Cortical Processing of Multimodal Sensory Learning in Human Neonates.

Following birth, infants must immediately process and rapidly adapt to the array of unknown sensory experiences associated with their new ex-utero environment. However, although it is known that unimodal stimuli induce activity in the corresponding primary sensory cortices of the newborn brain, it is unclear how multimodal stimuli are processed and integrated across modalities. The latter is essential for learning and understanding environmental contingencies through encoding relationships between sensory experiences; and ultimately likely subserves development of life-long skills such as speech and language. Here, for the first time, we map the intracerebral processing which underlies auditory-sensorimotor classical conditioning in a group of 13 neonates (median gestational age at birth: 38 weeks + 4 days, range: 32 weeks + 2 days to 41 weeks + 6 days; median postmenstrual age at scan: 40 weeks + 5 days, range: 38 weeks + 3 days to 42 weeks + 1 days) with blood-oxygen-level-dependent (BOLD) functional magnetic resonance imaging (MRI) and magnetic resonance (MR) compatible robotics. We demonstrate that classical conditioning can induce crossmodal changes within putative unimodal sensory cortex even in the absence of its archetypal substrate. Our results also suggest that multimodal learning is associated with network wide activity within the conditioned neural system. These findings suggest that in early life, external multimodal sensory stimulation and integration shapes activity in the developing cortex and may influence its associated functional network architecture.

Somatotopic Mapping of the Developing Sensorimotor Cortex in the Preterm Human Brain.

In the mature mammalian brain, the primary somatosensory and motor cortices are known to be spatially organized such that neural activity relating to specific body parts can be somatopically mapped onto an anatomical "homunculus". This organization creates an internal body representation which is fundamental for precise motor control, spatial awareness and social interaction. Although it is unknown when this organization develops in humans, animal studies suggest that it may emerge even before the time of normal birth. We therefore characterized the somatotopic organization of the primary sensorimotor cortices using functional MRI and a set of custom-made robotic tools in 35 healthy preterm infants aged from 31 + 6 to 36 + 3 weeks postmenstrual age. Functional responses induced by somatosensory stimulation of the wrists, ankles, and mouth had a distinct spatial organization as seen in the characteristic mature homunculus map. In comparison to the ankle, activation related to wrist stimulation was significantly larger and more commonly involved additional areas including the supplementary motor area and ipsilateral sensorimotor cortex. These results are in keeping with early intrinsic determination of a somatotopic map within the primary sensorimotor cortices. This may explain why acquired brain injury in this region during the preterm period cannot be compensated for by cortical reorganization and therefore can lead to long-lasting motor and sensory impairment.

Machine-learning to characterise neonatal functional connectivity in the preterm brain.

Brain development is adversely affected by preterm birth. Magnetic resonance image analysis has revealed a complex fusion of structural alterations across all tissue compartments that are apparent by term-equivalent age, persistent into adolescence and adulthood, and associated with wide-ranging neurodevelopment disorders. Although functional MRI has revealed the relatively advanced organisational state of the neonatal brain, the full extent and nature of functional disruptions following preterm birth remain unclear. In this study, we apply machine-learning methods to compare whole-brain functional connectivity in preterm infants at term-equivalent age and healthy term-born neonates in order to test the hypothesis that preterm birth results in specific alterations to functional connectivity by term-equivalent age. Functional connectivity networks were estimated in 105 preterm infants and 26 term controls using group-independent component analysis and a graphical lasso model. A random forest-based feature selection method was used to identify discriminative edges within each network and a nonlinear support vector machine was used to classify subjects based on functional connectivity alone. We achieved 80% cross-validated classification accuracy informed by a small set of discriminative edges. These edges connected a number of functional nodes in subcortical and cortical grey matter, and most were stronger in term neonates compared to those born preterm. Half of the discriminative edges connected one or more nodes within the basal ganglia. These results demonstrate that functional connectivity in the preterm brain is significantly altered by term-equivalent age, confirming previous reports of altered connectivity between subcortical structures and higher-level association cortex following preterm birth.

The effects of hemorrhagic parenchymal infarction on the establishment of sensori-motor structural and functional connectivity in early infancy.

INTRODUCTION: The objective of the study was to characterize alterations of structural and functional connectivity within the developing sensori-motor system in infants with focal perinatal brain injury and at high risk of cerebral palsy. METHODS: Functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI) data were used to study the developing functional and structural connectivity framework in six infants born prematurely at term equivalent age. This was first characterised in three infants without focal pathology, which was then compared to that derived from three infants with unilateral haemorrhagic parenchymal infarction and a subsequent focal periventricular white matter lesion who developed later haemiparesis. RESULTS: Functional responses to passive hand movement were in the contralateral perirolandic cortex, regardless of focal pathology. In infants with unilateral periventricular injury, afferent thalamo-cortical tracts appeared to have developed compensatory trajectories which circumvented areas of damage. In contrast, efferent corticospinal tracts showed marked asymmetry at term equivalent age following focal brain injury. Sensori-motor network analysis suggested that inter-hemispheric functional connectivity is largely preserved despite pathology and that impairment may be associated with adverse neurodevelopmental outcome. CONCLUSION: Following focal perinatal brain injury, altered structural and functional connectivity is already present and can be characterized with MRI at term equivalent age. The results of this small case series suggest that these techniques may provide valuable new information about prognosis and the pathophysiology underlying cerebral palsy.

Aperiodic and Hurst EEG exponents across early human brain development: A systematic review.

In electroencephalographic (EEG) data, power-frequency slope exponents (1/f_ß) can provide non-invasive markers of in vivo neural activity excitation-inhibition (E:I) balance. E:I balance may be altered in neurodevelopmental conditions; hence, understanding how 1/fß evolves across infancy/childhood has implications for developing early assessments/interventions. This systematic review (PROSPERO-ID: CRD42023363294) explored the early maturation (0-26 yrs) of resting-state EEG 1/f measures (aperiodic [AE], power law [PLE] and Hurst [HE] exponents), including studies containing ≥1 1/f measures and ≥10 typically developing participants. Five databases (including Embase and Scopus) were searched during March 2023. Forty-two studies were identified (Nparticipants=3478). Risk of bias was assessed using the Quality Assessment with Diverse Studies tool. Narrative synthesis of HE data suggests non-stationary EEG activity occurs throughout development. Age-related trends were complex, with rapid decreases in AEs during infancy and heterogenous changes thereafter. Regionally, AE maxima shifted developmentally, potentially reflecting spatial trends in maturing brain connectivity. This work highlights the importance of further characterising the development of 1/f measures to better understand how E:I balance shapes brain and cognitive development.

UNITY: A low-field magnetic resonance neuroimaging initiative to characterize neurodevelopment in low and middle-income settings.

Measures of physical growth, such as weight and height have long been the predominant outcomes for monitoring child health and evaluating interventional outcomes in public health studies, including those that may impact neurodevelopment. While physical growth generally reflects overall health and nutritional status, it lacks sensitivity and specificity to brain growth and developing cognitive skills and abilities. Psychometric tools, e.g., the Bayley Scales of Infant and Toddler Development, may afford more direct assessment of cognitive development but they require language translation, cultural adaptation, and population norming. Further, they are not always reliable predictors of future outcomes when assessed within the first 12-18 months of a child's life. Neuroimaging may provide more objective, sensitive, and predictive measures of neurodevelopment but tools such as magnetic resonance (MR) imaging are not readily available in many low and middle-income countries (LMICs). MRI systems that operate at lower magnetic fields (< 100mT) may offer increased accessibility, but their use for global health studies remains nascent. The UNITY project is envisaged as a global partnership to advance neuroimaging in global health studies. Here we describe the UNITY project, its goals, methods, operating procedures, and expected outcomes in characterizing neurodevelopment in sub-Saharan Africa and South Asia.

Computer-controlled stimulation for functional magnetic resonance imaging studies of the neonatal olfactory system.

AIM: Olfactory sensation is highly functional early in human neonatal life, with studies suggesting that odours can influence behaviour and infant-mother bonding. Due to its good spatial properties, blood oxygen level-dependent (BOLD) contrast functional magnetic resonance imaging (fMRI) has the potential to rapidly advance our understanding of the neural activity which underlies the development of olfactory perception in this key period. We aimed to design an 'olfactometer' specifically for use with neonatal subjects for fMRI studies of odour perception. METHODS: We describe a fully automated and programmable, fMRI compatible system capable of presenting odorant liquids. To prevent contamination of the system and minimize between-subject infective risk, the majority of the olfactometer is constructed from single-use, readily available clinical equipment. The system was used to present the odour of infant formula milk in a validation group of seven neonatal subjects at term equivalent postmenstrual age (median age 40 weeks). RESULTS: A safe, reliable and reproducible pattern of stimulation was delivered leading to well-localized positive BOLD functional responses in the piriform cortex, amygdala, thalamus, insular cortex and cerebellum. CONCLUSIONS: The described system is therefore suitable for detailed studies of the ontology of olfactory sensation and perception during early human brain development.

Somatosensory cortical activation identified by functional MRI in preterm and term infants.

Functional MRI (fMRI) has not previously been used systematically to investigate brain function in preterm infants. We here describe statistically robust and reproducible fMRI results in this challenging subject group using a programmable somatosensory stimulus synchronized with MR image acquisition which induced well-localized positive blood oxygen level dependent (BOLD) responses contralateral to the side of the stimulation in: 11 preterm infants (median post menstrual age 33 weeks and 4 days, range 29+1 to 35+3); 6 control infants born at term gestational age; and 18 infants born preterm (median gestational age at birth 30 weeks and 5 days, range 25+4 to 36+0) but studied at term corrected gestational age. Bilateral signals were identified in 8 of the ex-preterm infants at term age. Anatomical confirmation of appropriate activations was provided with diffusion tensor imaging (DTI) based tractography which identified connecting pathways from the regions of activation through the ipsilateral corticospinal tracts and posterior limb of the internal capsule. These results demonstrate that it is possible to reliably identify positive BOLD signals in the infant brain and that fMRI techniques can also be applied in the study of preterm infants.

Safety and efficacy of high-dose enteral, intravenous, and transdermal clonidine for the acute management of severe intractable childhood dystonia and status dystonicus: An illustrative case-series.

OBJECTIVE: Acute dystonia in children is distressing, painful and can progress to life-threatening status dystonicus. Typical management involves benzodiazepines which can result in respiratory depression requiring PICU admission. Clonidine is less respiratory-depressant, and by facilitating sleep, switches dystonia off. It can also be administered via enteral, continuous intravenous infusion, and transdermal slow release routes. We describe the dose range and safety profile of clonidine management in a case-series of children with severe acute exacerbation of dystonia in a tertiary hospital setting. METHODS: The management of 5 children (3 female, age range 8-14 years) suffering from an acute exacerbation of secondary dystonia requiring hospital admission at the Evelina London Children's Hospital was reviewed. The average and maximum dose of clonidine in mcg/kg/h and routes of administration were recorded for each day of hospital admission. Co-administration of any other medical treatments for dystonia and their route of administration were also recorded. Cardiovascular and respiratory clinical status were measured by recording the daily mean and maximum Paediatric Early Warning Scores (PEWS). RESULTS: Clonidine was administered via enteral, intravenous, and transdermal routes at a median dose of 2.5 mcg/kg/h (range 0.1-9 mcg/kg/h). Administration of high dose clonidine was associated with decreased use of benzodiazepines, morphine, and propofol: avoiding invasive respiratory support for ¾ cases during admission. Clonidine doses via all routes of administration did not correlate with poorer PEWS scores (p = 0.839). Both high dose intravenous and transdermal clonidine where found to be effective. CONCLUSIONS: High dose clonidine administered via different routes can be used in the acute management of severe exacerbations of dystonia. Its use in our cohort was not associated with significant cardio-respiratory depression even at doses as high as 9 mcg/kg/h.

A Simple fMRI Compatible Robotic Stimulator to Study the Neural Mechanisms of Touch and Pain.

This paper presents a simple device for the investigation of the human somatosensory system with functional magnetic imaging (fMRI). PC-controlled pneumatic actuation is employed to produce innocuous or noxious mechanical stimulation of the skin. Stimulation patterns are synchronized with fMRI and other relevant physiological measurements like electroencephalographic activity and vital physiological parameters. The system allows adjustable regulation of stimulation parameters and provides consistent patterns of stimulation. A validation experiment demonstrates that the system safely and reliably identifies clusters of functional activity in brain regions involved in the processing of pain. This new device is inexpensive, portable, easy-to-assemble and customizable to suit different experimental requirements. It provides robust and consistent somatosensory stimulation, which is of crucial importance to investigating the mechanisms of pain and its strong connection with the sense of touch.

An fMRI compatible wrist robotic interface to study brain development in neonates.

A comprehensive understanding of the mechanisms that underlie brain development in premature infants and newborns is crucial for the identification of interventional therapies and rehabilitative strategies. fMRI has the potential to identify such mechanisms, but standard techniques used in adults cannot be implemented in infant studies in a straightforward manner. We have developed an MR safe wrist stimulating robot to systematically investigate the functional brain activity related to both spontaneous and induced wrist movements in premature babies using fMRI. We present the technical aspects of this development and the results of validation experiments. Using the device, the cortical activity associated with both active and passive finger movements were reliably identified in a healthy adult subject. In two preterm infants, passive wrist movements induced a well localized positive BOLD response in the contralateral somatosensory cortex. Furthermore, in a single preterm infant, spontaneous wrist movements were found to be associated with an adjacent cluster of activity, at the level of the infant's primary motor cortex. The described device will allow detailed and objective fMRI studies of somatosensory and motor system development during early human life and following neonatal brain injury.

A patient care system for early 3.0 Tesla magnetic resonance imaging of very low birth weight infants.

BACKGROUND: Very low birth weight (VLBW) infants (weight <1500 g) are increasingly cared for without prolonged periods of positive pressure ventilation (PPV). AIMS: To develop a system for 3.0 T magnetic resonance (MR) image acquisition from VLBW infants who are not receiving PPV, and to test the clinical stability of a consecutive cohort of such infants. DESIGN: Seventy VLBW infants whose median weight at image acquisition was 940 g (590-1490) underwent brain MR imaging with the developed care system as participants in research. Twenty infants (29%) received nasal continuous positive airway pressure (nCPAP), 28 (40%) received supplemental oxygen by nasal cannulae, and 22 (31%) breathed spontaneously in air during the MR examination. RESULTS: There were no significant adverse events. Seventy-six percent had none or transient self-correcting oxygen desaturations. Desaturations that required interruption of the scan for assessment were less common among infants receiving nCPAP (2/20) or breathing spontaneously in air (2/22), compared with those receiving nasal cannulae oxygen (13/28), p=0.003. Sixty-four (91%) infants had an axillary temperature > or =36 degrees C at completion of the scan (lowest 35.7 degrees C), There was no relationship between weight (p=0.167) or use of nCPAP (p=0.453) and axillary temperature <36 degrees C. No infant became hyperthermic. CONCLUSION: VLBW infants who do not require ventilation by endotracheal tube can be imaged successfully and safely at 3.0 T, including those receiving nCPAP from a customised system.

Persistent infection by Helicobacter pylori down-modulates virus-specific CD8+ cytotoxic T cell response and prolongs viral infection.

To determine whether Helicobacter pylori infection affects clearance of a concomitant viral infection and cytotoxic T lymphocyte (CTL) and cytokine response to that infection, H. pylori-infected BALB/c mice were challenged with a recombinant vaccinia virus expressing human immunodeficiency virus type 1 gp160. Two H. pylori strains, a colonizing clinical isolate (KS612) and an established standard noncolonizing strain (NCTC11637), were compared. Clearance of recombinant vaccinia virus was reduced in KS612-infected mice compared with NCTC11637-infected and control mice. As a potential mechanism, in contrast to control or NCTC11637-infected mice, the H. pylori clinical isolate KS612 diminished gp160-specific and vaccinia virus-specific CTL activity, even in the presence of exogenous interleukin-2. Furthermore, KS612-infected mice had reduced Th1 cytokine responses to gp120 in vitro compared with control or NCTC11637-infected mice. These results have implications for possible effects of prevalent H. pylori infection on other human diseases.

Efficient class II major histocompatibility complex presentation of endogenously synthesized hepatitis C virus core protein by Epstein-Barr virus-transformed B-lymphoblastoid cell lines to CD4(+) T cells.

The induction of an efficient CD4(+) T-cell response against hepatitis C virus (HCV) is critical for control of the chronicity of HCV infection. The ability of HCV structural protein endogenously expressed in an antigen-presenting cell (APC) to be presented by class II major histocompatibility complex molecules to CD4(+) T cells was investigated by in vitro culture analyses using HCV core-specific T-cell lines and autologous Epstein-Barr virus-transformed B-lymphoblastoid cell lines (B-LCLs) expressing structural HCV antigens. The T- and B-cell lines were generated from peripheral blood mononuclear cells derived from HCV-infected patients. Expression and intracellular localization of core protein in transfected cells were determined by immunoblotting and immunofluorescence. By stimulation with autologous B-LCLs expressing viral antigens, strong T-cell proliferative responses were induced in two of three patients, while no substantial stimulatory effects were produced by B-LCLs expressing a control protein (chloramphenicol acetyltransferase) or by B-LCLs alone. The results showed that transfected B cells presented mainly endogenously synthesized core peptides. Presentation of secreted antigens from adjacent antigen-expressing cells was not enough to stimulate a core-specific T-cell response. Only weak T-cell proliferative responses were generated by stimulation with B-LCLs that had been pulsed beforehand with at least a 10-fold-higher amount of transfected COS cells in the form of cell lysate, suggesting that presentation of antigens released from dead cells in the B-LCL cultures had a minimal role. Titrating numbers of APCs, we showed that as few as 10(4) transfected B-LCL APCs were sufficient to stimulate T cells. This presentation pathway was found to be leupeptin sensitive, and it can be blocked by antibody to HLA class II (DR). In addition, expression of a costimulatory signal by B7/BB1 on B cells was essential for T-cell activation.

Characterization of anti-histone antibodies in patients with type 1 autoimmune hepatitis.

We have recently found that antibodies to total histones are common in a group of American patients with type 1 autoimmune hepatitis (AIH). In an attempt to determine the profile and clinical association of anti-histone antibody (AHA), 45 Japanese AIH patients were studied for serum isotypic reactivity with individual histones (H1, H2A, H2B, H3, H4) by enzyme-linked immunosorbent assay and western blotting. The results revealed that 40% of sera had reactivities with at least one of individual histones and that the antibodies were detected in all three classes of immunoglobulins (IgG, IgM, IgA). Immunoglobulin G type anti-H3 showed the dominant reactivity and it characterized 72% of sera with AHA. The titre of anti-H3 decreased significantly (P < 0.0075) after steroid therapy and the index of decrease for anti-H3 was correlated in individuals with that for serum aminotransferase. In general, patients with AHA showed higher serum level of alanine aminotransferase (P < 0.05), immunoglobulin G (P < 0.025), and higher frequency of A2-DR4 haplotype (53 vs 17%) than their seronegative counterparts. However, the titre of AHA was low in this disease condition and histone class-specific antibodies did not distinguish patients with distinctive clinical features, although patients with anti-H3 tended to be younger than those without AHA.

Approaches to improve engineered vaccines for human immunodeficiency virus and other viruses that cause chronic infections.

We used several approaches to develop enhanced vaccines for chronic viral infections such as human immunodeficiency virus (HIV) and hepatitis C virus (HCV). 1) Selected epitopes were used to avoid potentially harmful immune responses. 2) Linkage between helper and cytotoxic T-lymphocyte (CTL) epitopes was found to be important. 3) We developed an "epitope enhancement" approach modifying the sequences of epitopes to make more potent vaccines, including examples for HIV and HCV epitopes presented by murine class II and human class I major histocompatibility complex (MHC) molecules. 4) CTL avidity was found to be important for clearing viral infections in vivo, and the mechanism was examined. High-avidity CTLs, however, were found to undergo apoptosis when confronted with high-density antigen, through a mechanism involving tumor necrosis factor (TNF), TNF-RII, and a permissive state induced through the T-cell receptor. 5) We employed cytokines in the adjuvant to steer immune responses toward desired phenotypes, and showed synergy between cytokines. 6) Intrarectal immunization with peptide vaccine induced mucosal and systemic CTL. Local mucosal CTL were found to be critical for resistance to mucosal viral transmission and this resistance was enhanced with mucosally delivered interleukin-12. 7) We used an asymmetry in induction of mucosal and systemic immune responses to circumvent pre-existing vaccinia immunity for use of recombinant vaccinia vaccines.

CTL responses of HLA-A2.1-transgenic mice specific for hepatitis C viral peptides predict epitopes for CTL of humans carrying HLA-A2.1.

Vaccine development in animal models depends on ability to recognize epitopes seen by human T cells. In this work, we show that CTL responses in transgenic mice expressing human HLA-A2.1 prospectively predict the same four of 11 hepatitis C virus (HCV) structural protein-derived peptides, expressing a sequence motif for HLA-A2.1 binding, that are actually recognized by human A2.1-restricted CTLs. The CTLs also recognized targets endogenously expressing these proteins. Human CTLs from HCV-infected patients, tested by using the same peptides, revealed a virtually identical response repertoire. A highly conserved HCV core peptide was the most immunogenic, and may be a valuable component of a vaccine against a broad range of HCV isolates in HLA-A2-positive patients. These results suggest that, in spite of species differences, the T cell repertoire is plastic enough to allow a similar response when the same class I MHC molecule is presenting the peptide. Thus, the HLA molecule plays the primary role in determining which peptides are recognized by CTLs. This transgenic mouse model is important for the study of HLA-restricted CTL determinants and for an approach to design a potential HCV vaccine.

Reciprocal cytotoxic T lymphocyte cross-reactivity interactions between two major epitopes within HIV-1 gp160.

We have observed and analyzed an unexpected cross-reactivity of CD8+ CTL between two nonhomologous peptides of the HIV-1 IIIB gp160 envelope protein, P18 (residues 315-329) and HP53 (834-848, also called TH4.1), in the context of four different class I MHC molecules, Dd, Dp, Dq (or Lq), and H-2u. In strains expressing Dd, the cross-reactivity between peptides was bidirectional, whereas in other strains (H-2u, H-2p, and H-2q), the cross-reactivity was unidirectional; that is, P18-specific CTLs showed no killing against targets pulsed with HP53, although HP53 stimulated CTL showed cross-reactive lysis against P18-pulsed target cells. Cross-reactivity was also shown in immunization in vivo and with target cells endogenously expressing viral protein in vitro using two different recombinant vaccinia viruses expressing only the N-terminal portion of gp160, containing P18 but not HP53. Peptide cross-contamination was excluded. Cold target inhibition and single cell cloning experiments indicated that the same CTL was responding to both peptides. Using substituted and truncated peptides, we explored amino acid residues critical for cross-reactive CTL recognition, identified fine specificity similarities among all cross-reactive CTL lines but not non-cross-reactive lines, and mapped cross-reactivity to a 10-residue core of P18 and to an eight-residue core of HP53. A comparison of these peptide sequences and recent data on residues of P18 interacting with H-2Dd provided us with clues to residues involved in the interaction of the CTL with the MHC-peptide complex.

Use of intrinsic and extrinsic helper epitopes for in vivo induction of anti-hepatitis C virus cytotoxic T lymphocytes (CTL) with CTL epitope peptide vaccines.

The induction of virus-specific cytotoxic T lymphocytes (CTL) is an important part of vaccine strategy. CTL induction in vivo by two hepatitis C virus (HCV) peptides containing CTL epitopes, one from the NS5 region (P17) and one from the core (C7), was compared. P17 required covalent attachment of a helper peptide (PCLUS3 containing a cluster of epitopes from the human immunodeficiency virus envelope protein), whereas C7 did not. However, the minimal decapeptide of C7, C7A10, alone did not induce CTL. The helper cells induced by PCLUS3-17 or by C7 were shown to be CD4+ and to produce interleukin-2 (IL-2). Thus, help can be supplied by a natural helper epitope intrinsic to the CTL peptide, as in C7, or by attaching a helper epitope from another protein, as in the case of P17. The cluster peptides may be useful promiscuous helper peptides for a variety of CTL epitopes from diverse pathogens.

Prophylactic DNA vaccine for hepatitis C virus (HCV) infection: HCV-specific cytotoxic T lymphocyte induction and protection from HCV-recombinant vaccinia infection in an HLA-A2.1 transgenic mouse model.

DNA vaccines express antigens intracellularly and effectively induce cellular immune responses. Because only chimpanzees can be used to model human hepatitis C virus (HCV) infections, we developed a small-animal model using HLA-A2.1-transgenic mice to test induction of HLA-A2.1-restricted cytotoxic T lymphocytes (CTLs) and protection against recombinant vaccinia expressing HCV-core. A plasmid encoding the HCV-core antigen induced CD8(+) CTLs specific for three conserved endogenously expressed core peptides presented by human HLA-A2.1. When challenged, DNA-immunized mice showed a substantial (5-12 log(10)) reduction in vaccinia virus titer compared with mock-immunized controls. This protection, lasting at least 14 mo, was shown to be mediated by CD8(+) cells. Thus, a DNA vaccine expressing HCV-core is a potential candidate for a prophylactic vaccine for HLA-A2.1(+) humans.