Stable engraftment after megadose blood stem cell transplantation across the HLA barrier: the case for natural killer cells as graft-facilitating cells

Immune reconstitution after hematopoietic cell transplantation

PURPOSE OF REVIEW

Successful immune reconstitution is important for decreasing posthematopoietic cell transplant (post-HCT) infections, relapse, and secondary malignancy, without increasing graft-versus-host disease (GVHD). Here we review how different parts of the immune system recover, and the relationship between recovery and clinical outcomes.

RECENT FINDINGS

Innate immunity (e.g., neutrophils, natural killer cells) recovers within weeks, whereas adaptive immunity (B and T cells) recovers within months to years. This has been known for years; however, more recently, the pattern of recovery of additional immune cell subsets has been described. The role of these subsets in transplant complications like infections, GVHD and relapse is becoming increasingly recognized, as gleaned from studies of the association between subset counts or function and complications/outcomes, and from studies depleting or adoptively transferring various subsets.

SUMMARY

Lessons learned from observational studies on immune reconstitution are leading to new strategies to prevent or treat posttransplant infections. Additional knowledge is needed to develop effective strategies to prevent or treat relapse, second malignancies and GVHD.

Haploidentical allogeneic hematopoietic cell transplantation in adults using CD3/CD19 depletion and reduced intensity conditioning: a phase II study

BACKGROUND

We report a prospective multicenter phase II study of haploidentical hematopoietic stem cell transplantation using CD3/CD19-depleted grafts after reduced intensity conditioning with fludarabine, thiotepa, melphalan and OKT-3.

DESIGN AND METHODS

Sixty-one adults with a median age of 46 years (range 19-65 years) have been enrolled. Diagnoses were acute myeloid leukemia (n=38), acute lymphoblastic leukemia (n=8), non-Hodgkin's lymphoma (n=6), myeloma (n=4), chronic myeloid leukemia (n=3), chronic lymphatic leukemia (n=1) and myelodysplastic syndrome (n=1). Patients were considered high risk because of refractory disease (n=18), cytogenetics (n=6), complete remission (≥ 2) (n=9), chemosensitive relapse in partial remission (n=4) or relapse after prior hematopoietic stem cell transplantation (n=15 allogeneic, n=8 autologous, n=1 both). At haploidentical hematopoietic stem cell transplantation, 30 patients were in complete remission and 31 in partial remission. Grafts contained a median of 7.0 × 10(6) (range 3.2-22) CD34(+) cells/kg, 4.2 × 10(4) (range 0.6-44) CD3(+) T cells/kg and 2.7 × 10(7) (range 0.00-37.3) CD56(+) cells/kg.

RESULTS

Engraftment was rapid with a median of 12 days to granulocytes more than 0.5 × 10(9)/L (range 9-50 days) and 11 days to platelets more than 20 × 10(9) (range 7-38 days). Incidence of grade IIIV acute graft-versus-host-disease and chronic graft-versus-host-disease was 46% and 18%, respectively. Non-relapse mortality on Day 100 was 23% and 42% at two years. Cumulative incidence of relapse/progression at two years was 31%. Kaplan-Meier estimated 1-year and 2-year overall survival with median follow up of 869 days (range 181-1932) is 41% and 28%, respectively.

CONCLUSIONS

This regimen allows successful haploidentical hematopoietic stem cell transplantation with reduced intensity conditioning in high-risk patients lacking a suitable donor. (clinicaltrials.gov identifier:NCT00202917).

Breaking tolerance to self, circulating natural killer cells expressing inhibitory KIR for non-self HLA exhibit effector function after T cell-depleted allogeneic hematopoietic cell transplantation

Alloreactive natural killer (NK) cells are an important influence on hematopoietic stem cell transplantation (HSCT) outcome. In HLA-mismatched HSCT, alloreactivity occurs when licensed donor NK cells expressing inhibitory killer Ig-like receptors (KIR) for donor MHC class I ligands recognize the lack of the class I ligands in the mismatched recipient ("missing self"). Studies in HLA-matched HSCT, however, have also demonstrated improved outcome in patients lacking class I ligands for donor inhibitory KIR ("missing ligand"), indicating that classically nonlicensed donor NK cells expressing KIR for non-self MHC class I ligands may exhibit functional competence in HSCT. We examined NK function in 16 recipients of T cell-depleted allografts from HLA-identical or KIR-ligand matched donors after myeloablative therapy. After HSCT, nonlicensed NK cells expressing inhibitory KIR for non-self class I exhibit robust intracellular IFN-gamma and cytotoxic response to target cells lacking cognate ligand, gradually becoming tolerized to self by day 100. These findings could not be correlated with cytokine environment or phenotypic markers of NK development, nor could they be attributed to non-KIR receptors such as CD94/NKG2A. These findings confirm that NK alloreactivity can occur in HLA-matched HSCT, where tolerance to self is either acquired by the stem cell-derived NK cell after exiting the bone marrow or where tolerance to self can be temporarily overcome.

Haploidentical allogeneic hematopoietic cell transplantation in adults using CD3/CD19 depletion and reduced intensity conditioning: an update

Haploidentical hematopoietic cell transplantation (HHCT) after high dose conditioning with CD34-selected stem cells has been complicated by high regimen related toxicities, slow engraftment and delayed immune reconstitution leading to increased treatment related mortality (TRM). A new regimen using reduced intensity conditioning (RIC) and graft CD3/CD19 depletion with anti-CD3 and anti-CD19 coated microbeads on a CliniMACS device may allow HHCT with lower toxicity and faster engraftment. CD3/CD19 depleted grafts not only contain CD34+ stem cells but also CD34 negative progenitors, natural killer, graft facilitating and dendritic cells. RIC was performed with fludarabine (150-200 mg/m(2)), thiotepa (10 mg/kg), melphalan (120 mg/m(2)) and OKT-3 (5 mg/day, day -5 to +14) and no posttransplant immunosuppression. Twenty nine patients (median age=42 (range, 21-59) years) have been transplanted with this regimen. Diagnosis were AML (n=16), ALL (n=7), NHL (n=3), MM (n=2) and CML (n=1). Patients were "high risk" with refractory disease or relapse after preceding HCT. The CD3/CD19 depleted haploidentical grafts contained a median of 7.6x10(6) (range, 3.4-17x10(6)) CD34+ cells/kg, 4.4x10(4) (range, 0.006-44x10(4)) CD3+ T cells/kg and 7.2x10(7) (range, 0.02-37.3x10(7)) CD56+ cells/kg. Donor-recipient KIR-ligand-mismatch was found in 19 of 29 patients. The regimen was well tolerated with maximum acute toxicity being grade 2-3 mucositis. Because of severe neurotoxicity in 4 patients treated with 200 mg/m(2) fludarabine, the dose was reduced to 150 mg/m(2). Engraftment was rapid with a median time to >500 granulocytes/microL of 12 (range, 10-21) days, >20,000 platelets/microL of 11 (range, 7-38) days and full donor chimerism after 2-4 weeks in all patients. Incidence of grade II-IV degrees GVHD was 48% with grade II degrees =10, III degrees =2 and IV degrees =2. One patient, who received the highest T-cell dose, developed lethal grade IV GVHD. TRM in the first 100 days was 6/29 (20%) with deaths due to idiopathic pneumonia syndrome (n=1), mucormycosis (n=1), pneumonia (n=3) or GVHD (n=1). Overall survival is 9/29 patients (31%) with deaths due to infections (n=7), GVHD (n=1) and relapse (n=12) with a median follow-up of 241 days (range, 112-1271). In conclusion, this regimen is promising in high risk patients lacking a suitable donor, and a prospective phase I/II study is ongoing.

Haploidentical allogeneic hematopoietic cell transplantation in adults with reduced-intensity conditioning and CD3/CD19 depletion: Fast engraftment and low toxicity

OBJECTIVE

CD3/CD19 depletion may improve engraftment and immune reconstitution after haploidentical hematopoietic cell transplantation (HHCT) as grafts not only contain CD34+ stem cells but also CD34- progenitors and natural killer, dendritic, and facilitating cells.

PATIENTS AND METHODS

Ten consecutive patients received HHCT with CD3/CD19-depleted grafts. Reduced-intensity conditioning was performed with fludarabine (150-200 mg/m2), thiotepa (10 mg/kg), melphalan (120 mg/m2), and OKT-3 (5 mg/day, day -5 to +14) without additional posttransplant immunosuppression. Diagnoses were AML (n = 4), ALL (n = 3), NHL (n = 2), and multiple myeloma (n = 1). All patients were "high risk" with refractory disease or relapse after preceding HCT. The CD3/CD19-depleted haploidentical grafts contained a median of 7.8 x 10(6) (range, 5.2-17 x 10(6)) CD34+ cells/kg, 5.5 x 10(7) (range, 0.02-8.6 x 10(7)) CD56+ cells/kg, and 2.0 x 10(4) (range, 0.006-44 x 10(4)) CD3+ T cells/kg. Engraftment was rapid with median time to greater than 500 granulocytes/microL of 13 (range, 11-17) days, greater than 20,000 platelets/microL of 11 (range, 8-16) days, and full donor chimerism after 2 weeks in all patients. Six cases of grade II GVHD occurred. One patient, who received the highest T cell dose, developed lethal grade IV GVHD. Treatment-related mortality in the first 100 days was 3/10 (30%) with one death each due to idiopathic pneumonia syndrome, GVHD, and CMV disease. Two patients died after day 100, one due to relapse and one with systemic adenoviral infection. Overall survival is 5/10 patients (50%) with a median follow-up of 435 (range, 229-814) days.

CONCLUSION

This regimen is promising in high-risk patients lacking a suitable donor, and a prospective phase I/II study is ongoing.

Allogeneic peripheral blood stem cell graft composition affects early T-cell chimaerism and later clinical outcomes after non-myeloablative conditioning

We have studied the influence of cell subsets [CD34, CD3, CD4, CD8, CD14, CD20, natural killer (NK; CD3(-)/CD56(+)), NKT (CD3(+)/CD56(+)), DC1, and DC2 cells] of granulocyte colony-stimulating factor mobilized peripheral blood stem cells (PBSC) on early T-cell chimaerism and later clinical outcomes in 125 patients with haematological malignancies who received human leucocyte antigen (HLA)-matched related grafts after non-myeloablative conditioning. Conditioning consisted of 2 Gy total body irradiation (TBI) alone (n = 28), or 2 Gy TBI preceded by either 90 mg/m(2) fludarabine (n = 62) or planned autologous haematopoietic cell transplantation (HCT) (n = 35). Post-transplant immunosuppression included mycophenolate mofetil and ciclosporin. Multivariate analysis showed that higher numbers of grafted NK cells predicted higher early T-cell chimaerism (P = 0.03), while higher numbers of B cells were associated with better clinical outcomes and a higher risk for chronic graft-versus-host disease (P = 0.05). Higher numbers of CD14(+) cells were associated with worse overall survival (P = 0.03), while higher numbers of CD34(+) cells showed better survival (P = 0.03). The addition of fludarabine or autologous HCT predicted higher early T-cell chimaerism (P = 0.001), while advanced donor age predicted lower chimaerism (P < or = 0.02). Patients with aggressive diseases were at higher risk for relapse/disease progression, and shorter progression-free and overall survival (P < 0.01). These results suggest that the dosing of certain cellular subsets of PBSC products can influence important outcomes post-HCT after non-myeloablative conditioning.

Improved outcome in HLA-identical sibling hematopoietic stem-cell transplantation for acute myelogenous leukemia predicted by KIR and HLA genotypes

Inhibitory killer immunoglobulin (Ig)-like receptors (KIRs) recognize HLA-C and -B epitopes on target cells, thereby regulating natural killer (NK) cell activity. In 178 patients receiving T-cell-depleted HLA-identical sibling transplants for acute myelogenous leukemia (AML), chronic myelogenous leukemia (CML), acute lymphoblastic leukemia (ALL), or myelodysplastic syndrome (MDS), analysis of donor KIR genotype with HLA genotype demonstrated that 62.9% of the patients lacked an HLA ligand for donor-inhibitory KIR. Lack of HLA ligand for donor-inhibitory KIR (missing KIR ligand) had no effect on disease-free survival (DFS), overall survival (OS), or relapse in patients receiving transplants for CML and ALL. In patients with AML and MDS, however, there was a significant missing KIR ligand effect on DFS (P = .014; hazard ratio [HR], 0.53; 95% confidence interval [95% CI], 0.28-0.88) and OS (P = .03; HR, 0.53; 95% CI, 0.3-0.93). Incidence of relapse was also lower in patients with AML and MDS who lacked the HLA ligand for donor-inhibitory KIR (P = .04; HR, 0.41; 95% CI, 0.18-0.97). AML and MDS patients lacking 2 HLA ligands for donor-inhibitory KIR had the highest DFS (P = .002) and OS (P = .003). There was no significant contribution of donor-activating KIR to transplantation outcome in these patients. These data indicate that the absence of class I ligand in the recipient for donor-inhibitory KIR can be a prognostic factor for transplantation outcome in HLA-identical sibling transplantation and that the lack of HLA-C or -B ligands for donor-inhibitory KIR can contribute to improved outcomes for patients with AML and MDS.

Depletion of donor-reactive cells as a new concept for improvement of mismatched bone marrow engraftment using reduced-intensity conditioning

OBJECTIVE

New nonmyeloablative strategies to improve acceptance of mismatched bone marrow (BM) may compensate for the inadequate supply of compatible grafts. Recently we proposed to facilitate engraftment of mismatched BM by selective depletion of activated donor-reactive host cells with cyclophosphamide (CY). Here we have compared engraftment of allogeneic BM after depletion of antigen-activated host lymphocytes by CY, with BM engraftment following general immunosuppression by the same CY dose.

MATERIALS AND METHODS

Naive or mildly irradiated BALB/c mice were primed with C57BL/6 BM cells (day 0), treated with CY in order to deplete activated T cells (day 1), and transplanted with a second C57BL/6 BM inoculum (day 2) in order to achieve BM engraftment. Alternatively, mice received an equal dose of donor BM cells in a single injection one day after the same CY dose. Treated animals were repeatedly tested for persistence of donor cells in the blood.

RESULTS

Depletion of alloantigen-primed lymphocytes by 200 mg/kg CY provided stable GVHD-free engraftment of allogeneic BM in nonirradiated mice, while immunosuppressive treatment with the same CY dose alone resulted in BM rejection. Low-dose irradiation before priming with donor BM allowed the tolerance-inducing CY dose to be reduced to 100 mg/kg. Alloantigen-primed lymphocyte depletion (APLD) by a reduced CY dose resulted in engraftment of donor BM after a significantly lower irradiation dose than treatment with irradiation and CY alone.

CONCLUSION

Our results demonstrate that conditioning that focuses on APLD has a definite advantage over general immunosuppression with CY and radiation therapy.

Haploidentical transplantation for acute lymphoblastic leukemia in childhood

Haploidentical transplantation in childhood acute lymphoblastic leukemia (ALL) is a promising option for children lacking a suitable donor. We have updated our series of patients with ALL and report the results. Additionally, we reviewed the literature and try to embed our own experiences in the published results. We performed HLA-mismatched stem cell transplantations with megadoses of purified positively selected mobilized peripheral blood CD34+ progenitor cells (PBPC) from adult donors in 27 children with acute lymphoblastic leukemia (ALL) in first (CR1 n = 7), second (CR2 n = 10), or third (CR3 n = 4) complete remission, and in refractory state (NR n = 6). The patients received a mean number of 19.1+/-11.3 x 10(6)/kg purified CD34+ and a mean number of 15.5+/-24.2 x 10(3)/kg CD3+ T-cells. No additional graft-versus-host disease (GVHD) prophylaxis was used, except as short-term CSA in the first 3 patients. The myeloablative treatment was based on busulfan in 12 and on TBI in 14 patients. One patient was grafted with a non-myeloablative approach. Engraftment was rapid in 26 patients, with two patients suffering from a rejection. These two and one patient with initial non-engraftment had been successfully regrafted. The probability of survival of the total group is 0.34+/-0.09; the 12 patients transplanted in remission showed a probability of survival of 0.44+/-0.11. None of the patients transplanted in non-remission survived. There was no statistical difference in survival for patients with a 1, 2 or 3 antigen mismatched donor (out of 6 HLA antigens) or for patients in 1st, 2nd or 3rd remission. Causes of death were relapses in 10 patients, veno-occlusive disease (VOD) in 1, multi-organ failure (MOF) in 2 and infections in 4 patients. 3/24 evaluable patients without any additional GVHD-prophylaxis developed grade 1 or 2 GVHD. Ten patients were treated with additional donor lymphocyte infusion (DLI), from which 4 developed a maximum grade 3 GVHD. We conclude that the HLA barrier can be overcome by transplantation of megadoses of highly purified CD34+ PBPC and GVHD can effectively be prevented. This approach offers a promising treatment option for patients with acute lymphoblastic leukemia needing urgently transplantation but lacking a suitable donor.

Diagnostic chimerism analysis after allogeneic stem cell transplantation: new methods and markers

Analysis of chimerism after allogeneic hematopoietic cell transplantation is important for assessing engraftment and the early detection of graft failure. In addition, the monitoring of minimal residual disease and early detection of imminent relapse has also become an important issue. Novel transplant procedures, for example dose-reduced conditioning protocols, rely on chimerism analysis to guide intervention, i.e. the reduction of immunosuppression or infusion of donor lymphocytes. During the last 30 years, several methods for the analysis of chimerism after hematopoietic cell transplantation have been published. Currently, fluorescent in situ hybridization (XY-FISH) analysis of sex chromosomes after transplantation from a sex-mismatched donor or analysis of polymorphic DNA sequences, i.e. short tandem repeats (STR) or variable number of tandem repeats (VNTR), are the most widely used procedures used in the assessment of chimerism. Two major diagnostic fields can be defined for chimerism analysis: the period of engraftment and the detection of minimal residual disease. Although STR-PCR and FISH analysis are very useful in the diagnosis of engraftment and graft failure, they are only of limited use in the monitoring of minimal residual disease, largely because of its limited level of sensitivity (1-5% for the minor population). Several novel procedures to improve this level of detection have been reported in recent years. One focus has been the use of real-time PCR techniques based on analysis of the Y-chromosome or, more recently, single nucleotide polymorphism (SNPs). These procedures combine quantitative analysis with high sensitivity (10(-4) to 10(-6)), and hold great potential for the future. In addition, the combination of cell sorting based on leukemia-specific immunophenotype and STR-PCR has been successfully used for minimal residual disease detection. First clinical data using these procedures indicate that intervention (e.g. the reduction of immunosuppression or donor lymphocyte infusion) may be effective in the minimal residual disease situation, even in high risk diseases like acute myeloid leukemia and acute lymphoblastic leukemia. The optimal timing of these diagnostic interventions is a critical issue and has to be further optimized. Whether this will ultimately improve the survival of patients with leukemia after transplantation has to be shown in prospective studies.

Transplantation of hematopoietic stem cells for induction of unresponsiveness to organ allografts

Although it has been recognized since the early days of Owen and Medawar that engraftment of donor stem cells, induced in utero spontaneously or intentionally neonatally, results in life-long unresponsiveness to donor alloantigens. However, successful induction of transplantation tolerance in adult life still represents an unsolved problem. Engraftment of donor stem cells using conventional modalities involves intensive myeloablative or lymphoablative immunosuppression, which is associated with toxicity and mortality and such methods are not suitable for organ allograft recipients. In this chapter, we present an innovative approach for induction of donor-specific unresponsiveness to bone marrow and organ allografts without myeloablative conditioning. Our methods is based on cyclophosphamide-induced, alloantigen-primed lymphocyte depletion. Cyclophosphamide is administered 1 day following infusion of donor hematopoietic cells, thus eliminating predominantly host T lymphocytes reacting against donor cell challenge, and resulting in relative unresponsiveness to donor alloantigens. Subsequently, life-long tolerance to fully mismatched donor skin allografts can be accomplished by a second infusion of stem cells from the same donor, with donor T cells displacing residual alloreactive host cells that may have escaped deletion. Taken together, we believe that induction of true permanent and specific tolerance to organ allografts using donor hematopoietic cells could become a clinical reality in the foreseeable future.

Lineage-specific chimaerism after stem cell transplantation in children following reduced intensity conditioning: potential predictive value of NK cell chimaerism for late graft rejection

Chimaerism of FACS-sorted leucocyte subsets (CD14+, CD15+, CD3-/56+, CD3+/4+, CD3+/8+, CD19+) was monitored prospectively between days +14 and +100 in 39 children undergoing allogeneic stem cell transplantation with reduced intensity-conditioning regimens. Cell subsets exceeding 1% of nucleated cells were subject to cell sorting. Chimaerism was analysed by dual-colour FISH and/or by short tandem repeat-polymerase chain reaction. The chimaerism pattern on day +28 was evaluated with regard to its correlation with graft rejection. Of 39 patients, nine patients had donor chimaerism (DC) in all subsets. Mixed/recipient chimaerism (MC/RC) was detectable within T cells in 62%, within NK cells in 39% and within monocytes and granulocytes in 38% of the patients. The correlation of secondary graft rejection with the chimaerism pattern on day +28 revealed the strongest association between RC in NK-cells (P<0.0001), followed by T cells (P=0.001), and granulocytes and monocytes (P=0.034). Notably, patients with RC in T cells rejected their graft only if MC or RC was also present in the NK-cell subset. By contrast, none of the children with DC in NK cells experienced a graft rejection. These observations suggest that, in the presence of recipient T-cell chimaerism, the chimaerism status in NK-cells on day +28 might be able to identify patients at high risk for late graft rejection.

Dog leukocyte antigen nonidentical unrelated canine marrow grafts: enhancement of engraftment by CD4 and CD8 T cells

BACKGROUND

Previous studies have demonstrated that most marrow grafts from dog leukocyte antigen (DLA)-mismatched unrelated donors were rejected after 9.2 Gy total body irradiation (TBI), and that graft resistance could be overcome by infusing viable peripheral blood mononuclear cells (PBMCs) in addition to marrow.

METHODS

To investigate the donor cell populations that facilitate engraftment, we determined the minimal dose of PBMCs required to ensure stable engraftment. Nineteen dogs underwent transplantation with DLA-mismatched unrelated marrow and PBMCs in a dose de-escalation study. In subsequent studies, 12 dogs were given selected CD4 or CD8 cells in addition to marrow.

RESULTS

When 3 x 108 PBMC/kg were given in addition to a median of 4 x 108 marrow cells/kg, five of six animals engrafted. At a dose of 1 x 108 PBMC/kg, four of eight animals engrafted, and none of five dogs engrafted at a dose of 3 x 107 PBMC/kg. Accordingly, 12 dogs were given 9.2 Gy TBI, marrow grafts from DLA-mismatched unrelated dogs, and a median of 5.2 x 107 selected CD8 cells/kg or 10.4 x 107 selected CD4 cells/kg corresponding to the number of CD8 or CD4 cells contained in 3 x 108 PBMC/kg. Five of six dogs given CD8 cells and five of six dogs given CD4 cells engrafted.

CONCLUSION

Results indicate that at least 3 x 108 unmodified PBMC/kg are needed for stable engraftment of DLA-mismatched unrelated marrow, and that both CD4 and CD8 cell subpopulations are capable of facilitating engraftment.

Stable molecular remission induced by imatinib mesylate (STI571) in a patient with CML lymphoid blast crisis relapsing after allogeneic stem cell transplantation

We report the response to the ABL kinase inhibitor imatinib mesylate (STI571) in a patient with chronic myeloid leukemia (CML) who relapsed twice after dose-reduced allogeneic stem cell transplantation (alloSCT) for B lymphoid blast crisis (BC) and failed to develop an antileukemic response despite grade 3 graft-versus-host disease (GvHD). Complete hematologic, cytogenetic and molecular responses were achieved within 9 weeks of therapy and are maintained after 27 months. Extensive chronic skin GvHD necessitating immunosuppressive therapy developed after 14 months. This case illustrates the ability of imatinib to induce sustained hematologic and molecular remissions in some patients relapsing with advanced stage CML after alloSCT.

Antibody and pre- plus post-transplant prednisone treatments support T cell-depleted stem cell engraftment without drug-induced morbidity

Rigorous T cell depletion methods can now be used to reduce the risk of graft-versus-host disease (GVHD) associated with allogeneic, hematopoietic stem cell transplantation (HSCT). However, full T cell depletion is also associated with a significant risk of graft failure. Here we hypothesize that engraftment failures after T cell-depleted HSCT may be due, in part, to the absence of GVHD prophylaxis. To test this hypothesis, we used a haploidentical mouse model to systematically measure the effects of immunosuppressive drug treatments and anti-T cell antibodies on engraftment. Results showed that engraftment was supported in all animals when hosts were pre-treated with anti-T cell antibodies, but donor chimerism was significantly improved when hosts were also treated with prednisone. Interestingly, when hosts received only pre-HSCT prednisone treatments, engraftment was not improved; when hosts received only post-HSCT prednisone (initiated near the time of irradiation), the animals became extremely ill. Results therefore demonstrated the need for both pre- and post-HSCT prednisone treatments as a means to ensure engraftment without morbidity in all host animals.

Immunological aspects of haploidentical stem cell transplantation in children

Thirty-eight children with high-risk hematological malignancies underwent transplantation with megadoses of peripheral mobilized CD34+ cells from haploidentical parents (n = 24) or from matched unrelated donors (n = 14). The CD34+ cells were isolated to a purity of > 98% using magnetic-activated cell sorting. This high purity was associated with an almost complete depletion of T lymphocytes. No pharmacological prophylaxis for graft-versus-host disease (GvHD) was used, and significant primary GvHD was not seen. A final engraftment was seen in all patients. Sixteen patients are alive and disease-free with a median follow-up of 24 months. The immunological reconstitution was faster in the patients transplanted with CD34+ stem cells from the haploidentical donors compared to the matched unrelated donors, and the transplantation of large numbers of haploidentical CD34+ stem cells seems to be superior to that of the matched unrelated donors. The phenotypical and functional analysis of the immune reconstitution provided some insights into the biology of transplantation of highly purified CD34+ cells. In this article, we summarize our current results with the transplantation of highly purified stem cells and discuss possible implications for further antileukemic post-transplant therapeutic strategies.