In a phase 1 study (NCT04000165), we established proof of concept for activating pyruvate kinase (PK) in sickle cell disease (SCD) as a viable antisickling therapy. AG-348 (mitapivat), a PK activator, increased adenosine triphosphate (ATP) and decreased 2,3diphosphoglycerate levels while patients were on treatment, in line with the mechanism of the drug. We noted that the increased hemoglobin (Hb) persisted for 4 weeks after stopping AG-348 until the end of study (EOS). Here, we investigated the pathways modulated by activating PK that may contribute to the improved red blood cell (RBC) survival after AG-348 cessation. We evaluated frozen whole blood samples taken at multiple time points from patients in the phase 1 study, from which RBC ghosts were isolated and analyzed by western blotting for tyrosine phosphorylation of band 3 (Tyr-p-bd3), ankyrin-1, and intact (active) protein tyrosine phosphatase 1B (PTP1B) levels. We observed a significant dose-dependent decrease in mean Tyr-p-bd3 from baseline in the patients, accompanied by an increase in the levels of membrane-associated ankyrin-1 and intact PTP1B, all of which returned to near baseline by EOS. Because PTP1B is cleaved (inactivated) by intracellular Ca2'dependent calpain, we next measured the effect of AG-348 on ATP production and calpain activity and the plasma membrane Ca2' ATPase pump-mediated efflux kinetics in HbAA and HbSS erythrocytes. AG-348 treatment increased ATP levels, decreased calpain activity, and increased Ca2' efflux. Altogether, our data indicate that ATP increase is a key mechanism underlying the increase in hemoglobin levels upon PK activation in SCD. This trial was registered at www.clinicaltrials.gov as #NCT04000165.

Abstract Purpose Emerging research evidence suggests positive relationships between higher screen time and eating disorders. However, few studies have examined the prospective associations between screen use and eating disorder symptoms in early adolescents and how problematic screen use may contribute to symptom development. Methods We analyzed prospective cohort data from the Adolescent Brain Cognitive Development (ABCD) Study (N = 10,246, 2016–2020, ages 9–14). Logistic regression analyses were used to estimate the longitudinal associations between baseline self-reported screen time and eating disorder symptoms in year two. Logistic regression analyses were also used to estimate cross-sectional associations between problematic screen use in year two (either problematic social media or mobile phone use) and eating disorder symptoms in year two. Eating disorder symptoms based on the Kiddie Schedule for Affective Disorders and Schizophrenia (KSADS-5) included fear of weight gain, self-worth tied to weight, engaging in compensatory behaviors, binge eating, and distress with binge eating. Results Each additional hour of total screen time and social media use was associated with higher odds of fear of weight gain, self-worth tied to weight, compensatory behaviors to prevent weight gain, binge eating, and distress with binge eating two years later (odds ratio [OR] 1.05–1.55). Both problematic social media and mobile phone use were associated with higher odds of all eating disorder symptoms (OR 1.26–1.82). Conclusions Findings suggest greater total screen time, social media use, and problematic screen use are associated with more eating disorder symptoms in early adolescence. Clinicians should consider assessing for problem screen use and, when high, screen for disordered eating. Level of evidence Level III: Evidence obtained from well-designed cohort or case–control analytic studies.

AIM:The current study investigated the prospective relationships between parental monitoring, family conflict, and screen time across six screen time modalities in early adolescents in the USA. METHODS:We utilised prospective cohort data of children (ages 10-14 years) from the Adolescent Brain Cognitive Development (ABCD) Study (years baseline to Year 2 of follow-up; 2016-2020; N = 10 757). Adjusted coefficients (B) and 95% confidence intervals (CIs) were estimated using mixed-effect models with robust standard errors. RESULTS:A higher parental monitoring score was associated with less total screen time (B = -0.37, 95% CI -0.58, -0.16), with the strongest associations being with video games and YouTube videos. Conversely, a higher family conflict score was associated with more total screen time (B = 0.08, 95% CI 0.03, 0.12), with the strongest associations being with YouTube videos, video games, and watching television shows/movies in Years 1 and 2. CONCLUSION:The current study found that greater parental monitoring was associated with less screen time, while greater family conflict was linked to more screen time. These results may inform strategies to reduce screen time in adolescence, such as improving communication between parents and their children to strengthen family relationships.

OBJECTIVE:The objective of this study was to determine the association between cyberbullying and eating disorder symptoms in a national sample of 10-14-year-old early adolescents. METHOD:We analyzed cross-sectional data from the Adolescent Brain Cognitive Development (ABCD) Study (Year 2, 2018-2020, N = 10,258/11,875, 49% female, 46% non-White). Data were collected using multi-stage probability sampling. Modified Poisson regression analyses examined the association between cyberbullying and self-reported eating disorder symptoms based on the Kiddie Schedule for Affective Disorders and Schizophrenia (KSADS-5). RESULTS:Cyberbullying victimization was associated with worry about weight gain (prevalence ratio [PR] 2.41, 95% confidence interval [CI] 1.48-3.91), self-worth tied to weight (PR 2.08, 95% CI 1.33-3.26), inappropriate compensatory behavior to prevent weight gain (PR 1.95, 95% CI 1.57-2.42), binge eating (PR 1.95, 95% CI 1.59-2.39), and distress with binge eating (PR 2.64, 95% CI 1.94-3.59), in models adjusting for potential confounders. Cyberbullying perpetration was associated with worry about weight gain (PR 3.52, 95% CI 1.19-10.37), self-worth tied to weight (PR 5.59, 95% CI 2.56-12.20), binge eating (PR 2.36, 95% CI 1.44-3.87), and distress with binge eating (PR 2.84, 95% CI 1.47-5.49). DISCUSSION:Cyberbullying victimization and perpetration in early adolescence are associated with eating disorder symptoms. Clinicians may consider assessing for cyberbullying and eating disorder symptoms in early adolescence and provide anticipatory guidance. PUBLIC SIGNIFICANCE STATEMENT:Eating disorders often onset in adolescence and have among the highest mortality rates of any psychiatric disorder. In addition, cyberbullying has increased in prevalence among adolescents and significantly impacts mental health. In a national study of early adolescents, we found that cyberbullying victimization and perpetration are associated with eating disorder symptoms. Screening for and providing anticipatory guidance on cyberbullying and eating disorder symptoms in early adolescents may be warranted.

Purpose: The aim of this study is to determine the prospective associations between baseline screen time and obsessive-compulsive disorder (OCD) at 2-year follow-up in a national (United States) cohort of 9-to 10-year-old children. Methods: We analyzed prospective cohort data from the Adolescent Brain Cognitive Development study (n 1/4 9,208). Logistic regression analyses were used to determine the associations between baseline self-reported screen time (exposure) and OCD, based on the Kiddie Schedule for Affective Disorders and Schizophrenia (outcome), at 2-year-follow-up, adjusting for race/ethnicity, sex, household income, parent education, family history of psychopathology, and study site, excluding participants with baseline OCD. Results: The sample was 48.9% female and racially and ethnically diverse (43.5% non-White). Each additional hour of total screen time was prospectively associated with 1.05 higher odds of OCD at 2-year follow-up (95% confidence interval [CI] 1.01-1.09). For specific screen time modalities, each additional hour of playing video games (adjusted odds ratio 1.15, 95% CI 1.03-1.28) and watching videos (adjusted odds ratio 1.11, 95% CI 1.01-1.23) was associated with a subsequent OCD diagnosis. Conclusion: Video games and watching videos are prospectively associated with new-onset OCD in early adolescents. Future research should examine mechanisms linking these specific screen modalities to OCD development to inform future prevention and intervention efforts. (c) 2022 Society for Adolescent Health and Medicine. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Suicide is a leading cause of death among adolescents. Emerging literature has described relationships between excessive screen time and suicidal behaviors, though findings have been mixed. The objective of this study is to determine the prospective associations between screen time and suicidal behaviors two-years later in a national (U.S.) cohort of 9-11-year-old-children. We analyzed prospective cohort data from the Adolescent Brain Cognitive Development (ABCD) Study (N = 11,633). Logistic regression analyses were estimated to determine the associations between baseline self-reported screen time (exposure) and suicidal behaviors (outcome) based on the Kiddie Schedule for Affective Disorders and Schizophrenia (KSADS-5) at two-year-follow-up. Participants reported an average of 4.0 h of total screen time per day at baseline. At two-year-follow-up, 1.38% of the sample reported at least one suicidal behavior. Each additional hour of total screen time was prospectively associated with 1.09 higher odds of suicidal behaviors at 2-year-follow-up (95% CI 1.03-1.14), after adjusting for covariates. For specific screen time modalities, each additional hour of texting (aOR 1.36, 95% CI 1.06-1.74), video chatting (aOR 1.30, 95% CI 1.03-1.65), watching videos (aOR 1.21, 95% CI 1.04-1.39), and playing video games (aOR 1.18, 95% CI 1.01-1.38) was associated with higher odds of subsequent suicidal behaviors. Higher screen time is associated with higher odds of reporting suicidal behaviors at two-year-follow-up. Future research should seek to identify how specific screen time experiences may influence suicidal behaviors.

OBJECTIVE:Food insecurity is defined as lack of consistent access to adequate food for healthy living. The objective of this study was to determine the associations between food insecurity and binge-eating disorder in a national cohort of 9- to 14-year-old children.METHOD:We analyzed prospective cohort data from the Adolescent Brain Cognitive Development (ABCD) Study (N = 10,035, 2016-2020). Logistic regression analyses estimated the associations between food insecurity at baseline, year 1, or year 2 (exposure) and binge eating, subclinical binge-eating disorder (Other Specified Feeding and Eating Disorder-Binge-Eating Disorder [OSFED-BED]), and binge-eating disorder (BED) (outcome) based on the Kiddie Schedule for Affective Disorders and Schizophrenia (KSADS-5) at 2-year follow-up.RESULTS:The prevalence of food insecurity in the study was 15.8%. At 2-year follow-up, 1.71% of the sample received a diagnosis of BED or OSFED-BED, while 6.62% reported binge eating. Food insecurity was associated with 1.67 higher odds of BED or OSFED-BED (95% CI 1.04-2.69) and 1.31 higher odds of binge-eating symptoms (95% CI 1.01-1.71).DISCUSSION:Food insecurity in early adolescence is associated with higher odds of developing future binge-eating and BED or OSFED-BED. Clinicians may consider assessing for binge eating in adolescents with food insecurity and provide support in accessing appropriate food resources.PUBLIC SIGNIFICANCE:Prior research has shown that food insecurity is associated with disordered eating behaviors, including binge eating in adulthood. This study explored whether food insecurity in early adolescence increases risk for developing binge-eating disorder (BED). Targeted screening for BED in adolescents experiencing FI, and vice versa, may be warranted.

BACKGROUND:Cross-sectional studies have demonstrated associations between screen time and disruptive behavior disorders (conduct disorder and oppositional defiant disorder); however, prospective associations remain unknown. This study's objective was to determine the prospective associations of contemporary screen time modalities with conduct and oppositional defiant disorder in a national cohort of 9-11-year-old children.METHODS:We analyzed data from the Adolescent Brain Cognitive Development (ABCD) Study (N = 11,875). Modified Poisson regression analyses were conducted to estimate the associations between baseline child-reported screen time (total and by modality) and parent-reported conduct or oppositional defiant disorder based on the Kiddie Schedule for Affective Disorders and Schizophrenia (KSADS-5) at 1-year follow-up, adjusting for potential confounders.RESULTS:Participants reported an average of 4 hr of total screen time per day at baseline. Each hour of total screen time per day was prospectively associated with a 7% higher prevalence of conduct disorder (95% CI 1.03-1.11) and a 5% higher prevalence of oppositional defiant disorder (95% CI 1.03-1.08) at 1-year follow-up. Each hour of social media per day was associated with a 62% higher prevalence of conduct disorder (95% CI 1.39-1.87). Each hour of video chat (prevalence ratio [PR] 1.21, 95% CI 1.06-1.37), texting (PR 1.19, 95% CI 1.07-1.33), television/movies (PR 1.17, 95% CI 1.10-1.25), and video games (PR 1.14, 95% CI 1.07-1.21) per day was associated with a higher prevalence of the oppositional defiant disorder. When examining thresholds, exposure to >4 hr of total screen time per day was associated with a higher prevalence of conduct disorder (69%) and oppositional defiant disorder (46%).CONCLUSIONS:Higher screen time was prospectively associated with a higher prevalence of new-onset disruptive behavior disorders. The strongest association was between social media and conduct disorder, indicating that future research and interventions may focus on social media platforms to prevent conduct disorder.

Intracellular calcium is maintained at very low concentrations through the action of PMCA Ca++ extrusion pumps. Although much of our knowledge about these Ca++ extrusion pumps derives from studies with human erythrocytes, kinetic studies of Ca++ transport for these cells are limited to radioisotope flux measurements. Here, we developed a robust, microplate-based assay for erythrocyte Ca++ efflux using extracellular fluorescent Ca++ indicators. We optimized Ca++ loading with the A23187 ionophore, established conditions for removal of the ionophore, and adjusted fluorescent dye sensitivity by addition of extracellular EGTA to allow continuous tracking of Ca++ efflux. Efflux kinetics were accelerated by glucose and inhibited in a dose-dependent manner by the nonspecific inhibitor vanadate, revealing that Ca++ pump activity can be tracked in a 384-well microplate format. These studies enable radioisotope-free kinetic measurements of the Ca++ pump and should facilitate screens for specific inhibitors of this essential transport activity.

Background Adverse childhood experiences (ACEs) are common and linked to negative health outcomes. Previous studies have found associations between ACEs and binge-eating disorder (BED), though they have mainly focused on adults and use cross-sectional data. The objective of this study was to examine the associations between ACEs and BED in a large, national cohort of 9–14-year-old early adolescents in the US. Methods We analyzed prospective cohort data from the Adolescent Brain Cognitive Development (ABCD) Study (N = 10,145, 2016–2020). Logistic regression analyses were used to determine the associations between self-reported ACEs and BED based on the Kiddie Schedule for Affective Disorders and Schizophrenia at two-year follow-up, adjusting for sex, race/ethnicity, baseline household income, parental education, site, and baseline binge-eating disorder. Results In the sample, (49% female, 46% racial/ethnic minority), 82.8% of adolescents reported at least one ACE and 1.2% had a diagnosis of BED at two-year follow-up. The mean number of ACEs was higher in those with a diagnosis of BED compared to those without (2.6 ± 0.14 vs 1.7 ± 0.02). The association between number of ACEs and BED in general had a dose–response relationship. One ACE (adjusted odds ratio [aOR] 3.48, 95% confidence interval [CI] 1.11–10.89), two ACEs (aOR 3.88, 95% CI 1.28–11.74), and three or more ACEs (aOR 8.94, 95% CI 3.01–26.54) were all associated with higher odds of BED at two-year follow-up. When stratified by types of ACEs, history of household mental illness (aOR 2.18, 95% 1.31–3.63), household violence (aOR 2.43, 95% CI 1.42–4.15), and criminal household member (aOR 2.14, 95% CI 1.23–3.73) were most associated with BED at two-year follow-up. Conclusions Children and adolescents who have experienced ACEs, particularly household challenges, have higher odds of developing BED. Clinicians may consider screening for ACEs and providing trauma-focused care when evaluating patients for BED.

"Disparities Across Sexual Orientation in Obstructive Airway Disease among US Adults." Annals of the American Thoracic Society, 0(ja), pp. –

Purpose: The prevalence of eating disorders (EDs) in young children remains relatively unknown. Here, we aimed to assess the prevalence of anorexia nervosa (AN), bulimia nervosa (BN), binge ED (BED), and their subclinical derivatives, among 10-to 11-year-old children in the United States.Methods: Cross-sectional data from the year 1 sample of the nationwide Adolescent Cognitive Brain Development study were extracted, and unadjusted prevalence of EDs was reported, as per DSM-5 criteria.Results: Among 10-to 11-year-old children in the United States, no cases of AN were reported. The prevalence of BN was negligible, whereas the prevalence of BED was 1.1%. The prevalence of subclinical AN, BN, and BED was 6%, 0.2%, and 0.5%, respectively. Discussion: BED is the most prevalent ED subtype among preadolescent children in the United States, although subclinical markers for all ED subtypes are evident in this age range.(c) 2021 Society for Adolescent Health and Medicine. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

OBJECTIVE:To describe the relationship between parent and adolescent reports of adolescent recreational screen time and to determine sociodemographic predictors of recreational screen time reporting differences during the coronavirus disease 2019 pandemic. METHODS:We analyzed data from the Adolescent Brain Cognitive Development Study (N = 5335, ages 10-14) a national prospective cohort study in the United States collected in May 2020. We compared parent-reported, adolescent-reported, and a parent-adolescent differences in recreational screen time hours per day across 5 screen categories. RESULTS:Adolescents' total recreational screen time per day was reported as 4.46 hours by parents and 3.87 hours by adolescents. Parents reported higher levels of their child's texting, video chatting, and total recreational screen time, while adolescents reported higher multiplayer gaming and social media use. Larger discrepancies in total recreational screen time were found in older, Black, and Latino/Hispanic adolescents. Larger discrepancies in total recreational screen time were also found among unmarried/unpartnered parents. CONCLUSIONS:Given discrepancies in parent-adolescent recreational screen time reporting during the pandemic, a period of high screen use, pediatricians should encourage family discussions about adolescent media use through the development of a Family Media Use Plan. The digital media industry could provide more opportunities for parental monitoring of recreational screen time within product designs.

Proper timing of gene expression is central to lymphocyte development and differentiation. Lymphocytes often delay gene activation for hours to days after the onset of signaling components, which act on the order of seconds to minutes. Such delays play a prominent role during the intricate choreography of developmental events and during the execution of an effector response. Though a number of mechanisms are sufficient to explain timing at short timescales, it is not known how timing delays are implemented over long timescales that may span several cell generations. Based on the literature, we propose that a class of cis-regulatory elements, termed "timing enhancers," may explain how timing delays are controlled over these long timescales. By considering chromatin as a kinetic barrier to state switching, the timing enhancer model explains experimentally observed dynamics of gene expression where other models fall short. In this review, we elaborate on features of the timing enhancer model and discuss the evidence for its generality throughout development and differentiation. We then discuss potential molecular mechanisms underlying timing enhancer function. Finally, we explore recent evidence drawing connections between timing enhancers and genetic risk for immunopathology. We argue that the timing enhancer model is a useful framework for understanding how cis-regulatory elements control the central dimension of timing in lymphocyte biology.

Objective To determine sociodemographic correlates of contemporary screen time use among a diverse population-based sample of 9- and 10-year-old children. Study design In 2021, we analyzed cross-sectional baseline (2016-2018) data from the Adolescent Brain Cognitive Development study (n = 10 755). Multiple linear regression analyses were conducted to estimate associations between sociodemographic factors (sex, race/ethnicity, country of birth, household income, parental education) and 6 contemporary forms of screen time (television, videos [eg, YouTube], video games, social networking, texting, and video chat). Results On average, children reported 3.99 hours of screen time per day across 6 modalities, with the most time spent watching/streaming television shows/movies (1.31 hours), playing video games (1.06 hours), and watching/ streaming videos (1.05 hours). On average, Black children reported 1.58 more hours of screen time per day and Asian children reported 0.35 less hours of screen time per day compared with White children (mean 3.46 hours per day), and these trends persisted across most modalities. Boys reported higher overall screen time (0.75 hours more) than girls, which was primarily attributed to video games and videos. Girls reported more time texting, social networking, and video chatting than boys. Higher income was associated with lower screen time usage across all modalities except video chat. However, in high-income households, Latinx children reported 0.65 more hours of screen time per day than White children. Conclusions Given the sociodemographic differences in child screen use, guideline implementation strategies can focus on key populations, encourage targeted counseling by pediatricians, and adapt Family Media Use Plans for diverse backgrounds.

OBJECTIVE:To determine the prospective associations between contemporary screen time modalities in a nationally representative cohort of 9-10-year-old children and binge-eating disorder at one-year follow-up.METHOD:We analyzed prospective cohort data from the Adolescent Brain Cognitive Development (ABCD) Study (N = 11,025). Logistic regression analyses were conducted to estimate associations between baseline child-reported screen time (exposure) and parent-reported binge-eating disorder based on the Kiddie Schedule for Affective Disorders and Schizophrenia (KSADS-5, outcome) at one-year follow-up, adjusting for race/ethnicity, sex, household income, parent education, BMI percentile, site, and baseline binge-eating disorder.RESULTS:Each additional hour of total screen time per day was prospectively associated with 1.11 higher odds of binge-eating disorder at 1-year follow-up (95% CI 1.05-1.18) after adjusting for covariates. In particular, each additional hour of social networking (aOR 1.62, 95% CI 1.18-2.22), texting (aOR 1.40, 95% CI 1.08-1.82), and watching/streaming television shows/movies (aOR 1.39, 95% CI 1.14-1.69) was significantly associated with binge-eating disorder.DISCUSSION:Clinicians should assess screen time usage and binge eating in children and adolescents and advise parents about the potential risks associated with excessive screen time.

Background Community sample data indicate that weight control efforts in young adulthood may have associations with greater increases in body mass index (BMI) over time. Objective To determine the prospective associations between weight goals and behaviors in young adults and BMI trajectories over 15-year follow-up using a nationally representative sample. Design Longitudinal cohort data collected from 2001 to 2018 of the National Longitudinal Study of Adolescent to Adult Health. Participants Young adults aged 18–26 years old at baseline stratified by gender and BMI category. Main Measures Predictors: weight goals, any weight loss/maintenance behaviors, dieting, exercise, disordered eating behaviors. Outcomes: BMI at 7- and 15-year follow-up. Key Results Of the 12,155 young adults in the sample (54% female, 32% non-White), 33.2% reported a goal to lose weight, 15.7% to gain weight, and 14.6% to maintain weight. In unadjusted models, all groups have higher mean BMI at 7- and 15-year follow-up. In mixed effect models, goals to lose weight in men with BMI < 18.5 (5.94 kg/m 2 ; 95% CI 2.58, 9.30) and goals to maintain weight in men with BMI ≥ 25 (0.44; 95% CI 0.15, 0.72) were associated with greater BMI increase compared to no weight goal. Engaging in disordered eating behaviors was associated with greater BMI increase in men with BMI < 18.5 (5.91; 2.96, 8.86) and women with 18.5 ≤ BMI < 25 (0.40; 0.16, 0.63). Dieting (− 0.24; − 0.41, − 0.06) and exercise (− 0.31; − 0.45, − 0.17) were associated with lower BMI increase in women with 18.5 ≤ BMI < 25. In women with BMI < 18.5, dieting was associated with greater BMI increase (1.35; 0.33, 2.37). Conclusions Weight control efforts may have variable effects on BMI over time by gender and BMI category. These findings underscore the need to counsel patients on the effectiveness of weight control efforts and long-term weight management.

During normal T cell development in the thymus, alpha beta TCRs signal immature thymocytes to differentiate into mature T cells by binding to peptide-MHC ligands together with CD4/CD8 coreceptors. Conversely, in MHC and CD4/CD8 coreceptor-deficient mice, the thymus generates mature T cells expressing MHC-independent TCRs that recognize native conformational epitopes rather than linear antigenic-peptides presented by MHC. To date, no structural information of MHC-independent TCRs is available, and their structural recognition of non-MHC ligand remains unknown. To our knowledge in this study, we determined the first structures of two murine MHC-independent TCRs (A11 and B12A) that bind with high nanomolar affinities to mouse adhesion receptor CD155. Solution binding demonstrated the V alpha beta-domain is responsible for MHC-independent B12A recognition of its ligand. Analysis of A11 and B12A sequences against various MHC-restricted and -independent TCR sequence repertoires showed that individual V-genes of A11 and B12A did not exhibit preference against MHC-restriction. Likewise, CDR3 alone did not discriminate against MHC binding, suggesting VDJ recombination together with V alpha/V beta pairing determine their MHC-independent specificity for CD155. The structures of A11 and B12A TCR are nearly identical to those of MHC-restricted TCR, including the conformations of CDR1 and 2. Mutational analysis, together with negative-staining electron microscopy images, showed that the CDR regions of A11 and B12A recognized epitopes on Dl domain of CD155, a region also involved in CD155 binding to poliovirus and Tactile in human. Taken together, MHC-independent TCRs adopt canonical TCR structures to recognize native Ags, highlighting the importance of thymic selection in determining TCR ligand specificity.

The αβ T cell receptor (TCR) repertoire on mature T cells is selected in the thymus, but the basis for thymic selection of MHC-restricted TCRs from a randomly generated pre-selection repertoire is not known. Here we perform comparative repertoire sequence analyses of pre-selection and post-selection TCR from multiple MHC-sufficient and MHC-deficient mouse strains, and find that MHC-restricted and MHC-independent TCRs are primarily distinguished by features in their non-germline CDR3 regions, with many pre-selection CDR3 sequences not compatible with MHC-binding. Thymic selection of MHC-independent TCR is largely unconstrained, but the selection of MHC-specific TCR is restricted by both CDR3 length and specific amino acid usage. MHC-restriction disfavors TCR with CDR3 longer than 13 amino acids, limits positively charged and hydrophobic amino acids in CDR3β, and clonally deletes TCRs with cysteines in their CDR3 peptide-binding regions. Together, these MHC-imposed structural constraints form the basis to shape VDJ recombination sequences into MHC-restricted repertoires.