Race and Ethnic Group Enrollment and Outcomes for Wilms Tumor: Analysis of the Current Era Children's Oncology Group Study, AREN03B2

BACKGROUND

To review race and ethnic group enrollment and outcomes for Wilms tumor (WT) across all 4 risk-assigned therapeutic trials from the current era Children's Oncology Group Renal Tumor Biology and Risk Stratification Protocol, AREN03B2.

STUDY DESIGN

For patients with WT enrolled in AREN03B2 (2006 to 2019), disease and biologic features, therapeutic study-specific enrollment, and event-free (EFS) and overall (OS) 4-year survival were compared between institutionally reported race and ethnic groups.

RESULTS

Among 5,146 patients with WT, no statistically significant differences were detected between race and ethnic groups regarding subsequent risk-assigned therapeutic study enrollment, disease stage, histology, biologic factors, or overall EFS or OS, except the following variables: Black children were older and had larger tumors at enrollment, whereas Hispanic children had lower rates of diffuse anaplasia WT and loss of heterozygosity at 1p. The only significant difference in EFS or OS between race and ethnic groups was observed among the few children treated for diffuse anaplasia WT with regimen UH-1 and -2 on high-risk protocol, AREN0321. On this therapeutic arm only, Black children showed worse EFS (hazard ratio = 3.18) and OS (hazard ratio = 3.42). However, this finding was not replicated for patients treated with regimen UH-1 and -2 under AREN03B2 but not on AREN0321.

CONCLUSIONS

Race and ethnic group enrollment appeared constant across AREN03B2 risk-assigned therapeutic trials. EFS and OS on these therapeutic trials when analyzed together were comparable regarding race and ethnicity. Black children may have experienced worse stage-specific survival when treated with regimen UH-1 and -2 on AREN0321, but this survival gap was not confirmed when analyzing additional high-risk AREN03B2 patients.

Magnetic resonance neurography techniques in the pediatric population

The use of magnetic resonance imaging (MRI) in the evaluation of the central extracranial nervous system, namely the brachial and lumbosacral plexuses, is well established and has been performed for many years. Only recently after numerous advances in MRI, has image quality been sufficient to properly visualize small structures, such as nerves in the extremities. Despite the advances, peripheral MR Neurography remains a complex and difficult examination to perform, especially in the pediatric patient population, in which the risk for motion artifact and compliance is always of concern. Thus, technical aspects of the MR imaging protocol must be flexible but robust, to balance image quality with scan time, in a patient population of varying sizes. An additional important step for reliably performing a successful MR Neurography examination is the non-technical pre-imaging preparation, which includes patient/family education and open communication with referring teams. This paper will discuss in detail the individual technical and non-technical/operational aspects of peripheral MR Neurography, to help guide in building a successful program in the pediatric population.

Imaging of pediatric renal tumors: A COG Diagnostic Imaging Committee/SPR Oncology Committee White Paper focused on Wilms tumor and nephrogenic rests

Malignant renal tumors account for approximately 6% of pediatric malignancies, with Wilms tumor (WT) representing approximately 90% of pediatric renal tumors. This paper provides consensus-based imaging guidelines for the initial evaluation of a child with suspected WT and follow-up during and after therapy co-developed by the Children's Oncology Group (COG) Diagnostic Imaging and Society for Pediatric Radiology (SPR) oncology committees. The guidelines for Wilms Tumor Imaging in the Society of International Pediatric Oncology (SIOP) are briefly discussed to highlight some of the differences in imaging approach.

Imaging of pediatric bone tumors: A COG Diagnostic Imaging Committee/SPR Oncology Committee White Paper

Malignant primary bone tumors are uncommon in the pediatric population, accounting for 3%-5% of all pediatric malignancies. Osteosarcoma and Ewing sarcoma comprise 90% of malignant primary bone tumors in children and adolescents. This paper provides consensus-based recommendations for imaging in children with osteosarcoma and Ewing sarcoma at diagnosis, during therapy, and after therapy.

Volumetric and multispectral DWI near metallic implants using a non-linear phase Carr-Purcell-Meiboom-Gill diffusion preparation

PURPOSE

DWI near metal implants has not been widely explored due to substantial challenges associated with through-slice and in-plane distortions, the increased encoding requirement of different spectral bins, and limited SNR. There is no widely adopted clinical protocol for DWI near metal since the commonly used EPI trajectory fails completely due to distortion from extreme off-resonance ranging from 2 to 20 kHz. We present a sequence that achieves DWI near metal with moderate b-values (400-500 s/mm² ) and volumetric coverage in clinically feasible scan times.

THEORY AND METHODS

Multispectral excitation with Cartesian sampling, view angle tilting, and kz phase encoding reduce in-plane and through-plane off-resonance artifacts, and Carr-Purcell-Meiboom-Gill (CPMG) spin-echo refocusing trains counteract T2* effects. The effect of random phase on the refocusing train is eliminated using a stimulated echo diffusion preparation. Root-flipped Shinnar-Le Roux refocusing pulses permits preparation of a high spectral bandwidth, which improves imaging times by reducing the number of excitations required to cover the desired spectral range. B₁ sensitivity is reduced by using an excitation that satisfies the CPMG condition in the preparation. A method for ADC quantification insensitive to background gradients is presented.

RESULTS

Non-linear phase refocusing pulses reduces the peak B₁ by 46% which allows RF bandwidth to be doubled. Simulations and phantom experiments show that a non-linear phase CPMG pulse pair reduces B₁ sensitivity. Application in vivo demonstrates complementary contrast to conventional multispectral acquisitions and improved visualization compared to DW-EPI.

CONCLUSION

Volumetric and multispectral DW imaging near metal can be achieved with a 3D encoded sequence.

Zero echo time pediatric musculoskeletal magnetic resonance imaging: initial experience

BACKGROUND

Projection radiography (XR) is often supplemented by both CT (to evaluate osseous structures with ionizing radiation) and MRI (for marrow and soft-tissue assessment). Zero echo time (ZTE) MR imaging produces a "CT-like" osseous contrast that might obviate CT.

OBJECTIVE

This study investigated our institution's initial experience in implementing an isotropic ZTE MR imaging sequence for pediatric musculoskeletal examinations.

MATERIALS AND METHODS

Pediatric patients referred for extremity MRI at 3 tesla (T) underwent ZTE MR imaging to yield images with contrast similar to that of CT. A radiograph-like image was also created with ray-sum image processing. We assessed ZTE-CT/XR anatomical image quality (S_anat) from 0 (nondiagnostic) to 5 (outstanding). Further, we made image comparisons on a 5-point scale (S_comp) (range of -2 = conventional CT/XR greater anatomical delineation to +2 = ZTE-CT/XR greater anatomical delineation; 0=same) for three cohorts: (1) ZTE-XR to conventional radiography, (2) ZTE-CT to conventional CT and (3) pathological lesion assessment on ZTE-XR to conventional radiography. We measured cortical thickness of ZTE-XR and ZTE-CT and compared these with conventional imaging. We calculated confidence interval of proportions, Wilcoxon rank sum test and intraclass correlation coefficients for inter-reader agreement.

RESULTS

Cohorts 1, 2 and 3 consisted of 40, 20 and 35 cases, respectively (age range 0.6-23.0 years). ZTE-CT versus CT and ZTE-XR versus radiography of cortical thicknesses were not significantly different (P=0.55 and P=0.31, respectively). Cortical delineation was rated diagnostic or better (score of 3, 4 or 5) in all cases (confidence interval of proportions = 100%) for ZTE-CT/XR. Similarly, intramedullary cavity delineation was rated diagnostic or better in all cases for ZTE-CT, and ZTE-XR was at least diagnostic in 58-63% of cases. For cohort 2, cortex and intramedullary cavity S_comp for ZTE-CT was comparable to those of conventional CT, with confidence interval of proportion (sum of score of -1 to +2) of 93-100% and 95%, respectively. Pathology visualized on ZTE-CT/XR was comparable; S_comp confidence interval of proportions was 95%/97-100%, with improved delineation of non-displaced fractures on ZTE-XR. Readers had moderate to near-perfect intraclass correlation coefficient (range=0.60-0.93).

CONCLUSION

Implementation of a diagnostic-quality ZTE MRI sequence in the pediatric population is feasible and can be performed as a complementary pulse sequence to enhance musculoskeletal MRI studies. Compared to conventional CT, ZTE has comparable cortical delineation, intramedullary cavity and pathology visualization. While not intended as a replacement for conventional radiography, ZTE-XR provides similar visualization of pathology.

Ultrasound shear wave elastography: does it add value to gray-scale ultrasound imaging in differentiating biliary atresia from other causes of neonatal jaundice?

BACKGROUND

Neonatal/infantile jaundice is relatively common, and most cases resolve spontaneously. However, in the setting of unresolved neonatal cholestasis, a prompt and accurate assessment for biliary atresia is vital to prevent poor outcomes.

OBJECTIVE

To determine whether shear wave elastography (SWE) alone or combined with gray-scale imaging improves the diagnostic performance of US in discriminating biliary atresia from other causes of neonatal jaundice over that of gray-scale imaging alone.

MATERIALS AND METHODS

Infants referred for cholestatic jaundice were assessed with SWE and gray-scale US. On gray-scale US, two radiology readers assessed liver heterogeneity, presence of the triangular cord sign, hepatic artery size, presence/absence of common bile duct and gallbladder, and gallbladder shape; associated interobserver correlation coefficients (ICC) were calculated. SWE speeds were performed on a Siemens S3000 using 6C2 and 9 L4 transducers with both point and two-dimensional (2-D) SWE US. Both univariable and multivariable analyses were performed, as were receiver operating characteristic curves (ROC) and statistical significance tests (chi-squared, analysis of variance, t-test and Wilcoxon rank sum) when appropriate.

RESULTS

There were 212 infants with biliary atresia and 106 without biliary atresia. The median shear wave speed (SWS) for biliary atresia cases was significantly higher (P<0.001) than for non-biliary-atresia cases for all acquisition modes. For reference, the median L9 point SWS was 2.1 m/s (interquartile range [IQR] 1.7-2.4 m/s) in infants with biliary atresia and 1.5 m/s (IQR 1.3-1.9 m/s) in infants without biliary atresia (P<0.001). All gray-scale US findings were significantly different between biliary-atresia and non-biliary-atresia cohorts (P<0.001), intraclass correlation coefficient (ICC) range 0.7-1.0. Triangular cord sign was most predictive of biliary atresia independent of other gray-scale findings or SWS - 96% specific and 88% sensitive. Multistep univariable/multivariable analysis of both gray-scale findings and SWE resulted in three groups being predictive of biliary atresia likelihood. Abnormal common bile duct/gallbladder and enlarged hepatic artery were highly predictive of biliary atresia independent of SWS (100% for girls and 95-100% for boys). Presence of both the common bile duct and the gallbladder along with a normal hepatic artery usually excluded biliary atresia independent of SWS. Other gray-scale combinations were equivocal, and including SWE improved discrimination between biliary-atresia and non-biliary-atresia cases.

CONCLUSION

Shear wave elastography independent of gray-scale US significantly differentiated biliary-atresia from non-biliary-atresia cases. However, gray-scale findings were more predictive of biliary atresia than elastography. SWE was useful for differentiating biliary-atresia from non-biliary-atresia cases in the setting of equivocal gray-scale findings.

Near-Silent and Distortion-Free Diffusion MRI in Pediatric Musculoskeletal Disorders: Comparison With Echo Planar Imaging Diffusion

BACKGROUND

Diffusion-weighted imaging (DWI) is common for evaluating pediatric musculoskeletal lesions, but suffers from geometric distortion and intense acoustic noise.

PURPOSE

To investigate the performance of a near-silent and distortion-free DWI sequence (DW-SD) relative to standard echo-planar DWI (DW-EPI) in pediatric extremity MRI.

STUDY TYPE

Prospective validation study.

SUBJECTS

Thirty-nine children referred for extremity MRI.

FIELD STRENGTH/SEQUENCE

DW-EPI and DW-SD, based on a rotating ultrafast sequence modified with sinusoidal diffusion preparation gradients, at 3T.

ASSESSMENT

DW-SD image quality (S_anat ) was assessed from 0 (nondiagnostic) to 5 (outstanding) and comparative image quality (S_comp ) (from -2 = DW-EPI more delineated to +2 = DW-SD more delineated, 0 = same). ADC measured by DW-SD and DW-EPI were compared in bone marrow, muscle, and lesions.

STATISTICAL TESTS

Wilcoxon rank-sum test and confidence interval of proportions (CIOP) were calculated for S_comp , Student's t-test, coefficient of variation (COV), and Bland-Altman analysis for ADC values, and intraclass correlation coefficient (ICC) for interreader agreement.

RESULTS

DW-SD and DW-EPI ADC values for bone marrow, muscle, and lesions were not significantly different (P = 0.3, P = 0.2, and P = 0.27, respectively) and had an overall ADC COV of 14.8% (95% confidence interval: 12.3%, 16.9%) and no significant proportional bias on Bland-Altman analysis. S_anat CIOP was rated diagnostic or better (score of 3, 4, or 5) in 72-98% of cases for bone marrow, muscle, and soft tissues. DW-SD was equivalent to or preferred over DW-EPI in muscles and soft tissues, with CIOP 86-93% and 93%, respectively. Lesions were equally visualized on DW-SD and DW-EPI in 40-51%, with DW-SD preferred in 44-56% of cases. DW-SD was rated significantly better than DW-EPI across all comparative variables that included bone marrow, muscle, soft tissue, cartilage, and lesions (P < 0.05). Readers had moderate to near-perfect (ICC range = 0.45-0.85).

DATA CONCLUSION

DW-SD of the extremities provided similar ADC values and improved image quality compared with conventional DW-EPI. Level of Evidence 2 Technical Efficacy Stage 2 J. MAGN. RESON. IMAGING 2021;53:504-513.

Targeted rapid knee MRI exam using T2 shuffling

BACKGROUND

MRI is commonly used to evaluate pediatric musculoskeletal pathologies, but same-day/near-term scheduling and short exams remain challenges.

PURPOSE

To investigate the feasibility of a targeted rapid pediatric knee MRI exam, with the goal of reducing cost and enabling same-day MRI access.

STUDY TYPE

A cost effectiveness study done prospectively.

SUBJECTS

Forty-seven pediatric patients.

FIELD STRENGTH/SEQUENCE

3T. The 10-minute protocol was based on T2 Shuffling, a four-dimensional acquisition and reconstruction of images with variable T2 contrast, and a T1 2D fast spin-echo (FSE) sequence. A distributed, compressed sensing-based reconstruction was implemented on a four-node high-performance compute cluster and integrated into the clinical workflow.

ASSESSMENT

In an Institutional Review Board-approved study with informed consent/assent, we implemented a targeted pediatric knee MRI exam for assessing pediatric knee pain. Pediatric patients were subselected for the exam based on insurance plan and clinical indication. Over a 2-year period, 47 subjects were recruited for the study and 49 MRIs were ordered. Date and time information was recorded for MRI referral, registration, and completion. Image quality was assessed from 0 (nondiagnostic) to 5 (outstanding) by two readers, and consensus was subsequently reached.

STATISTICAL TESTS

A Wilcoxon rank-sum test assessed the null hypothesis that the targeted exam times compared with conventional knee exam times were unchanged.

RESULTS

Of the 49 cases, 20 were completed on the same day as exam referral. Median time from registration to exam completion was 18.7 minutes. Median reconstruction time for T2 Shuffling was reduced from 18.9 minutes to 95 seconds using the distributed implementation. Technical fees charged for the targeted exam were one-third that of the routine clinical knee exam. No subject had to return for additional imaging.

DATA CONCLUSION

The targeted knee MRI exam is feasible and reduces the imaging time, cost, and barrier to same-day MRI access for pediatric patients.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 6 J. Magn. Reson. Imaging 2019.

Case 253: Thrombosed Umbilical Venous Varix in an Infant

History A 3-month-old previously healthy girl presented to an outside institution with a 4-day history of low-grade fever, irritability, and a tender "knot" in the upper abdomen. Ultrasonography (US) was performed at an outside hospital. US images were not available for review; however, they showed a mass in the left hepatic lobe, per the outside report, and the patient was referred to our institution for further evaluation. Her parents reported a normal full-term pregnancy, with regular prenatal care and normal prenatal US findings. The baby was born after an uncomplicated gestation. She was delivered at term via an uncomplicated cesarean section due to a maternal history of cesarean section. The perinatal course was uncomplicated, and there was no history of umbilical catheterization, per the parents. On arrival at our institution, the patient had a temperature of 38.2°C. All other vital signs were normal. Palpation revealed a tender and firm mass in the periumbilical region; otherwise, physical examination findings were normal. Results of laboratory work-up were normal, except for elevated white blood cell count (26 600/mm³ [26.6 × 10⁹/L]; normal, 6000-17 500/mm³ [6-17.5 × 10⁹/L]). The patient underwent US followed by intravenous contrast material-enhanced (10 mL ioversol, Optiray 320; Medtronic, Santa Rosa, Calif) computed tomography (CT) on the same day.

Imaging of renal medullary carcinoma in children and young adults: a report from the Children's Oncology Group

BACKGROUND

Renal medullary carcinoma is a rare renal malignancy of childhood. There are no large series describing the imaging appearance of renal medullary carcinoma in children.

OBJECTIVE

To characterize the clinical and imaging features of pediatric renal medullary carcinoma at initial presentation.

MATERIALS AND METHODS

We retrospectively analyzed images of 25 pediatric patients with renal medullary carcinoma enrolled in the Children's Oncology Group renal tumors classification, biology and banking study (AREN03B2) from March 2006 to August 2016. Imaging findings of the primary mass, and patterns of locoregional and distant spread were evaluated in correlation with pathological and surgical findings.

RESULTS

Median age at presentation was 13 years (range: 6-21 years), with a male predominance (3.2:1). The overall stage of disease at initial presentation was stage 1 in 1, stage 2 in 2 and stage 4 in 22. Maximum diameter of the primary renal mass ranged from 1.6 to 10.3 cm (mean: 6.6 cm) with a slight right side predilection (1.5:1). Enlarged (>1 cm short axis) retroperitoneal lymph nodes were identified at initial staging in 20/25 (80%) cases, 10 of which were histologically confirmed while the others did not undergo surgical sampling. Enlarged lymph nodes were also identified in the mediastinum (14/25; 56%) and supraclavicular regions (4/25; 16%). Metastatic disease was present in the lungs in 19/25 (76%) and liver in 6/25 (24%). The pattern of lung metastases was pulmonary lymphangitic carcinomatosis: 10 cases (9 bilateral, 1 unilateral), pulmonary nodules with indistinct margins: 6 cases, pulmonary nodules with distinct margins: 2 cases, while 1 case had pulmonary nodules with both indistinct and distinct margins. Pulmonary lymphangitic carcinomatosis was pathologically confirmed in 4/10 cases. All cases with pulmonary lymphangitic carcinomatosis had associated enlarged mediastinal lymph nodes.

CONCLUSION

Renal medullary carcinoma in children and young adults presents at an advanced local and distant stage in the majority of patients. The diagnosis of renal medullary carcinoma should be considered when a child or young adult presents with a poorly defined/infiltrative, centrally located renal mass, especially in the setting of known sickle cell hemoglobinopathy. Distant metastases are common at initial presentation in the lungs, distant lymph nodes and liver and often involve multiple sites simultaneously. Pulmonary lymphangitic carcinomatosis, a distinctive and uncommon form of lung metastasis in children, is common in this patient population.

Vitamin D treatment modulates immune activation in cystic fibrosis

Persistent inflammatory response in cystic fibrosis (CF) airways is believed to play a central role in the progression of lung damage. Anti-inflammatory treatment may slow lung disease progression, but adverse side effects have limited its use. Vitamin D has immunoregulatory properties. We randomized 16 CF patients to receive vitamin D2, vitamin D3 or to serve as controls, and investigated the effect of vitamin D supplementation on soluble immunological parameters, myeloid dendritic cells (mDCs) and T cell activation. Three months of vitamin D treatment were followed by two washout months. Vitamin D status at baseline was correlated negatively with haptoglobin, erythrocyte sedimentation rate and immunoglobulin A concentration. Total vitamin D dose per kg bodyweight correlated with the down-modulation of the co-stimulatory receptor CD86 on mDCs. Vitamin D treatment was associated with reduced CD279 (PD-1) expression on CD4⁺ and CD8⁺ T cells, as well as decreased frequency of CD8⁺ T cells co-expressing the activation markers CD38 and human leucocyte antigen D-related (HLA-DR) in a dose-dependent manner. There was a trend towards decreased mucosal-associated invariant T cells (MAIT) cell frequency in patients receiving vitamin D and free serum 25-hydroxyvitamin D (free-s25OHD) correlated positively with CD38 expression by these cells. At the end of intervention, the change in free-s25OHD was correlated negatively with the change in CD279 (PD-1) expression on MAIT cells. Collectively, these data indicate that vitamin D has robust pleiotropic immunomodulatory effects in CF. Larger studies are needed to explore the immunomodulatory treatment potential of vitamin D in CF in more detail.

Percutaneous Cryoablation for the Treatment of Recurrent Malignant Pleural Mesothelioma: Safety, Early-Term Efficacy, and Predictors of Local Recurrence

PURPOSE

To determine safety and early-term efficacy of CT-guided cryoablation for treatment of recurrent mesothelioma and assess risk factors for local recurrence.

MATERIALS AND METHODS

During the period 2008-2012, 24 patients underwent 110 cryoablations for recurrent mesothelioma tumors in 89 sessions. Median patient age was 69 years (range, 48-82 y). Median tumor size was 30 mm (range, 9-113 mm). Complications were graded using Common Terminology Criteria for Adverse Events version 4.0 (CTCAE v4.0). Recurrence was diagnosed on CT or positron emission tomography/CT by increasing size, nodular enhancement, or hypermetabolic activity and analyzed using the Kaplan-Meier method. Cox proportional hazards model was used to determine covariates associated with local tumor recurrence.

RESULTS

Median duration of follow-up was 14.5 months. Complications occurred in 8 of 110 cryoablations (7.3%). All but 1 complication were graded CTCAE v4.0 1 or 2. No procedure-related deaths occurred. Freedom from local recurrence was observed in 100% of cases at 30 days, 92.5% at 6 months, 90.8% at 1 year, 87.3% at 2 years, and 73.7% at 3 years. Tumor recurrence was diagnosed 4.5-24.5 months after cryoablation (mean 5.7 months). Risk of tumor recurrence was associated with a smaller ablative margin from the edge of tumor to iceball ablation margin (multivariate hazard ratio 0.68, CI 0.48-0.95, P = .024).

CONCLUSIONS

CT-guided cryoablation is safe for local control of recurrent mesothelioma, with a low rate of complications and promising early-term efficacy. A smaller ablative margin may predispose to tumor recurrence.

Assessing splenomegaly: automated volumetric analysis of the spleen

RATIONALE AND OBJECTIVES

To define systematic volumetric thresholds to identify and grade splenomegaly and retrospectively evaluate the performance of radiologists to assess splenomegaly in computed tomography (CT) image data.

MATERIALS AND METHODS

A clinical tool was developed to segment spleens from 172 contrast-enhanced clinical CT studies. There were 45 normal and 127 splenomegaly cases confirmed by radiological reports. Spleen volumes were compared to manual measurements using overlap/error. Volumetric thresholds for mild/massive splenomegaly were defined at 1/2.5 standard deviations above the average splenic volume of the healthy population. The thresholds were validated against consensus reports. The performance of radiologists in assessing splenomegaly was retrospectively evaluated.

RESULTS

The automated segmentation of spleens was robust with volume overlap/error of 95.2/3.3%. There were no significant differences (P > .2) between manual and automated segmentations for either normal/splenomegaly subgroups. Comparable correlations between interobserver and manual-automated measurements were found (r = 0.99 for all). The average volume of normal spleens was 236.89 ± 77.58 mL. For splenomegaly, average volume was 1004.75 ± 644.27 mL. Volumetric thresholds of 314.47/430.84 mL were used to define mild/massive splenomegaly (±18.86 mL, 95% CI). Radiologists disagreed in 23.25% (n = 40) of the diagnosed cases. The area under the receiver operating characteristic curve of the volumetric criterion for splenomegaly detection was 0.96. Using the volumetric thresholds as the reference standard, the sensitivity of radiologists in detecting all/mild/massive splenomegaly was 95.0/66.6/99.0% at 78.0% specificity, respectively.

CONCLUSION

Thresholds for the identification and grading of splenomegaly from automatic volumetric spleen assessment were introduced. The volumetric thresholds match well with clinical interpretations for splenomegaly and may improve splenomegaly detection compared with splenic cephalocaudal height measurements or visual inspection commonly used in current clinical practice.

Assessing hepatomegaly: automated volumetric analysis of the liver

RATIONALE AND OBJECTIVES

The aims of this study were to define volumetric nomograms for identifying hepatomegaly and to retrospectively evaluate the performance of radiologists in assessing hepatomegaly.

MATERIALS AND METHODS

Livers were automatically segmented from 148 abdominal contrast-enhanced computed tomographic scans: 77 normal livers and 71 cases of hepatomegaly (diagnosed by visual inspection and/or linear liver height by radiologists). Quantified liver volumes were compared to manual measurements using volume overlap and error. Liver volumes were normalized to body surface area, from which hepatomegaly nomograms were defined (H scores) by analyzing the distribution of liver sizes in the healthy population. H scores were validated against consensus reports. The performance of radiologists in diagnosing hepatomegaly was retrospectively evaluated.

RESULTS

The automated segmentation of livers was robust, with volume overlap and error of 96.2% and 2.2%, respectively. There were no significant differences (P > .10) between manual and automated segmentation for either the normal or the hepatomegaly subgroup. The average volumes of normal and enlarged livers were 1.51 ± 0.25 and 2.32 ± 0.75 L, respectively. One-way analysis of variance found that body surface area (P = .004) and gender (P = .02), but not age, significantly affected normal liver volume. No significant effects were observed for two-way and three-way interactions among the three variables (P > .18). H-score cutoffs of 0.92 and 1.08 L/m2 were used to define mild and massive hepatomegaly (95% confidence interval, ± 0.02 L/m2). Using the H score as the reference standard, the sensitivity of radiologists in detecting all, mild, and massive hepatomegaly was 84.4%, 56.7%, and 100.0% at 90.1% specificity, respectively. Radiologists disagreed on 20.9% of the diagnosed cases (n = 31). The area under the receiver-operating characteristic curve of the H-score criterion for hepatomegaly detection was 0.98.

CONCLUSIONS

Nomograms for the identification and grading of hepatomegaly from automatic volumetric liver assessment normalized to body surface area (H scores) are introduced. H scores match well with clinical interpretations for hepatomegaly and may improve hepatomegaly detection compared with height measurements or visual inspection, commonly used in current clinical practice.

NK cells and CD1d-restricted NKT cells respond in different ways with divergent kinetics to IL-2 treatment in primary HIV-1 infection

Cytokine immunotherapy is being evaluated as adjunct treatment in infectious diseases. The effects on innate and adaptive immunity in vivo are insufficiently known. Here, we investigate whether combination treatment with antiretroviral therapy (ART) and Interleukin-2 (IL-2) of patients with primary HIV-1 infection induces sustained increases in circulating NKT cell and NK cell numbers and effector functions and investigate how changes are coordinated in the two compartments. Patients with primary HIV-1 infection starting ART were analyzed for numbers, phenotype and function of NKT cells, NK cells and dendritic cells (DC) in peripheral blood before, during and after IL-2 treatment. NKT cells expanded during IL-2 treatment as expected from previous studies. However, their response to α-galactosyl ceramide antigen were retained but not boosted. Myeloid DC did not change their numbers or CD1d-expression during treatment. In contrast, the NK cell compartment responded with rapid expansion of the CD56(dim) effector subset and enhanced IFNγ production. Expansions of NKT cells and NK cells retracted back towards baseline values at 12 months after IL-2 treatment ended. In summary, NKT cells and NK cells respond to IL-2 treatment with different kinetics. Effects on cellular function are distinct between the cell types and the effects appear not to be sustained after IL-2 treatment ends. These results improve our understanding of the effects of cytokine immunotherapy on innate cellular immunity in early HIV-1 infection.

Automated segmentation and quantification of liver and spleen from CT images using normalized probabilistic atlases and enhancement estimation

PURPOSE

To investigate the potential of the normalized probabilistic atlases and computer-aided medical image analysis to automatically segment and quantify livers and spleens for extracting imaging biomarkers (volume and height).

METHODS

A clinical tool was developed to segment livers and spleen from 257 abdominal contrast-enhanced CT studies. There were 51 normal livers, 44 normal spleens, 128 splenomegaly, 59 hepatomegaly, and 23 partial hepatectomy cases. 20 more contrast-enhanced CT scans from a public site with manual segmentations of mainly pathological livers were used to test the method. Data were acquired on a variety of scanners from different manufacturers and at varying resolution. Probabilistic atlases of livers and spleens were created using manually segmented data from ten noncontrast CT scans (five male and five female). The organ locations were modeled in the physical space and normalized to the position of an anatomical landmark, the xiphoid. The construction and exploitation of liver and spleen atlases enabled the automated quantifications of liver/spleen volumes and heights (midhepatic liver height and cephalocaudal spleen height) from abdominal CT data. The quantification was improved incrementally by a geodesic active contour, patient specific contrast-enhancement characteristics passed to an adaptive convolution, and correction for shape and location errors.

RESULTS

The livers and spleens were robustly segmented from normal and pathological cases. For the liver, the Dice/Tanimoto volume overlaps were 96.2%/92.7%, the volume/height errors were 2.2%/2.8%, the root-mean-squared error (RMSE) was 2.3 mm, and the average surface distance (ASD) was 1.2 mm. The spleen quantification led to 95.2%/91% Dice/Tanimoto overlaps, 3.3%/ 1.7% volume/height errors, 1.1 mm RMSE, and 0.7 ASD. The correlations (R2) with clinical/manual height measurements were 0.97 and 0.93 for the spleen and liver, respectively (p < 0.0001). No significant difference (p > 0.2) was found comparing interobserver and automatic-manual volume/height errors for liver and spleen.

CONCLUSIONS

The algorithm is robust to segmenting normal and enlarged spleens and livers, and in the presence of tumors and large morphological changes due to partial hepatectomy. Imaging biomarkers of the liver and spleen from automated computer-assisted tools have the potential to assist the diagnosis of abdominal disorders from routine analysis of clinical data and guide clinical management.

Atlas-based automated segmentation of spleen and liver using adaptive enhancement estimation

The paper presents the automated segmentation of spleen and liver from contrast-enhanced CT images of normal and hepato/splenomegaly populations. The method used 4 steps: (i) a mean organ model was registered to the patient CT; (ii) the first estimates of the organs were improved by a geodesic active contour; (iii) the contrast enhancements of liver and spleen were estimated to adjust to patient image characteristics, and an adaptive convolution refined the segmentations; (iv) lastly, a normalized probabilistic atlas corrected for shape and location for the precise computation of each organ's volume and height (mid-hepatic liver height and cephalocaudal spleen height). Results from test data demonstrated the method's ability to accurately segment the spleen (RMS error = 1.09 mm; DICE/Tanimoto overlaps = 95.2/91) and liver (RMS error = 2.3 mm, and DICE/Tanimoto overlaps = 96.2/92.7). The correlations (R2) with clinical/manual height measurements were 0.97 and 0.93 for the spleen and liver respectively.

Limited Magnitude and Breadth in the HLA-A2-Restricted CD8 T-Cell Response to Nef in Children with Vertically Acquired HIV-1 Infection

CD8 T cells are believed to play a key role in the immune control of human immunodeficiency virus-1 (HIV-1) infection in children as well as in adults. We have used an enhanced EliSpot (AmpliSpot) assay to quantitate CD8 T-cell responses directed to five human leucocyte antigen (HLA)-A2-presented HIV-1 epitopes derived from the key viral antigen Nef. Responses were assayed in one group of 21 children with vertically acquired HIV infection and one group of 19 adult subjects with chronic infection. The paediatric group displayed significantly weaker and more narrowly focused CD8 T-cell responses as compared with the adult subjects. Two epitopes stood out as the most frequently and strongly recognized, suggesting that they should be considered immunodominant in the CD8 T-cell response to HIV-1 Nef. Interestingly, the most frequently and strongly recognized epitope in both adults and children was previously identified in HLA-A2-transgenic mice, demonstrating the usefulness of such mice in finding natural viral epitopes. These findings indicate significant weakness in strength and breadth of the CD8 T-cell response to the target protein Nef in infected children and prompt renewed efforts into the immunology of vertically acquired HIV-1 infection.

Functional heterogeneity of cytokines and cytolytic effector molecules in human CD8+ T lymphocytes

CD8(+) T cells use a number of effector mechanisms to protect the host against infection. We have studied human CD8(+) T cells specific for CMV pp65(495-503) epitope, or for staphylococcal enterotoxin B, for the expression patterns of five cytokines and cytolytic effector molecules before and after antigenic stimulation. Ex vivo, the cytolytic molecule granzyme B was detected in a majority of circulating CMV-specific CD8(+) T cells, whereas perforin was rarely expressed. Both were highly expressed after Ag-specific activation accompanied by CD45RO up-regulation. TNF-alpha, IFN gamma, and IL-2 were sequentially acquired on recognition of Ag, but surprisingly, only around half of the CMV-specific CD8(+) T cells responded to antigenic stimuli with production of any cytokine measured. A dominant population coexpressed TNF-alpha and IFN-gamma, and cells expressing TNF-alpha only, IFN-gamma only, or all three cytokines together also occurred at lower but clearly detectable frequencies. Interestingly, perforin expression and production of IFN-gamma and TNF-alpha in CD8(+) T cells responding to staphylococcal enterotoxin B appeared to be largely segregated, and no IL-2 was detected in perforin-positive cells. Together, these data indicate that human CD8(+) T cells can be functionally segregated in vivo and have implications for the understanding of human CD8(+) T cell differentiation and specialization and regulation of effector mechanisms.

CD8+ T cells rapidly acquire NK1.1 and NK cell-associated molecules upon stimulation in vitro and in vivo

NKT cells express both NK cell-associated markers and TCR. Classically, these NK1.1+TCRalphabeta+ cells have been described as being either CD4+CD8- or CD4-CD8-. Most NKT cells interact with the nonclassical MHC class I molecule CD1 through a largely invariant Valpha14-Jalpha281 TCR chain in conjunction with either a Vbeta2, -7, or -8 TCR chain. In the present study, we describe the presence of significant numbers of NK1.1+TCRalphabeta+ cells within lymphokine-activated killer cell cultures from wild-type C57BL/6, CD1d1-/-, and Jalpha281-/- mice that lack classical NKT cells. Unlike classical NKT cells, 50-60% of these NK1.1+TCRalphabeta+ cells express CD8 and have a diverse TCR Vbeta repertoire. Purified NK1.1-CD8alpha+ T cells from the spleens of B6 mice, upon stimulation with IL-2, IL-4, or IL-15 in vitro, rapidly acquire surface expression of NK1.1. Many NK1.1+CD8+ T cells had also acquired expression of Ly-49 receptors and other NK cell-associated molecules. The acquisition of NK1.1 expression on CD8+ T cells was a particular property of the IL-2Rbeta+ subpopulation of the CD8+ T cells. Efficient NK1.1 expression on CD8+ T cells required Lck but not Fyn. The induction of NK1.1 on CD8+ T cells was not just an in vitro phenomenon as we observed a 5-fold increase of NK1.1+CD8+ T cells in the lungs of influenza virus-infected mice. These data suggest that CD8+ T cells can acquire NK1.1 and other NK cell-associated molecules upon appropriate stimulation in vitro and in vivo.

Enhancement of human immunodeficiency virus type 1-specific CD4 and CD8 T cell responses in chronically infected persons after temporary treatment interruption

Immunologic and virologic outcomes of treatment interruption were compared for 5 chronically human immunodeficiency virus (HIV)-infected persons who have maintained antiretroviral therapy-mediated virus suppression, as compared with 5 untreated controls. After a median interruption of 55 days of therapy accompanied by rebound of virus, reinitiated therapy in 4 of 5 subjects resulted in suppression of 98.86% of plasma virus load by 21-33 days and no significant decrease in CD4 T cell percentage from baseline. Increased T helper responses against HIV-1 p24 antigen (P=. 014) and interferon-gamma-secreting CD8 T cell responses against HIV-1 Env (P=.004) were present during interruption of therapy and after reinitiation of treatment. The remaining subject whose treatment was interrupted did not resume treatment and continued to have a low virus load (<1080 HIV-1 RNA copies/mL) and persistent antiviral cell-mediated responses. In summary, cellular immunity against autologous HIV-1 has the potential to be acutely augmented in association with temporary treatment interruption in chronically infected persons.

Human immunodeficiency virus type 1 Nef epitopes recognized in HLA-A2 transgenic mice in response to DNA and peptide immunization

We investigated the immune response against a human immunodeficiency virus type 1 (HIV-1) nef DNA sequence administered epidermally in mice transgenic for the human major histocompatibility complex (MHC) class I molecule HLA-A201. Ten potential HLA-A2 binding 9-mer Nef peptides were identified by a computer-based search algorithm. By a cell surface MHC class I stabilization assay, four peptides were scored as good binders, whereas two peptides bound weakly to HLA-A2. After DNA immunization, cytotoxic T lymphocyte (CTL) responses were predominantly directed against the Nef 44-52, 81-89, and 85-93 peptides. Interestingly, the 44-52 epitope resides outside the regions of Nef where previously described CTL epitopes are clustered. Dominance among Nef-derived peptides did not strictly correlate with HLA-A2 binding, in that only one of the high-affinity binding peptides was targeted in the CTL response. The 44-52, 85-93, and 139-147 peptides also generated specific CTLs in response to peptide immunization. T helper cell proliferation was detected after stimulation with 20-mer peptides in vitro. Three Nef regions (16-35, 106-125, and 166-185) dominated the T helper cell proliferation. The implications of these results for the development of DNA-based vaccines against HIV is discussed.

T cell tolerance based on avidity thresholds rather than complete deletion allows maintenance of maximal repertoire diversity

Given the flexible nature of TCR specificity, deletion or permanent disabling of all T cells with the capacity to recognize self peptides would severely limit the diversity of the repertoire and the capacity to recognize foreign Ags. To address this, we have investigated the patterns of CD8+ CTL reactivity to a naturally H-2Kb-presented self peptide derived from the elongation factor 1alpha (EF1alpha). EF1alpha occurs as two differentially expressed isoforms differing at one position of the relevant peptide. Low avidity CTLs could be raised against both variants of the EF1alpha peptide. These CTLs required 100-fold more peptide-H-2Kb complexes on the target cell compared with CTLs against a viral peptide, and did not recognize the naturally expressed levels of EF1alpha peptides. Thus, low avidity T cells specific for these self peptides escape tolerance by deletion, despite expression of both EF1alpha isoforms in dendritic cells known to mediate negative selection in the thymus. The low avidity in CTL recognition of these peptides correlated with low TCR affinity. However, self peptide-specific CTLs expressed elevated levels of CD8. Furthermore, CTLs generated against altered self peptide variants displayed intermediate avidity, indicating cross-reactivity in induction of tolerance. We interpret these data, together with results previously published by others, in an avidity pit model based on avidity thresholds for maintenance of both maximal diversity and optimal self tolerance in the CD8+ T cell repertoire.

Immunization with dendritic cells breaks immunodominance in CTL responses against minor histocompatibility and synthetic peptide antigens

We have examined the mechanisms involved in immunodominance in two different experimental models: the cytotoxic T lymphocyte (CTL) response in B6 mice against minor histocompatibility antigens of BALB.B mice, and the response of B6 mice against a mixture of five synthetic peptides corresponding to well-defined immunogenic epitopes. The CTL responses in these models focus on a few dominant epitopes, whereas no or only weak responses can be detected against other subdominant epitopes. Neither of these immunodominance phenomena can be explained by insufficient presentation of subdominant epitopes in the presence of the dominant ones. Immunodominance could also be demonstrated in an in vitro system, in which B6 splenocytes primed with BALB.B could interfere with the CTL response against subdominant antigens. This interference was dependent on CD8+ T cells and on the simultaneous presentation of dominant and subdominant antigens on the same antigen-presenting cell, suggesting T cell competition around the antigen-presenting cell as a potential explanation. The immunodominance in both systems could be broken by immunization with dendritic cells (from BALB.B or from B6 loaded with peptides). This procedure allowed detection of CTL responses against both dominant and previously subdominant antigens.

T cell competition for the antigen-presenting cell as a model for immunodominance in the cytotoxic T lymphocyte response against minor histocompatibility antigens

We recently demonstrated that spleen cells primed against dominant BALB.B antigens can inhibit the cytotoxic T lymphocyte (CTL) response against subdominant antigens in vitro. In this study, we show that this interference is dependent on CD8+, but not CD4+, T cells directed against dominant antigens. Similar to immunodominance in vivo, T cell interference in vitro required presentation of dominant and subdominant antigens by the same antigen-presenting cell. In vivo priming with cells expressing dominant and subdominant antigens did not induce long-lasting unresponsiveness against the latter. These results support a model in which immunodominance is mediated by T cell competition. In line with this, we found that the immunodominance effects in the CTL response against these minor histocompatibility antigens could be broken by immunization with live bone marrow-derived dendritic cells.

Generation and function of bone marrow-derived dendritic cells from CD4/CD8(-/-) double-knockout mice

We present a novel, simple and straightforward method to obtain mouse bone marrow-derived dendritic cells (DC) from C57Bl/6 CD4/CD8(-/-) double knock-out mice. This new method, involving culture of bone marrow cells in medium supplemented with Interleukin 4 and Granulocyte-Macrophage Colony-Stimulating Factor, does not involve negative immunodepletion of CD4+ and CD8+ populations, or extensive prior manipulations of the starting population. The resulting, loosely adherent cell population, exhibited the morphological characteristics and typical surface markers of DCs, and were endowed with the functional activities characteristic of bone marrow-derived DCs of wild-type mice. Interestingly, LCMV GP33-41 peptide-loaded CD4/CD8(-/-) DCs were efficiently lysed by peptide-specific activated CTLs in vitro. Furthermore, these peptide-loaded CD4/CD8(-/-) DCs induced a peptide-specific CTL response upon immunization of wild-type C57Bl/6 mice.

Recognition of the major histocompatibility complex restriction element modulates CD8(+) T cell specificity and compensates for loss of T cell receptor contacts with the specific peptide

Triggering of a T cell requires interaction between its specific receptor (TCR) and a peptide antigen presented by a self-major histocompatibility complex (MHC) molecule. TCR recognition of self-MHC by itself falls below the threshold of detection in most systems due to low affinity. To study this interaction, we have used a read-out system in which antigen-specific effector T cells are confronted with targets expressing high levels of MHC compared with the selecting and priming environment. More specifically, the system is based on CD8(+) T cells selected in an environment with subnormal levels of MHC class I in the absence of beta2-microglobulin. We observe that the MHC restriction element can trigger viral peptide-specific T cells independently of the peptide ligand, provided there is an increase in self-MHC density. Peptide-independent triggering required at least four times the natural in vivo level of MHC expression. Furthermore, recognition of the restriction element at expression levels below this threshold was still enough to compensate for lack of affinity to peptides carrying alanine substitutions in major TCR contact residues. Thus, the specificity in TCR recognition and T cell activation is fine tuned by the avidity for self-MHC, and TCR avidities for peptide and MHC may substitute for each other. These results demonstrate a functional role for TCR avidity for self-MHC in tuning of T cell specificity, and support a role for cross-reactivity on "self" during T cell selection and activation.

MHC class I mosaic mice reveal insights into control of Ly49C inhibitory receptor expression in NK cells

We have analyzed lymphocyte development in natural MHC class I chimeric mice, generated through a transgenic approach in beta2-microglobulin (beta2m)-/- mice. In these mice, MHC class I+ cells coexist with an equal proportion of MHC class I-deficient cells. These MHC class I mosaic mice had normal numbers of CD8+ T cells, which had a target cell specificity similar to that of wild-type mice. Consequently, the mice did not develop any signs of autoimmunity. They also had normal numbers of NK cells. This allowed an examination of the MHC class I influence on the expression of the Ly49C inhibitory receptor on NK cells. This receptor binds to H-2Kb. It is expressed at low levels on NK cells in wild-type mice of the H-2b haplotype, but at markedly higher levels on NK cells in beta2m-/- mice and other strains of mice lacking expression of H-2Kb. Relatively little is known about how MHC class I molecules affect expression of the Ly49 receptors. Through the analysis of the present MHC class I mosaic mice, we demonstrate that the expression levels of Ly49C on NK cells is a consequence not only of MHC class I expression in the environment, but also of the expression of MHC class I molecules by the NK cells themselves. These findings are discussed in relation to the biological role of the calibration of the Ly49 inhibitory receptor expression in relation to self-MHC class I.

Immunodominance in the CTL response against minor histocompatibility antigens: interference between responding T cells, rather than with presentation of epitopes

We have investigated mechanisms involved in immunodominance of the CTL response of C57BL/6 (B6) mice against cells of BALB.B origin. This transplantation barrier consists of at least 40 minor histocompatibility (H) Ags. Insufficient presentation of nondominant epitopes in the presence of dominant epitopes was investigated as a possible mechanism for immunodominance. Ag presentation was assessed by recognition of dendritic cells of BALB.B origin, MLC restimulatory capacity, and quantification of cell surface presentation by peptide elution from intact cells. Cells from BALB.B mice, which fail to elicit CTL against nondominant epitopes, presented nondominant epitopes to a similar extent as cells from minor H congenic mice; the latter do elicit CTL against nondominant minor H Ags. Nevertheless, presentation of nondominant and dominant epitopes by the same APC appeared to be an important factor for immunodominance to occur, since simultaneous immunization with the epitopes on separate cells elicited CTL against both types of epitopes. This suggested that immunodominance is determined in the interaction between different responding T cells and the APC. Support for this was obtained in an in vitro model in which the CTL response against a nondominant epitope was inhibited by the concomitant response against a dominant epitope. This study suggests that immunodominance in the CTL response against certain minor H Ags results from interference between T cell responses and not from insufficient presentation of peptide epitopes. The study also provides an in vitro model for further investigations of the immunodominance phenomenon.

Superdominance among immunodominant H-2Kb-restricted epitopes and reversal by dendritic cell-mediated antigen delivery

To examine possible interference patterns between immunodominant CTL Ags, we analyzed the response to mixtures of five well-characterized H-2Kb-restricted epitopes, each of which had earlier been described as immunodominant within its antigenic system. Clear patterns of dominance were observed between peptides in the mixture, with the CTL response focusing on the Sendai virus nucleoprotein 324-332 and vesicular stomatitis virus nucleoprotein 52-59 epitopes. The dominance of these epitopes correlated with high CTL availability. Subdominance of the OVA(257-264) and the MCF1233 murine leukemia virus envelope 574-581 peptides could not be explained by inferior ability to bind and stabilize MHC class I molecules. Interestingly, immunodominance was broken if the peptide mixture was pulsed on bone marrow-derived dendritic cells, a mode of immunization allowing efficient recognition of a broader set of specificities. Our results show that immunodominance is neither an absolute feature of a given epitope nor does it apply only in relation to other epitopes within the same protein, micro-organism, or cell. Novel "superdominant" hierarchies emerge in the response against multiple "dominant" epitopes. A T cell competition model to explain the data in terms of a balance influenced by CTL frequencies and available APC capacity is discussed.

Generation of CD8+ T cells specific for transporter associated with antigen processing deficient cells

Cells with impaired transporter associated with antigen processing (TAP) function express low levels of cell surface major histocompatibility complex (MHC) class I molecules, and are generally resistant to lysis by MHC class I restricted cytotoxic T lymphocytes (CTLs). Here we report the generation of MHC class I restricted CD8(+) CTLs that surprisingly require target cell TAP deficiency for efficient recognition. C57BL/6 (B6) mice immunized with syngenic B7-1 (CD80) expressing TAP-deficient cells generated a potent CTL response against both TAP-deficient RMA-S tumor cells and TAP-deficient Con A blasts, whereas the corresponding TAP-expressing target cells were considerably less susceptible or resistant to lysis. The CTL epitopes recognized were expressed also by the human TAP-deficient cell line T2, transfected with appropriate MHC class I molecules. B6 mice immunized with B7-1-transfected TAP-deficient RMA-S cells were protected from outgrowth of a subsequent RMA-S tumor challenge. These findings are discussed in relation to the biochemical nature of MHC class I dependent CTL epitopes associated with impaired TAP function, as well as implications for immunotherapy and autoimmunity.

TAP1-deficient mice select a CD8+ T cell repertoire that displays both diversity and peptide specificity

Mice deficient in the gene encoding the transporter associated with antigen processing 1 (TAP1) are defective in providing major histocompatibility complex (MHC) class I molecules with cytosolic peptides. Consequently, these mice express reduced levels of MHC class I glycoproteins on the cell surface, and have reduced numbers of CD8+ T cells in the periphery. In the present study, we have addressed the diversity and specificity of the peripheral CD8+ T cell population in TAP1 -/- mice. CD8+ T cells were polyclonal with regard to T cell receptor (TCR) V beta expression. Overall, V beta usage in TAP1 -/- mice appear to be very similar to that in wild-type mice, with significantly reduced levels of V beta 5.1/5.2-expressing CD8+ T cells as the only clear exception. This polyclonal population of CD8+ T cells readily mounted epitope-specific CTL responses against four out of five well-defined MHC class I-restricted peptides. In contrast to allospecific CTL, peptide-specific CTL from TAP1 -/- mice did not cross-react on cells expressing normal levels of H-2b class I. The present results demonstrate that a polyclonal CD8+ T cell repertoire, displaying both diversity and peptide specificity, is positively selected in mice devoid of a functional peptide transporter. These observations imply that TAP-dependent peptides are not absolutely required for positive selection of a functionally diverse repertoire of CD8+ T cells.