Lymphoblastic Leukemia: The Emerging Role of
Peritransplantation Minimal Residual Disease/
Chimerism Monitoring and Novel Chemotherapeutic,
Molecular, and Immune Approaches Aimed at
Michael A. Pulsipher,1
Laurence J. N. Cooper3 Although improved donor sources and supportive care have decreased transplantation-related mortality over the past decade, relapse remains the principal cause of failure after allogeneic transplantation for high-risk pediatric acute lymphoblastic leukemia (ALL). Emerging tools of minimal residual disease (MRD) and chimerism monitoring before and after transplantation have defined those children at highest risk for relapse and provide the opportunity for intervention to prevent relapse. Specific methods aimed at decreas-ing relapse include the use of intensive treatment before transplantation to increase the percentage of pa-tients undergoing the procedure with negative MRD, optimal transplantation preparative regimens, and posttransplantation interventions with targeted or immunologic therapy. Early data demonstrate decreased relapse with the use of sirolimus for all types of ALL and imatinib for ALL with the Philadelphia chromosome (Ph1ALL) after transplantation. Patients with increasing chimerism or MRD have been shown to benefit from early withdrawal of immune suppression or donor lymphocyte infusion. Finally, various targeted immu-nologic therapies, including monoclonal antibodies, killer cell immunoglobulin-like receptor mismatching, natural killer cell therapy, and targeted T cell therapies, are emerging that also could have an affect on relapse and improve survival after transplantation for pediatric ALL.
Biol Blood Marrow Transplant 15: 62-71 (2009) Ó 2009 American Society for Blood and Marrow Transplantation KEY WORDS: Pediatric Allogeneic Transplantation, Acute Lymphoblastic Leukemia, Minimal Residual Disease, Immunotherapy
Intensive, risk-based chemotherapy approaches cure more than 80% of children with acute lymophoblastic leukemia (ALL)[1-3]. Studies have identified several cy-togenetic and response-based risk factors associated with poor outcome that can be defined at presentation
or after relapse. Although allogeneic transplantation has been shown to benefit many children with these risk factors, relapse rates after such transplantation remain high, especially for the highest-risk disease states (eg, very early relapse, CR31) [5,6]. Recently, the persis-tence of minimal residual disease (MRD), measured by flow cytometry or molecular methods, has been used to further identify poor outcomes from chemotherapy [7-9]. Preliminary studies have suggested that a similar link exists between outcome and MRD values before transplantation[10,11]; larger studies should more fully illuminate the predictive power of pretransplantation MRD in the next few years. These tools have spurred in-vestigation into the use of approaches before and after transplantation to achieve and sustain MRD negativity. Such approaches include intensive chemotherapeutic blocks using promising new agents before transplanta-tion and such therapies as imatinib and sirolimus after transplantation. Other approaches include withdrawal of immune suppression and donor lymphocyte infusion
From the1Division of Hematology/BMT, Primary Children’s
Med-ical Center, University of Utah School of Medicine, Salt Lake City, Utah;2Department of Stem Cell Transplantation,
Chil-dren’s Hospital of the J.W. Goethe University, Frankfurt, Ger-many; and 3Division of Pediatrics, MD Anderson Cancer
Center, Children’s Cancer Hospital, Houston, TX. Financial disclosure: See Acknowledgments on page 69.
Correspondence and reprint requests: Michael Pulsipher, MD, Huntsman Cancer Institute, University of Utah/Primary Chil-dren’s Medical Center, 50 North Medical Drive, Salt Lake City, UT 84132 (e-mail:firstname.lastname@example.org). 1083-8791/09/151S-0001$36.00/0
(DLI) in selected patients with MRD or persistent chimerism and immune-based treatments, such as anti-body and cellular therapies. Emerging evidence suggests that improved outcomes after transplantation for pediat-ric ALL will be seen in the next few years as clinicians (1) define very-high-risk patients early and perform transplantation when performance scores are high, (2) minimize disease burden before transplantation with cytotoxic and targeted agents, (3) use total body ir-radiation (TBI)-based regimens that decrease the risk of relapse for the patients at greatest risk, and (4) closely monitor patients with MRD and/or chimerism after transplantation, treating as necessary to minimize the risk of relapse.
INDICATIONS FOR ALLOGENEIC
TRANSPLANTATION IN PEDIATRIC ACUTE LYMOPHOBLASTIC LEUKEMIA
Indications for transplantation for pediatric ALL have varied through the years, based on improvements in chemotherapy and outcomes for different risk groups. Although international groups vary somewhat, the Children’s Oncology Group (COG) Acute Lym-phocytic Leukemia and Hematopoietic Stem Cell Transplant Committees currently recommend alloge-neic transplantation for the indications outlined in Table 1. Indications for transplantation at first com-plete remission (CR1) will be modified as the data ma-ture regarding the predictive power of MRD (COG
uses flow cytometry, sensitive to 10-4) and gene array profiles. COG investigators presented data at the American Society of Hematology’s 2007 meeting showing outstanding early outcomes using intensive chemotherapy plus imatinib; thus, the role of CR1 transplantation in this group soon may be limited to selected high-risk patients. Transplantation for infant ALL remains controversial; COG investigators have seen improved outcomes for both MLL rearranged and non-MLL–rearranged infants with intensive che-motherapy approaches . Outcomes for the youn-gest infants with MLL rearrangements remain poor, however, and transplantation has been offered with reasonable rates of cure when remission is obtained . The second complete remission (CR2) indica-tions listed in Table 1 reflect the fact that some pa-tients with late bone marrow relapse and isolated extramedullary relapses can be successfully treated with chemotherapy. But the relatively low morbid-ity and rates of graft-versus-host disease (GVHD) after matched sibling transplantation, along with reports or equivalent or better outcomes, make transplantation a reasonable option for these patients, especially those who cannot receive salvage chemotherapy of adequate intensity.
INTENSITY OF CHEMOTHERAPY BEFORE TRANSPLANTATION
It has been argued that the biology of ALL at re-lapse determines the outcome, and that patients with sensitive disease will do well and those with resistant disease will do poorly regardless of the pretransplan-tation chemotherapeutic approach. Recent data from the COG refutes this argument, showing that 71% of patients who achieve CR after induction chemo-therapy but have MRD positivity will experience de-creased disease burden with subsequent intensive blocks of chemotherapy, with nearly half of them achieving MRD-negative status after 2 consolidation blocks. Because achieving MRD negativity before transplantation improves long-term survival, the ma-jor focus in the COG relapsed ALL group is to intro-duce agents into reinduction protocols that will increase a patient’s chance of becoming MRD-nega-tive before transplantation. Early promising data are emerging with the inclusion of clofarabine and epra-tuzumab (anti-CD22 antibody) in induction/consoli-dation approaches [14,15], and protocols including bortezomib and temsirolimus are in the planning stages. Optimal pretransplantation induction regi-mens for relapsed ALL patients should maximize the number of patients achieving MRD-negative sta-tus, while minimizing transplantation-limiting com-plicatons, such as organ damage and invasive fungal infections.
Table 1. COG Indications for Transplantation in ALL Remission Status Indication
CR1 Alternative or sibling donors: Ph+patients requiring transplantation
Extreme hypodiploidy (< 44 chromosomes or DNA index < 0.81)
11q23 (MLL) plus slow early response (M2/ M3 at day 14 or MRD > 0.1% at day 29) Primary induction failure: M3 bone marrow at
day 29 or M2 bone marrow or MRD > 1% at day 29 who then fail after consolidation with M2 or M3 bone marrow or MRD > 1% at day 43 CR2 Matched sibling donor only:
B-lineage ALL after late bone marrow relapse ($ 36 months from diagnosis)
B-lineage ALL in after a very early isolated extramedullary relapse (< 18 months from diagnosis)
Alternative or sibling donors:
B-lineage ALL after early bone marrowe relapse (< 36 months from diagnosis) T-lineage ALL after bone marrow relapse at
Ph+ALL bone marrow relapse at any time
T-lineage ALL after very early isolated extramedullary relapse (< 18 months from the initiation of primary chemotherapy) CR3 Alternative or sibling donors:
Any B- or T-lineage patient with any type of relapse
TOTAL BODY IRRADIATION–BASED
TRANSPLANTATION REGIMENS FOR ACUTE LYMOPHOBLASTIC LEUKEMIA
Fractionated TBI-based preparative regimens, generally containing at least 1200 cGy, have been shown to decrease the risk of relapse in high-risk CR2 ALL transplantation recipients by as much as 27% compared with non-TBI regimens . Relapse can be further decreased by slightly higher doses of TBI (total 1300 to 1400 cGy) or the addition of VP-16 or thiotepa to a 1200-cGy regimen . Although concerns surround the growth and develop-mental implications of TBI use in infants or children under age 3, several groups have been using this approach with low rates of relapse and acceptable growth and developmental outcomes . Whether TBI is necessary for MRD-negative, intermediate-risk patients, in whom relapse intermediate-risk is less of an issue, is unknown.
PREVENTING RELAPSE WITH TARGETED POSTTRANSPLANTATION THERAPIES
The use of cytotoxic therapies to maintain ALL remission after transplantation is infeasible because of hematologic toxicity and abrogation of the graft-versus-leukemia (GVL) effect. However, noncytotoxic agents (eg, molecular targeted therapies, immune approaches) can be used successfully to help achieve and sustain remission after transplantation. Two exam-ples with early data are posttransplantation imatinib for Ph1ALL and sirolimus. Carpenter et al.reported
efficacy with acceptable toxicity in early administration of imatinib after transplantation for patients with Ph1 ALL. All 9 of the pediatric patients in their cohort (me-dian follow-up, 1 year) are alive, with 1 relapse. COG investigators found a reasonable toxicity profile with imatinib started 6 months after transplantation, but with some relapses, making the benefits of this approach unclear. Studies evaluating the posttransplantation use of dasatinib and nilotinib are currently underway through the COG and Fred Hutchinson Cancer Research Center in Seattle.
Sirolimus is a mammalian target of rapamycin (mTOR) inhibitor that has been used for more than 15 years as an immunosuppressive agent for solid or-gan transplantation. Several groups have reported effi-cacy using sirolimus for GVHD prevention, and because sirolimus has been shown to have a significant antileukemic effect on human ALL in nonobese diabetic–severe combined immune deficiency (NOD-SCID) models, it has been used by COG investi-gators to both control GVHD and prevent relapse. A pilot study using sirolimus-based GVHD prophy-laxis in pediatric ALL has shown lower rates of relapse than expected in all risk groups (Figure 1), and a Phase
III trial comparing sirolimus-based GVHD prophy-laxis with standard approaches is ongoing.
PERITRANSPLANTATION MINIMAL RESIDUAL DISEASE/CHIMERISM MONITORING
Analysis of chimerism peritransplantation allows assessment for the rare possibility of imminent graft rejection and also can serve as an indicator for recur-rence of underlying malignancy. In addition, character-ization of MRD before and after transplantation has defined those patients at greatest risk for relapse. Consecutive posttransplantation MRD monitoring to-gether with chimerism analysis allows the detection of impending relapse in a substantial percentage of children undergoing transplantation for ALL. Conse-quently, for some groups, these analyses have served as the basis for treatment intervention to avoid graft rejection, maintain engraftment, and treat imminent relapse.
DONOR LYMPHOCYTE INFUSION
Kolb et al.first described the efficacy of DLI in patients with chronic myelogenous leukemia (CML) re-lapsing after allogeneic stem cell transplantation. DLI initiated during frank hematologic relapse was found to induce complete remission in 8% of patients with ALL and in 22% of patients with acute myeloid leukemia (AML). When tumor burden was reduced by che-motherapy before DLI, the rate of complete response was improved significantly (to 33% in ALL and 37% in AML). These results suggest that immunotherapy provides the greatest benefit to patients with acute 5.00 4.00 3.00 2.00 1.00 0.00
Time from Transplant (yrs)
1.0 0.8 0.6 0.4 0.2 0.0 Cumulative Survival CR1 IR CR2 CR3 HR CR2
Figure 1. Early outcome data for rapamycin-based GVHD prophylaxis after TBI-based transplantation for ALL (50 patients; median follow-up, 21 months). Two-year event-free survival: CR1, 92% 6 8%; CR2 IR (bone marrow relapse . 36 months from diagnosis), 93% 6 6%; CR2 HR (bone marrow relapse, B cell, \ 36 months from diagnosis, all T cell), 49% 6 14%; CR3, 67% 6 16%.
leukemia after stem cell transplantation when adminis-tered before hematologic relapse occurs. Thus, it is de-sirable to identify those patients at greatest risk for relapse early, to maximize the chance of successful im-munotherapy.
POSTTRANSPLANTATION MONITORING Two different techniques are currently available for posttransplantation surveillance of disease remission: characterization of posttransplantation chimerism and specific detection of MRD. MRD assessment mea-sures the malignant clone directly, whereas chimerism assessment characterizes the origin of posttransplanta-tion hematopoiesis. Over the past decade, methods of detecting MRD and chimerism have improved greatly, with techniques ranging from polymerase chain reac-tion (PCR) to immunophenotypic analysis (flow cytometry) of aberrant cell surface molecules[23-25]. Chimerism in Leukemia
Molecular evidence of persistent or reappearing recipient cells may be a reflection of survival of leuke-mic cells, survival of normal host hematopoietic cells, or a combination of both. Surviving host hematopoi-etic cells may in turn facilitate the reemergence of a ma-lignant cell clone by inhibiting immunocompetent donor effector cells. It has been shown that in patients with CML, the reappearance of host hematopoietic cells in the mononuclear cell fraction precedes hema-tologic relapse; thus, mixed chimerism has been considered to reduce the GVL effect[26,27].
Several early studies left the question of whether patients with acute leukemias and mixed chimerism have an increased risk of relapse unanswered. In the mid-1990s, it was realized that evolution of chimerism is a dynamic process and that chimerism analysis should be done serially in short time intervals. Using short tan-dem repeat (STR)-PCR– based serial analysis of micro-satellite regions identified patients with rapidly increasing mixed chimerism as being at the greatest risk for relapse[22,28-31]. Investigations of subpopu-lations of patients with acute leukemia indicated a pos-sible difference between adult and pediatric patients. Guimond et al.demonstrated that mixed chime-rism in T and natural killer (NK) cell subpopulations is frequently found in pediatric patients with leukemia relapse but not in those in remission, and also not in adult patients with relapse. Studies performed by the German group demonstrated that persistent mixed chi-merism in the early posttransplantation period is caused predominantly by normal recipient hemato-poietic cells. An increase in recipient chimerism pre-cedes the reappearance of the underlying disease. These findings support the hypothesis that a state of mixed hematopoietic chimerism may reduce the
clini-cal GVL effect of alloreactive donor-derived effector cells in patients with acute leukemias and thus facilitate the proliferation of residual malignant cells that may have survived the preparative regimen. Barrios et al. showed that patients with acute leukemias with in-creasing mixed chimerism have a significantly elevated risk of relapse. Based on the foregoing studies, several trials were initiated to evaluate the efficacy of prevent-ing relapse through preemptive immunotherapy on the basis of chimerism analysis in patients with acute leuke-mias [29,30]. A German pediatric transplantation group showed that STR-based chimerism analysis per-formed at regular intervals after transplantation was able to define a subset of children with impending re-lapse. Furthermore, overt relapse was prevented in a portion of patients by preemptive immunotherapy (withdrawal of immune suppression or DLI) on the ba-sis of increasing mixed chimerism. One challenge of this approach was that it is not possible to define im-pending relapse in all patients, because the time interval between the conversion of chimerism and relapse can be very short. Chimerism analysis provides informa-tion about the alloreactivity and/or tolerance inducinforma-tion of the graft and thus serves as more of a prognostic fac-tor than an indirect marker for MRD. It is important to stress that because of its low sensitivity (about 1%), chi-merism analysis is not a reliable means of detecting MRD. Consequently, basing the decision to initiate immunotherapy solely on the results of chimerism test-ing may result in starttest-ing immunotherapy too late.
Studies performed by the German group retro-spectively investigated the MRD load of patients who received DLI on the basis of increasing mixed chime-rism. They found that patients with an MRD load of .10-3did not benefit from the intervention, but pa-tients with an MRD load \ 10-3at the start of immu-notherapy had a survival rate of just below 40% at 2 years (Figure 2). 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 EFS 0,0 1,0 2,0 3,0 4,0 Time [Years] MRD <10-3; n= 18; pEFS = 36% MRD > 10-3; n=7; pEFS = 0.0%
Figure 2. The level of MRD at the time point of further immunotherapy determines the success rate for avoiding hematologic relapse in children with ALL after allogeneic stem cell transplantation. Retrospective assess-ment of quantitative characterization of MRD levels are shown in chil-dren with ALL who received preemptive DLI on the basis of increasing mixed chimerism.
Minimal Residual Disease
A number of retrospective reports have shown that MRD before conditioning is the strongest predictor for relapse posttransplantation [10,11]. The Bristol group showed that patients entering the transplanta-tion with a high MRD load of .10-3did not survive their disease, whereas a portion of patients with low-level disease (\10-3) survived, and MRD-negative pa-tients had the best chance of survival . Using the same semiquantitative technique, a retrospective anal-ysis of German data largely confirmed these results in a series of 45 children. Patients with high-level MRD at the time of transplantation (.10-3) were cured only rarely; however, even in these high-risk patients, additional immunotherapy posttransplantation can eradicate residual disease. As shown in Figure 3, a 15-year-old boy entered transplantation with an MRD load of 1 10-3before transplantation. After
en-graftment, he had mixed chimerism, although MRD was undetectable at day 130. Posttransplantation im-munosuppression was withdrawn. Because chimerism could not be converted to complete donor status, addi-tional DLI was given. Despite the DLI, MRD began to increase; in response, a second DLI was given 50 days later. As a consequence of these 2 DLI treatments, the patient developed grade I GVHD, followed by a de-crease in MRD, and eventually became MRD-negative 3 months after the second DLI (Figure 3).
Methods for MRD monitoring in ALL include dis-ease-specific PCR techniques such as T cell receptor (TCR) repertoire or immune globulin gene rearrange-ments [35,36]. When a disease-specific marker is not available, chimerism analysis in cell subpopulations can be used as a surrogate marker for MRD. Thiede et al.showed that mixed chimerism in CD341cells
is predictive of relapse in patients with AML and ALL in peripheral blood. They noted that increasing
autolo-gous cells within this subset precedes relapse with a me-dian interval of 52 days (range, 12 to 97 days). To enrich this rare subpopulation, however, 50 mL of blood was needed, which limits the applicability of this procedure in pediatric patients. Mattsson et al.  performed a prospective analysis in 30 patients with AML and MDS. They evaluated chimerism in CD331, CD71, and CD451cells and found significantly more relapses in patients whose subpopulations had mixed chimerism compared with those who had complete donor chime-rism. In ALL patients, several studies have investi-gated the impact of mixed chimerism after enrichment of the cell population carrying the leukemic phenotype and found a remarkable correlation between MRD and mixed chimerism in the respective subset . Large studies in ALL patients assessing the predictive value of mixed chimerism in enriched cell populations have not yet been published, however.
Serial quantitative analysis of chimerism in whole blood by STR-PCR allows the identification of those patients at greatest risk for relapse. Unfortunately, however, only a percentage of patients who relapse can be detected, because of the often very short time interval between the onset of mixed chimerism and re-lapse. Therefore, it is essential to that these analyses be performed frequently during the first 100 to 200 days after transplantation, when most relapses occur. Per-forming chimerism analysis in cell subpopulations can significantly increase the sensitivity of this ap-proach. In this setting, chimerism analysis can be con-sidered a surrogate marker for MRD. Combining chimerism and MRD analysis allows for documenta-tion of engraftment and surveillance of posttransplan-tation remission status, thus providing a rational basis for individual preemptive immunotherapy strategies to prevent recurrence of the underlying disease.
IMMUNOTHERAPY FOR PEDIATRIC B CELL ACUTE LYMOPHOBLASTIC LEUKEMIA
Various immune-based therapeutic approaches currently under development may decrease relapse in ALL, especially B-lineage ALL. Over the next decade, the use of these therapies before (to deepen remission) or after (to maintain remission) allogeneic transplanta-tion procedures may produce a cure in patients resistant to cytotoxic chemotherapy without the mor-bidity associated with GVHD.
Therapeutic monoclonal antibodies (mAbs) have been developed that target B-lineage cell surface anti-gens. The COG recently reported that epratuzumab, a humanized mAb targeting CD22, administered in combination with reinduction chemotherapy, re-sulted in some early responses in children with
Time [Days] Detection limit Quantification limit -50 0 50 100 150 200 250 300 1E-5 1E-3 0,1 20 40 60 80 100 Chimerism % Dono r CSA WD DLI GVHDI MRD
Figure 3. The course of a 15-year-old boy with ALL who underwent transplantation in CR2 from a HLA-identical unrelated donor. Cyclo-sporin was stopped when increasing mixed chimerism developed. Al-though autologous chimerism decreased, MRD became positive. This led to a DLI of 1 105
/kg recipient weight, followed by a second infusion of 1 106
/kg after MRD did not disappear. Finally, the patient developed grade I GVHD and become MRD-negative. The patient remains in remis-sion.
relapsed B-ALL. Anti-CD19 mAb also has shown promise in mice with B-ALL xenografts, and early clinical trials using CD19-specific mAb are underway for B-lineage lymphoma. Although targeting CD20 (with, eg, rituximab) typically is not considered for treatment of B-ALL, CD20 is expressed in a subgroup of de novo precursor B-ALL associated with an infe-rior outcome compared with CD20neg B-ALL . Other mAbs with potential therapeutic efficacy are be-coming available for B-ALL, such as targeting HLA DR and BR3, which blocks B cell-activating factor, leading to loss of B cells [43-45]. Combinations of mAbs (eg, anti-CD20 and anti-CD22) or mAbs with chemotherapy may be used to improve therapeutic po-tential for B-lineage malignancies [41,46,47]. The therapeutic benefit of mAbs for B-ALL may be further improved when they are used for targeted delivery of radioimmunotherapy , conjugated to toxins (eg, recombinant immunotoxin BL22, an anti-CD22 Fv fragment fused to truncated Pseudomonas exotoxin) [48-50]or chemotherapy[51,52], or fused to cytokines (immunocytokine). Instead of conjugating mAbs to toxins, investigators have ‘‘coupled’’ mAbs to T cells. Recently, low doses of a bispecific (BiTE) mAb (blinatumomab, MT103/MEDI-538) with a dual specificity for CD19 on tumor cells and CD3 on unsti-mulated T cells were found to induce an antitumor re-sponse in patients with B-lineage non-Hodgkin lymphoma . This promising therapy also might be applicable to the treatment of pediatric B-ALL.
Natural Killer Cells
Human NK cells are a subset of CD561 and/or CD161 peripheral blood lymphocytes that lack the TCR (ie, CD3). As part of the innate immune re-sponse, NK cells kill tumor cells independent of recog-nition of the major histocompatibility complex (MHC). The lack of NK cell inhibitory receptor mis-matching (eg, killer cell immunoglobulin-like recep-tors [KIRs] recognizing self-antigens at HLA-A, -B, or -C loci) leads to inefficient killing of autologous tu-mor cells, which may be overcome by engraftment of allogeneic NK cells that sense missing expression of donor KIR ligand(s) in the recipient and mediate alloreactions. But for adult patients with B-ALL, relapse-free survival after T cell–depleted HLA-mis-matched (HLA-haploidentical) hematopoietic stem cell transplantation (HSCT) typically has not been im-proved by KIR-ligand mismatches between donor and recipient. This may reflect the presence of inhib-itory MHC molecules on B-ALL blasts and/or inher-ent deficiinher-ent NK cell activation [58,59]. Recent data suggest that pediatric patients with B-ALL treated with high proportions of alloreactive NK cells may be sensitive to NK cell alloreactivity. To harness potential donor-versus-recipient NK cell
alloreacitiv-ity, new investigational treatments (at, eg, M.D. Anderson Cancer Center) are combining haplotype-mismatched NK cells and epratuzumab after lympho-depleting chemotherapy given to achieve homeostatic expansion and prevention of immune-mediated rejec-tion of infused allogeneic NK cells.
Approaches to children with high-risk B-ALL un-dergoing allogeneic HSCT , including umbilical cord blood transplantation (UCBT), use immuno-genetics to shape the GVL effect mediated by T cells against B-ALL[56,63,64]. For example, HLA matching and mismatching can be used to reduce GVHD, and may improve the GVL effect after UCBT. In addi-tion, genotyping of unrelated hematopoietic stem cell donors outside the MHC loci (eg, absence in the recip-ient of single-nucleotide polymorphisms of the nucleo-tide-binding oligomerization domain 2/caspase recruitment domain 15 gene) can positively affect im-mune function, leading to improved prognoses for pa-tients with acute leukemias, especially ALL. Other donor–recipient factors besides immunogenetics can in-fluence the GVL effect. A recent retrospective analysis provided preliminary evidence that persistent microchi-merism of maternal cells in individuals leads to improved relapse-free survival (due to decreased GVHD) when the mother is preferentially used as the haploidentical hematopoietic stem cell donor. This is especially true for malignancies that are not susceptible to NK cell al-loreactivity. The beneficial effect of donor sex in haplo-type-mismatched parent-to-child HSCT is added to, and as pronounced as, the survival advantage associated with haploidentical HSCT from NK cell alloreactive donors for NK-sensitive diseases (eg, AML, pediatric ALL) . The benefit of a T cell response against B-ALL also is seen in data demonstrating rapid lympho-cyte recovery after chemotherapy or HSCT [67,68]. DLIs, which are readily available, contain T cells with a polyclonal TCR repertoire and haves been given to re-cipients with relapsed B-ALL[69,70], but DLI toxicities of acute or chronic GVHD or marrow aplasia often out-weigh the antitumor effect. However, combining DLI with small molecules (eg, imatinib or nilotinib) after lymphodepleting chemotherapy [71,72], intrabone in-jections, graded doses of lymphocytes, DLI from granulocyte colony-stimulating factor–mobilized do-nors with immunosuppression[74,75], or depleting T cell subsetsmay be justified in clinical trials, because curative options are rarely available for B-ALL that re-lapses early (eg, less than 6 months) after allogeneic HSCT[77-81].
To improve the therapeutic index of DLI, investiga-tors from around the world have transduced T cells to express thymidine kinase (TK) derived from herpes sim-plex virus[82,83]. In the event of toxicity, conditional
ablation of the TK1T cells can be achieved using
gan-ciclovir. To improve specificity, allogeneic CD81and
CD41T cells that recognize tumor-associated antigens
(TAAs) in context of MHC through endogenous ab TCR have been selectively propagated to target B-line-age antigens, such as intracellular HB-1 . Others have enriched T cells reactive to minor histocompatibil-ity antigens that are present on recipient blasts, but not normal donor hematopoietic cells [85-88]. Immune tolerance typically precludes the ability of autologous T cells to recognize TAAs in the context of MHC in B-ALL. In addition, antigen-specific T cells may not be able lyse B-ALL blasts. Consequently, many investi-gators have redirected T cell specificity through the ge-netic introduction of chimeric antigen receptors (CARs) that recognize CD19 expressed on B-ALL, including cancer stem cellsand normal B cells[90-92]. Phase I trials are now reporting on the safety and feasibility of this gene therapy approach, and clinical trials of next-generation CARs and gene-transfer approaches for B-ALL are being planned.
MHC class I and II molecules on allogeneic B-ALL blasts can directly present TAAs to donor-derived CD81 and CD41 T cells, respectively. For example,
the leukemia-associated Wilms’ tumor antigen (WT1) is immunogenic, raising the possibility that WT1 vaccines can be used to augment immunity[94,95]. In addition, fusion genes, such as BCR-ABL in P1
B-ALL, may serve as a neoantigen that can be targeted by tumor-specific T cells. B-ALL blasts generally lack costimulatory molecules, such as CD80/CD86, to complete a fully competent T cell activation signal. To augment the immune response, investigators have ex-plored active immunization for B-ALL using autolo-gous dendritic cells pulsed with peptides from TAAs derived from B-ALL[97,98]. Alternatively, autologous tumor cells themselves, possibly infused with accessory
cells such as skin fibroblasts, can be genetically modified to enforce expression of costimulatory molecules, such as CD40 ligand (and interleukin-2), to serve as a source of antigen-presenting cells[99,100].
FUTURE OF IMMUNOTHERAPY FOR PEDIATRIC ALL
Despite the irony that a dysregulated immune re-sponse to infection early in life may lead to childhood B-ALL, pediatric oncologists now have many op-portunities to manipulate the immune system (Figure 4) to achieve a lasting anti-B-ALL effect. Immunotherapy may be more effective during a MRD state after bone marrow transplantation, and allogeneic transplantation offers an opportunity to target specific minor histocom-patibility antigens or to use KIR antigen mismatching to direct immune reactions to cancer cells. To move im-mune-based treatments into widespread practice, coop-erative groups and biopharmaceutical companies need to emphasize the systematic testing of promising immu-notherapies in children with high-risk or relapsed B-ALL, and pediatric oncology fellowships need to in-corporate a thorough understanding of the immune sys-tem into their training programs. This approach will help build a future in which immunotherapies can be de-signed rationally and combinations of therapies can be delivered to improve overall survival and decrease the cost of survivorship of pediatric B-ALL.
As chemotherapy for ALL in children improves, the patients with ALL who benefit from transplanta-tion will have a disease that is largely resistant to cyto-toxic approaches. Minimizing the disease burden, administering optimal regimens, closely following pa-tients with MRD techniques, and treating when neces-sary with targeted and immune-based approaches may Antibody Unconjugated Conjugated Bi-specific Vaccine Dendritic cell
Autologous tumor rendered immunogenic Lymphocyte
Allogenic T cells recognizing tumor by endogenous αβ TCR T cells recognizing tumor by introduced CAR
Haploindetical NK cells
Immunotherapies for B-ALL
decrease relapse and improve survival. As these ap-proaches improve, children unable to obtain remission who currently benefit little from transplantation ap-proaches also may experience improved survival.
Financial disclosure: Dr Bader’s studies of MRD/ chimerism have been supported by the Wilhelm Sander Stiftung, Mu¨nchen, Germany and the Deut-sche Krebshilfe, Bonn, Germany.
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