Paradoxical Roles of Leucine-Rich α2-Glycoprotein-1 in Cell Death and Survival Modulated by Transforming Growth Factor-Beta 1 and Cytochrome c

Leucine-rich α₂-glycoprotein-1 (LRG1) has been shown to impact both apoptosis and cell survival, pleiotropic effects similar to one of its known ligands, transforming growth factor-beta 1 (TGF-β1). Recent studies have given insight into the TGF-β1 signaling pathways involved in LRG1-mediated death versus survival signaling, i.e., canonical or non-canonical. Interaction of LRG1 with another ligand, extracellular cytochrome c (Cyt c), promotes cell survival, at least for lymphocytes. LRG1 has been shown to bind Cyt c with high affinity, higher than it binds TGF-β1, making it sensitive to small changes in the level of extracellular Cyt c within a microenvironment that may arise from cell death. Evidence is presented here that LRG1 can bind TGF-β1 and Cyt c simultaneously, raising the possibility that the ternary complex may present a signaling module with the net effect of signaling, cell death versus survival, determined by the relative extent to which the LRG1 binding sites are occupied by these two ligands. A possible role for LRG1 should be considered in studies where extracellular effects of TGF-β1 and Cyt c have been observed in media supplemented with LRG1-containing serum.

The proapoptotic function of Noxa in human leukemia cells is regulated by the kinase Cdk5 and by glucose

The BH3-only protein, Noxa, is induced in response to apoptotic stimuli, such as DNA damage, hypoxia, and proteasome inhibition in most human cells. Noxa is constitutively expressed in proliferating cells of hematopoietic lineage and required for apoptosis in response to glucose stress. We show that Noxa is phosphorylated on a serine residue (S(13)) in the presence of glucose. Phosphorylation promotes its cytosolic sequestration and suppresses its apoptotic function. We identify Cdk5 as the Noxa kinase and show that Cdk5 knockdown or expression of a Noxa S(13) to A mutant increases sensitivity to glucose starvation, confirming that the phosphorylation is protective. Both glucose deprivation and Cdk5 inhibition promote apoptosis by dephosphorylating Noxa. Paradoxically, Noxa stimulates glucose consumption and may enhance glucose turnover via the pentose phosphate pathway rather than through glycolysis. We propose that Noxa plays both growth-promoting and proapoptotic roles in hematopoietic cancers with phospho-S(13) as the glucose-sensitive toggle switch controlling these opposing functions.

Leucine-rich alpha-2-glycoprotein-1 is upregulated in sera and tumors of ovarian cancer patients

BACKGROUND

New biomarkers that replace or are used in conjunction with the current ovarian cancer diagnostic antigen, CA125, are needed for detection of ovarian cancer in the presurgical setting, as well as for detection of disease recurrence. We previously demonstrated the upregulation of leucine-rich alpha-2-glycoprotein-1 (LRG1) in the sera of ovarian cancer patients compared to healthy women using quantitative mass spectrometry.

METHODS

LRG1 was quantified by ELISA in serum from two relatively large cohorts of women with ovarian cancer and benign gynecological disease. The expression of LRG1 in ovarian cancer tissues and cell lines was examined by gene microarray, reverse-transcriptase polymerase chain reaction (RT-PCR), Western blot, immunocytochemistry and mass spectrometry.

RESULTS

Mean serum LRG1 was higher in 58 ovarian cancer patients than in 56 healthy women (89.33 ± 77.90 vs. 42.99 ± 9.88 ug/ml; p = 0.0008) and was highest among stage III/IV patients. In a separate set of 193 pre-surgical samples, LRG1 was higher in patients with serous or clear cell ovarian cancer (145.82 ± 65.99 ug/ml) compared to patients with benign gynecological diseases (82.53 ± 76.67 ug/ml, p < 0.0001). CA125 and LRG1 levels were moderately correlated (r = 0.47, p < 0.0001). LRG1 mRNA levels were higher in ovarian cancer tissues and cell lines compared to their normal counterparts when analyzed by gene microarray and RT-PCR. LRG1 protein was detected in ovarian cancer tissue samples and cell lines by immunocytochemistry and Western blotting. Multiple iosforms of LRG1 were observed by Western blot and were shown to represent different glycosylation states by digestion with glycosidase. LRG1 protein was also detected in the conditioned media of ovarian cancer cell culture by ELISA, Western blotting, and mass spectrometry.

CONCLUSIONS

Serum LRG1 was significantly elevated in women with ovarian cancer compared to healthy women and women with benign gynecological disease, and was only moderately correlated with CA125. Ovarian cancer cells secrete LRG1 and may contribute directly to the elevated levels of LRG1 observed in the serum of ovarian cancer patients. Future studies will determine whether LRG1 may serve as a biomarker for presurgical diagnosis, disease recurrence, and/or as a target for therapy.

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.

Cytochrome C release from CNS mitochondria and potential for clinical intervention in apoptosis-mediated CNS diseases

Apoptosis is critical for normal development and tissue homeostasis. However, its abnormal occurrence has been implicated in a number of disorders, including neurodegenerative diseases and stroke. Translocation of cytochrome c (Cyt c) from mitochondria to the cytoplasm is a key step in the initiation and/or amplification of apoptosis. Here we discuss Cyt c release in apoptosis with its impact on the CNS and review our studies of Cyt c release from isolated rat brain mitochondria in response to several insults. Calcium-induced Cyt c release, as occurs in neurons during stroke and ischemia, involves rupture of the mitochondrial outer membrane (MOM) and can be blocked by inhibitors of the mitochondrial permeability transition (mPT). Thus, inhibitors of the mPT have shown efficacy in animal models of ischemia. In contrast, proapoptotic proteins, such as BID, BAX, and BAK, induce Cyt c release independently of the mPT without lysing the MOM. Several inhibitors of BAX-induced Cyt c release have shown promise in models of CNS apoptosis. Because of their distinct mechanisms for Cyt c release, both the mPT and proapoptotic proteins should be targeted for effective clinical intervention in CNS disorders involving apoptosis.

Activation of calcium-independent phospholipase A2 (iPLA2) in brain mitochondria and release of apoptogenic factors by BAX and truncated BID

Cleaved or truncated BID (tBID) is known to oligomerize both BAK and BAX. Previously, BAK and BAX lacing the C-terminal fragment (BAXDeltaC) were shown to induce modest cytochrome c (Cyt c) release from rat brain mitochondria when activated by tBID. We now show that tBID plus monomeric full-length BAX induce extensive release of Cyt c, Smac/DIABLO, and Omi/HtrA2 (but not endonuclease G and the apoptosis inducing factor) comparable to the release induced by alamethicin. This occurs independently of the permeability transition without overt changes in mitochondrial morphology. The mechanism of the release may involve formation of reactive oxygen species (ROS) and activation of calcium-independent phospholipase A(2) (iPLA(2)). Indeed, increased ROS production and activated iPLA(2) were observed prior to massive Cyt c release. Furthermore, the extent of inhibition of Cyt c release correlated with the degree of suppression of iPLA(2) by the inhibitors propranolol, dibucaine, 4-bromophenacyl bromide, and bromenol lactone. Consistent with a requirement for iPLA(2) in Cyt c release from brain mitochondria, synthetic liposomes composed of lipids mimicking the outer mitochondrial membrane (OMM) but lacing iPLA(2) failed to release 10 kDa fluorescent dextran (FD-10) in response to tBID plus BAX. We propose that tBID plus BAX activate ROS generation, which subsequently augments iPLA(2) activity leading to changes in the OMM that allow translocation of certain mitochondrial proteins from the intermembrane space.

Two pathways for tBID-induced cytochrome c release from rat brain mitochondria: BAK- versus BAX-dependence

The mechanisms of truncated BID (tBID)-induced Cyt c release from non-synaptosomal brain mitochondria were examined. Addition of tBID to mitochondria induced partial Cyt c release which was inhibited by anti-BAK antibodies, implicating BAK. Immunoblotting showed the presence of BAK, but not BAX, in brain mitochondria. tBID did not release Cyt c from rat liver mitochondria, which lacked both BAX and BAK. This indicated that tBID did not act independently of BAX and BAK. tBID plus monomeric BAX produced twice as much Cyt c release as did tBID or oligomeric BAX alone. Neither tBID alone nor in combination with BAX induced mitochondrial swelling. In both cases Cyt c release was insensitive to cyclosporin A plus ADP, inhibitors of the mitochondrial permeability transition (mPT). Recombinant Bcl-xL inhibited Cyt c release induced by tBID alone or in combination with monomeric BAX. Koenig's polyanion, an inhibitor of VDAC, suppressed tBID-induced Cyt c release from brain mitochondria mediated by BAK but not by BAX. Thus, tBID can induce mPT-independent Cyt c release from brain mitochondria by interacting with exogenous BAX and/or with endogenous BAK that may involve VDAC. In contrast, neither adenylate kinase nor Smac/DIABLO was released from isolated rat brain mitochondria via BAK or BAX.

Calcium-induced Cytochrome c release from rat brain mitochondria is altered by digitonin

To determine if calcium could release Cytochrome c (Cyt c) from brain mitochondria without activating the permeability transition (mPT), brain mitochondria were prepared in two different ways. Digitonin was used to lyse synaptosomes and release synaptosomal mitochondria or a Percoll gradient was used to separate non-synaptosomal mitochondria from the synaptosomes. In gradient-purified mitochondria, low levels of added digitonin produced swelling and Cyt c release. Digitonin augmented Ca(2+)-induced Cyt c release that was insensitive to the mPT inhibitors, cyclosporin A CsA and ADP. Similarly, in mitochondria prepared with digitonin, these inhibitors also failed to prevent Ca(2+)-induced Cyt c release. Thus the mPT-independent, Ca(2+)-induced Cyt c release pathway was attributable to alteration of the permeability properties of the outer mitochondrial membrane by digitonin.

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.

Calcium-induced cytochrome c release from CNS mitochondria is associated with the permeability transition and rupture of the outer membrane

The mechanisms of Ca2+-induced release of Cytochrome c (Cyt c) from rat brain mitochondria were examined quantitatively using a capture ELISA. In 75 or 125 mm KCl-based media 1.4 micromol Ca2+/mg protein caused depolarization and mitochondrial swelling. However, this resulted in partial Cyt c release only in 75 mm KCl. The release was inhibited by Ru360, an inhibitor of the Ca2+ uniporter, and by cyclosporin A plus ADP, a combination of mitochondrial permeability transition inhibitors. Transmission electron microscopy (TEM) revealed that Ca2+-induced swelling caused rupture of the outer membrane only in 75 mm KCl. Koenig's polyanion, an inhibitor of mitochondrial porin (VDAC), enhanced swelling and amplified Cyt c release. Dextran T70 that is known to enhance mitochondrial contact site formation did not prevent Cyt c release. Exposure of cultured cortical neurons to 500 microM glutamate for 5 min caused Cyt c release into the cytosol 30 min after glutamate removal. MK-801 or CsA inhibited this release. Thus, the release of Cyt c from CNS mitochondria induced by Ca2+ in vitro as well as in situ involved the mPT and appeared to require the rupture of the outer membrane.

Cytochrome-C localizes in secretory granules in pancreas and anterior pituitary

We used quantitative immunogold electron microscopy to evaluate the subcellular distribution of cytochrome-c in normal rat tissues, employing a wide variety of monoclonal and polyclonal antibodies against mammalian cytochrome-c. Immunogold labeling of tissues embedded in the acrylic resin LR Gold shows highly specific labeling of mitochondria in all tissues examined, including adrenal gland, cerebellum, cerebral cortex, heart, kidney, liver, pituitary, pancreas, skeletal muscle, spleen and thyroid. In pancreatic acinar cells and anterior pituitary, however, there was also strong cytochrome-c reactivity in zymogen granules and growth hormone granules, respectively. In the pancreas, strong immunoreactivity is also detected in condensing vacuoles and in the acinar lumen. Immunocytochemical controls included (i) use of monoclonal antibodies to horse cytochrome-c which recognize an epitope not present in rat cytochrome-c, (ii) preadsorption of antibodies with purified cytochrome-c, and (iii) omission of the primary antibody. The indicated presence of cytochrome-c outside mitochondria in certain tissues under normal physiological conditions raises interesting questions concerning translocation mechanisms and the cellular functions of cytochrome-c.

Kinetic and genetic bases for the heteroclitic recognition of mouse cytochrome c by mouse anti-pigeon cytochrome c monoclonal antibodies

The B lymphocyte response to pigeon cytochrome c (CYT) in BALB/c mice was previously shown to initiate as a heteroclitic response specific for the self antigen mouse CYT. As the immune response progressed, the mAb that were produced became less heteroclitic and often bound pigeon CYT with higher affinity than they bound mouse CYT [Minnerath, J. et al., 1995. Proc. Natl. Acad. Sci. USA 92, 12379-12383]. To study the basis for heterocliticity and its loss in this system, the H and L chain amino acid sequences of anti-pigeon CYT mAb obtained from the primary and secondary Ab responses were first compared. The most frequent somatic mutations and Ig gene joints were then introduced into an engineered single-chain Fv (scFv) that expressed the germline-encoded V(H) and V(L) amino acid sequences. The effects of those changes on the on- rate, off-rate, and affinity constants in binding both mouse and pigeon CYT were determined by surface plasmon resonance. In this system, the heterocliticity of primary mAb was largely due to a decreased on-rate constant for binding pigeon CYT relative to mouse CYT. Various combinations of the three frequently occurring H chain somatic mutations (H31, H56, and H58) increased the on-rate constant to maximal levels. Only one of the mutations (H58) decreased the off-rate constant when in combination with the other mutations and the effect of H58 was greater for scFv binding mouse CYT than pigeon CYT. Consequently, the mutated scFv and many secondary mAb remained heteroclitic, although their affinities for pigeon CYT increased. Secondary mAb that were no longer heteroclitic expressed non-canonical amino acid sequences in the V(H)-D-J(H) joint in the context of the canonical V genes or expressed different V genes. In addition to providing insight into the molecular basis for heterocliticity, our findings confirm that both faster on-rate and slower off-rate constants are favored during affinity maturation of the Ab response.

Immunoglobulin gene joints compensate for reduced on-rates imposed by somatic mutations in a V(H) gene

Affinity maturation in the B lymphocyte response to a protein epitope appears to be largely due to a decrease in the off-rate constant of the antibodies (Ab) resulting from somatic mutation without a significant increase in the on-rate constant. Here, we show by site-directed mutagenesis of a germline encoded single-chain Fv that somatic mutations frequently selected in the Ab response to mouse cytochrome c (CYT) at heavy (H) chain positions 31 and 58 actually cause a two and three-fold decrease, respectively, in the on-rate constant as well as a two and five-fold decrease, respectively, in the off-rate constant and together cause nearly an eight-fold decrease in the off-rate. However, additional selection for a tyrosine residue at position 96 in the V(kappa)-J(kappa) joint compensates for the decreased on-rate imposed by the somatic mutations. This allows for an increase in the affinity of Ab during the secondary response. Certain sequences at the V(H)-D-J(H) joint were also shown to maintain a normal on-rate constant in the context of the common H chain mutations and, in addition, to reduce the off-rate, thus increasing the affinity. The results support the idea that both faster on-rates and slower off-rates for B lymphocyte antigen-specific receptors are favored during the maturation of the Ab response to mouse CYT.

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.

Characterization of monoclonal antibodies to an immunodominant protein of the etiologic agent of human granulocytic ehrlichiosis

Immunodominant proteins in the range of 42-45 kD are important for the serodiagnosis of human granulocytic ehrlichiosis (HGE). Antigens from human isolates of the etiologic agent of HGE cultivated in HL-60 cells were used to immunize BALB/c mice and generate a panel of hybridomas secreting monoclonal antibodies. Using an enzyme immunoassay, an immunofluorescent assay (IFA), and Western blotting, we showed that culture supernatants and ascites of these hybridomas were reactive with human isolates of the etiologic agent of HGE, Ehrlichia equi and E. phagocytophila. Following screening and subcloning, we selected three stable hybridomas, R1B10, R5E4, and R5A9, which were determined to be of the isotypes IgG3, IgG1, and IgG2a, respectively. These results suggest that the epitopes of the 42-45-kD protein recognized by these three monoclonal antibodies are conserved among E. equi, E. phagocytophila, and the etiologic agent of HGE. Western blot analysis showed reactivity with the 44-kD protein of human isolates of the HGE agent. None of the monoclonal antibodies were reactive with HL-60 cells that were not infected with the HGE agent. No cross-reactivity with related intracellular pathogens could be detected when undiluted supernatants from hybridoma cultures were allowed to react by IFA with antigens from E. chaffeensis, E. risticii, E. platys, Rickettsia rickettsii, R. prowazekii, or Coxiella burnetii. The additivity index of two antibodies, R5E4 and R1B10 was near zero, suggesting that these two antibodies may compete for the same epitope of the 44-kD protein, while monoclonal antibody R5A9 appears to interact with a different epitope. The antibodies secreted by these hybridomas may be useful as immunologic agents in serodiagnostic, immunohistochemical, and other studies of the etiologic agent of HGE.

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.

Regulated targeting of BAX to mitochondria

The proapoptotic protein BAX contains a single predicted transmembrane domain at its COOH terminus. In unstimulated cells, BAX is located in the cytosol and in peripheral association with intracellular membranes including mitochondria, but inserts into mitochondrial membranes after a death signal. This failure to insert into mitochondrial membrane in the absence of a death signal correlates with repression of the transmembrane signal-anchor function of BAX by the NH2-terminal domain. Targeting can be instated by deleting the domain or by replacing the BAX transmembrane segment with that of BCL-2. In stimulated cells, the contribution of the NH2 terminus of BAX correlates with further exposure of this domain after membrane insertion of the protein. The peptidyl caspase inhibitor zVAD-fmk partly blocks the stimulated mitochondrial membrane insertion of BAX in vivo, which is consistent with the ability of apoptotic cell extracts to support mitochondrial targeting of BAX in vitro, dependent on activation of caspase(s). Taken together, our results suggest that regulated targeting of BAX to mitochondria in response to a death signal is mediated by discrete domains within the BAX polypeptide. The contribution of one or more caspases may reflect an initiation and/or amplification of this regulated targeting.

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.

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.

A cytosolic factor is required for mitochondrial cytochrome c efflux during apoptosis

Treatment of HL-60 cells with staurosporine (STS) induced mitochondrial cytochrome c efflux into the cytosol, which was followed by caspase-3 activation and apoptosis. Consistent with these observations, in vitro experiments demonstrated that, except for cytochrome c, the cytosol of HL-60 cells contained sufficient amounts of all factors required for caspase-3 activation. In contrast, treatment of HCW-2 cells (an apoptotic-resistant HL-60 subclone) with STS failed to induce significant amounts of mitochondrial cytochrome c efflux, caspase-3 activation, and apoptosis. In vitro assays strongly suggested that a lack of cytochrome c in the cytosol was the primary limiting factor for caspase-3 activation in HCW-2 cells. To explore the mechanism which regulates mitochondrial cytochrome c efflux, we developed an in vitro assay which showed that cytosolic extracts from STS-treated, but not untreated, HL-60 cells contained an activity, which we designated 'CIF' (cytochrome c-efflux inducing factor), which rapidly induced cytochrome c efflux from HL-60 mitochondria. In contrast, there was no detectable CIF activity in STS-treated HCW-2 cells although the mitochondria from HCW-2 cells were responsive to the CIF activity from STS-treated HL-60 cells. These experiments have identified a novel activity, CIF, which is required for cytochrome c efflux and they indicate that the absence of CIF is the biochemical explanation for the impaired ability of HCW-2 cells to activate caspase-3 and undergo apoptosis.

Involvement of a caspase-3-like cysteine protease in 1-methyl-4-phenylpyridinium-mediated apoptosis of cultured cerebellar granule neurons

Exposure of various neuronal cells or cell lines to high concentrations of 1-methyl-4-phenylpyridinium (MPP+), the active metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), results in cell death. Recently, it has been reported that low concentrations of MPP+ induce apoptosis in susceptible neurons. We have further characterized MPP+-mediated toxicity of cultured cerebellar granule neurons (CGNs) and found that exposure of CGNs to relatively low concentrations of MPP+ results in apoptosis, whereas higher concentrations result in necrosis. Cotreatment of CGNs with MPP+ and the tetrapeptide inhibitor of caspase-3-like proteases, acetyl-DEVD-CHO, markedly attenuates apoptotic but not necrotic death of these neurons. The more specific inhibitor of caspase-1-like proteases, acetyl-YVAD-CHO, however, was ineffective against MPP+ neurotoxicity. Moreover, cytoplasmic extracts prepared from MPP+-treated CGNs contain markedly increased protease activity that cleaves the caspase-3 substrate acetyl-DEVD-p-nitroaniline. Finally, the cytoplasmic concentration of the apoptogenic protein cytochrome c was increased in a time-dependent fashion in MPP+-treated CGNs before the onset of apoptosis. Our data confirm that the neurotoxicity of MPP+ is due to both necrosis and apoptosis and suggest that the latter is mediated by activation of a caspase-3-like protease.

B lymphocyte recognition of the self antigen mouse cytochrome C in different mouse strains: targeting of the same dominant epitope by naturally-occurring cells expressing distinct VH genes

Previously we reported that, early in the antibody response of BALB/c mice to several cytochromes c (CYT) coupled to ovalbumin (OVA), B cells responding to the self antigen mouse CYT recognized a single site on mouse CYT and were in much higher frequency than B cells responding to foreign CYT. In the present study these B cells were shown by in vitro activation of primary splenocytes to be present in naive BALB/c mice, i.e. prior to exposure to exogenous CYT. The higher frequency of B cells responsive to self versus foreign CYT was also shown in this study to occur in the early antibody response to CYT-OVA in C57BL/6 mice. The same dominant site was recognized in BALB/c mice (IgHa), C57BL/6 mice (IgHb) and the congenic strains BC-17 (IgHa on the C57BL/6 background) and CB-20 (IgHb on the BALB/c background). However, anti-mouse CYT mAbs produced in IgHb mice were shown to derive from the VH gene 5.54.4 while mAbs in IgHa mice derive from the VH gene 19.1.2. The polypeptides encoded by these VH genes, which differ by only five amino acid residues, paired with polypeptides encoded by the same Vk genes (R9 and 2G5). In both VH 19.1.2- and VH 5.54.4-derived mAbs H3 and the Vk-Jk join were variable. The affinity for mouse CYT was reduced in the VH gene 5.54.4-derived mAb due to a faster off-rate constant. This difference in affinity may relate to the lower frequency of B cells responding to mouse CYT in C57BL/6 mice than in BALB/c mice. The results show that naturally-occurring CYT-specific autoreactive B cells occur normally in more than one mouse strain and that self antigen recognition by those cells appears to be atypical involving mostly the immunoglobulin V gene-encoded segments.

Maturation of the antibody response to the major epitope on the self antigen mouse cytochrome c. Restricted V gene usage, selected mutations, and increased affinity

Most B lymphocytes recognizing the major epitope on the self Ag mouse cytochrome c (CYT) express one particular VH gene (19.1.2) in combination with one of two Vkappa genes (R9 or 2G5). This restriction made it possible to observe changes in the primary structure of mouse CYT-specific mAb as the B cell response to mouse CYT (coupled to OVA) progressed. Thus, somatic mutations at positions 31 and 58 in the VH gene complementarity-determining regions (CDR) 1 and 2, respectively, and tyrosine at position 96 in the Vkappa-Jkappa join were frequently selected during the response. The affinity constant (ka), which increased as much as 90-fold due to a decrease in the off-rate constant, correlated with the number of somatic mutations but not strictly with the commonly selected changes. There appears to be little selection for a particular H chain CDR3 in the mouse CYT-specific mAb since it is extremely variable, in sequence and length, throughout the response. This variability may reflect that mouse CYT is a self Ag. Otherwise, the Ab response to this protein Ag is consistent with the paradigm for affinity maturation that is well established for Ab responses to haptens.

Maturation of the antibody response to the major epitope on the self antigen mouse cytochrome c. Restricted V gene usage, selected mutations, and increased affinity

Most B lymphocytes recognizing the major epitope on the self Ag mouse cytochrome c (CYT) express one particular VH gene (19.1.2) in combination with one of two Vkappa genes (R9 or 2G5). This restriction made it possible to observe changes in the primary structure of mouse CYT-specific mAb as the B cell response to mouse CYT (coupled to OVA) progressed. Thus, somatic mutations at positions 31 and 58 in the VH gene complementarity-determining regions (CDR) 1 and 2, respectively, and tyrosine at position 96 in the Vkappa-Jkappa join were frequently selected during the response. The affinity constant (ka), which increased as much as 90-fold due to a decrease in the off-rate constant, correlated with the number of somatic mutations but not strictly with the commonly selected changes. There appears to be little selection for a particular H chain CDR3 in the mouse CYT-specific mAb since it is extremely variable, in sequence and length, throughout the response. This variability may reflect that mouse CYT is a self Ag. Otherwise, the Ab response to this protein Ag is consistent with the paradigm for affinity maturation that is well established for Ab responses to haptens.

Induction of apoptotic program in cell-free extracts: requirement for dATP and cytochrome c

A cell-free system based on cytosols of normally growing cells is established that reproduces aspects of the apoptotic program in vitro. The apoptotic program is initiated by addition of dATP. Fractionation of cytosol yielded a 15 kDa protein that is required for in vitro apoptosis. The absorption spectrum and protein sequence revealed that this protein is cytochrome c. Elimination of cytochrome c from cytosol by immunodepletion, or inclusion of sucrose to stabilize mitochondria during cytosol preparation, diminished the apoptotic activity. Adding back cytochrome c to the cytochrome c-depleted extracts restored their apoptotic activity. Cells undergoing apoptosis in vivo showed increased release of cytochrome c to their cytosol, suggesting that mitochondria may function in apoptosis by releasing cytochrome c.

The BALB/c mouse B-cell response to pigeon cytochrome c initiates as a heteroclitic response specific for the self antigen mouse cytochrome c

Direct evidence is presented in support of the longstanding but unproven hypothesis that B lymphocytes specific for self antigens (Ags) can be used in the immune response to foreign Ags. We show that the B cells in BALB/c mic responding early to pigeon cytochrome c (CYT) produce antibodies that recognize and bind the major antigenic site on mouse CYT with greater affinity than they bind pigeon CYT i.e., they are heteroclitic for the self Ag. Furthermore, these B cells express the same combination of immunoglobulin variable region (V) genes that are known to be used in B-cell recognition of mouse CYT. Over time, the response to pigeon CYT becomes more specific for the foreign Ag through the recruitment of B cells expressing different combinations of V genes and, possibly, somatic mutation of the mouse CYT specific B cells from early in the response. Cross-recognition of pigeon CYT by mouse CYT-specific B cells results from the sharing of critical amino acid residues by the two Ags. Although B-cell recognition of the self Ag, mouse CYT, is very specific, which limits the extent to which foreign Ags can cross-activate the autoreactive B cells, it is possible that polyreactive B cells to other self Ags may be used more frequently in response to foreign Ags.

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.

Rapid refolding of native epitopes on the surface of cytochrome c

Refolding of surface epitopes on horse cytochrome c has been measured by monoclonal antibody binding. Two antibodies were used to probe re-formation of native-like surface structure: one antibody (2B5) binds to native cytochrome c near a type II turn (residue 44) while the other (5F8) binds to a different epitope on the opposite face of the protein near the amino terminus of an alpha-helical segment (residue 60). The results show that within the first approximately 100 ms of refolding all of the unfolded protein collapses to native-like folding intermediates that contain both antibody binding sites. All three absorbance/fluorescence-detected kinetic phases in the folding of cytochrome c (k1 approximately 5 s-1, k2 approximately 0.4 s-1, k3 approximately 0.03 s-1) are slower than the rates of re-formation of the antibody binding sites (k(obs) > 10.0 s-1), suggesting that the formation of antibody binding sites precedes the refolding reactions observed in kinetically resolved optically-detected refolding phases. Kinetically unresolved folding processes account for 79% and 19% of the total fluorescence change and absorbance change, respectively, observed in equilibrium unfolding. Thus, kinetically unresolved folding reactions appear to be responsible for re-formation of the MAb binding sites within partially folded intermediate species. These species are non-native (incompletely folded) in that their optical properties are in between those of the unfolded and the fully folded protein. As a test of whether antibody binding to folding intermediate(s) perturbs further folding, the rate of the absorbance-detected slow refolding phase has been measured for folding intermediate(s) of cytochrome c complexed with antibodies.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallization of two monoclonal Fab fragments of similar amino-acid sequence bound to the same area of horse cytochromecand interacting by potentially distinct mechanisms

The mouse monoclonal antibodies (mAb), 2E5.G10 and 1F5.D1, are specific for horse cytochrome c and appear to bind the same epitope, since their heavy (H) and light (L) chains are functionally interchangeable. Comparison of the amino-acid sequences suggests that slightly different interactions may be involved in antigen recognition. In addition, the H chains differ at only a few amino-acid residues from the H chain of a rat cytochrome c-specific mAb suggesting that specificity for one protein over another may be determined by these amino-acid differences. To address these possibilities, the three-dimensional structures of the Fab portions of the mAb bound to cytochrome c are being determined by X-ray diffraction analysis. Here we describe the preparation and crystallization of the two complexes with horse cytochrome c. The complex of the Fab fragment of 2E5.G10 with horse cytochrome c yielded crystals of X-ray diffraction quality under two sets of conditions; in both the space group was P2(1). The corresponding complex of 1F5.D1 under one of these conditions crystallized in the P2(1)2(1)2(1) space group. Three-dimensional X-ray data for these two complexes have been collected with nominal resolutions of 2.86 and 2.48 A, respectively.

The murine immune response to the male-specific antigen mouse testicular cytochrome c

Male and female A/J mice were examined for their ability to elicit T lymphocyte and antibody (Ab) responses to the male-specific Ag, mouse testicular cytochrome c (Mt cyt). T lymphocytes from both male and female mice primed in vivo responded to the Ag in in vitro proliferation assays, and the dose-response curves were statistically indistinguishable. In addition, similar levels of Ab to Mt cyt were observed in immunized male and female mice. The B cells producing the Ab had switched isotypes to IgG1 and IgG2a, indicating that the self-reactive T helper (Th) cells in male mice were functional. Thus, male mice do not appear to be immunologically tolerant to Mt cyt, at least at the Th and B lymphocyte levels. No evidence for disease was found in male mice primed with Mt cyt. Major histocompatibility complex (MHC) class II-positive antigen-presenting cells are present in the testes and these were shown in vitro to process and present Mt cyt to a T cell hybridoma specific for the synthetic peptide Mt cyt 93-104. However, the hybridoma was not activated in the absence of exogenous Mt cyt 93-104 or Mt cyt, indicating that endogenous Mt cyt is not normally processed in sufficient quantity to effectively load MHC class II molecules with this particular Mt cyt-derived peptide. Notwithstanding any immunologic privilege of the testes, the lack of tolerance to Mt cyt and its failure to elicit an autoimmune disease could extend from the low levels of processed Mt cyt Ag available for T cell recognition. The T cell response elicited by Mt cyt contrasts the lack of response to mouse somatic cytochrome c which differs from Mt cyt at 13 amino acid residues and is expressed in most tissues and at higher levels.

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.

A single amino acid substitution in a cytochrome c T cell stimulatory peptide changes the MHC restriction element from one isotype (I-Ak) to another (I-Ek)

The binding sites of class II major histocompatibility complex (MHC) molecules can accommodate many seemingly diverse peptides. In the case of mouse class II molecules, it appears that in general, the I-A and I-E isotypes associate with different peptides. In this study we report an example where a single amino acid substitution in an I-Ak restricted peptide changes the restriction element to I-Ek. A T cell hybridoma, F6.A10, specific for the peptide 93-104 from mouse testicular cytochrome c (Mt cyt 93-104) was found to be restricted by I-Ak using class II molecule specific blocking monoclonal antibodies (mAb). The activation of this hybridoma by Mt cyt 93-104 was competitively inhibited by other peptides that bind to the I-Ak molecule but not by the peptide Mt cyt 93-104(A96) in which lysine at position 96 was substituted by alanine. This single amino acid substitution resulted in the ability of Mt cyt 93-104(A96) to activate the pigeon cytochrome c specific, I-Ek restricted, T cell hybridoma 2B4.11. The activation of 2B4.11 by Mt cyt 93-104(A96) was inhibited by peptides which bind to the I-Ek molecule but not by Mt cyt 93-104 and by mAb specific for I-Ek but not by mAb specific for I-Ak. These results suggest that the amino acid at position 96 may be an important anchor residue for both I-Ak and I-Ek binding but that peptides with different amino acid side chains are accommodated at that position by one or the other MHC class II isotype. Thus, in this particular case a single amino acid residue in the peptides determines the MHC class II isotype specificity.

Diffusion-limited rates for monoclonal antibody binding to cytochrome c

The kinetic and spectroscopic changes accompanying the binding of two monoclonal antibodies to the oxidized form of horse heart cytochrome c have been investigated. The two epitopes recognized by the antibodies are distinct and noninteracting: antibody 2B5 binds to native cytochrome c near a type II turn (residue 44) while antibody 5F8 binds on the opposite face of the protein near the amino terminus of an alpha-helical segment (residue 60). Antibody-cytochrome c binding obeys a simple bimolecular reaction mechanism with second-order rate constants approaching those expected for diffusion-limited protein-protein interactions. The association rate constants have small activation enthalpies and are inversely dependent on solvent viscosity, as expected for diffusion-controlled reactions. There is a moderate ionic strength dependence of the rate of association between the 2B5 antibody and cytochrome c, with the rate constant increasing about 4-fold as the ionic strength is varied between 0.14 and 0 M. Comparison of the rates for antibody-cytochrome c complex formation for binding to the reduced-native, oxidized-native, and alkaline conformations shows that for MAb 2B5 the forward rate constant depends slightly on cytochrome c conformation. Investigation of the pH-induced transition between the native and alkaline conformational states for free cytochrome c and for antibody-cytochrome c complexes shows that antibody binding stabilizes the native form of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Tolerance induction in resting memory B cells specific for a protein antigen

Resting memory B lymphocytes specific for the model protein Ag cytochrome c have been shown to be susceptible to tolerance induction in in vitro splenic fragment cultures. This induction of nonresponsiveness is dependent upon the strength of the interaction between surface Ig and specific Ag, where concentration, valency, affinity, and time of exposure all appear to be important factors, as is the case for tolerance induction in immature or primary B cells. The induction of nonresponsivenes in greater than 80% of Ag-specific memory B cells was achieved by incubation with 1 microM cytochrome polymer for 24 h in the absence of T cell help. Not only were memory B cells unresponsive to specific Ag, they were also unable to become activated through nonspecific uptake and presentation of an Ag to which T cells have been primed, demonstrating that the induction of nonresponsiveness involves more than a modulation or blockade of surface Ig receptors. Although soluble factors collected from activated T cells failed to prevent memory B cells from becoming nonresponsive after surface Ig cross-linking, the direct activation of T cells within splenic fragment cultures did partially inhibit tolerance induction in splenic fragment memory B cells. In addition, the induction of tolerance was partially blocked by protein tyrosine kinase inhibitors, suggesting a physiologic change within the B cells associated with the state of nonresponsiveness and resulting from tyrosine-specific phosphorylation.

Tolerance induction in resting memory B cells specific for a protein antigen

Resting memory B lymphocytes specific for the model protein Ag cytochrome c have been shown to be susceptible to tolerance induction in in vitro splenic fragment cultures. This induction of nonresponsiveness is dependent upon the strength of the interaction between surface Ig and specific Ag, where concentration, valency, affinity, and time of exposure all appear to be important factors, as is the case for tolerance induction in immature or primary B cells. The induction of nonresponsivenes in greater than 80% of Ag-specific memory B cells was achieved by incubation with 1 microM cytochrome polymer for 24 h in the absence of T cell help. Not only were memory B cells unresponsive to specific Ag, they were also unable to become activated through nonspecific uptake and presentation of an Ag to which T cells have been primed, demonstrating that the induction of nonresponsiveness involves more than a modulation or blockade of surface Ig receptors. Although soluble factors collected from activated T cells failed to prevent memory B cells from becoming nonresponsive after surface Ig cross-linking, the direct activation of T cells within splenic fragment cultures did partially inhibit tolerance induction in splenic fragment memory B cells. In addition, the induction of tolerance was partially blocked by protein tyrosine kinase inhibitors, suggesting a physiologic change within the B cells associated with the state of nonresponsiveness and resulting from tyrosine-specific phosphorylation.

Different functional boundaries for the major antigenic region of two cytochromes c

The antigenic sites of horse and rat cytochromes c in the major antigenic region were compared using a panel of variant cytochromes c and a large number of BALB/c mouse monoclonal antibodies (mAbs) in competitive ELISAs. The major antigenic region of cytochrome c is located on the surface opposite of that containing the exposed heme crevice. mAbs specific for this region on rat cytochrome c were affected in binding by amino acid substitutions at positions 62 and at one or more of positions 3, 100, 103, and 104 but not by a substitution at position 89. In contrast, mAbs specific for the same region on horse cytochrome c were affected by amino acid substitutions at positions 62, (probably) 60, and 89. Some, but not all, of the anti-horse cytochrome c mAbs were affected by amino acid substitutions at one or more of positions 3, 100, 103, and 104. Thus, the functional boundaries of this antigenic region are different for these two cytochromes c, as shown primarily by the differential effects of residue 89. This distinction is most likely due to one or more of the four amino acid differences between horse and rat cytochromes c on the antigenic surface that result in different physicochemical properties for the horse and rat proteins. The results suggest that as yet unidentified physicochemical parameters, possibly surface topography and/or charge distribution, influence the focusing of antibodies onto the surface of a protein antigen.

Relative frequencies of secondary B cells activated by cognate vs. other mechanisms

Three distinct mechanisms for the activation of secondary B cells to antibody-forming cells have been examined in splenic fragment cultures. The clonal response to a protein antigen [cytochrome c (cyt)] was quantified in terms of both the number of B cells activated and the amount of antibody produced by each clone. The vast majority of memory B cells required surface immunoglobulin (sIg) receptor-mediated uptake of antigen followed by cognate interactions with an antigen-specific T helper cell. Nevertheless, a significant number (as many as 12%) could be activated via soluble factor-mediated bystander activation, involving occupation of neither the sIg receptor nor class II major histocompatibility complex (MHC) molecules. This pathway, however, was relatively inefficient in that individual clones secreted less than half as much antibody as clones activated as a result of cognate collaboration with a T helper cell. A similar number of secondary B cells were activated following nonspecific uptake of high concentrations of antigen for which splenic fragment T cells had been primed [i.e., hemocyanin (Hy)], independent of sIg receptor occupancy. Antibody levels were similar to those in cultures where B cells were activated in a cognate manner with sIg receptor occupancy. When Hy-stimulated fragment cultures were supplemented with polymerized cyt, the frequency of activation via this latter pathway increased fourfold, despite the fact that no cyt-primed T cells were present. This observation supports the idea that receptor-mediated uptake of antigen serves not just to focus antigen but also provides an important signal in activating B cells. Bystander B cell activation, however, was not enhanced by providing a sIg cross-linking signal with polymerized cyt. The lower level of antibody production by B cells activated in a bystander fashion and the inability to enhance their frequency of activation with sIg receptor occupancy suggest that there is indeed a fundamental difference between soluble factor-mediated bystander activation and activation via T cell determinant-mediated cognate T helper-B cell interactions, perhaps involving signaling through class II MHC molecules.

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

To examine if there are common physicochemical features among antibodies binding the same antigenic region of a protein, B cell hybridomas were prepared against the two major antigenic regions on mammalian cytochromes c, and the nucleotide sequences encoding the monoclonal antibody (mAb) heavy (H) and light (L) chains were determined and compared. Although the genetic elements used were somewhat diverse, similarities among mAbs to a given antigenic region were observed. In particular, mAbs binding in a region situated at a bend in the antigen around residues 44 and 47 had longer complementarity-determining regions (4-5 additional amino acid residues in L1 and 1-2 in H3) than mAbs binding the other region around residues 60 and 62 located on a relatively flat surface. These observations indicate that the topography of an antigenic site and the lengths of certain complementarity-determining regions are important physicochemical properties determining, at least in part, which antibodies (B cells) will participate in an immune response to a particular site on a protein antigen.

A monoclonal antibody specific for a cytochrome c T cell stimulatory peptide inhibits T cell responses and affects the way the peptide associates with antigen-presenting cells

A monoclonal antibody (mAb) specific for the 93-104 segment of pigeon cytochrome c (cyt) was shown to block interleukin 2 production and proliferation by pigeon cyt-specific T cells in response to the pigeon cyt 81-104 peptide using either the LK35.2 B cell hybridoma or normal splenocytes as antigen-presenting cells (APC). The mAb inhibited the response to soluble peptide antigen presented by metabolically inactive paraformaldehyde-fixed APC but not the response to APC that were pre-pulsed with Ag. These results suggest that the mAb blocked the formation of peptide-major histocompatibility complex (MHC) class II molecule complexes at the cell surface but did not displace the peptide once bound to the MHC class II molecule. As determined by direct binding experiments using labeled peptide, the major means of free peptide association with live APC was fluid-phase endocytosis. No free peptide associated directly with the MHC class II molecule at the cell surface near 0 degrees C since APC pulsed with peptide on ice did not activate cyt-specific T cells. The mAb enhanced the association of the radiolabeled peptide with APC at 4 degrees C apparently by binding of the peptide-mAb complex to Fc receptors. By stripping molecules from the LK35.2 cell surface using a nonspecific protease it was shown that the peptide-mAb complexes were not internalized either at 4 degrees C or 37 degrees C. Since the mAb was found to stably bind the peptide at pH levels below that of endosomes (pH 5.5-6.2) even if the peptide-mAb complexes were taken up by fluid-phase endocytosis, it is likely that the peptide would not be able to associate with MHC class II molecules inside the APC. This mAb appears to inhibit T cell activation by blocking the formation of peptide-MHC class II molecule complexes at the cell surface and by interfering with uptake of the peptide into endosomes. Therefore, it is different from other antibodies that have been reported to block T cell receptor recognition of preformed peptide/MHC class II molecule complexes.

The specificity of human anti-cytochrome c autoantibodies that arise in autoimmune disease

Cytochromes c (cyt c) are among the best characterized model Ag because their amino acid sequences and tertiary structures are well defined. One unique aspect of cyt c as an immunogen is its ability to induce autoantibody responses in animal models, although no pathology resulting from these responses has been reported. In this study, the presence and specificity of autoantibodies to cyt c were investigated in patients with SLE and related connective tissue diseases. Anti-cyt c antibodies were found in approximately 7% of patient sera and were statistically associated with the expression of antimitochondrial antibodies but were not statistically associated with any disease subset among those represented. Anti-cyt c was not associated with the presence of autoantibodies to DNA, histones, Ro, La, or Sm autoantigens. Most of the autoantibodies were specific for native or native-like forms of cyt c but antibodies to denatured forms were also apparent. Autoantibody binding was shown to be directed predominantly at selected sites of evolutionary variability within cyt c. The specificity of the human anti-cyt c autoantibodies appear to be similar to that of mouse anti-human cyt c antibodies and to autoantibodies elicited in mice against rat (mouse) cyt c.

The specificity of human anti-cytochrome c autoantibodies that arise in autoimmune disease

Cytochromes c (cyt c) are among the best characterized model Ag because their amino acid sequences and tertiary structures are well defined. One unique aspect of cyt c as an immunogen is its ability to induce autoantibody responses in animal models, although no pathology resulting from these responses has been reported. In this study, the presence and specificity of autoantibodies to cyt c were investigated in patients with SLE and related connective tissue diseases. Anti-cyt c antibodies were found in approximately 7% of patient sera and were statistically associated with the expression of antimitochondrial antibodies but were not statistically associated with any disease subset among those represented. Anti-cyt c was not associated with the presence of autoantibodies to DNA, histones, Ro, La, or Sm autoantigens. Most of the autoantibodies were specific for native or native-like forms of cyt c but antibodies to denatured forms were also apparent. Autoantibody binding was shown to be directed predominantly at selected sites of evolutionary variability within cyt c. The specificity of the human anti-cyt c autoantibodies appear to be similar to that of mouse anti-human cyt c antibodies and to autoantibodies elicited in mice against rat (mouse) cyt c.

Fine manipulation of antibody affinity for synthetic epitopes by altering peptide structure: antibody binding to looped peptides*

Linear peptides weakly imitate antibody binding sites on globular proteins when the peptides are shown to be effective at all. As a step toward enhancing the ability of peptides to mimic epitopes, we have examined the effects of various alterations in peptide structure on antibody binding. Synthetic peptides containing the core amino acid sequence of residues 41 to 48 from horse cytochrome c were examined for their ability to bind antibodies elicited against the 41-48 peptide coupled to bovine serum albumin (BSA). Since residues 41-48 in native cytochrome c are part of an omega loop, in some peptides cysteines were incorporated for intrachain disulfide bonding to stabilize loop structure. In additional cases, glycine was incorporated as a spacer between the natural sequence and the cysteine residues with the intent of relaxing loop structure slightly. Eleven analogues containing the 41-48 sequence were tested. These included native cytochrome c and the 1-80 and 1-65 cyanogen bromide-cleaved fragments. The native protein did not bind the anti-41-48 antibodies. The other analogues differed by over three orders of magnitude in their binding. The affinity of binding was inversely related to the extent of predicted loop structure indicating that the antibodies were elicited against the 41-48 sequence in a more unfolded conformation despite the Pro Gly sequence at positions 44 and 45 that generally favors a beta turn. Surprisingly, the immunizing peptide, containing residues 41-48 only, was the poorest binding peptide. The relative impotence of 41-48 was shown to be largely due to differences at the amino terminus between the free and BSA-coupled peptides as the antibodies were elicited against the latter. The distinctions among the synthetic peptides containing the 41-48 sequence show the exquisite sensitivity of antibody binding to amino acid changes that may occur outside of an epitope and suggest modifications in peptide structure at the periphery of an epitope that can lead to desired changes in antibody affinity.

Affinity consideration in the design of synthetic vaccines intended to elicit antibodies

As a model for synthetic vaccines BALB/c mice were injected with a large cyanogen bromide cleaved fragment of horse cytochrome c, containing residues 1-80 of the 104 residue long polypeptide chain; then individual B cells specific for the peptide were challenged in vitro in splenic fragment cultures, with either the fragment or intact cytochrome c, both coupled to hemocyanin. The splenic environment in which the B cells were cultured contained hemocyanin-primed T cells, which provided equivalent T cell help for both the peptide and protein immunogens. In two experiments, intact cytochrome c-hemocyanin activated a total of only five peptide-primed B cells, compared to 66 that were activated by the peptide-hemocyanin conjugate. Furthermore, antibodies from the few cells that appeared to be activated by the protein did not bind the native protein with appreciable affinity in competitive ELISA. Of the mAbs elicited by the peptide, 51 were shown to have detectable affinity for native cytochrome c, but their affinity was dramatically less than that previously observed for antibodies elicited by protein-primed B cells and also less than that for the peptide. Thus, although the Ig receptors on many of the peptide-primed B cells did bind the protein to some extent, most such B cells were not activated. These results demonstrate that, in the development of synthetic vaccines, the affinity of a protein for peptide-primed antibodies (Ig receptors) is an important criterion to be considered. Qualitative examination of the binding of an anti-peptide antibody to a protein antigen, especially in denaturing conditions such as Western blots or in potentially denaturing conditions such as ELISA, is not an accurate indication of the efficacy of the peptide to prime B cells that can be activated upon challenge from the native protein.

Clonal analysis of the BALB/c secondary B cell repertoire specific for a self-antigen, cytochrome c

mAb to rat cytochrome c (cyt c), totaling 556, were produced by individual clones of secondary B lymphocytes from nine groups of five BALB/c mice each in vitro using the splenic focus culture system. Inasmuch as rat and mouse cyt c are identical, these B cells can be considered specific for a self-antigen. The mAb were categorized into specificity groups based on their reactivities with a panel of seven cyts c that differ at two to six amino acid residues. The number of distinct specificities for the native protein was restricted to fewer than 20. Different groups of mice expressed the same specificities at comparable frequencies, including a single dominant one, and the total number of secondary cyt c-specific B cells was constant among groups of mice. This suggests that the acquisition of the secondary B cell specificity repertoire for this self-antigen is regulated. However, it is indeed possible that each specificity group may comprise a number of distinct mAb molecules that have arisen stochastically. Specificities expressed by as few as 1% of the total mAb were observed. Thus, it is likely that the identified specificities reflect the secondary B cell specificity repertoire for rat cyt c. The dominant specificity expressed by 50% of the mAb was characterized by elimination of antigen recognition as a result of replacement of aspartic acid by glutamic acid at position 62. Minor specificities expressed by 19% of the mAb were characterized by more subtle affects of an amino acid change at position 62 and/or an amino acid substitution from rat cyt c at position 60. Antibodies in other specificity groups reacted with epitopes in the region of residues 44 and 47. Whereas substitutions at positions 44, 47, 60, and 62 eliminated recognition by most of the mAb, changes at position 92 and at 103 also appeared to affect the binding of some mAb in the region around residues 60 and 62. The amino acid residues implicated in the recognition by murine mAb of murine cyt c have been shown previously to be involved in the epitopes of foreign mammalian cyt c. Therefore, self-tolerance cannot fully explain the restriction of the epitopes to these regions on foreign mammalian cyt c.