Safety and efficacy of donor lymphocyte infusions following mismatched stem cell transplantation.

The use of a mismatched allograft necessitates T cell depletion for prevention of uncontrolled graft-versus-host disease (GVHD), thus impairing a graft-versus-leukemia effect. Data on donor lymphocyte infusion (DLI) after mismatched stem cell transplantation are lacking. Our experience with 28 patients (treated with 59 mismatched DLIs; range, 1-7) is described. The procedure was prophylactic in 6 patients (9 DLIs) and therapeutic in 22 (50 DLIs). DLI dose ranged from 10(2) to 1.5 x 10(9) T cells/kg. In the 6 patients receiving prophylactic DLI, complete remission was maintained in 5; however, 2 died from GVHD. Clinical response to therapeutic DLI was seen in 6 of 22 (27.3%) patients; a greater tumor burden produced a lower response. GVHD appeared in 13 of 28 patients. Surprisingly, a greater HLA mismatch was associated with a lower risk of GVHD, with 3 of 19 DLIs in 3/6 matching and 16 of 29 DLIs in 5/6 matching with similar follow-up. Nevertheless, no correlation between efficacy and HLA mismatching was noted. Death was frequent and usually related to the basic disease rather than to DLI complications. We conclude that mismatched DLI is feasible and may be effective, especially if given soon after transplantation. Future developments using cell therapy with selective or targeted anticancer activity are warranted, with special attention to prophylactic treatment of T cell depleted mismatched allografts recipients.     

Graft-versus-host reactions and the effectiveness of donor lymphocyte infusions.

We retrospectively analyzed 83 consecutive recipients of donor lymphocyte infusions (DLI) after allogeneic transplantation for factors associated with disease response and graft-versus-host disease (GVHD). DLI was highly effective in relapsed chronic phase chronic myeloid leukemia (CML), with 71% of patients achieving durable complete remissions (CR). In relapsed acute myeloid leukemia, DLI led to durable CRs in 31% of patients; the rate was <20% in all other diseases. Achieving full donor chimerism and GVHD were predictive of CR. Grade II or higher acute or chronic GVHD occurred in 36 (43%) patients and contributed to death in 13 (16%). Even more patients, 33 (40%), died of their underlying malignancy, including 10 who developed active GVHD. In relapsed CML, most durable CRs occurred without clinically apparent GVHD, yet all responders achieved full donor chimerism, including 6 with coincident normal host hematopoiesis at the time of DLI. Thus, in CML, potent lymphohematopoietic graft-versus-host reactions occurred even in the absence of clinically apparent GVHD; this confirms the ability to dissociate these processes and argues against a leukemia-specific immunologic effect. DLI clearly has efficacy in the treatment of relapsed disease after allogeneic transplantation. However, with the exception of CML, most patients die of their underlying disease because of insufficient antitumor activity even with active GVHD.     

Randomized trial of CD8+ T-cell depletion in the prevention of graft-versus-host disease associated with donor lymphocyte infusion.

The application of DLI is limited by the potential development of GVHD. Results of single-arm trials suggest that CD8+ depletion of DLI may reduce the incidence of GVHD while still inducing pathologic and cytogenetic remissions. To test the impact of CD8 depletion on GVHD, we initiated a randomized trial comparing outcome among patients receiving unselected donor lymphocytes or CD8+-depleted cells. DLI was administered to patients with disease in remission to prevent relapse 6 months after T-cell-depleted allogeneic BMT. CD8 depletion was performed with monoclonal antibody and rabbit complement. Donor lymphocytes obtained from the original donor were infused fresh without cryopreservation. Infusions were adjusted so that all patients received 1.0 x 10(7) CD4+ cells/kg. Patients randomized to CD8 depletion received a median of 0.7 x 10(5) versus 32.0 x 10(5) CD8+ cells/kg in the unmanipulated cohort. Six (67%) of 9 patients receiving unselected DLI developed acute GVHD compared with 0 (0%) of 9 recipients of CD8-depleted DLI (P = .009). In the unselected group, 2 patients died while the disease was in remission, and 3 patients had relapses. In the CD8-depleted cohort, there were no toxic deaths and only 1 relapse. Measures of immunologic reconstitution by T-cell receptor excision circle analysis and T-cell receptor spectratyping demonstrated similar patterns of T-cell recovery in both the CD8-depleted and the unselected cohorts. Both groups converted from mixed to full donor hematopoietic chimerism after DLI. Our results indicate that CD8 depletion reduces the incidence of GVHD associated with DLI without adversely affecting conversion to donor hematopoiesis or immunologic recovery.     

Leucyl-leucine methyl ester-treated haploidentical donor lymphocyte infusions can mediate graft-versus-leukemia activity with minimal graft-versus-host disease risk.

L-leucyl-L-leucine methyl ester (LLME) prevents GVHD in several animal models by depleting dipeptidyl peptidase I (DPPI)-expressing cytotoxic cellular subsets. However, clinical application has been hampered by difficulties in stem cell engraftment following treatment of donor bone marrow inocula with LLME at the concentrations necessary to purge DPPI-expressing T-cells. Noting that T-cells can mediate graft-versus-leukemia (GVL) responses via both perforin (usually co-expressed in cytotoxic granules with DPPI) and Fas ligand pathways in a murine model, we hypothesized that LLME might be useful for treatment of delayed DLIs for potential GVL activity with a decreased risk of GVHD induction. In regard to the clinical setting, the ex vivo use of LLME for this purpose would circumvent any toxicity issues for donor stem cells, because by that time patients would have already achieved successful engraftment. For our preclinical studies, we used the haploidentical C57BL/6 (B6) (H2b) --> ((B6 x DBA/2)F1 (H(2b/d)) murine model with lethally irradiated hosts that had received transplants of T-cell-depleted bone marrow cells and were challenged with the MMD2-8 myeloid leukemia line (H2d) of DBA/2 origin. A DLI of LLME-treated donor splenocytes, from B6 mice presensitized to recipient alloantigens, was administered in varying doses 14 days post-marrow transplantation, and the potential for both GVHD and GVL activity was assessed. All mice that received any dose of LLME-treated DLI survived indefinitely, without evidence of cachexia nor B-cell hypoplasia, in contrast to the severe and lethal GVHD induced by mock-treated DLI. Histological analysis largely correlated with the symptomatic findings and revealed no GVHD-like lesions in the spleens of LLME-treated DLI recipients, although some mice displayed various degrees of hepatic mononuclear infiltration. Most notably, mice given LLME-treated DLI also experienced DLI dose-dependent increases in survival against the challenge with the MMD2-8 leukemia. LLME-treated splenocytes remained immunocompetent, as these cells could proliferate in response to mitogens and to restimulation with ovalbumin when used as a recall antigen. In conclusion, LLME-treated DLI possesses immune potential and, in particular, GVL activity without inducing clinically evident GVHD.     

Conversion to full donor chimerism following donor lymphocyte infusion is associated with disease response in patients with multiple myeloma.

Donor lymphocyte infusions (DLIs) have been demonstrated to induce clinical responses in patients with relapsed multiple myeloma after allogeneic bone marrow transplantation, but the immunologic mechanisms involved have not been well characterized. In patients with chronic myelocytic leukemia (CML), remissions following DLI are invariably associated with conversion to complete donor hematopoiesis, suggesting that the target antigens of this response are expressed on both normal and CML-derived hematopoietic stem cells. In the present study, we examined hematopoietic chimerism and the complexity of the T-cell receptor (TCR) repertoire in 4 patients with relapsed multiple myeloma who received infusions of donor CD4+ lymphocytes. Three of 4 patients had a clinical response that began 1 to 2 months after DLI. All 3 responding patients developed lymphocytosis at the initiation of response that was due to a 2- to 4.5-fold increase in the number of CD3+ T cells. In 1 patient, this was due primarily to increases in CD3+ and CD8+ cells; in 2 patients, to increased numbers of CD3+ and CD8+ and CD3+ and CD4+ T cells. In all responding patients, conversion to complete donor hematopoiesis occurred in the first 2 months after DLI. The single nonresponding patient remained it 100% recipient hematopoiesis. The TCR repertoire complexity was examined by polymerase chain reaction amplification of complementary-determining region 3 (CDR3) derived from 24 Vbeta gene subfamilies. In 2 patients, the initiation of myeloma response and conversion to complete donor hematopoiesis was associated with normalization of TCR complexity. Complete donor chimerism and normal TCR complexity remained stable in all patients and did not change with subsequent relapse or development of graft-versus-host disease (GVHD). Thus, conversion to full donor chimerism was temporally associated with the antimyeloma effect of DLI but not with the development of GVHD. Nevertheless, the maintenance of stable donor hematopoiesis did not prevent disease relapse and was not associated with prolonged remission. The selective relapse of myeloma cells without concomitant return of mixed hematopoietic chimerism suggests that myeloma tumor cells in some patients develop resistance to immune destruction.     

Reduced-intensity conditioning stem cell transplantation: comparison of double umbilical cord blood and unrelated donor grafts

There are little data comparing umbilical cord blood (UBC) and conventional stem cell sources for reduced-intensity conditioning (RIC) hematopoietic stem cell transplantation (HSCT). We performed a retrospective analysis of RIC HCST using double UCB (dUCB) grafts and RIC HSCT using unrelated donor (URD) grafts. The study included 64 dUCB transplantations and 221 URD transplantations performed at Dana-Farber Cancer Institute and Massachusetts General Hospital between 2004 and 2008. The cumulative incidence of grade II-IV acute graft-versus-host disease (GVHD) was 14.1% for dUCB and 20.3% for URD (P = .32). The 2-year cumulative incidence of chronic GVHD was significantly lower in dUCB compared with URD (21.9% versus 53.9%; P < .0001). The 2-year cumulative incidence of nonrelapse mortality was significantly higher in dUCB (26.9% versus 10.4%; P = .0009). In our analysis, dUCB HSCT and URD HSCT had comparable 3-year overall survival (46% in dUCB and 50% in URD; P = .49) and progression-free survival (30% in dUCB and 40% in URD; P = .47). dUCBT was associated with greater nonrelapse mortality despite less chronic GVHD. Our findings suggest that the use of 2 partially matched UCB units appears to be a suitable alternative for patients undergoing RIC HSCT without an HLA-matched donor.     

The effect of unrelated donor marrow transplantation on health-related quality of life: a report of the unrelated donor marrow transplantation trial (T-cell depletion trial).

The primary objective of this study was to compare health-related quality of life (HRQL) in adult patients undergoing either ex vivo T cell-depleted bone marrow transplantation or conventional marrow transplantation. Data on patients' HRQL were gathered as part of a multicenter randomized trial comparing the effect of ex vivo T-cell depletion versus methotrexate and cyclosporine immunosuppression on disease-free survival. HRQL assessments were conducted at baseline, day +100, 6 months, 1 year, and 3 years. There were no treatment arm differences 1 year after transplantation on the Functional Assessment of Cancer Therapy, Bone Marrow Transplantation, the Medical Outcomes Study Short-Form 36, and the Centers for Epidemiological Studies of Depression. The lack of treatment differences was robust across types of data analyses that took baseline functioning into account and that recognized the sensitivity of outcome measures to assumptions concerning missing data. The trajectory of recovery revealed an initial decrease in function and then a recovery to pretreatment levels that were similar for both treatment arms. Furthermore, the patients in both treatment groups returned to a functional level that approximated general US population norms. Even though the incidence of acute graft-versus-host disease was slightly higher in the conventional treatment arm, T-cell depletion did not differentially affect HRQL at 1 year after transplantation.     

The role of disease stage in the response to donor lymphocyte infusions as treatment for leukemic relapse.

Between 1991 and 1999, 44 leukemic patients received donor lymphocyte infusions (DLIs) at our center (22 patients with chronic myelogenous leukemia [CML]; 10 with acute myelogenous leukemia; 11 with acute lymphatic leukemia; and 1 with myelodysplastic syndrome). Seventeen patients received graft-versus-host disease (GVHD) prophylaxis with methotrexate (MTX) at the time of DLI. In CML patients, 15 of 22 (68%) re-entered complete remission after DLI. At 3 years post-DLI, patients with cytogenetic (n = 10) or molecular (n = 3) relapse had a current leukemia-free survival (cLFS) rate of 85% compared with 0% for patients with hematologic relapse (P < .001). Among 15 CML patients who initially responded to DLI, 4 patients relapsed within the first 2 years. Four of 16 patients (25%) with acute leukemia had an initial response with complete remission after DLI. Two of them subsequently relapsed within 1 year. Patients with acute leukemia who relapsed within 1 year of hematopoietic stem cell transplantation (n = 9) had 0% cLFS at 18 months; patients with later relapse had 29% cLFS (P = .015). The overall probability of cLFS at 3 years for CML patients was 46%. For other diseases, cLFS was 13% at 18 months after DLI. Patients who developed chronic GVHD secondary to DLI showed a 3-year cLFS of 51% compared with 18% for patients without chronic GVHD (P = .022). This study emphasizes the importance of early disease stage and presence of chronic GVHD for effective DLI.     

Long-Term follow-up of recipients of CD8-depleted donor lymphocyte infusions for the treatment of chronic myelogenous leukemia relapsing after allogeneic progenitor cell transplantation.

Donor lymphocyte infusions (DLIs) are an effective treatment for relapsed Chronic myeloid leukemia (CML) after allogeneic transplantation but are limited by the occurrence of GVHD. CD8+ T lymphocytes are involved in the pathogenesis of GVHD but may not be essential for the graft-versus-leukemia (GVL) effect in CML. We have treated 26 CML patients with posttransplantation relapse with CD8-depleted DLI. Thirteen of 15 patients (87%) who relapsed in early-phase CML achieved complete cytogenetic response, but only 1 of 11 who relapsed in advanced-phase disease achieved complete response. Acute GVHD occurred in 2 patients (8%), and extensive chronic GVHD occurred in 2 patients (11%). Treatment-related mortality was 11.5%. Responses were durable; with a median follow-up of 4.2 years (1-7.5 years), only 1 responding patient relapsed (7%). CD8-depleted DLI was equally effective and safe after unrelated donor transplants and sibling transplants. Cytogenetic clonal evolution at the time of DLI was not predictive of treatment failure unless associated with hematologic criteria for disease acceleration. CD8 depletion is an effective method to separate GVL from GVHD for posttransplantation relapsed CML. This strategy is associated with durable complete remissions and a low rate of complications and therefore merits further investigation in larger-scale comparative trials.     

Effect of in vivo T-cell depletion on the effector T-cell function of immunity to Eimeria falciformis.

BALB/c mice were exposed to the enteric parasite Eimeria falciformis to produce a natural acquired immunity. The mice were then depleted of their effector T-cell function by in vivo administration of a cytotoxic Thy-1.2 mouse monoclonal antibody. T-cell depletion was demonstrated by a reduction in concanavalin A-induced proliferation of splenic lymphocytes in treated mice compared with that in controls. Twenty-four hours following T-cell depletion, the mice were challenged with 5,000 oocysts of E. falciformis. Daily total oocyst counts were done for each mouse from days 6 to 21 following challenge. Our studies demonstrated that depleting mice of their effector T-cell function following establishment of immunity caused an abrogation of protective immunity to this parasite.     

Reduced-intensity stem cell transplantation in children and adolescents: the Mexican experience.

A group of 21 consecutive patients aged 4-20 (median 13) years was prospectively allografted using a reduced intensity preparative regimen. The group included both malignant (acute lymphoblastic leukemia, acute myelogenous leukemia, and chronic myelogenous leukemia) and nonmalignant (aplastic anemia, Diamond-Blackfan anemia, thalassemia major and adrenoleukodystrophy) conditions. Follow-up times ranged between 16 and 1038 days. Four of 21 patients (9.5%) developed acute graft-versus-host disease, and 2 of them died, whereas limited chronic graft-versus-host disease was observed in 2 of 15 cases. The 100-day mortality was 19%. Median overall survival was above 1038 days, whereas the 34-month survival was 55%. These data show that reduced intensity stem cell transplantation in children permits rapid engraftment from siblings with little toxicity.     

Effects of in vivo T lymphocyte subset depletion on mycobacterial infections in mice.

The relative importance of CD4+ and CD8+ T cell subsets in the expression of acquired resistance to systemic infection by Mycobacterium kansasii was determined. T cell subsets were depleted in thymectomized C57BL/6 mice by the intravenous administration of monoclonal antibodies directed against the relevant T cell determinants. Depletion of the CD4+ subset exacerbated the severity of the infection in intravenously challenged mice. This effect was apparent in the first 2 weeks of the infection and persisted throughout the 12 weeks of the study. On the other hand, depletion of the CD8+ cells had no apparent effect on the growth curves. Infections by Mycobacterium tuberculosis Erdman or bacille Calmette-Guérin (BCG) Pasteur were also substantially enhanced by CD4 depletion, but not by the depletion of CD8+ cells. The effect of subset depletion on infections by M. tuberculosis and BCG was examined in both innately susceptible C57BL/6 mice and innately resistant B6D2 mice.     

Increased adjuvant efficacy in stimulation of antibody responses after macrophage elimination in vivo.

To investigate whether macrophages influence the efficacy of adjuvants, we locally eliminated lymph node macrophages with dichloromethylene diphosphonate containing-liposomes before primary immunization. After macrophage elimination, animals were immunized with a soluble antigen (TNP-KLH; 2,4,6-trinitrophenyl-keyhole limpet haemocyanin) either in phosphate-buffered saline (PBS), in complete Freund's adjuvant (CFA), or in Specol is a water-in-oil emulsion, considered to be less aggressive than CFA, but equally effective. A secondary immunization followed with TNP-KLH. In vivo macrophage elimination before Specol/TNP-KLH immunization resulted in increased adjuvant efficacy as measured by (primary) antibody responses. This suggests that the activity of Specol is not primarily mediated through macrophages but rather through other antigen-presenting cell types (e.g. dendritic cells, B cells, fibroblasts). The overall quality of produced antibodies, in terms of isotype distribution and affinity maturation, remained the same. After primary injection, CFA/TNP-KLH immunization resulted in significantly higher antibody responses in macrophage-depleted animals and antibody responses did not increase significantly after secondary immunization. However, a booster effect was found when macrophages were not eliminated before CFA/TNP-KLH immunization, suggesting that the presence of macrophages during the first weeks of the primary immune response is essential for the induction of an effective secondary response in CFA immunizations. In conclusion, macrophage depletion before immunization with a soluble T-cell-dependent antigen combined with Specol may enhance specific antibody responses without changing the isotype or affinity of the antibodies.     

Rapid loss of feedback suppressor T-cell activity after priming in vivo.

T cells from antigen-primed mice have a diminished capacity to mediate feedback suppression when compared to T cells from unprimed mice. This was demonstrated using an in vitro model of B-cell induced feedback suppression in which spleen cells from mice primed with sheep erythrocytes (SRBC) activate feedback suppressor T-cell precursors to mediate suppression of the primed spleen cell response. The addition of splenic T cells from unprimed mice to cultures of spleen cells from SRBC-primed mice resulted in suppression of the secondary IgM and IgG anti-SRBC response. In contrast, no suppression was detected when T cells from mice primed with SRBC were added to the primed spleen cell cultures. The loss of suppression by T cells from primed mice was antigen-specific and was detectable by 24 hr after priming, coinciding with the appearance after priming of T-cell enhancing activity. The reduced suppressive activity could be due to changes in the active T-cell subset itself or to the appearance of cells or factors within the T-cell population that block or mask detection of feedback suppression. In either case, the present finding suggests that priming of a host not only activates feedback suppression induction mechanisms, but also rapidly affects the ability of the T-cell population to develop effective feedback suppression.     

C3- and T-cell-dependent adjuvant activity of in vivo formed immune complexes.

The effects of polyclonal antibodies to mouse serum components on the primary humoral immune response of mice in vivo were studied. It was observed that rabbit IgG to complement component C3 and albumin and mouse IgG to C5, but also heat-aggregated non-immune rabbit IgG, enhanced the agglutinating antibody response to sheep erythrocytes (SRBC). Since the increase in response was only observed when antigen and antibodies were administered via the same route (i.p.), immunological adjuvant activity was implicated. Ineffectiveness of anti-C5 IgG in C5-deficient mice indicated that the antibody-induced adjuvant activity is mediated by in vivo formed immune complexes (IC). The adjuvant activity of IC was reduced by selective C3-depletion of animals, pointing to a requirement of C3. The effect of variations in other parameters was studied with anti-C3 and anti-C5 IgG as immunoadjuvant. The immunostimulatory effect was most pronounced when the antibodies were administered simultaneously with or shortly before antigen. Treatment of animals with antibodies one or two days before antigen, however, resulted in a suppression of the response. The response to thymus-independent antigens was not enhanced by anti-C3 nor by anti-C5 IgG. Optimal adjuvant activity of anti-C3 IgG was observed at low antigen doses. Nude mice were insensitive to the immunopotentiating effect of anti-C3 and so was the F1 progeny of BALB/c male and CBA/N female mice expressing a B-cell maturation defect. C5 deficiency and lipopolysaccharide (LPS) non-responsiveness did not affect the adjuvant activity of in vivo formed C3-anti-C3 IC.     

Reduced-intensity allogeneic stem cell transplantation in adults and children with malignant and nonmalignant diseases: end of the beginning and future challenges.

During the last 10 years, multiple studies using reduced-intensity (RI) conditioning followed by allogeneic stem cell transplantation (AlloSCT) have been reported in adult and, less so, pediatric recipients. RI AlloSCT allegedly eradicates malignant cells through a graft-versus-leukemia/graft-versus-tumor effect provided by alloreactive donor T lymphocytes, natural killer cells, or both. Various studies have clearly demonstrated a graft-versus-leukemia/graft-versus-tumor effect in hematologic malignancies and solid tumors. Acute short-term toxicity, including infection and organ decompensation after myeloablative conditioning therapy, can result in a significant incidence of early transplant-related mortality. More importantly, long-term late effects-including growth retardation, infertility, and secondary malignancies-are major complications after myeloablative conditioning therapy, especially in vulnerable children, who are more susceptible to these complications. Recent results comparing RI conditioning with myeloablative conditioning followed by HLA-matched sibling AlloSCT have demonstrated a significant reduction in use of blood products, risk of infections, transplant-related mortality, length of hospitalization, and feasibility of conditioning therapy in outpatient settings. Despite the success of RI AlloSCT, large prospective randomized multicenter studies are necessary to define the appropriate patient population, optimal conditioning regimens and pretransplantation immunosuppression, role of donor lymphocyte infusions, duration of hospitalization, overall survival, cost-benefit ratio, and differences in long-term effects to evaluate the role of RI AlloSCT more fully. We review the recent experience of RI AlloSCT in adults and children with both malignant and nonmalignant diseases and discuss the challenges for the future.     

The efficacy of high-dose melphalan with autologous peripheral blood stem cell transplantation in patients with multiple myeloma

Although high-dose therapy (HDT) with autologous hematopoietic stem cell transplantation (ASCT) is widely accepted as an effective and safe consolidation therapy for multiple myeloma (MM), few reports on its efficacy are available in Korea. We present the results of a prospective phase II study, involving 33 patients with MM treated with HDT with ASCT. The treatment consisted of 4 courses of VAD (vincristine, adriamycin, dexamethasone) induction, peripheral blood stem cell collection, and high-dose melphalan with stem cell infusion. The overall response rate was 93%, with 45% of patients having complete responses. The toxicity was predictable and tolerable. With a median follow-up of 27.6 months, the 2-year event free survival rate was 43%. At the time of writing, the median overall survival duration had not been reached with 2-year survival and projected 3-year survival rates of 81% and 74%, respectively. The overall survival was significantly better than that of the historical control patients (N=82) treated with conventional chemotherapy at our institution. The results suggest that HDT with ASCT is a valuable first or second-line treatment for patients with MM.     

Chronic graft-versus-host disease after granulocyte colony-stimulating factor-mobilized allogeneic stem cell transplantation: the role of donor T-cell dose and differentiation.

The use of granulocyte colony-stimulating factor (G-CSF)-mobilized peripheral blood as a source of stem cells has resulted in a high incidence of severe chronic graft-versus-host disease (cGVHD), which compromises the outcome of clinical allogeneic stem cell transplantation. We have studied the effect of G-CSF on both immune complex and fibrotic cGVHD directed to major (DBA/2 --> B6D2F1) or minor (B10.D2 --> BALB/c) histocompatibility antigens. In both models, donor pretreatment with G-CSF reduced cGVHD mortality in association with type 2 differentiation. However, after escalation of the donor T-cell dose, scleroderma occurred in 90% of the recipients of grafts from G-CSF-treated donors. In contrast, only 11% of the recipients of control grafts developed scleroderma, and the severity of hepatic cGVHD was also reduced. Mixing studies confirmed that in the presence of high donor T-cell doses, the severity of scleroderma was determined by the non-T-cell fraction of grafts from G-CSF-treated donors. These data confirm that the induction of cGVHD after donor treatment with G-CSF is dependent on the transfer of large numbers of donor T cells in conjunction with a putatively expanded myeloid lineage, providing a further rationale for the limitation of cell dose in allogeneic stem cell transplantation.