Everyday-like memory and its cognitive correlates in healthy older adults and in young patients with traumatic brain injury: a pilot study based on virtual reality.

UNLABELLED: PURPOSE STATE: A pilot-study with a comparison approach between aging and traumatic brain injury (TBI) is proposed to investigate everyday object memory patterns using a virtual HOMES test. METHODS: Sixteen young controls, 15 older adults and 15 TBI patients underwent the HOMES test and traditional tests. RESULTS: Older adults and TBI patients exhibited similar HOMES performances: poor recall, a greater recognition benefit, high false recognitions, but intact clustering and proactive interference effects. The age-related differences for HOMES measures were mainly mediated by executive functioning, while the HOMES performances in the TBI group were correlated with memory measures. CONCLUSION: The differential cognitive mediating effects for a similar everyday-like memory pattern are discussed by highlighting the need for more cautious interpretations of cognitive mechanisms behind similar behavioral patterns in different populations especially in clinical and rehabilitation settings.

De novo everolimus-based therapy in renal transplant recipients: effect on proteinuria and renal prognosis.

BACKGROUND: In large-scale clinical trials, the proliferation signal inhibitor (PSI) everolimus (EVL) combined with cyclosporine (CsA) and steroids, has been shown to be efficacious among de novo renal transplant recipients. Development of proteinuria has been shown to be an important predictor of renal dysfunction after conversion from CsA to a PSI-based regimen, and a key marker of allograft disease progression. Whether EVL de novo treatment is associated with a similar proteinuric effect is still under investigation. METHODS: We compared the development of proteinuria among a cohort of 24 renal transplant recipients who were prescribed EVL (3 mg/d; n = 12; high-dose group) or 1.5 mg/d (n = 12; standard-dose group), in association with CsA, versus third control cohort of 12 patients who received mycophenolate mofetil (control group). EVL doses were adjusted to achieve trough blood levels of 3-8 ng/mL and 8-12 ng/mL among the standard and high-dose groups, respectively. We assessed renal function and protein excretion over a 2-year observation. RESULTS: The high-dose group showed a trend toward greater proteinuria than the standard-dose on control groups. They showed significantly greater proteinuria from 9 months until 2 years; 0.86 +/- 0.5, 0.5 +/- 0.3, 0.47 +/- 0.2 g/24 h (P = .03 and P = .02, respectively, at 24 months). Mean proteinuria significantly correlated with mean EVL doses (n = .73; P = .0001). Concomitantly, the estimated glomerular filtration rate (eGFR) was significantly lower among patients treated with EVL 3.0 versus 1.5 mg/d (53.7 +/- 24 vs 73.04 +/- 17.6 mL/min; P = .037). Among patients in the standard-dose, the eGFR was consistently higher than the control group (62.6 +/- 29 mL/min). CONCLUSION: EVL/CsA therapy is a safe alternative regimen for de novo renal transplant recipients. Higher EVL doses are correlated with greater increases in proteinuria. The standard EVL dose seems to be useful treatment strategy to prevent acute rejection episodes, with a better renal prognosis in the long term.

An African perspective on mucosal immunity and HIV-1.

HIV prevention mandates an understanding of the mechanisms of mucosal immunity with attention to some unique features of the epidemic and mucosal environment in the developing world. An effective vaccine will have to induce mucosal protection against a highly diverse virus, which is equipped with a number of immune evasion strategies. Its development will require assessment of mucosal immune responses, and it will have to protect a mucosal environment where inflammation and altered immune responses are common because of the presence of other mucosal infections, such as sexually transmitted infections and parasites, and where nutritional status may also be compromised. Ideally, not only prevention methods would protect adults but also provide cover against gastrointestinal transmission through maternal milk. Prevention might also be complemented by microbicides and circumcision, two alternative approaches to mucosal protection. It seems unlikely that a single solution will work in all instances and intervention might have to act at multiple levels and be tailored to local circumstances. We review here some of the mucosal events associated with HIV infection that are most relevant in an African setting.

Selective modifications in GAD67 mRNA levels in striatonigral and striatopallidal pathways correlate to dopamine agonist priming in 6-hydroxydopamine-lesioned rats.

The present study investigated long-term alterations in striatal gene expression after single exposure of unilaterally 6-hydroxydopamine-lesioned rats to different dopamine agonists (priming). Rats were primed with the D1 agonist SKF38393 (10 mg/kg), the D2/D3 agonist quinpirole (0.2 mg/kg), the dopamine precursor L-DOPA (50 mg/kg) or with vehicle (drug-naive), and GAD67, dynorphin and enkephalin mRNAs were evaluated in the striatum by in situ hybridization, 3 days after priming. To evaluate GAD67 mRNA in striatonigral and striatopallidal neurons, identified as enkephalin (-) and (+) neurons, double-labelling in situ hybridization was used. Drug-naive lesioned rats showed an increase in GAD67 mRNA in enkephalin (-) and (+) neurons, an increase in enkephalin and a decrease in dynorphin mRNAs. Priming with either SKF38393 or quinpirole further increased GAD67 mRNA in enkephalin (-) and (+) neurons, however, while SKF38393 produced a high and unbalanced activation toward enkephalin (-) neurons, after quinpirole the increase was of low intensity and similar in the two pathways. Dynorphin mRNA was increased by SKF38393 but not by quinpirole, whereas enkephalin mRNA was not changed by either priming. L-DOPA produced a high and similar increase in GAD67 mRNA in enkephalin (-) and (+) neurons. Priming differentially affected peptides and GAD67 mRNA in striatopallidal and striatonigral neurons depending on the dopamine agonist used. The degree of enduring overactivity of the striatopallidal and striatonigral pathways may be related to the ability of L-DOPA and D1 or D2/D3 receptor agonists to prime motor behavioural responses and to produce dyskinetic side-effects.

Enhanced IL-4 responses in children with a history of respiratory syncytial virus bronchiolitis in infancy.

Infants who recover from respiratory syncytial virus (RSV)-induced bronchiolitis are at high risk of developing asthma and recurrent wheezing. It is not known whether severe RSV infection itself causes persistent effects or is a marker of a "wheezy" predisposition. To determine the long-term immunological correlates of infantile bronchiolitis, interleukin (IL)-4 and interferon (IFN)-gamma responses to a panel of antigens were studied in a well-characterised cohort of 7-8-yr-old children with a history of severe RSV bronchiolitis in infancy. Peripheral blood lymphocytes from 37 children who were hospitalised with RSV bronchiolitis in infancy and from 69 age-, sex- and location-matched controls were stimulated in vitro with RSV, house-dust mite, birch and cat antigens. Cellular proliferation, and enzyme-linked immunoSPOT IFN-gamma and IL-4 production were measured. IL-4 producing T-cells responding to RSV and cat antigens were significantly more frequent in exbronchiolitics. Other responses (including the IFN-gamma response to RSV) were equally strong in exbronchiolitics and controls. Respiratory syncytial virus infection primes memory T-cells that make interferon-gamma, but virus and aeroallergen-specific and interleukin-4 producing T-cells are also frequently primed in bronchiolitics. Respiratory syncytial virus bronchiolitis in infancy may increase the risk of allergic sensitisation by providing a local interleukin-4-rich environment, in which airborne allergens are first encountered.

Increased aeroallergen-specific interleukin-4-producing T cells in asthmatic adults.

BACKGROUND: Asthma, atopy and some forms of respiratory syncytial virus (RSV) disease are thought to be caused by T cells making IL-4 (Th2 cells). However, not all patients with similar patterns of clinical disease have the same underlying pathogenesis and the ability to detect immunopathogenic T cells by examination of the peripheral blood remains in doubt. With the prospect of specific immunotherapy for diseases caused by T cell subsets, it is important to determine whether peripheral blood mononuclear cell (PBMC) reactivity can be used to establish the presence of immunopathogenic responses and therefore to predict therapeutic effects. OBJECTIVE: To detect IL-4 and IFN-gamma production as markers of Th1 and Th2 responses in the peripheral blood of atopic and asthmatic adults. METHODS: PBMC from 22 adult asthmatics (18 of whom were atopic) and 21 non-asthmatic volunteers (ten of whom were atopic) were stimulated with cat, birch and house dust mite allergens, human rhinovirus, RSV and recombinant chimaeric F/G protein from RSV in vitro. ELISPOT assays were used to enumerate cells producing IL-4 and IFN-gamma. RESULTS: Asthmatics had a sixfold increase in frequencies of IL-4-producing cells to cat and birch allergen (median values: 37 vs. 7 per million PBMC, P < 0.01 and 20 vs. 3 per million PBMC, P < 0.04, respectively) compared to non-asthmatics. By contrast, non-asthmatic atopics showed no specific increase in antigen-specific IL-4 responses and there was no evident correlation between skin prick test reactivity and ELISPOT results. Atopics had significantly more IFN-gamma-producing cells specific for FG than nonatopics. while IFN-gamma and IL-4 responses to other antigens were not significantly different. CONCLUSION: Enhanced IL-4 responses to non-viral aeroallergens are seen in adults with asthma, while enhanced IFN-gamma responses to viral antigen FG were see in atopics. In practical terms, ELISPOT assays for specific cytokines may provide a method that could be used to monitor antigen-specific T cell responses in peripheral blood.

Flow cytometric measurement of intracellular cytokines.

The identification of distinct T helper lymphocyte subsets (Th1/2) with polarised cytokine production has opened up new fields in immunobiology. Of the several alternative methods of monitoring cytokine production, flow cytometric analysis of intracellular staining has distinct advantages and pitfalls. It allows high throughput of samples and multiparameter characterisation of cytokine production on a single cell basis without the need for prolonged in vitro culture and cloning. However, these methods may cause important changes in cell surface phenotype which can make interpretation difficult.

Single cell analysis of cytokine expression kinetics by human CD4+ T-cell clones during activation or tolerance induction.

Exposure to optimal peptide antigen concentrations induces human CD4+ T-cell clones to proliferate and secrete various cytokines. Higher (> 10-fold optimal) antigen concentrations cause long-term proliferative unresponsiveness, which can be reversed by exogenous interleukin-2 (IL-2). We call this condition 'tolerance'. We used intracellular cytokine staining and flow cytometric analysis to investigate the kinetics of interferon-gamma, tumour necrosis factor-alpha, IL-4 and IL-5 production during the initial phase of tolerance induction. Single cell analysis of interferon-gamma and IL-4 or IL-5 coexpression showed functional heterogeneity of cloned human CD4+ T cells. Superstimulation with phorbol 12-myristate 13-acetate and ionomycin (PI) revealed enhanced responsiveness shortly after tolerizing treatment, followed by reduced responsiveness. Both tolerized and activated T cells had similarly reduced cytokine responses when further stimulated with antigen during the following 48 hr, with limited enhancement following additional stimulation with PI. We conclude that cytokine induction is normally followed by a refractory phase, but that the expression of cytokines is enhanced in the initial phase of tolerance induction.

Characterization of the DNA-binding domains from the yeast cell-cycle transcription factors Mbp1 and Swi4.

The minimal DNA-binding domains of the Saccharomyces cerevisiae transcription factors Mbp1 and Swi4 have been identified and their DNA binding properties have been investigated by a combination of methods. An approximately 100 residue region of sequence homology at the N-termini of Mbp1 and Swi4 is necessary but not sufficient for full DNA binding activity. Unexpectedly, nonconserved residues C-terminal to the core domain are essential for DNA binding. Proteolysis of Mbp1 and Swi4 DNA-protein complexes has revealed the extent of these sequences, and C-terminally extended molecules with substantially enhanced DNA binding activity compared to the core domains alone have been produced. The extended Mbp1 and Swi4 proteins bind to their cognate sites with similar affinity [K(A) approximately (1-4) x 10(6) M(-)(1)] and with a 1:1 stoichiometry. However, alanine substitution of two lysine residues (116 and 122) within the C-terminal extension (tail) of Mbp1 considerably reduces the apparent affinity for an MCB (MluI cell-cycle box) containing oligonucleotide. Both Mbp1 and Swi4 are specific for their cognate sites with respect to nonspecific DNA but exhibit similar affinities for the SCB (Swi4/Swi6 cell-cycle box) and MCB consensus elements. Circular dichroism and (1)H NMR spectroscopy reveal that complex formation results in substantial perturbations of base stacking interactions upon DNA binding. These are localized to a central 5'-d(C-A/G-CG)-3' region common to both MCB and SCB sequences consistent with the observed pattern of specificity. Changes in the backbone amide proton and nitrogen chemical shifts upon DNA binding have enabled us to experimentally define a DNA-binding surface on the core N-terminal domain of Mbp1 that is associated with a putative winged helix-turn-helix motif. Furthermore, significant chemical shift differences occur within the C-terminal tail of Mbp1, supporting the notion of two structurally distinct DNA-binding regions within these proteins.

Host genetic determinants of vaccine-induced eosinophilia during respiratory syncytial virus infection.

In BALB/c mice, sensitization with the attachment protein (G) of respiratory syncytial virus (RSV) leads to CD4+ T cell-mediated lung eosinophilia during subsequent challenge with RSV. To determine the host genetic influences on this model of lung eosinophilia, we tested 15 different inbred mouse strains. Eosinophilia developed in all H-2d (BALB/c, DBA/2n, and B10.D2), but not in H-2k (CBA/Ca, CBA/J, C3H, BALB.K, or B10.BR) mouse strains. Among H-2b mice, 129 and BALB.B developed eosinophilia, whereas C57BL/6 and C57BL/10 did not. Testing first generation crosses between sensitive and resistant strains showed that eosinophilia developed in all H-2dxk (n = 5), irrespective of background genes, but not in H-2dxb (n = 2) mice. In vivo depletion of CD8+ T cells or IFN-gamma rendered C57BL/6, but not BALB.K mice, susceptible to eosinophilia. Analysis of B10 recombinant mice showed that the Dd allele (in B10.A(5R) mice) prevented CD8+ T cell accumulation in the lung, resulting in intense lung eosinophilia. However, the Db allele (in B10.A(2R) and B10.A(4R) mice) supported CD8+ T cell expansion and prevented eosinophilia. Intracellular cytokine staining showed that lung eosinophilia correlated with reduced IFN-gamma and increased IL-10 expression in lung T cells. These results are compatible with the unifying model that Th2 cells mediate the disease but can be inhibited by CD8+ T cells secreting IFN-gamma. Our findings have important implications for the development of protective, nonpathogenic vaccines for RSV disease.

Enumeration of lymphokine-secreting cells as a quantitative measure for cellular immune responses in rhesus macaques.

We investigated whether enumeration of lymphokine-secreting T cells can be used as a quantitative measure to determine the immunogenicity of foreign proteins in rhesus monkeys. In addition, it was assessed whether this approach can supplement and/or substitute for the well-established lymphoproliferation assay. Two candidate vaccine proteins (e.g., HIV-1 gp120 and HSV-2gD) were used as model antigens for immunization. PBMCs from immunized animals were antigenically stimulated and evaluated on their proliferative capacity and lymphokine release at the single cell level. The experiments showed a close quantitative correlation between antigen-triggered proliferative responses and the antigen-induced generation of IL-2 and IFN-gamma producing cells (pc). IL-4pc were found to appear relatively late after the initiation of antigen exposure. The data indicate that ELISPOT assays provide valuable tools for the assessment of the antigenicity of foreign proteins in vivo.

HIV-1 envelope-elicited neutralizing antibody titres correlate with protection and virus load in chimpanzees.

In an attempt to compare the protective effect of vaccination with two forms of envelope antigens, and to define immunological correlates of protection against HIV infection, chimpanzees were vaccinated with either recombinant gp160 or gp120. Homologous HIV challenge was performed 3 weeks after the fourth immunization. The animal with the highest level of serum neutralizing antibodies (gp160 immunogen) was protected against HIV infection. All other chimpanzees became infected, but displayed various levels of infected PBMCs. The postchallenge data gave rise to the following conclusions: (1) protection correlated with the level of the serological immune response, but not with the nature of immunogen (gp120 versus gp160); (2) the virus-neutralizing titre at day of challenge correlated with protection from infection; (3) the relative magnitude of the lymphoproliferative T-cell response at day of challenge did not correlate with any protective effect; (4) the peak numbers of virus-infected PBMCs in vaccinated animals were lower than those observed in control animals, and this effect was correlated with the intensity of the antibody response at day of challenge. This raises the possibility that a beneficial effect of HIV vaccination may be achieved in a situation where sterile immunity is not consistently obtained.

Hepatitis B virus-specific cytotoxic T lymphocyte responses in patients with acute and chronic hepatitis B virus infection.

An in vitro model was developed that allowed the analysis of hepatitis B virus-specific cytotoxic T lymphocyte responses in patients suffering from acute and chronic hepatitis B virus infections. Since virus-specific cytotoxic T lymphocytes recognize endogenously synthesized and processed antigen only when it is presented in the context of autologous HLA class I molecules and since hepatitis B virus does not infect human cells in vitro, a panel of recombinant vaccinia viruses was constructed to induce the expression of hepatitis B virus envelope and nucleocapsid proteins in cultured primary cells or cell lines derived from the patients to be studied. In order for a cytotoxic T lymphocyte response to be detectable with the currently available techniques, a sufficient number of activated cytotoxic T lymphocytes is required. To meet this requirement, lymphocytes freshly isolated from venous blood were stimulated in vitro with recombinant vaccinia-infected and formaldehyde-fixed autologous T lymphoblasts. The presence of hepatitis B virus-specific cytotoxic T lymphocytes, amplified and activated during this induction culture, was demonstrated in a microcytotoxicity assay using 51Cr-labeled, recombinant vaccinia-infected Epstein-Barr virus-immortalized, autologous B lymphocytes as target cells. Using this in vitro model, we were able to demonstrate the presence of hepatitis B virus envelope- and nucleocapsid-specific cytotoxic T lymphocytes in venous blood from one subject who had recently recovered from an acute hepatitis B virus infection and in three patients suffering from chronic hepatitis B virus infections. No hepatitis B virus-specific cytotoxic T lymphocytes activity was discernible in the venous blood from two vaccine recipients.(ABSTRACT TRUNCATED AT 250 WORDS)

Recognition of oligonucleotide-encoded T cell epitopes introduced into a gene unrelated to the original antigen.

We have previously demonstrated that H-2Kd-restricted CTL specific for HLA-CW3 or HLA-A24 can recognize synthetic peptides corresponding to residues 170-182 of the HLA molecules. Synthetic oligonucleotides encoding region 170-182 of CW3 or A24 were inserted into the influenza nucleoprotein (NP) gene. We demonstrate herein that P815 (H-2d) cells transfected with the NP-oligo recombinant genes are specifically lysed by HLA-specific Kd-restricted CTL clones. Our results imply that there must be a high degree of flexibility for the expression of T cell epitopes in different molecular contexts.

Antigen recognition by H-2-restricted cytolytic T lymphocytes: inhibition of cytolysis by anti-CD8 monoclonal antibodies depends upon both concentration and primary sequence of peptide antigen.

While it is generally agreed that the specificity of the interaction between cytolytic T lymphocytes (CTL) and their target cells is controlled mainly by antigen-specific T cell receptors (TcR), the molecular role of cell surface CD8 molecules in this interaction is less well understood. In the present study we have reinvestigated the apparent contribution of CD8 molecules to the overall avidity of interaction between CTL and their targets by using a recently developed system of major histocompatibility complex (MHC) class I-restricted CTL clones that recognize defined peptide antigens. We demonstrate that under conditions where the density of MHC, TcR and accessory molecules remains constant, the susceptibility of CD8+ CTL to inhibition of cytolysis by anti-CD8 antibodies is highly dependent upon the concentration and primary structure of the peptide antigen. Although the precise role of the CD8 molecule remains unknown, our results are compatible with models that suggest its contribution to the overall avidity of the CTL-target cell interaction particularly in cases where the affinity between the TcR and antigen-MHC is low.

Competition between unrelated peptides recognized by H-2-Kd restricted T cells.

P815 (H-2d) target cells incubated with synthetic peptides corresponding to region 170-182 of HLA or to region 141-161 of influenza nucleoprotein (NP) are lysed by DBA/2 derived cytolytic T cells (CTL) specific for HLA or by BALB/c derived CTL-specific for NP, respectively. Both peptide Ag are recognized in the context of Kd. We show herein that these unrelated, nonhomologous peptides clearly compete reciprocally for recognition by the appropriate Kd restricted CTL. In contrast, different NP peptides that are recognized by other CTL restricted by HLA-B37, H-2-Db or KK, either failed to compete or were much less efficient as competitors than NP peptides recognized in the context of Kd. The efficiency of a peptide as a competitor correlated with its potency as an Ag. The most efficient competitor was a variant peptide of NP 147-158 with R156 deleted, which had been previously shown to be 1,000 times more efficient as an Ag than its natural homolog. Our results suggest that peptides recognized by CTL in the context of the same MHC class I restriction element may bind to the same or interdependent site(s) on the restriction molecule.

Synthetic peptides as antigens and competitors in recognition by H-2-restricted cytolytic T cells specific for HLA.

The specificity of peptide recognition by a number of Kd-restricted CTL clones specific for HLA-CW3 or HLA-A24 was investigated. The CTL clones were derived from DBA/2 (H-2d) mice immunized with syngeneic P815 mouse cells transfected with genes encoding HLA-CW3 or HLA-A24 class I molecules. We had previously shown that CTL clones that lysed P815-CW3 transfectant target cells could lyse P815 (HLA-) target cells incubated with synthetic CW3 peptides corresponding to the COOH-terminal end of the alpha 2 domain. In the present study, we found that Kd-restricted CTL clones that lysed P815-A24 transfectant target cells recognized a synthetic peptide from the same region (residues 170-182) of the A24 molecule. CW3 and A24 differ by only one amino acid within this region. Recognition of CW3 or A24 peptides corresponded exactly with lysis of P815-HLA transfectants both for clones that mutually exclusively lysed CW3 or A24 transfectant target cells and for CW3/A24 crossreactive CTL clones. The latter CTL clones that lysed both CW3 and A24 transfectant target cells showed a clear preference for the peptide corresponding to the immunizing HLA allele. The homologous CW3 and A24 peptides could compete with each other for recognition, in contrast to a peptide from the same region of HLA-B7. Peptides from the corresponding region of the endogenous Kd and Dd/Ld molecules could also inhibit recognition of CW3 and A24 peptides. Competition with peptides apparently occurred at the level of the target cell. These results are consistent with a model whereby MHC class I molecules position protein fragments or peptides for specific recognition by T cells.

Mapping of HLA epitopes recognized by H-2-restricted cytotoxic T lymphocytes specific for HLA using recombinant genes and synthetic peptides.

Immunization of DBA/2 (H-2d) mice with syngeneic P815 tumor cell transfectants that express HLA class I genes elicits CTL that recognize HLA in the context of H-2Kd molecules. Anti-HLA-CW3 CTL cross-react to a variable extent on the related alleles A3 and A24. Using a panel of target cells expressing native or recombinant HLA genes, we could map the epitope recognized by a CTL clone specific for CW3 to the second external (alpha 2) domain of CW3. Moreover, the epitope recognized by this clone could be mimicked by incubating P815 (HLA negative) target cells with a synthetic peptide corresponding to the C-terminal 12 amino acids of the CW3 alpha 2 domain (residues 171 to 182). Other independent anti-CW3 CTL clones with different fine specificities recognized the same CW3 peptide. In contrast, CTL clones specific for HLA-A24 or HLA-A3 that did not lyse P815-CW3 transfectants did not recognize this peptide. The CW3 peptide could be recognized on other tumor cell targets that were also of H-2d origin, but not on those of H-2b or H-2k origin. The requirement for the expression of H-2Kd by the target cells was directly demonstrated using L cell Kd transfectants. Our results suggest that the CTL response of DBA/2 mice immunized with P815-CW3 transfectants is predominantly Kd restricted and focused on epitopes contained within the 12 C-terminal amino acids of the alpha 2 domain.

H-2-restricted cytolytic T cells specific for HLA can recognize a synthetic HLA peptide.

It is generally accepted that T lymphocytes recognize antigens in the context of molecules encoded by genes in the major histocompatibility complex (MHC). MHC class II-restricted T cells usually recognize degraded or denatured rather than native forms of antigen on the surface of class II-bearing antigen presenting cells. It has recently been shown that short synthetic peptides corresponding to mapped antigenic sites of the influenza nucleoprotein (NP) can render uninfected target cells susceptible to lysis by NP-specific class I-restricted cytolytic T cells (CTL). These and earlier experiments that showed specific recognition of NP deletion mutant transfectants suggest that class I-restricted recognition might also involve processed antigenic fragments. One important issue arising from these studies is whether the model applies not only to viral proteins that are expressed internally (such as NP) but also to antigens normally expressed as integral membrane proteins at the cell surface. We have recently isolated class I-restricted mouse CTL clones that recognize class I gene products of the human MHC (HLA) as antigens in mouse cell HLA-transfectants. Here we show that these anti-HLA CTL can lyse HLA-negative syngeneic mouse cells in the presence of a synthetic HLA peptide. These results suggest that the model applies generally.