 'innate-like' in function, which can respond in both a T cell receptor (TCR)-independent and also major histocompatibility complex (MHC)-unrestricted TCR-dependent manner. While the relative importance of TCR recognition had remained unclear, recent studies revealed that human Vδ1 T cells display unexpected parallels with adaptive αβ T cells. Vδ1 T cells undergo profound and highly focussed clonal expansion from an initially diverse and private TCR repertoire, most likely in response to specific immune challenges. Concomitantly, they differentiate from a Vδ1 T cell naïve (Tnaïve) to a Vδ1 T cell effector (Teffector) phenotype, marked by the downregulation of lymphoid homing receptors and upregulation of peripheral homing receptors and effector markers. This suggests that an adaptive paradigm applies to Vδ1 T cells, likely involving TCR-dependent but MHC-unrestricted responses to microbial and non-microbial challenges.

Connexin 43 hemichannels contribute to the propagation of apoptotic cell death in a rat C6 glioma cell model

Gap junctions (GJs) have been demonstrated to communicate cell death signals from apoptotic to healthy cells, thereby spatially extending apoptosis. Before being incorporated into GJs, hemichannels (hemi-GJs) are normally closed but recent evidence suggests that they can be opened by various messengers and conditions, thereby forming a pore through which molecules can enter or leave the cell potentially leading to cell death. The aim of this study was to determine the contribution of GJs and hemichannels in the communication of apoptosis toward surrounding cells. We induced apoptosis in C6 glioma cells stably transfected with connexin (Cx)43, with cytochrome C (cytC) using in situ electroporation and found that healthy surrounding cells underwent apoptotic transformation. Work with various cell death markers, wild-type (WT) and Cx43-expressing cells, inhibitors of GJs and/or hemichannels, and Cx43 gene silencing showed that GJs contribute to the spread of apoptosis in a zone next to where apoptosis was triggered whereas hemichannels also promoted cell death beyond this area. Buffering cytoplasmic Ca(2+) changes inhibited the spread of apoptosis in both cases. We conclude that Cx43 hemichannels, in concert with their GJ counterparts, play a role in communicating cytC-induced apoptotic cell death messages.

Tumor necrosis factor regulation of apoptosis in mouse preimplantation embryos and its antagonism by transforming growth factor alpha/phosphatidylionsitol 3-kinase signaling system

Survival and apoptosis of cells in preimplantation embryos are fundamental for successful pregnancy. Relevant to these processes, tumor necrosis factor (TNF) and transforming growth factor alpha (TGFA) are produced by mammalian oviducts and uteri. In early embryos, TNF induces apoptosis, whereas TGFA could act as a survival factor. Here we investigated the TNF regulation of apoptosis in early mouse embryos and its antagonism by TGFA. TNF receptor superfamily, member 1a mRNA was detectable throughout early embryonic stages, with an increase after the early blastocyst stage, whereas the expression of TNF receptor superfamily, member 1b transcripts were detected only at the expanded blastocyst stage. Although pregnant uteri produced TNF, physiologic levels were low during the preimplantation period. Treatment with TNF inhibited the development of two-cell stage embryos to blastocysts showing decreased proliferation and increased apoptosis both in vitro and in vivo. These detrimental effects of TNF on early embryo development and survival were blocked by a neutralizing anti-TNF antibody. In addition to the death receptor-mediated pathway, TNF-induced apoptosis was further mediated by disruption of mitochondrial functions, characterized by release of cytochrome c and activation of caspase 9. The proapoptotic effects of TNF in blastocysts were counteracted by cotreatment with TGFA. The antagonistic effect of TGFA on TNF-induced apoptosis was blocked by phosphatidylionsitol 3-kinase (PI3K) inhibitors. The present findings demonstrate the stage-selective susceptibility to the apoptosis-inducing effect of TNF in mouse preimplantation embryos and that the TGFA/PI3K signaling system has an important role in the control of TNF-induced apoptosis in blastocysts.

Serum leucine-rich alpha-2-glycoprotein-1 binds cytochrome c and inhibits antibody detection of this apoptotic marker in enzyme-linked immunosorbent assay

Cytochrome c (Cyt c) has been implicated as a serum marker for aberrant apoptosis and, thus, has considerable clinical potential. Using a sandwich enzyme-linked immunosorbent assay (ELISA) we found that the sensitivity of Cyt c detection is reduced in the presence of serum. The inhibitory factor responsible was purified from both fetal bovine serum and human serum employing standard chromatography procedures followed by affinity chromatography on Affi-Gel 10-bound Cyt c. In SDS-PAGE, bands at 44 kD and 50 kD were observed for the bovine and human proteins, respectively. Mass spectrometry analysis identified the serum inhibitory factor as leucine-rich alpha-2-glycoprotein-1 (LRalpha2GP1). This identification may lead to a modified ELISA to quantify total Cyt c in patients' sera. LRalpha2GP1 is the first extracellular ligand for Cyt c that has been identified. A physiological function associated with binding is suggested.

Modeling the binding sites of anti-hen egg white lysozyme antibodies HyHEL-8 and HyHEL-26: an insight into the molecular basis of antibody cross-reactivity and specificity

Three antibodies, HyHEL-8 (HH8), HyHEL-10 (HH10), and HyHEL-26 (HH26) are specific for the same epitope on hen egg white lysozyme (HEL), and share >90% sequence homology. Their affinities vary by several orders of magnitude, and among the three antibodies, HH8 is the most cross-reactive with kinetics of binding that are relatively invariable compared to HH26, which is highly specific and has quite variable kinetics. To investigate structural correlates of these functional variations, the Fv regions of HH8 and HH26 were homology-modeled using the x-ray structure of the well-characterized HH10-HEL complex as template. The binding site of HH26 is most charged, least hydrophobic, and has the greatest number of intramolecular salt bridges, whereas that of HH8 is the least charged, most hydrophobic and has the fewest intramolecular salt bridges. The modeled HH26-HEL structure predicts the recently determined x-ray structure of HH26, (Li et al., 2003, Nat. Struct. Biol. 10:482-488) with a root-mean-square deviation of 1.03 A. It is likely that the binding site of HH26 is rendered rigid by a network of intramolecular salt bridges whereas that of HH8 is flexible due to their absence. HH26 also has the most intermolecular contacts with the antigen whereas HH8 has the least. HH10 has these properties intermediate to HH8 and HH26. The structurally rigid binding site with numerous specific contacts bestows specificity on HH26 whereas the flexible binding site with correspondingly fewer contacts enables HH8 to be cross-reactive. Results suggest that affinity maturation may select for high affinity antibodies with either "lock-and-key" preconfigured binding sites, or "preconfigured flexibility" by modulating combining site flexibility.

Cytochrome c, released from cerebellar granule cells undergoing apoptosis or excytotoxic death, can generate protonmotive force and drive ATP synthesis in isolated mitochondria

In rat cerebellar granule cells, cytochrome c release takes place during glutamate toxicity and apoptosis due to deprivation of depolarising levels of potassium. We show that, as in necrosis, the released cytochrome c present in the cytosolic fraction obtained from cerebellar granule cells undergoing apoptosis can operate as a reactive oxygen species (ROS) scavenger and as a respiratory substrate. The capability of the cytosolic fraction containing cytochrome c, obtained from cerebellar granule cells undergoing either necrosis or apoptosis, to energise coupled mitochondria isolated by the same cells is also investigated. We show that, in both cases, the cytosolic fraction containing cytochrome c, added to mitochondria, can cause proton ejection, and membrane potential generation and can drive ATP synthesis and export in the extramitochondrial phase, as photometrically measured via the ATP detecting system. Cytochrome c, separated immunologically from the cytosolic fraction of apoptotic cells when added to mitochondria, is found to cause proton ejection to generate membrane potential and to drive ATP synthesis and export in a manner not sensitive to the further addition of the cytosolic fraction depleted of cytochrome c, which failed to do this. In the light of these findings we propose that in apoptosis the released cytochrome c can contribute to provide ATP required for the cell programmed death to occur.

Radiographic contrast media-induced tubular injury: evaluation of oxidant stress and plasma membrane integrity

BACKGROUND

Experimental and clinical investigations suggest that oxidant stress is a critical determinant of radiocontrast nephropathy (RCN), and that N acetyl cysteine (NAC) can prevent this damage. This study addresses these issues directly at the tubular cell level. Potential alternative mechanisms for RCN have also been sought.

METHODS

Isolated mouse proximal tubule segments (PTS), or cultured proximal tubule (HK-2) cells, were subjected to radiocontrast media (RCM) (Ioversol, Optiray 320) exposure, followed by assessments of cellular viability [% lactate dehydrogenase (LDH) release, tetrazolium dye (MTT), uptake] and lipid peroxidation. These experiments were conducted in the absence or presence of a variety of antioxidants [NAC, glutathione (GSH), superoxide dismutase, catalase] or pro-oxidant (GSH depletion, heme oxygenase inhibition) strategies. RCM effects on mitochondrial and plasma membrane integrity were also assessed.

RESULTS

RCM exposure did not induce PTS lipid peroxidation. Neither antioxidant nor pro-oxidant interventions mitigated or exacerbated RCM-induced tubular cell injury, respectively. RCM impaired mitochondrial integrity, as assessed by ouabain-resistant ATP reductions, and by cytochrome c release (before cell death). RCM also induced plasma membrane damage, as indicated by loss of key resident proteins (NaK-ATPase, caveolin) and by increased susceptibility to phospholipase A2 (PLA2) attack (increase of >/=2 times in free fatty acid and NaK-ATPase release). Hyperosmolality could not account for RCM's toxic effects.

CONCLUSION

RCM toxicity can be dissociated from tubular cell oxidant stress. Alternative mechanisms may include mitochondrial injury/cytochrome c release and plasma membrane damage. The latter results in critical protein loss, as well as a marked increase in plasma membrane susceptibility to exogenous/endogenous PLA2 attack.

Release of intact, monomeric cytochrome c from apoptotic and necrotic cells

Cytochrome c (Cyt c) has been shown to translocate from mitochondria to the cytoplasm of cells early in apoptosis. In this study sandwich ELISAs for Cyt c were used to determine if Cyt c is ultimately released from apoptotic and necrotic cells. Gel-filtration and cation-exchange chromatographies, in conjunction with immunoreactivity in ELISA, and Western blotting were employed to examine the integrity of the released Cyt c. The results show that Cyt c is released from both apoptotic and necrotic cells in an intact, monomeric form. The release of Cyt c from apoptotic splenocytes began within 2 h following apoptotic insult, while Cyt c was immediately released following induction of necrosis by heat shock. These findings may be relevant to understanding how Cyt c becomes a target for antibody production in some patients with systemic autoimmune diseases.

Molecular characterization of a monoclonal murine anti-blood group A antibody

A mouse monoclonal anti-human blood group A antigen (AC12, mu, kappa) has been generated and sequenced in order to analyze the immunoglobulin genes used to generate antibodies with anti-human blood group A specificity. Mice were immunized with human type A RBC. Anti-A producing hybridomas were detected by agglutination against human type A RBC. Total cellular RNA was extracted from hybridomas cells. PCR amplification and sequencing of anti-A heavy and light chain cDNAs were performed. The VH and VK sequences of antibody AC12 were shown to be very homologous to that used by other antibodies recognizing carbohydrates as well as glycoproteins, peptides or haptens constituting self antigens as well as nonself antigens. The VH sequence of antibody AC12 presented important homology with a previously reported monoclonal anti-blood group B antibody. The antibody AC12 also presented homology with the VH and VK sequences of a previously reported human anti-blood group A antibody which contributes additional evidence in favor of a restricted usage of V segments by antibodies directed against red blood antigens.

Mechanisms of HIV-associated lymphocyte apoptosis

Infection with the human immunodeficiency virus (HIV) is associated with a progressive decrease in CD4 T-cell number and a consequent impairment in host immune defenses. Analysis of T cells from patients infected with HIV, or of T cells infected in vitro with HIV, demonstrates a significant fraction of both infected and uninfected cells dying by apoptosis. The many mechanisms that contribute to HIV-associated lymphocyte apoptosis include chronic immunologic activation; gp120/160 ligation of the CD4 receptor; enhanced production of cytotoxic ligands or viral proteins by monocytes, macrophages, B cells, and CD8 T cells from HIV-infected patients that kill uninfected CD4 T cells; and direct infection of target cells by HIV, resulting in apoptosis. Although HIV infection results in T-cell apoptosis, under some circumstances HIV infection of resting T cells or macrophages does not result in apoptosis; this may be a critical step in the development of viral reservoirs. Recent therapies for HIV effectively reduce lymphoid and peripheral T-cell apoptosis, reduce viral replication, and enhance cellular immune competence; however, they do not alter viral reservoirs. Further understanding the regulation of apoptosis in HIV disease is required to develop novel immune-based therapies aimed at modifying HIV-induced apoptosis to the benefit of patients infected with HIV.

Mechanisms of HIV-associated lymphocyte apoptosis

Infection with the human immunodeficiency virus (HIV) is associated with a progressive decrease in CD4 T-cell number and a consequent impairment in host immune defenses. Analysis of T cells from patients infected with HIV, or of T cells infected in vitro with HIV, demonstrates a significant fraction of both infected and uninfected cells dying by apoptosis. The many mechanisms that contribute to HIV-associated lymphocyte apoptosis include chronic immunologic activation; gp120/160 ligation of the CD4 receptor; enhanced production of cytotoxic ligands or viral proteins by monocytes, macrophages, B cells, and CD8 T cells from HIV-infected patients that kill uninfected CD4 T cells; and direct infection of target cells by HIV, resulting in apoptosis. Although HIV infection results in T-cell apoptosis, under some circumstances HIV infection of resting T cells or macrophages does not result in apoptosis; this may be a critical step in the development of viral reservoirs. Recent therapies for HIV effectively reduce lymphoid and peripheral T-cell apoptosis, reduce viral replication, and enhance cellular immune competence; however, they do not alter viral reservoirs. Further understanding the regulation of apoptosis in HIV disease is required to develop novel immune-based therapies aimed at modifying HIV-induced apoptosis to the benefit of patients infected with HIV.

Autocrine gastrins in colon cancer cells Up-regulate cytochrome c oxidase Vb and down-regulate efflux of cytochrome c and activation of caspase-3

Suppression of the gastrin gene in human colon cancer cells by stably expressing antisense (AS) gastrin RNA results in significant growth suppression of AS cells. To understand mechanisms mediating the growth effects of autocrine gastrins, differential expression of transcripts by AS and control (C) clones of a representative cell line (HCT-116) was analyzed to identify target genes of autocrine gastrins. Six differentially expressed transcripts were confirmed and sequenced. Of these, the RNA and protein levels of cytochrome c oxidase (COX) Vb were significantly higher in C versus AS cells. The expression of COX Vb by colon cancer cells was proportional to the expression of gastrin. Higher levels of COX Vb coprecipitated with cytochrome c in the mitochondria of C versus AS cells. Treatment of mitochondria with digitonin resulted in a 2-fold higher release of cytochrome c from AS versus C mitochondria. As a corollary, the cytosolic levels of cytochrome c were significantly higher in AS versus C cells, which correlated with approximately 2- and approximately 3-fold higher activation of caspase-9 and -3, respectively, in AS versus C cells in response to camptothecin. Thus, autocrine gastrins may support growth/survival of cells by up-regulating COX Vb, which may decrease the sensitivity of the cancer cells to apoptotic stimuli by increasing retention of cytochrome c in mitochondria.

Antibody-detected folding: kinetics of surface epitope formation are distinct from other folding phases

The rate of macromolecular surface formation in yeast iso-2 cytochrome c and its site-specific mutant, N52I iso-2, has been studied using a monoclonal antibody that recognizes a tertiary epitope including K58 and H39. The results indicate that epitope refolding occurs after fast folding but prior to slow folding, in contrast to horse cytochrome c where surface formation occurs early. The antibody-detected (ad) kinetic phase accompanying epitope formation has k(ad) = 0.2 s(-1) and is approximately 40-fold slower than the fastest detectable event in the folding of yeast iso-2 cytochrome c (k2f approximately 8 s(-1)), but occurs prior to the absorbance- and fluorescence-detected slow folding steps (k1a approximately 0.06 s(-1); k1b approximately 0.09 s(-1)). N5I iso-2 cytochrome c exhibits similar kinetic behavior with respect to epitope formation. A detailed dissection of the mechanistic differences between the folding pathways of horse and yeast cytochromes c identifies possible reasons for the slow surface formation in the latter. Our results suggest that non-native ligation involving H33 or H39 during refolding may slow down the formation of the tertiary epitope in iso-2 cytochrome c. This study illustrates that surface formation can be coupled to early events in protein folding. Thus, the rate of macromolecular surface formation is fine tuned by the residues that make up the surface and the interactions they entertain during refolding.

Structural differences among monoclonal antibodies with distinct fine specificities and kinetic properties

The mAbs HyHEL-8, HyHEL-26 (HH8, and HH26, respectively) recognize epitopes on hen egg-white lysozyme (HEL) highly overlapping with the structurally defined HH10 epitope, while the structurally related XRPC-25 is specific for DNP and does not bind HEL. All four Abs appear to use the same Vk23 germ line gene, and all but HH8 use the same VH36-60 germ line gene. Of the three anti-HEL Abs, the sequences of HH26 variable regions are closest to those encoded by the respective germ line sequences. HH8 utilizes a different member of the VH36-60 gene family. Thus, the same Vk and VH genes, combined with somatically derived sequence differences, are used to recognize the unrelated Ags HEL and DNP. In contrast, different VH36-60 germ line genes are used to bind the same antigen (e.g. HH8 vs HH10 and HH26). While the affinities of HH10, HH8, and HH26 for HEL vary by less than 10-fold, their affinities for mutated Ag vary over several orders of magnitude. Analyses of Fab binding kinetics with natural species variants and site-directed mutants of lysozyme indicate that these cross-reactivity differences reflect the relative sensitivities of both the association and dissociation rates to antigenic mutation: HH8 has relatively mutation-insensitive association and dissociation rates, HH10 has a relatively mutation-sensitive association rate but more variable dissociation rates, and HH26 has variable association and dissociation rates. Only a few amino acid differences among the antibodies produce the observed differences in the robustness of the association and dissociation rates. Our results suggest that association and dissociation rates and mutation sensitivity of these rates may be independently modulated during antibody repertoire development.

Antibody immunodiversity: a study on the marked specificity difference between two anti-yeast iso-1 cytochrome c monoclonal antibodies whose epitopes are closely related

Anti-yeast iso-1 cytochrome c (cyt. c) monoclonal antibodies 2-96-12 and 4-74-6 have closely related epitopes (antigenic determinants). However, while the specificity of 4-74-6 is stringent, 2-96-12 cross-reacts with many evolutionarily related cytochromes c. Such a marked difference in specificity of antibodies with overlapping epitopes may represent unique antibody immunodiversity. Thus, we constructed Fv fragment models consisting of the variable domains of the heavy and light chains of 2-96-12 and 4-74-6 and that of another anti-iso-1 cyt. c as a control to gain insight into the origin of this difference in specificity. Our models show that 4-74-6 and 2-96-12 contain five and two aromatic side chains, respectively, in or near the central area of the antigen-combining site. The side chains of Arg95H (heavy chain) in 2-96-12 and Arg91L (light chain) in 4-74-6 project toward the central area of the combining site in our model. Antigen docking to our Fv models, combined with previous immunological studies, suggests that iso-1 cyt. c Asp60 may interact with Arg95H in 2-96-12 and Arg91L in 4-74-6 and that both epitopes of 2-96-12 and 4-74-7 may include iso-1 cyt. c Leu58, Asp60, Asn62, and Asn63. The effect of the Arg95H to Lys mutation on the antigen binding is also in accord with our model. The difference in specificity may be partly explained by a greater degree of conformational flexibility in and around the central area of the combining site in 2-96-12 compared to 4-74-6 due to differences in aromatic side chain packing.

Pseudo-native motifs in the noncovalent heme-apocytochrome c complex. Evidence from antibody binding studies by enzyme-linked immunosorbent assay and microcalorimetry

When beef heart apocytochrome c is unfolded, it folds upon noncovalent heme binding (Dumont, M. E., Corin, A. F., and Campbell, G.A. (1994) Biochemistry, 33, 7368-7378). Here, the conformation of the heme-apocytochrome noncovalent complex is compared with that of holocytochrome c. A purification method was designed for obtaining in large amounts apocytochrome c that was shown by amino acid analysis and mass spectroscopy to be chemically intact. The apoprotein and its noncovalent complex were characterized by absorption, fluorescence, circular dichroism, and sedimentation velocity, confirming previous reports. Sedimentation-diffusion equilibrium showed that the apoprotein and its noncovalent complex with heme were monomeric. Surprisingly, whereas apocytochrome c was quite soluble, the noncovalent complex slowly formed heavy aggregates, thus precluding experiments at the concentrations needed for structural studies. Two monoclonal antibodies that bind strongly to distinct antigenic sites on native holocytochrome were used to probe the noncovalent complex conformation. For both antibodies, the affinity for the noncovalent complex was only about 5-10-fold smaller than that for native holocytochrome c, and about 50-100-fold larger than that for apocytochrome c. These results indicate that the noncovalent complex, although not entirely native, carries some pseudo-native structural motifs.

A conformational change in cytochrome c of apoptotic and necrotic cells is detected by monoclonal antibody binding and mimicked by association of the native antigen with synthetic phospholipid vesicles

By flow cytometry, a conformational change in mouse cytochrome c (cyt c) of apoptotic and necrotic T hybridoma cells was detected using a monoclonal antibody (mAb) that recognizes the region around amino acid residue 44 on a non-native form of the protein. The conformational change in cyt c is an early event in apoptosis, which can be identified in pre-apoptotic cells that are negative for other indicators of apoptosis. Since the mAb did not bind fixed and permeabilized live cells and did not immunoprecipitate soluble cyt c extracted with detergent from dead cells, it appears to recognize cyt cbound in a detergent-sensitive complex to other cellular components. Coincidentally, the mAb was also shown by competitive enzyme-linked immunosorbent assay to bind cyt c associated with synthetic phosphatidic acid vesicles. This suggests that the conformational change of cyt c in dying cells could be due to its association with intracellular membranes that are, perhaps, altered in cell death. By immunofluorescent confocal microscopy, conformationally altered cyt c in post-apoptotic T hybridoma cells showed a punctate distribution, indicating that it remained associated with mitochondria. Furthermore, the heavy membrane fraction of post-apoptotic cells but not of live cells was functional in caspase activation. This suggests that membrane-bound cyt c is the relevant caspase coactivation factor in the T hybridoma cells.

Altered cytochrome c display precedes apoptotic cell death in Drosophila

Drosophila affords a genetically well-defined system to study apoptosis in vivo. It offers a powerful extension to in vitro models that have implicated a requirement for cytochrome c in caspase activation and apoptosis. We found that an overt alteration in cytochrome c anticipates programmed cell death (PCD) in Drosophila tissues, occurring at a time that considerably precedes other known indicators of apoptosis. The altered configuration is manifested by display of an otherwise hidden epitope and occurs without release of the protein into the cytosol. Conditional expression of the Drosophila death activators, reaper or grim, provoked apoptogenic cytochrome c display and, surprisingly, caspase activity was necessary and sufficient to induce this alteration. In cell-free studies, cytosolic caspase activation was triggered by mitochondria from apoptotic cells but identical preparations from healthy cells were inactive. Our observations provide compelling validation of an early role for altered cytochrome c in PCD and suggest propagation of apoptotic physiology through reciprocal, feed-forward amplification involving cytochrome c and caspases.

B Cell Tolerance to a Minor, But Not to a Major, Antigenic Surface of the Self Antigen, Cytochrome c

To study B cell tolerance to the mitochondrial protein cytochrome c (CYT), the B cell response to pigeon CYT (PCC) was examined in mice transgenic for PCC. PCC was coupled to OVA to provide T cell help, since PCC-specific T cells in PCC-transgenic mice are deleted in the thymus. The frequency of secondary B cells responding to the minor antigenic surface around residue 44 on PCC was decreased about 10-fold in native PCC-transgenic mice compared with that in control mice or in transgenic mice expressing an altered form of PCC that lacked the heme and had a different amino acid sequence at the N-terminus. A similar decrease has been observed in the frequency of B cells in normal mice recognizing the site around residue 44 on mouse CYT compared with the frequency of B cells recognizing the corresponding site on foreign CYT. There were no major decreases but apparently were compensatory increases in the frequencies of B cells recognizing other sites on PCC in the native PCC-transgenic mice compared with those in other mice. These results indicate that B cells in mice are only partially tolerant to self CYT. A possible basis for this partial tolerance relating to the fate of CYT in cell death is discussed. This may be the first example of the use of a transgenic system to study B cell tolerance to a homologous self Ag.

Major and minor epitopes on the self antigen mouse cytochrome c mapped by site-directed mutagenesis

Variants of rat (mouse) cytochrome c, prepared by site-directed mutagenesis or represented by closely-related cytochromes c from different species, were employed to map the functional boundaries of a number of mouse monoclonal antibodies (mAb) specific for the major antigenic region on the self antigen (Ag) around residue 62 and the minor antigenic region around residue 44. The recombinant mouse cytochromes c tested were, unlike the tissue-derived Ag, trimethylated at position 72, and included the wild-type which was acetylated at the amino terminus, a variant that was unacetylated at the amino terminus, and variants with the following single amino acid residue replacements: V11I (valine to isoleucine at position 11), Q12M, A15S, A44P, F46Y, D50A, T58I and G89E. Of these, only the A44P variant affected the binding of mAb to the region previously localized to the vicinity of residue 44, thus confirming that assignment. Loss of the acetyl group at the amino terminus affected the binding of most of the mAb to the region around residue 62. The other mutations had little, if any, affect on mAb binding. The epitopes of mAb binding the region around residue 62 were shown in this study to have similar functional boundaries. This site on the self Ag, which encompasses at least three discontinuous segments of the polypeptide chain, is comparable in size to epitopes on other protein Ag that have been mapped by X-ray crystallography and is similar to an epitope in the corresponding region of the foreign Ag, horse cytochrome c, that has been mapped by hydrogen-deuterium exchange. In addition to the mAb binding the regions around residues 44 and 62, a third group of mouse cytochrome c-specific mAb known to be broadly reactive with mammalian cytochromes c and that represents a minor portion of the mAb was tested for binding the site-directed mutants of mouse cytochrome c. None of these mAb were affected by the mutations, indicating the presence of at least one more antigenic region on the self Ag in an area not encompassed by these mutations that is structurally highly conserved.

B lymphocyte recognition of cytochrome c: higher frequency of cells specific for self versus foreign antigen early in the immune response and V gene usage in the response to self antigen

To study immunoglobulin gene usage in the antibody response of mice to the self antigen (Ag) mouse cytochrome c (cyt), B cell hybridomas were prepared from splenic B cells of immunized BALB/c mice prior to the onset of somatic mutation, i.e. 3 days after injecting ovalbumin (OVA)-primed mice with mouse cyt coupled to OVA. Monoclonal antibodies (mAb) from all of the seven primary hybridomas we obtained were sensitive to a single amino acid substitution from aspartic acid to glutamic acid at position 62 in mouse cyt. This is the specificity of the vast majority of B cells responding to mouse cyt as determined from assays of B cells activated in splenic fragment cultures. Six of the mAb derive from the 19.1.2 J558 VH gene which is also used in the response to alpha (1-->6) dextran and three of these mAb derive from the R9 V kappa gene, a member of the V kappa Ox-1 family. The other mAb derive from distinct, although similar, V kappa genes. Attempts to obtain hybridomas secreting primary (unmutated) mAb specific for cyt foreign to mice have been hampered by the much lower frequency of B cells responding early to foreign cyt in comparison to the self Ag. This suggests that, contrary to expectation of tolerance mechanisms, in naive BALB/c mice B lymphocytes specific for a single epitope on self cyt are present in higher frequency than B lymphocytes specific for similar epitopes on foreign cyt. Possible explanations for this result include biased expression in the B cell repertoire of the particular combination of V genes encoding mouse cyt-specific mAb or to positive selection of developing B lymphocytes by endogenous Ag.

Differences in heavy chain amino acid sequences affecting the specificity of antibodies for variants of cytochrome c

In a previous study [Goshorn et al. (1991) J. biol. Chem. 266, 2134-2142], several mAb specific for the same region on different cytochromes c were shown to have similar H or L chains. To determine the effect of differences in individual chains on antigenic variant specificity in the present study, chimeric mAb were prepared by recombining the H and L chains of mAb having the same or a different cytochrome c specificity. The H and L chains of two mAb to the region around residue 60 on horse cytochrome c (1F5.D1 and 2E5.G10) were functionally interchangeable even though the H chain differed by 11 amino acid residues in the complementarity-determining regions (CDR) and 15 amino acids overall in the variable regions. The L chains only differed by four amino acid residues in the CDR (five residues overall). Neither the H nor L chain of a mAb binding the same region of rat cytochrome c (6H2.B4) was functionally interchangeable with the chains of the two horse cytochrome c-specific mAb. The L chain of this mAb is very different from the other L chains which were derived from a different V kappa family, but the H chain is nearly as similar to the horse cytochrome c-specific H chains as they are to each other. Most of the differences occur in CDR3 and result from the use of a distinct DH segment. The results indicate that, in some cases, the specificity of a mAb for a particular variant of a protein Ag, at least in regard to the H chain, is determined by only a few amino acid differences. The differences in the sequences of the H chains of the three mAb in this study and in the structures of their specific Ag provide insight into a possible molecular basis for the specificity of these mAb.

Protein—protein interactions: structural motifs and molecular recognition

The past year has brought new insights into common structural motifs used for protein-protein interactions by DNA-binding proteins. In addition, there have been significant advances in our understanding of antibody-protein complexes.