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Post-transplant cyclophosphamide (PTCY) is increasingly used as effective graft-versus-host disease (GvHD) prophylaxis in allogeneic hematopoietic-cell transplantation (allo-HCT). However, PTCY is associated with toxicities. Whether patients with specific comorbidities are more vulnerable to cyclophosphamide-induced toxicity is unclear. We retrospectively evaluated the impact of individual organ dysfunctions for non-relapse mortality (NRM) risk and overall survival (OS) among 5888 adults who underwent PTCY-based allo-HCT for acute myeloid leukemia between 2010 and 2023. In multivariable analyses 5 of the comorbidities (renal, moderate/severe hepatic, cardiac including arrhythmia/valvular disease, severe pulmonary, infection) were independently associated with adverse NRM and OS without influencing relapse rate. A simplified model using the absence (n = 4390), presence of 1 (n = 1229) or presence of 2 or 3 (n = 269) of the comorbidities which were determined individually to contribute to NRM stratified patients into 3 NRM risk (16.2% vs. 21.6% vs. 36%, retrospectively) and OS categories (64% vs. 56% vs. 36.4%, retrospectively). In Cox model, recipients with 2 or 3 comorbidities had an increased hazard ratio for NRM of 2.38 (95% confidence interval [CI], 1.89-3) and for OS of 1.96 (95% CI 1.64-2.33). Whether patients with concomitant diagnoses, as determined here, may benefit from a reduced PTCY dose remains to be evaluated in prospective clinical trials., Competing Interests: Competing interests: The authors declare no competing interests., (© 2025. The Author(s), under exclusive licence to Springer Nature Limited.)

Isocitrate dehydrogenase 1 and 2 (IDH1 and IDH2) mutations have uncertain prognostic implications in AML. We investigate the impact IDH1 and IDH2 mutations in AML patients undergoing allogeneic hematopoietic cell transplantation (allo-HCT) in first complete remission (CR1). In total, 1515 adult patients were included, 15.91% (n = 241) carried IDH1 mutation (mIDH1), and 26.27% (n = 398) IDH2 mutation (mIDH2) and 57.82% (n = 876) had no-IDH mutation. NPM1 was frequently encountered with IDH1 mutation (no-IDH group, n = 217, 24.8%, mIDH1, n = 103, 42.7%, mIDH2, n = 111, 27.9%, p < 0.0001). At day 180, the cumulative incidence (CI) of grade II-IV acute graft-versus-host disease (GVHD) was significantly lower in mIDH1 and mIDH2 compared to no-IDH groups (Hazard ratio [HR] = 0.66 (95% CI 0.47-0.91), p = 0.011; HR = 0.73 (95% CI 0.56-0.96), p = 0.025, respectively). In the mIDH1 group, overall survival (OS) was improved compared to no-IDH (HR = 0.68 (95% CI 0.48-0.94), p = 0.021), whereas mIDH2 was associated with lower incidence of relapse (HR = 0.49 (95% CI 0.34-0.7), p < 0.001), improved leukemia free survival (LFS) (HR = 0.7 (95% CI 0.55-0.9), p = 0.004) and OS (HR = 0.74 (95% CI 0.56-0.97), p = 0.027). In the subgroup of NPM1 wild type, only IDH2 was associated with improved outcomes. In conclusion, our data suggest that IDH1 and IDH2 mutations are associated with improved outcomes in patients with AML undergoing allo-HCT in CR1., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

Allogeneic hematopoietic cell transplantation (allo-HCT) remains the best consolidation strategy for acute myeloid leukemia (AML) with complex karyotype (CK). However, CK is a heterogenous and highly diverse entity. Numerical abnormalities have been associated with a controversial prognosis and AML with only multiple numerical abnormalities known as pure hyperdiploid karyotype (HDK) may have a distinct prognosis after allo-HCT compared to non-pure HDK CK AML. A total of 236 patients were identified within the EBMT registry as having HDK comprising 95 pure (pHDK) and 141 with other cytogenetic abnormalities (HDK+). The 2-year probability of leukemia-free survival (LFS) was 50% for pHDK and 31% for HDK+ (p = 0.003). The 2-year probability of overall survival (OS) was 57% for pHDK and 36% for HDK+ (p = 0.007). The 2-year cumulative incidence of relapse (RI) was 22% for pHDK and 44% for HDK+ (p = 0.001). The 2-year probability of graft-versus-host disease (GvHD)-free and relapse-free survival (GRFS) was 36% for pHDK and 21% for HDK+ (p = 0.01). On multivariate analysis, pHDK remained associated with significantly better LFS, OS and GRFS and lower RI (all p-values <0.004). pHDK AML constitutes probably a distinct cytogenetic entity from HDK+ or other non-hyperdiploid CK AML with better outcomes after allo-HCT., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

Anti-CD19 chimeric antigen receptor T cells (CART) has rapidly been adopted as the standard third-line therapy to treat aggressive B-cell lymphomas (ABCL) after failure of second-line therapy despite the lack of direct comparisons with allogeneic hematopoietic cell transplantation (alloHCT)-based strategies. Using the Grupo Español de Trasplante y Terapia Celular (GETH-TC) registry, we selected patients with the following characteristics: CART or alloHCT performed between 2016 and 2021; ≥18 years old; ABCL diagnosis; ≥2 lines of therapy; and either anti-CD19 CART or alloHCT as therapy at relapse. The analysis included a total of 316 (CART = 215, alloHCT = 101) patients. Median follow-up was 15 and 36 months for the CART and alloHCT cohorts, respectively. In the multivariate analysis, CART was confirmed to be similar to alloHCT for the primary study endpoint (progression-free survival) (hazard ratio [HR] 0.92, CI95%:0.56-1.51, p = 0.75). Furthermore, when the analysis was limited to only patients with chemo-sensitive diseases (complete and partial response) at infusion (CART = 26, alloHCT=93), no differences were reported (progression-free survival at month +18: 65% versus 55%, p = 0.59). However, CART had lower non-relapse mortality (HR 0.34, 95% CI: 0.13-0.85, p = 0.02). Given the lower toxicity and similar survival outcomes, these results suggest the use of CART before alloHCT., (© 2023. The Author(s), under exclusive licence to Springer Nature Limited.)

We report on 318 patients with acute leukemia, receiving donor lymphocyte infusion (DLI) in complete hematologic remission (CHR) after allogeneic stem cell transplantation (alloSCT). DLI were applied preemptively (preDLI) for minimal residual disease (MRD, n = 23) or mixed chimerism (MC, n = 169), or as prophylaxis in high-risk patients with complete chimerism and molecular remission (proDLI, n = 126). Median interval from alloSCT to DLI1 was 176 days, median follow-up was 7.0 years. Five-year cumulative relapse incidence (CRI), non-relapse mortality (NRM), leukemia-free and overall survival (LFS/OS) of the entire cohort were 29.1%, 12.7%, 58.2%, and 64.3%. Cumulative incidences of acute graft-versus-host disease (aGvHD) grade II-IV°/chronic GvHD were 11.9%/31%. Nineteen patients (6%) died from DLI-induced GvHD. Age ≥60 years (p = 0.046), advanced stage at transplantation (p = 0.003), shorter interval from transplantation (p = 0.018), and prior aGvHD ≥II° (p = 0.036) were risk factors for DLI-induced GvHD. GvHD did not influence CRI, but was associated with NRM and lower LFS/OS. Efficacy of preDLI was demonstrated by decreasing MRD/increasing blood counts in 71%, and increasing chimerism in 70%. Five-year OS after preDLI for MRD/MC was 51%/68% among responders, and 37% among non-responders. The study describes response and outcome of DLI in CHR and helps to identify candidates without increased risk of severe GvHD., (© 2021. The Author(s).)

Acute myeloid leukemia with runt-related transcription factor 1 gene mutation (RUNX1+ AML) is associated with inferior response rates and outcome after conventional chemotherapy. We performed a retrospective, registry-based analysis to elucidate the prognostic value of RUNX1 mutation after allogeneic stem cell transplantation (alloSCT). All consecutive adults undergoing alloSCT for AML in first complete remission (CR1) between 2013 and 2019 with complete information on conventional cytogenetics and RUNX1 mutational status were included. Endpoints of interest were cumulative relapse incidence, non-relapse mortality, overall and leukemia-free survival (OS/LFS), and GvHD-free/relapse-free survival. A total of 674 patients (183 RUNX1+, 491 RUNX1-) were identified, with >85% presenting as de novo AML. Median follow-up was 16.4 (RUNX1+) and 21.9 (RUNX1-) months. Survival rates showed no difference between RUNX1+ and RUNX1- patients either in univariate or multivariate analysis (2-year OS: 67.7 vs. 66.1%, p = 0.7; 2-year LFS: 61.1 vs. 60.8%, p = 0.62). Multivariate analysis identified age, donor type and poor cytogenetics as risk factors for inferior outcome. Among patients with RUNX+ AML, older age, reduced intensity conditioning and minimal residual disease at alloSCT predicted inferior outcome. Our data provide evidence that the negative influence of RUNX1 mutations in patients with AML can be overcome by transplantation in CR1., (© 2021. The Author(s).)

We performed a retrospective multicenter study including 140 patients with relapsed/refractory (R/R) diffuse large B cell lymphoma (DLBCL) who underwent allogeneic hematopoietic stem cell transplantation (allo-SCT) from March 1995 to November 2018. Our objective was to analyze long term outcomes. Seventy-four percent had received a previous auto-SCT (ASCT) and the median number of lines pre-allo-SCT was 3 (range 1-9). Three year-event free survival (EFS) and overall survival (OS) were 38% and 44%, respectively. Non-relapse mortality (NRM) at day 100 was 19%. Cumulative incidence of grade III-IV acute graft versus host disease (GVHD) at day 100 was 16% and moderate/severe chronic GVHD at 3 years 34%. Active disease at allo-SCT (HR 1.95, p = 0.039) (HR 2.19, p = 0.019), HCT-CI ≥ 2 (2.45, p = 0.002) (HR 2.33, p = 0.006) and donor age >37 years (HR 2.75, p = 0.014) (HR 1.98, p = 0.043) were the only independent variables both for PFS and OS, respectively. NRM was significantly modified by HCT-CI ≥ 2 (HR 4.8, p = 0.008), previous ASCT (HR 4.4, p = 0.048) and grade III-IV acute GVHD on day 100 (HR 6.13, p = 0.016). Our data confirmed that allo-SCT is a curative option for patients with R/R DLBCL, displaying adequate results for fit patients with chemosensitive disease receiving an allo-SCT from a young donor., (© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)

The presence of IKZF1 gene mutations is associated with poor prognosis of B-cell precursor acute lymphoblastic leukemia (BCP-ALL). The goal of this retrospective study was to evaluate outcome of allogeneic hematopoietic cell transplantation (allo-HCT) in this population. Ninety-five patients transplanted in first (n = 75) or second (n = 20) complete remission (CR) from either HLA-matched sibling (n = 32), unrelated (n = 47) or haploidentical (n = 16) donor were included in the analysis. The probabilities of the overall survival (OS) and leukemia-free survival (LFS) at 2 years were 55% and 47%, respectively. Relapse incidence (RI) was 32% while non-relapse mortality (NRM), 21%. The incidence of grade II-IV acute graft-versus-host disease (GVHD) and chronic GVHD was 34% and 30%, respectively. The probability of GVHD and relapse-free survival (GRFS) was 35%. In a multivariate analysis positive minimal residual disease (MRD) status was associated with decreased chance of LFS (HR = 3.15, p = 0.004) and OS (HR = 2.37, p = 0.049) as well as increased risk of relapse (HR = 5.87, p = 0.003). Disease stage (CR2 vs. CR1) affected all, LFS, OS, GRFS, RI, and NRM. Results of allo-HCT for patients with BCP-ALL and IKZF1 mutations are generally improving, however, individuals with detectable MRD have poor prognosis and require additional intervention prior to transplantation.

No studies have been reported so far on bridging treatment with idelalisib for patients with chronic lymphocytic leukemia (CLL) prior to allogeneic hematopoietic cell transplantation (alloHCT). To study potential carry-over effects of idelalisib and to assess the impact of pathway-inhibitor (PI) failure we performed a retrospective EBMT registry-based study. Patients with CLL who had a history of idelalisib treatment and received a first alloHCT between 2015 and 2017 were eligible. Data on 72 patients (median age 58 years) were analyzed. Forty percent of patients had TP53 mut/del CLL and 64% had failed on at least one PI. No primary graft failure occurred. Cumulative incidences of acute GVHD °II-IV and chronic GVHD were 51% and 39%, respectively. Estimates for 2-year overall survival (OS), progression-free survival (PFS), and cumulative incidences of relapse/progression (CIR) and non-relapse mortality NRM were 59%, 44%, 25%, and 31%. In univariate analysis, drug sensitivity was a strong risk factor. For patients who had failed neither PI treatment nor chemoimmunotherapy (CIT) the corresponding 2-year estimates were 73%, 65%, 15%, and 20%, respectively. In conclusion, idelalisib may be considered as an option for bridging therapy prior to alloHCT. Owing to the high risk for acute GVHD intensified clinical monitoring is warranted.

Detectable measurable residual disease (MRD) is a key prognostic factor in both acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML) patients. Thus, we conducted a survey in EBMT transplant centers focusing on pre- and post-allo-HCT MRD. One hundred and six centers from 29 countries responded. One hundred had a formal strategy for routine MRD assessment, 91 for both ALL and AML. For ALL (n = 95), assessing MRD has been routine practice starting from 2010 (range, 1990-2019). Techniques used for MRD assessment consisted of PCR techniques alone (n = 27), multiparameter flow cytometry (MFC, n = 16), both techniques (n = 43), next-generation sequencing (NGS) + PCR (n = 2), or PCR + MFC + NGS (n = 7). The majority of centers assessed MRD every 2-3 months for 2 (range, 1-until relapse) years. For AML, assessing MRD was routine in 92 centers starting in 2010 (range 1990-2019). Assessment of MRD was by PCR (n = 23), MFC (n = 13), both PCR and MFC (n = 39), both PCR and NGS (n = 3), and by all three techniques (n = 14). The majority assesses MRD for AML every 2-3 months for 2 (range, 1-until relapse) years. This survey is the first step in the aim to include MRD status as a routine registry capture parameter in acute leukemia.

To address limitations of the currently used reduced-intensity/myeloablative conditioning (RIC/MAC) classification scheme we aimed to develop a tool that can capture more standardized the conditioning intensity of allogeneic hematopoietic cell transplantation (HCT). We assigned intensity weight scores for frequently used conditioning regimen components and used their sum to generate the transplant conditioning intensity (TCI) score. We retrospectively tested the impact of TCI on 8255 adult (45-65 years) acute myeloid leukemia patients who underwent HCT in first complete remission. A Cox model for early nonrelapse mortality (NRM) yielded a 3-group TCI risk scheme (low, intermediate, high) with respective TCI scores of [1-2], [2.5-3.5] and [4-6]. On multivariate modeling, TCI grouping was highly and better predictive for early (day 100 and 180) NRM, 2-year NRM and relapse (REL) as compared with the RIC/MAC classification. Validation was done on 200 bootstrap samples. Moreover, TCI scoring enabled the identification of a distinct subgroup of RIC and MAC conditioning regimens with an intermediate TCI [2.5-3.5] score that had identical outcomes and which are frequently referred as "reduced toxicity conditioning". TCI scheme provides an improvement of the RIC/MAC classification. We propose TCI as a new tool to define and measure the conditioning regimen intensity.

Preliminary data suggest that allogeneic stem cell transplantation (allo-SCT) may be effective in T-prolymphocytic leukemia (T-PLL). The purpose of the present observational study was to assess the outcome of allo-SCT in patients aged 65 years or younger with a centrally confirmed diagnosis of T-PLL. Patients were consecutively registered with the EBMT at the time of transplantation and followed by routine EBMT monitoring but with an extended dataset. Between 2007 and 2012, 37 evaluable patients (median age 56 years) were accrued. Pre-treatment contained alemtuzumab in 95% of patients. Sixty-two percent were in complete remission (CR) at the time of allo-SCT. Conditioning contained total body irradiation with 6 Gy or more (TBI6) in 30% of patients. With a median follow-up of 50 months, the 4-year non-relapse mortality, relapse incidence, progression-free (PFS) and overall survival were 32, 38, 30 and 42%, respectively. By univariate analysis, TBI6 in the conditioning was the only significant predictor for a low relapse risk, and an interval between diagnosis and allo-SCT of more than 12 months was associated with a lower NRM. This study confirms for the first time prospectively that allo-SCT can provide long-term disease control in a sizable albeit limited proportion of patients with T-PLL.

Allogeneic hematopoietic stem cell transplantation (HSCT) in first complete remission is a standard of care for adult patients with Philadelphia chromosome (Ph)-negative acute lymphoblastic leukemia (ALL) and high risk of relapse. However, the stratification systems vary among study groups. Inadequate response at the level of minimal residual disease is the most commonly accepted factor indicating the need for alloHSCT. In this consensus paper on behalf of the European Working Group for Adult Acute Lymphoblastic Leukemia and the Acute Leukemia Working Party of the European Society for Blood and Marrow Transplantation, we summarize available evidence and reflect current clinical practice in major European study groups regarding both indications for HSCT and particular aspects of the procedure including the choice of donor, source of stem cells and conditioning. Finally, we propose recommendations for daily clinical practice as well as for planning of prospective trials.

The purpose of this retrospective registry study was to investigate the outcome of autoSCT for primary mediastinal B-cell lymphoma (PMBCL) in the rituximab era, including the effects of eventual post-transplant radiotherapy (RT) consolidation. Patients with PMBCL aged between 18 and 70 years who were treated with a first autoSCT between 2000 and 2012 and registered with the EBMT were eligible. Eighty-six patients with confirmed PMBCL and the full data set required for this analysis were evaluable. Sixteen patients underwent autoSCT in remission after first-line therapy (CR/PR1), 44 patients were transplanted with chemosensitive relapsed or primary refractory disease (CR/PR >1), and 24 patients were chemorefractory at the time of autoSCT. With a median follow-up of 5 years, 3-year estimates of relapse incidence, progression-free survival, and overall survival were 6%, 94%, and 100% for CR/PR1; 31%, 64%, and 85% for CR/PR >1; and 52%, 39%, and 41% for REF, respectively. Whilst there was no significant benefit of post-transplant RT in the CR/PR >1 group, RT could completely prevent disease recurrence post d100 in the refractory group. In conclusion, autoSCT with or without consolidating RT is associated with excellent outcome in chemoimmunotherapy-sensitive PMBCL, whereas its benefits seem to be limited in chemoimmunotherapy-refractory disease.

Systemic sclerosis (SSc) is a rare disabling autoimmune disease with a similar mortality to many cancers. Two randomized controlled trials of autologous hematopoietic stem cell transplantation (AHSCT) for SSc have shown significant improvement in organ function, quality of life and long-term survival compared to standard therapy. However, transplant-related mortality (TRM) ranged from 3-10% in patients undergoing HSCT. In SSc, the main cause of non-transplant and TRM is cardiac related. We therefore updated the previously published guidelines for cardiac evaluation, which should be performed in dedicated centers with expertize in HSCT for SSc. The current recommendations are based on pre-transplant cardiopulmonary evaluations combining pulmonary function tests, echocardiography, cardiac magnetic resonance imaging and invasive hemodynamic testing, initiated at Northwestern University (Chicago) and subsequently discussed and endorsed within the EBMT ADWP in 2016.

This study shows the long-term updated outcomes of a multicenter retrospective study which analyzed 843 patients with myelodysplastic syndrome (MDS) who underwent transplantation with an HLA-identical sibling donor with either reduced-intensity conditioning (RIC) in 213 patients, or standard myeloablative conditioning (MAC) in 630 patients. In multivariate analysis, the 13-year relapse rate was significantly increased after RIC (31% after MAC vs 48% in RIC; HR, 1.5; 95% CI, 1.1-1.9; P=0.04), but with no differences in overall survival (OS) (30% after MAC vs 27% in RIC; P=0.4) and PFS (29 vs 21%, respectively, P=0.3). Non-relapse mortality was higher in MAC (40 vs 31%; P=0.1), especially in patients older than 50 years (50 vs 33%, P<0.01). In addition, long-term follow-up confirms the importance of other variables on 13-year OS, mainly MDS risk category, disease phase, cytogenetics and receiving a high donor cell dose, irrespective of the conditioning regimen used.

Allogeneic hematopoietic stem cell transplantation (allo-SCT) remains the therapeutic method with the most potent anti-leukemic activity mediated by the graft versus leukemia effect. However, a significant proportion of patients with AML will relapse after allo-SCT. The prognosis for these patients is dismal, with a probability of long-term survival of <20%. Data from previous studies have shown that disease-specific prognostic factors, are in general, the same as those in patients treated with conventional chemotherapy. Minimal residual disease (MRD) and chimerism status monitoring after allo-SCT may be used as predictors of impending relapse and should be part of routine follow-up for AML patients. A significant number of studies have shown that pre-emptive administration of donor lymphocyte infusion (DLI) based on MRD and chimerism monitoring, as well as prophylactic DLI in AML patients at high risk of relapse is effective in preventing relapse. In this review, we discuss strategies for the identification of high-risk patients, review current therapeutic options and provide our recommendations for the management of post-SCT AML.

Sinusoidal obstruction syndrome or veno-occlusive disease (SOS/VOD) is a potentially life-threatening complication of hematopoietic SCT (HSCT). This review aims to highlight, on behalf of the European Society for Blood and Marrow Transplantation, the current knowledge on SOS/VOD pathophysiology, risk factors, diagnosis and treatments. Our perspectives on SOS/VOD are (i) to accurately identify its risk factors; (ii) to define new criteria for its diagnosis; (iii) to search for SOS/VOD biomarkers and (iv) to propose prospective studies evaluating SOS/VOD prevention and treatment in adults and children.

The Accreditation Sub-Committee of the EBMT regularly publishes special reports on current practice of haemopoietic stem cell transplantation for haematological diseases, solid tumours and immune disorders. Major changes have occurred since the last report in 1998. Haemopoietic stem cell transplantation today includes allogeneic and autologous stem cells derived from bone marrow, peripheral blood and cord blood. With reduced intensity conditioning regimens in allogeneic transplantation, the age limit has increased, permitting the inclusion of older patients. New indications have emerged, such as autoimmune disorders and AL amyloidosis for autologous, and solid tumours for allogeneic transplants. Other indications, such as autologous transplantation for breast cancer have been challenged. An updated report with revised tables and operating definitions is presented here.

The results of 33 allogeneic peripheral blood progenitor cells transplants (allo-PBPCT) in adult patients with hematologic malignancies were analyzed in a retrospective and multicenter study. In 21 of 33 cases (63%) the disease was refractory or in advanced stage and eight of the 33 cases (24%) were second transplants after relapse. Donors were treated with a median of 10 (4-16) micrograms/kg/day of rhG-CSF subcutaneously for 5-7 days. Three required a central venous line for harvesting. Peripheral blood leukapheresis product contained a median of 5.9 (1.8-13) 10(6)/kg CD34+ cells and a median of 309.5 (153-690) 10(6)/kg CD3+ cells. After a myeloablative regimen, all patients received PBPC from HLA-identical donors as the sole source of progenitor cells. Cyclosporin A (CsA) alone (n = 2), CsA and steroids (n = 9), and CsA and methotrexate (MTX) (n = 22) were used for GVHD prophylaxis. Growth factors post-transplant were given to 11 patients (33%). The median follow-up of the patients was 3 months. Actuarial median day for hemopoietic recovery was: neutrophils to >0.5 (>1) x 10(9)/l, day 14 (15); platelets to >20 (>50) x 10(9)/l, day 14 (21). The quantity of CD34+ cells infused did not significantly affect the engraftment kinetics, from a starting cutoff of 2.5 x 10(6)/kg. The speed of neutrophil recovery seemed to be influenced strongly by using rhG-CSF post-transplant and marginally by the type of GVHD prophylaxis. Actuarial probability for grade II-IV acute GVHD of the whole group was 37% (95% Cl, 20-54%).

We evaluated proliferative responses in mixed lymphocyte cultures (MLC) following bone marrow transplantation (BMT) in 14 recipients of T cell-depleted haplo-compatible parental marrow: 11 for the treatment of severe combined immunodeficiency (SCID), 2 for leukemia and 1 for Wiskott-Aldrich syndrome (WAS). We compared the results obtained in 9 SCID patients and 1 WAS patient with split chimerism (T cells of donor origin, B cells and monocytes of recipient origin) to 4 patients (2 SCID and 2 leukemias) who were full chimeras (T, B and monocytes of donor origin). In the full chimeras, as with the fresh donor PBMC, fresh donor T cells did not proliferate in the MLC to recipient non-T cells (E-). In this group there were no differences (p > 0.2) between the responses of engrafted T and fresh donor T to recipient E- cells. We found tolerance of engrafted donor T cells to residual mismatched T cell-depleted (E-) recipient cells in the split chimera group. In this group the engrafted T cells had low or no responses in MLC to HLA mismatched E- host cells compared with fresh donor cells (p < 0.001). In 3 of 8 split chimera patients that we tested the addition of small numbers (5000-10,000) of freshly isolated donor T cells, irradiated or not, resulted in a two fold increase in the engrafted T cell response to recipient E- cells. In contrast, in 3 of 3 full chimeras tested, the addition of fresh donor T cells had no demonstrable effect on the response of engrafted T cells to recipient E-.(ABSTRACT TRUNCATED AT 250 WORDS)