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T cell fitness has emerged as a critical determinant of the efficacy and persistence of Chimeric Antigen Receptor (CAR)-T cell therapy. Defined by the capacity of T cells to proliferate, resist exhaustion, persist in vivo , and exert sustained effector functions, T cell fitness reflects the integration of a dynamic network of intrinsic and extrinsic regulatory mechanisms. In this review, we present a comprehensive synthesis of the molecular and cellular foundations underlying T cell fitness, emphasizing the influence of differentiation trajectories, signaling pathways, metabolic reprogramming, and epigenetic modifications. We further discuss the impact of patient-specific conditions such as age as well as disease biology, prior therapeutic exposures, and timing and quality of T cell collection, on the phenotypic and functional efficacy of CAR-T cell products. Beyond delineating these determinants, we highlight emerging strategies aimed at enhancing T cell fitness. Importantly, we propose T cell fitness as an integrated, multi-layered systems property emerging from the interaction between differentiation state, signaling architecture, metabolic–mitochondrial competence, epigenetic stability, and host-specific inflammatory and treatment-related pressures. We introduce a mechanistic framework that links these layers across the CAR-T therapeutic timeline from leukapheresis to post-infusion tumor engagement and outline how this framework can be operationalized into measurable parameters to guide patient stratification, manufacturing decisions, and rational therapeutic interventions.

Abstract Background: Haploidentical allogeneic stem cell transplantation (haplo-SCT) has been a transformative option for patients lacking matched donors. Post-transplant cyclophosphamide (PTCy) at 100 mg/kg total dose (50 mg/kg on Day +3 and +4) is a standard for graft-versus-host diseases (GvHD) prophylaxis. Efforts to decrease toxicity while maintaining efficacy have informed pursuits of dose de-escalation. However, data regarding the efficacy and safety of reduced-dose PTCy (<100 mg/kg) remain scattered and variably reported. Methods: We performed a systematic review registered on PROSPERO (CRD420250650291) and conducted per PRISMA guidelines. Six databases (Medline, Embase, Web of Science, Scopus, Cinahl, Cochrane) were searched initially up to May 2024. An updated search was conducted to include studies till June 2025. Eligible studies included prospective or retrospective cohorts or randomized trials evaluating low-dose PTCy in haplo-SCT recipients reporting on acute/chronic GvHD, overall survival (OS), relapse-free survival (RFS), immune reconstitution, or complications such as hemorrhagic cystitis or infections. Non-comparative studies, reviews, and abstracts were excluded. Data was extracted independently by two reviewers, and risk-of-bias (ROB) was assessed using ROBINS-I and Cochrane RoB-2 tools. Results: Thirty-three studies, including 31 observational studies and 2 randomized controlled trials (RCTs), encompassing over 2500 patients, met our eligibility criteria. Most studies originated from Asia, with smaller contributions from Europe, the United States, India and Latin America. Low-dose PTCy regimens ranged from 14.5 to 80 mg/kg, commonly administered as 50 mg/kg ×1 on day +3 or 40 mg/kg ×2 on days +3 and +4. Comparators included standard-dose PTCy (100 mg/kg), ATG-based prophylaxis, or other regimens. Median follow-up ranged from 4 months to over 5 years. Acute GvHD (grades II–IV) rates in low-dose PTCy arms ranged from 9% to 34%. Most observational data showed comparable or reduced incidence of aGvHD compared to controls. Chronic GvHD outcomes varied from 8-46% and showed no clear benefit of dose reduction in most studies.Two-year OS ranged widely from 47% to 100%, with some observational studies favoring low-dose PTCy but others showing no survival advantage. RFS was similarly variable. Hemorrhagic cystitis occurred in 3–33% of patients, with no clear dose-dependent pattern. Infections were inconsistently reported, but, when data were available, viral infections, including BK virus, CMV, and EBV reactivation, were common, with no consistent dose-dependent trend. Median time to immune reconstitution was slightly shorter in some cohorts, but clinical relevance remains uncertain. Two studies reported significantly reduced incidence of cardiac complications with lower PTCy exposure. One randomized controlled trial comparing 80 mg/kg PTCy + ATG with the Beijing Protocol reported significantly reduced aGvHD (11.5% vs. 39.3%, p=0.001) and better OS (75.4% vs. 54.1%, p = 0.021) and RFS ((72.7% vs. 55.0%, p = 0.044)) in the experimental arm. In contrast, the only other randomized study used ultra-low-dose PTCy (14.5 mg/kg ×2) vs. ATG and found no significant difference in acute GvHD (18.2% vs. 18.2%, p=0.996) or survival and was stopped early because of futility. Both RCTs showed comparable rates of cGvHD, toxicity, and immune reconstitution.Importantly, 87% of observational studies had a serious or critical ROB. Both RCTs had some concerns of bias due to inadequate allocation concealment and open-label design but were otherwise at low ROB. Conclusions: Low-dose PTCy for GvHD prophylaxis in haplo-SCT carries comparable or lower rates of acute GvHD, similar chronic GvHD incidence, and non-inferior survival outcomes in many studies. Marked heterogeneity in regimens, patient populations, high ROB, and selective reporting preclude definitive conclusions. Randomized trials must focus on identifying subgroups benefiting from de-escalated PTCy. Until then, low-dose PTCy remains a promising but investigational alternative in haploidentical transplantation.

BACKGROUND: Patients with arrhythmogenic mitral valve prolapse syndrome are at increased risk for life-threatening ventricular arrhythmias, but studies have been limited by small sample sizes. We sought to assemble an international arrhythmogenic mitral valve prolapse syndrome registry to delineate the clinical, imaging, and treatment characteristics of patients with arrhythmogenic mitral valve prolapse syndrome who survived sudden cardiac arrest (SCA) or had sustained ventricular tachycardia (VT) or ventricular fibrillation. METHODS: In this descriptive registry, we characterized patients with arrhythmogenic mitral valve prolapse syndrome who survived SCA, sustained VT, or ventricular fibrillation. Deidentified data were abstracted locally and combined centrally. RESULTS: We included 148 patients who had SCA or VT/ventricular fibrillation. Patients had a mean age of 43.7±15.4 years; 68% were women, 73% had bileaflet prolapse, 65% had mitral annular disjunction, 67% had nonsustained VT, and 59% had inferolateral T-wave inversions. Syncope (n=54, 48%) and anterolateral T-wave inversion (n=26, 22%) were relatively common. Catheter ablation was performed in 50 (35%) patients for premature ventricular complexes and in 18 (17.7%) patients for VT. Sites of origin for arrhythmias were commonly in the papillary muscles, fascicles, mitral annulus, and inferior/inferolateral left ventricle. CONCLUSIONS: In this international descriptive registry of patients with arrhythmogenic mitral valve prolapse syndrome and SCA, patients were young, women, and had bileaflet mitral valve prolapse, mitral annular disjunction, inferolateral T-wave inversions, and nonsustained VT. A history of syncope and anterolateral T-wave inversions was relatively common in patients who survived SCA or sustained VT/ventricular fibrillation.

Introduction: Although most patients with mitral valve prolapse (MVP) have a benign prognosis, there is a subgroup with bileaflet MVP syndrome (biMVPS) at increased risk of idiopathic ventricular fibrillation (VF). The phenotypic features of this syndrome include myxomatous bileaflet prolapse, complex ventricular ectopy (VE), T-wave inversion in inferolateral ECG leads, and female predominance. The mechanism of ventricular arrhythmia in these patients is unknown. Methods: We reviewed our center’s ablation experience of 14 consecutive patients with biMVPS patients between February 2007 and October 2013 (n=13 female, median [range] age at index ablation 33.8 [21.0 - 58.7] years, left ventricular ejection fraction [EF] 60 [45-67] %, all ≤ moderate mitral regurgitation [MR]). We compared findings from 6 with prior sudden cardiac arrest (SCA) and recurrent ICD shocks for drug refractory VF with 8 who had symptomatic drug-refractory complex VE but without prior SCA. Results: All patients with prior SCA had a predominant VE trigger for the VF. There was no difference between the two groups in EF, MR severity, VE burden, and number of different VE morphologies or foci ablated. Sites of successful VE ablation at index procedure were the LV papillary muscles (PM, n=6 anterior, 6 posterior), fascicular system (n=5 anterior, 4 posterior), mitral annulus (n=2), outflow tracts (3 RVOT, 1 LVOT), and coronary sinus (n=1). All patients had at least one LV papillary or fascicular VE focus. Purkinje origin VE was identified as the VF trigger in 6/6 SCA patients (3 from PM, 3 from LV fascicles) and only 4/8 (none from PM) non-SCA patients (p=0.04). Acute success was seen in 17/19 procedures with no complications. VF storm occurred within 24 hours of ablation in a single patient. At 310 (39 - 2099) days follow-up, 1 SCA patient received a single shock (p=0.031 compared to pre-ablation), and symptoms from VE were reduced in 12/14. Conclusions: BiMVPS is characterized by VE from the LV fascicles and papillary muscles that trigger VF. The VE is invariably multifocal and may also involve the outflow tracts and mitral annulus. Ablation of clinically dominant VE foci alleviates symptoms and reduces the number of VF-terminating ICD shocks.

Introduction: Although the vast majority of mitral valve prolapse (MVP) is benign, women with bileaflet MVP (biMVP), complex ventricular ectopy (VE), and abnormal T waves may comprise the recently described malignant biMVP syndrome. The mechanism of ventricular arrhythmia is unknown. To further characterize the arrhythmic substrate, we reviewed our center’s ablation experience in 6 biMVP patients with prior cardiac arrest and recurrent ICD shocks for drug refractory ventricular fibrillation (VF). Methods and Results: Six women with biMVP (median age 31.5 [range 24.2 - 58.7] years, EF 65 [45 - 67]%, all ≤moderate mitral regurgitation) experienced 6 (3 - 25) appropriate ICD shocks over 4.8 (2.8 - 10.7) years and underwent index ablation between 2/2007 - 10/2013. All had multiple VE morphologies (median 7 [3 - 24]) with variable coupling intervals but with a predominant VE trigger for the VF. A median 2 (1 - 4) VE foci were ablated. Sites of successful ablation of VF-triggering and other dominant VE were left ventricular papillary muscles [PM] (1 anterior, 1 posterior, 1 both), fascicles (1 anterior, 1 posterior), or both (1 both PM and posterior fascicle). Outflow tract VE was also present and targeted (1 left, 1 right)i. Two underwent repeat ablation (288 and 312 days) for recurrent complex VE without shocks, with different foci to the index ablation (1 posterior fascicle, 1 both fascicles). The VF-triggering VE in all patients was confirmed as originating from within the left fascicular system, which in 3/6 was at a papillary muscle. Acute procedural success was seen in all with no complications to date. A VF storm occurred within 24 hours of ablation in a single patient. At follow-up of a mean 662 (47 - 2099) days, 1 patient received a single shock (p=0.03 vs. preablation). Symptomatic VE was reduced in all; while 3/6 continue Class 1c antiarrhythmics and 5/6 have beta blockade. Conclusion: Malignant biMVP syndrome is characterized by fascicular and papillary muscle PVCs that trigger ventricular fibrillation, yet in all patients, the VE is multifocal. Ablation of at least one focus appears to improve symptoms and reduce shocks.

AbstractSecondary acute myeloid leukemia (sAML) includes AML as a complication of an antecedent hematological disorder or a therapy‐related AML. Large registry‐based data identified sAML as an independent poor‐outcome type of AML post allogeneic hematopoietic cell transplantation (allo‐HCT). In our study, we tried to define factors affecting outcomes of sAML post allo‐HCT, and identify patients with sAML who may truly benefit from allo‐HCT. We retrospectively analyzed the data of 64 patients aged (14‐61 years) with sAML who received allo‐HCT between September 2010 and February 2018 at our institute. Most of the patients were transplanted from matched related donors (MRD; 54, 84.4%). Our results showed that poor‐risk cytogenetics were identified in 31 patients (48.4%), and their presence was an indicator of poor overall survival (OS) and disease‐free survival (DFS; P‐value = .009, and .004, respectively). The cumulative incidence of chronic graft‐versus‐host disease (cGVHD) was significantly lower in sAML patients with poor‐risk cytogenetics (P‐value = .003) resulting in a high risk of death without cGVHD in this group of patients (P‐value = .02). Besides, GVHD relapse‐free survival (GRFS) analysis showed that most of our studied patients experienced either relapse or debilitating grade II‐IV cGVHD in the first 2 years post allo‐HCT. We conclude that sAML patients with poor‐risk cytogenetics have a significantly lower DFS post allo‐HCT with a high risk of death without active cGVHD.

Background— Although the vast majority of mitral valve prolapse (MVP) is benign, a small subset of patients, predominantly women, with bileaflet prolapse, complex ventricular ectopy (VE), and abnormal T waves comprise the recently described bileaflet MVP syndrome. We compared findings on electrophysiological study in bileaflet MVP syndrome patients with and without cardiac arrest to identify factors that may predispose to malignant ventricular arrhythmia. Methods and Results— Fourteen consecutive bileaflet MVP syndrome patients (n=13 women; median [limits], age at index ablation, 33.8 [21.0–58.7] years; ejection fraction, 60% [45%–67%]; all ≤ moderate mitral regurgitation; n=6 with previous cardiac arrest and implantable cardioverter defibrillator shocks for ventricular fibrillation; and n=8 without implantable cardioverter defibrillator although with symptomatic complex VE) were included. The 2 groups had similar baseline echocardiographic and electrocardiographic characteristics. All patients had at least 1 left ventricular papillary or fascicular VE focus. Purkinje origin VE was identified as the ventricular fibrillation trigger in 6 of 6 cardiac arrest patients (4 from papillary muscle) and Purkinje origin of dominant VE was seen in 5 of 8 (3 from papillary muscle) nonarrest patients. Acute success was seen in 17 of 19 procedures, and a ventricular fibrillation storm occurred within 24 hours of ablation in a single patient. Repeat ablation for recurrent symptomatic arrhythmia was performed in 6 patients. At 478 (39–2099) days of follow-up, 2 cardiac arrest patients received appropriate shocks. Symptoms from VE were reduced in 12 of 14. Conclusions— Bileaflet MVP syndrome is characterized by fascicular and papillary muscle VE that triggers ventricular fibrillation. Ablation of clinically dominant VE foci improves symptoms and reduces appropriate implantable cardioverter defibrillator shocks.

Direct Pulmonary Vein Ablation With Stenosis Prevention TherapyIntroductionThe dominant location of electrical triggers for initiating atrial fibrillation (AF) originates from the muscle sleeves inside pulmonary veins (PVs). Currently, radiofrequency ablation (RFA) is performed outside of the PVs to isolate, rather than directly ablate these tissues, due to the risk of intraluminal PV stenosis.MethodsIn 4 chronic canine experiments, we performed direct PV muscle sleeve RFA ± postablation drug‐coated balloon (DCB) treatment with paclitaxel/everolimus. Of the 4 PVs, 2 PVs were ablated and treated with DCB, 1 PV was ablated without DCB treatment (positive control), and 1 PV was left as a negative control. Local electrograms were assessed in PVs for near‐field signals and were targeted for ablation. After 12–14 weeks survival, PVs were interrogated for absence of near‐field PV potentials, and each PV was assessed for stenosis.ResultsAll canines survived the study period without cardiorespiratory complications, and remained ambulatory. In all canines, PVs that were ablated and treated with DCB remained without any significant intraluminal stenosis. In contrast, PVs that were ablated and not treated with DCB showed near or complete intraluminal stenosis. At terminal study, PV potentials remained undetectable. A blinded, histologic analysis demonstrated that ablated PVs without DCB treatment had extensive thrombus, fibrin, mineralization, and elastin disruption.ConclusionOur chronic canine data suggest that direct PV tissue ablation without subsequent stenosis is feasible with the use of postablation DCBs.

Arrhythmogenic Substrate of Ventricular Arrhythmias at the AMCIntroductionCatheter ablation of ventricular arrhythmia (VA) at the fibrous aortic mitral continuity (AMC) has been described, yet the nature of the arrhythmogenic substrate remains unknown.MethodsProcedural records of 528 consecutive patients undergoing ablation of VA at Mayo Clinic, Rochester, MN, were reviewed. The electrocardiographic and electrophysiologic characteristics of patients with successful ablation at the AMC were analyzed to characterize the underlying arrhythmogenic substrate.ResultsOf the 21 patients (mean age 53.2 ± 13.4 years, 47.6% male) who underwent ablation of VA at the AMC with acute success, prepotentials (PPs) were found at the ablation sites preceding the ventricular electrogram (VEGM) during arrhythmias in 13 (61.9%) patients and during sinus rhythm in 7 (53.8%) patients. VAs with PPs were associated with a significantly higher burden of premature ventricular complexes (PVCs; 26.1 ± 10.9% vs. 14.9 ± 10.1%, P = 0.03), shorter VEGM to QRS intervals (9.0 ± 28.5 milliseconds vs. 33.1 ± 8.8 milliseconds, P = 0.03), lower pace map scores (8.7 ± 1.6 vs. 11.4 ± 0.8, P = 0.001), and a trend toward shorter V‐H intervals during VA (32.1 ± 38.6 milliseconds vs. 76.3 ± 11.1 milliseconds, P = 0.06) as compared to those without PP. A strong and positive correlation was found between V‐H interval and QRS duration during arrhythmia in those with PPs (B = 2.11, R2 = 0.97, t = 13.7, P < 0.001) but not in those without PPs.ConclusionLocal EGM characteristics and relative activation time of the His bundle suggest the possibility of conduction tissue as the origin for VA arising from the fibrous AMC. Specific identification and targeting of PPs when ablating VAs at this location may improve procedural success.