Characterization and quantitation of peptide-MHC complexes produced from hen egg lysozyme using a monoclonal antibody

Here we describe generation of Aw3.18, a monoclonal antibody that recognizes peptide residues 48-62 of hen egg lysozyme (HEL) bound to the MHC class II molecule I-Ak. Epitope mapping revealed that Aw3.18 detects a change in the solvent-exposed surface of this peptide-MHC complex upon substitution of the peptide side chain at position P1. Furthermore, Aw3.18 blocked recognition by some, but not all, of the HEL 48-62-reactive T cell hybridomas tested, suggesting a heterogeneity in the T cell response toward this complex. Finally, using Aw3.18, it was possible to determine the fraction of I-Ak molecules loaded with 48-62 peptide after culture of an antigen-presenting cell in medium containing HEL.

Recognition of the mother's face by six-month-old infants: a neurobehavioral study

Event-related potentials (ERPs) were recorded from 6-month-olds as each watched pictures of the mother's face and a stranger's face. The ERPs differed for the 2 faces, but the pattern of neural activity elicited depended on whether the mother and stranger looked different (Experiment 1, n = 22) or alike (Experiment 3, n = 22). In contrast, when different 6-month-olds were each shown 1 of these 44 pairs of faces their ERPs did not differ between the 2 faces (Experiment 2, n = 22, and Experiment 4, n = 22). In a visual preference test of recognition, infants showed no evidence of recognizing the mother's face (Experiment 5, n = 32). Together, these results suggest that infants are able to recognize their mothers' faces but (1) the neural processes accompanying recognition depend on the difficulty with which mother can be discriminated from stranger and (2) under the conditions investigated in this study, ERPs are a more sensitive measure of recognition than is looking time.

High level object recognition without an anterior inferior temporal lobe

Seven individuals with unilateral anterior inferior temporal (AIT) lobectomies performed two types of shape recognition tasks with line drawing of 3D objects briefly presented in either the left or the right visual field. In one task, subjects named familiar objects in a name priming paradigm. In the other task, subjects judged whether two objects, presented sequentially with an intervening mask, were the same or different in shape, disregarding differences in orientation of up to 60 degrees in depth. They could not use names or basic level concepts to do the matching as the stimuli were either nonsense objects or, if familiar objects, were of same name-different-shaped exemplars on different trials. The disadvantage of presenting an image to the lobectomized hemisphere was negligible in both tasks. Two non-exclusive possibilities are suggested by this result: (a) Object recognition is completed posterior to AIT, likely at the temporal-occipital boundary, with no deleterious retrograde effects on object recognition from the AIT section, or (b) Callosal transfer of object information prior to AIT is completely efficient. These results, along with results of single unit recording and lesion experiments in the monkey, PET and MRI imaging in humans, and a plausibility argument based on the pattern of callosal connections suggest both are correct. Rather than mediating real-time object recognition, AIT may code representations for visual episodes and scenes.

Amino-terminal trimming of peptides for presentation on major histocompatibility complex class II molecules

Major histocompatibility complex (MHC) class II molecules bind antigenic peptides for display to T lymphocytes. Although the enzymes involved remain to be identified, it is commonly believed that class II associated peptides are released from intact antigens through a series of proteolytic steps carried out inside antigen presenting cells. We have examined the effect of amino acid substitutions on proteolytic processing of the model antigen hen-egg lysozyme (HEL). Altered HEL molecules, engineered by site-directed mutagenesis of a HEL cDNA, were expressed as separate stable transfectants in a B cell lymphoma line. Each transfectant processed a different mutant HEL protein for presentation on MHC class II. We purified the resulting class II-associated peptides and analyzed them by mass spectrometry. Our results strongly support the hypothesis that antigen processing continues after peptide binding to the MHC class II molecule and are most consistent with a scenario in which long peptides first bind to MHC class II and are then trimmed by exopeptidase.

Visual event-related brain potentials in 4-month-old infants at risk for neurodevelopmental impairments

The recording of event-related potentials (ERPs) is an electrophysiologic technique that has been used to evaluate the functional maturation of neural pathways responsible for recognition memory systems in infants and children. The purpose of this study was to evaluate ERP correlates of visual recognition memory in 4-month-old infants at risk for later cognitive impairments. We compared ERPs using a test of shape recognition at 4 months of age (adjusted for prematurity) in 16 high-risk, neonatal intensive care unit (NICU) survivors and 16 healthy full-term infants. ERPs were recorded while infants were familiarized with one stimulus (a red cross, 15 trials), then tested with 60 trials of this familiar stimulus and a novel stimulus (a red corkscrew). Both the NICU and control groups' ERPs demonstrated evidence of differential processing of the two stimuli, but the NICU groups' ERP patterns were distinctly different from those of the control group. In the NICU group, the novel stimulus elicited parietal positivity at 1000-1700 ms poststimulus, whereas in the control group the novel stimulus elicited occipital and frontal negativity at 500-1700 ms poststimulus. The ERP pattern demonstrated by the NICU group was atypical as it has not been previously described in healthy infants. The results of the study indicate that the ERP technique can be used to demonstrate altered patterns of neural activity during tasks of visual recognition memory in high-risk infants. We speculate that the atypical ERP patterns described in this study may indicate that patterns of synaptic organization were altered by neonatal events.

Development of preterm and full-term infant ability on AB, recall memory, transparent barrier detour, and means-end tasks

10 preterm and 10 full-term infants were tested longitudinally from 28 to 60 weeks of age on a modified version of the AB task, a nonreaching AB task, a Barrier Detour task, a Means-End task, and Perseveration in the Means-End task. Results show that age-corrected (age since conception) premature infants tolerated longer delays than full-term infants on the modified and nonreaching AB tasks. However, when compared by chronological age (age since birth), there were no group differences on either the reaching or nonreaching AB task. No group differences were found on Barrier Detour, Means-End, or Perseveration in either the age-corrected or chronological age comparisons. The results suggest that the function that mediates modified AB performance is one of memory and not of perseveration or means-end ability. Further, these findings suggest that current proposals about brain development based on single samples of infants may be tenuous. Finally, the results of this study suggest that development of the brain structure(s) that mediate modified AB performance is strongly influenced by experience in the postnatal environment.

Neural correlates of infants' visual responsiveness to facial expressions of emotion

An extensive literature documents the infant's ability to recognize and discriminate a variety of facial expressions of emotion. However, little is known about the neural bases of this ability. To examine the neural processes that may underlie infants' responses to facial expressions, we recorded event-related potentials (ERPs) while 7-month-olds watched pictures of a happy face and a fearful face (Experiment 1) or an angry face and a fearful face (Experiment 2). In both experiments an early positive component, a middle-latency negative component and a later positive component were elicited. However, only when the infants saw the happy and fearful faces did the components differ for the two expressions. These results are discussed in the context of the neurobiological processes involved in preceiving facial expressions.

TCR-mediated adhesion of T cell hybridomas to planar bilayers containing purified MHC class II/peptide complexes and receptor shedding during detachment

T cell recognition of foreign Ag/MHC class II complexes is sensitive down to approximately 100 complexes per cell or approximately 0.2 complexes/micron2. To better understand the physical basis of the recognition stage of Ag presentation, we examined adhesion of the lysozyme- specific T cell hybridoma, 3A9, to artificial bilayers containing covalent MHC class II/peptide complexes or adhesion molecules. Adhesion of 3A9 cells required a superphysiologic density of the MHC class II/peptide complex and was partly dependent on CD4; cells adhered but did not crawl. No adhesion was observed to bilayers containing MHC class II molecules without the lysozyme peptide. Activated 3A9 cells adhered and crawled on bilayers containing ICAM-1. The physical strength of contacts was tested with fluid shear. 3A9 cells adherent to bilayers containing MHC class II/peptide complexes shed their contact, which remained on the substrate and contained TCR. In contrast, 3A9 cells peeled from the ICAM-1 bilayer, and held firmly on LFA-1 bilayers; in a manner dependent on filamentous actin. When ICAM-1 and the MHC/peptide complexes were combined, the 3A9 cells adhered tightly and spread, but did not crawl, on the bilayers and TCR clustered at the center of the contact area. Physiologically, the TCR is unlikely to directly initiate adhesion. TCR clusters formed with the assistance of adhesion mechanisms may have to be shed to allow de-adhesion, and this may contribute to TCR down-regulation.

TCR-mediated adhesion of T cell hybridomas to planar bilayers containing purified MHC class II/peptide complexes and receptor shedding during detachment

T cell recognition of foreign Ag/MHC class II complexes is sensitive down to approximately 100 complexes per cell or approximately 0.2 complexes/micron2. To better understand the physical basis of the recognition stage of Ag presentation, we examined adhesion of the lysozyme- specific T cell hybridoma, 3A9, to artificial bilayers containing covalent MHC class II/peptide complexes or adhesion molecules. Adhesion of 3A9 cells required a superphysiologic density of the MHC class II/peptide complex and was partly dependent on CD4; cells adhered but did not crawl. No adhesion was observed to bilayers containing MHC class II molecules without the lysozyme peptide. Activated 3A9 cells adhered and crawled on bilayers containing ICAM-1. The physical strength of contacts was tested with fluid shear. 3A9 cells adherent to bilayers containing MHC class II/peptide complexes shed their contact, which remained on the substrate and contained TCR. In contrast, 3A9 cells peeled from the ICAM-1 bilayer, and held firmly on LFA-1 bilayers; in a manner dependent on filamentous actin. When ICAM-1 and the MHC/peptide complexes were combined, the 3A9 cells adhered tightly and spread, but did not crawl, on the bilayers and TCR clustered at the center of the contact area. Physiologically, the TCR is unlikely to directly initiate adhesion. TCR clusters formed with the assistance of adhesion mechanisms may have to be shed to allow de-adhesion, and this may contribute to TCR down-regulation.

A negatively charged anchor residue promotes high affinity binding to the MHC class II molecule I-Ak

An allele-specific peptide-binding motif for the murine MHC class II molecule I-Ak has proven elusive. Here we demonstrate that the I-Ak molecule preferentially binds peptides that contain negatively charged amino acids at the primary anchor position (Asp or Glu at P1), and that I-Ak can also bind peptides with polar residues at P1 (Cys, Ser, Asn, Gin, or Thr), although with lower affinity. This preference for a negatively charged anchor residue is so pronounced that polyalanine peptides containing a single Asp can bind to I-Ak. Eight naturally processed peptides were found to use an Asp, as demonstrated by a drop in the I-Ak binding affinity of these peptides after Ala substitution. The chemical identity of the amino acid in the anchor position was also important in determining the ability of peptide-I-Ak complexes to resist denaturation on SDS-polyacrylamide gels. The P1 binding pockets of HLA-DR and H-2E molecules are reported to be large and hydrophobic, and these class II molecules prefer to bind peptides with large aliphatic or aromatic side chains at P1. Our results suggest that the structure of the I-Ak P1 binding pocket is different. Based on sequence comparisons, we suggest that the P1 binding pockets of H-2A molecules may prove more polymorphic than the P1 binding pockets of H-2E molecules, and that this additional polymorphism will cause H-2A molecules to display larger intra-allelic differences in peptide binding specificities.

A negatively charged anchor residue promotes high affinity binding to the MHC class II molecule I-Ak

An allele-specific peptide-binding motif for the murine MHC class II molecule I-Ak has proven elusive. Here we demonstrate that the I-Ak molecule preferentially binds peptides that contain negatively charged amino acids at the primary anchor position (Asp or Glu at P1), and that I-Ak can also bind peptides with polar residues at P1 (Cys, Ser, Asn, Gin, or Thr), although with lower affinity. This preference for a negatively charged anchor residue is so pronounced that polyalanine peptides containing a single Asp can bind to I-Ak. Eight naturally processed peptides were found to use an Asp, as demonstrated by a drop in the I-Ak binding affinity of these peptides after Ala substitution. The chemical identity of the amino acid in the anchor position was also important in determining the ability of peptide-I-Ak complexes to resist denaturation on SDS-polyacrylamide gels. The P1 binding pockets of HLA-DR and H-2E molecules are reported to be large and hydrophobic, and these class II molecules prefer to bind peptides with large aliphatic or aromatic side chains at P1. Our results suggest that the structure of the I-Ak P1 binding pocket is different. Based on sequence comparisons, we suggest that the P1 binding pockets of H-2A molecules may prove more polymorphic than the P1 binding pockets of H-2E molecules, and that this additional polymorphism will cause H-2A molecules to display larger intra-allelic differences in peptide binding specificities.

Targeting of glucose transport proteins for tumor imaging: is it feasible?

If glucose transport proteins (Glut) are elevated in tumors they may be good targets for tumor imaging. For targeting, the overexpression of Glut should be a general characteristic of tumors. Moreover agents which bind to Glut should accumulate selectively in tumors.

METHODS

To test this, we quantitated Glut in isolated membranes from three human tumor xenografts, two murine tumor models and normal murine tissues using direct binding studies. Additionally, the biodistribution of two compounds which bind to Glut, 7-[[(2-(3-(125I-p-hydroxyphenyl)propionyl)aminoethyl)amino]carbonyl]-7-+ ++desacetyl-forskolin([125I]HPP forskolin) and [3H]cytochalasin B, were studied in a tumor model which overexpressed Glut.

RESULTS

There were multiple classes of binding sites for [3H]cytochalasin B and a percentage of these sites were competitive with D-glucose but not L-glucose. The rank potency and IC50 values for [3H]cytochalasin B binding were: 2-deoxy-D-glucose (4.5 mM) > or = D-glucose (7 mM) > mannose (25 mM) > galactose (35 mM) > rhamnose (1-3 mM) > sorbitol (1-3 mM) and were similar to reported values for transport. The average density of Glut in four tumor models and normal tissues was between 0.7 and 4 pmole/mg protein, but Kd values were not significantly different (69 nM). In LX-1 human lung tumor xenograft (LX-1) Glut were 10-to-20-fold higher than other tissues (21.6 +/- 0.6 pmole/mg protein, p<0.01). Immunostaining of Glut-1 was more prominent in LX-1 than other xenograft tumors, consistent with the binding data. Glut density was highest in poorly vascularized regions suggesting that Glut upregulation was related to a biofeedback mediated event. Iodine-125 HPP-forskolin and [3H]cytochalasin B did not localize in LX-1 tumors.

CONCLUSION

Glut overexpression was not a common characteristic of the five tumors tested. Iodine-125 HPP-forskolin and [3H]cytochalasin B did not localize in LX-1 tumors, indicating that these agents did not target tumors with upregulated Glut. Results suggest that Glut are not a promising target for tumor imaging.

The interaction among glucose transport, hexokinase, and glucose-6-phosphatase with respect to 3H-2-deoxyglucose retention in murine tumor models

The development of new diagnostic/therapeutic modalities for cancer requires a specific understanding of how tumors differ from normal tissues. Though the key components involved in the selective accumulation of 2-deoxy-D-glucose (2-DG) analogs in tumors are known, the relative importance of each is controversial. For this reason glucose transport protein (GLUT) density, hexokinase/glucose-6-phosphatase (GP) activity, and 2-DG biodistribution were measured together in four tumor models and normal murine tissues. Direct binding studies with 3H-cytochalasin B showed that GLUT density was elevated 20-fold in LX-1 tumors. Immunohistochemically in all tumors, the expression of GLUT-1 was highest in the necrotic/ perinecrotic foci and similar in cells not adjacent to necrotic foci. As the retention of 3H-2-DG was similar in all tumors, these data suggest that the GLUT-1 in perinecrotic tumor cells were not rate limiting for 3H-2-DG uptake. Kidney, liver, and lung had high GP activity and rapid clearance of 3H-2-DG. Sodium orthovanadate (5 mumol), a GP inhibitor, increased the concentration of 3H-2-DG in these tissues, suggesting that GP is a rate-limiting enzyme for 3H-2-DG clearance. All tumor homogenates had low GP activity, and hexokinase activity was not elevated compared to normal tissues. Thus, in the tumors studied, the selective accumulation of 3H-2-DG consistently occurred in the absence of significant GP activity without the marked overexpression of hexokinase or GLUT.

Age-related changes in the electrophysiological response to visual stimulus novelty: a topographical approach

The relationship of task relevance and stimulus probability to P300 morphology, latency and distribution was assessed. Eight year olds and adults completed visual oddball tasks of recognition memory with frequent non-target (60%), infrequent target (20%), and infrequent novel (20%) stimuli. Stimuli consisted of 2 female faces posing neutral expressions, and 40 trial unique novel photographs depicting scenes, animals, objects or abstract patterns. Event-related potentials were recorded from 17 electrodes over frontal, central and parietal scalp, including lateral temporal sites. All stimuli elicited P300 responses at parietal electrodes, with the largest responses to the target stimuli (relevant and infrequent). The P300 responses of adults and children were morphologically dissimilar, with children showing broader peaks and latency shifts across electrodes. In addition, the eight year olds displayed a frontal negativity to novel stimuli which was absent in the responses of adult participants. Results suggest that different anatomical or functional circuitry may be involved in the processing of novelty for adults as compared to eight year olds.

Complexes generated by the binding of free peptides to class II MHC molecules are antigenically diverse compared with those generated by intracellular processing

We investigated the specificity of T cell hybridomas isolated from mice immunized with synthetic peptides identical in sequence with the dominant, naturally processed, I-Ak-restricted peptides of hen egg lysozyme (HEL). Surprisingly, the majority of hybridomas showed little or no recognition of intact HEL after processing by different APCs. This was not an artifact caused by the use of synthetic peptides since the peptide-specific hybridomas responded to a tryptic digest of HEL or to naturally processed HEL peptides extracted from I-Ak. Thus, the interaction of free peptides with class II MHC molecules can generate complexes that are antigenically dissimilar to those resulting from intracellular processing of intact Ag. This has important implications both for the interpretation of experimental studies that involve peptide immunization and for the efficacy of peptide vaccination as a strategy for intervention in human disease.

Complexes generated by the binding of free peptides to class II MHC molecules are antigenically diverse compared with those generated by intracellular processing

We investigated the specificity of T cell hybridomas isolated from mice immunized with synthetic peptides identical in sequence with the dominant, naturally processed, I-Ak-restricted peptides of hen egg lysozyme (HEL). Surprisingly, the majority of hybridomas showed little or no recognition of intact HEL after processing by different APCs. This was not an artifact caused by the use of synthetic peptides since the peptide-specific hybridomas responded to a tryptic digest of HEL or to naturally processed HEL peptides extracted from I-Ak. Thus, the interaction of free peptides with class II MHC molecules can generate complexes that are antigenically dissimilar to those resulting from intracellular processing of intact Ag. This has important implications both for the interpretation of experimental studies that involve peptide immunization and for the efficacy of peptide vaccination as a strategy for intervention in human disease.

Amino acid residues on the I-Ak alpha-chain required for the binding and stability of two antigenic peptides

The class II molecules of the MHC bind processed Ag fragments (peptides) for presentation to T cells, but the role of individual MHC residues in binding these peptides has not been entirely defined. A panel of 27 mutant I-Ak transfectants was analyzed for the capacity to bind 2 unrelated peptides. The main peptides examined were hen egg lysozyme residues 48-62 and heat shock protein (hsp70) to residues 28-41. Alanine substitutions of sites in the alpha-helical region of the I-Ak alpha-chain altered the ability of this class II protein to bind both peptides. Of the 27 substitutions tested, nine caused a decrease in peptide binding while only three caused an increase in peptide binding. The stabilities of these altered I-Ak-peptide complexes were also examined on SDS-Page. Complexes with lowered stabilities were observed after only four substitutions, and in all four cases this loss of stability was accompanied by a loss in hen egg lysozyme or hsp70 peptide-binding ability. Further, three of these residues lie in the short extended strand at the N terminus of the alpha-helix of the alpha 1 domain, suggesting that this region of I-Ak molecule may be critical for the formation of stable peptide-MHC complexes.

Amino acid residues on the I-Ak alpha-chain required for the binding and stability of two antigenic peptides

The class II molecules of the MHC bind processed Ag fragments (peptides) for presentation to T cells, but the role of individual MHC residues in binding these peptides has not been entirely defined. A panel of 27 mutant I-Ak transfectants was analyzed for the capacity to bind 2 unrelated peptides. The main peptides examined were hen egg lysozyme residues 48-62 and heat shock protein (hsp70) to residues 28-41. Alanine substitutions of sites in the alpha-helical region of the I-Ak alpha-chain altered the ability of this class II protein to bind both peptides. Of the 27 substitutions tested, nine caused a decrease in peptide binding while only three caused an increase in peptide binding. The stabilities of these altered I-Ak-peptide complexes were also examined on SDS-Page. Complexes with lowered stabilities were observed after only four substitutions, and in all four cases this loss of stability was accompanied by a loss in hen egg lysozyme or hsp70 peptide-binding ability. Further, three of these residues lie in the short extended strand at the N terminus of the alpha-helix of the alpha 1 domain, suggesting that this region of I-Ak molecule may be critical for the formation of stable peptide-MHC complexes.

Decisions about life-sustaining treatment. Impact of physicians' behaviors on the family

BACKGROUND

Despite the growing availability of advance directives, most patients in the intensive care unit lack written directives, and, therefore, consultation with families about treatment decisions remains the rule. In the context of decision making about withdrawing life-sustaining treatments, we investigated which physician and nurse behaviors families find supportive and which behaviors increase the family's burden.

METHODS

We conducted intensive 1- to 2-hour-long individual interviews using a semistructured interview protocol with 32 family members of patients without advance directives whose deaths followed a stay in the intensive care unit and withdrawal of treatment. We analyzed more than 700 pages of verbatim interview data using content analysis techniques and achieved more than 90% interrater agreement on data codes.

RESULTS

Themes emerged as families identified selected physician and nursing behaviors as helpful: encouraging advanced planning, timely communication, clarification of families' roles, facilitating family consensus, and accommodating family's grief. Behaviors that made families feel excluded or increased their burden included postponing discussions about treatment withdrawal, delaying withdrawal once scheduled, placing the full burden of decision making on one person, withdrawing from the family, and defining death as a failure.

CONCLUSIONS

Study findings provide an increased understanding of the unmet needs of families and serve to guide physicians and nurses in reducing actions that increase families' burdens as they participate in treatment withdrawal decisions.

Identification of a major I-Ek-restricted determinant of hen egg lysozyme: limitations of lymph node proliferation studies in defining immunodominance and crypticity

We have chemically analyzed the peptides presented by I-Ek molecules after processing of hen egg lysozyme (HEL) by a murine B-lymphoma line or by splenocytes. In both cases, the identified peptides were derived from a single region of HEL, containing the core residues 85-96 with heterogeneous N and C termini. This was a surprising result because this determinant had previously been described as cryptic--i.e., not presented after processing of intact HEL. Examination of the specificities of T hybridomas isolated after immunization with either HEL or 84-96 peptide (p84-96) provided an explanation for this controversy. Whereas hybridomas induced by immunization with HEL responded equally well to HEL and p84-96, those induced by peptide immunization showed a marked preference for p84-96 over intact HEL. In other words, hybridomas isolated after p84-96 immunization responded poorly to forms of the 84-96 determinant produced by natural processing, leading to the possible erroneous interpretation that 84-96 is a hidden determinant. We conclude that (i) p84-96 is efficiently presented on I-Ek molecules after processing of HEL, (ii) the explanation for the weak lymph node response to this epitope after immunization with HEL lies at the level of the T cell, not the antigen-presenting cell, and (iii) crypticity cannot be defined on the basis of T-cell proliferation studies alone.