Mesenchymal stem cell therapy in proteoglycan induced arthritis.

OBJECTIVES: To explore the immunosuppressive effect and mechanism of action of intraperitoneal (ip) and intra-articular (ia) mesenchymal stem cell (MSC) injection in proteoglycan induced arthritis (PGIA). METHODS: MSC were administered ip or ia after establishment of arthritis. We used serial bioluminescence imaging (BLI) to trace luciferase-transfected MSC. Mice were sacrificed at different time points to examine immunomodulatory changes in blood and secondary lymphoid organs. RESULTS: Both ip and local ia MSC injection resulted in a beneficial clinical and histological effect on established PGIA. BLI showed that MSC ip and ia in arthritic mice are largely retained for several weeks in the peritoneal cavity or injected joint respectively, without signs of migration. Following MSC treatment pathogenic PG-specific IgG2a antibodies in serum decreased. The Th2 cytokine IL-4 was only upregulated in PG-stimulated lymphocytes from spleens in ip treated mice and in lymphocytes from draining lymph nodes in ia treated mice. An increase in production of IL-10 was seen with equal distribution. Although IFN-γ was also elevated, the IFN-γ/IL-4 ratio in MSC treated mice was opposite to the ratio in (untreated) active PGIA. CONCLUSIONS: MSC treatment, both ip and ia, suppresses PGIA, a non-collagen induced arthritis model. MSC are largely retained for weeks in the injection region. MSC treatment induced at the region of injection a deviation of PG-specific immune responses, suggesting a more regulatory phenotype with production of IL-4 and IL-10, but also of IFN-γ, and a systemic decrease of pathogenic PG-specific IgG2a antibodies. These findings underpin the potential of MSC treatment in resistant arthritis.

Human versus porcine mesenchymal stromal cells: phenotype, differentiation potential, immunomodulation and cardiac improvement after transplantation.

Although mesenchymal stromal cells (MSCs) have been applied clinically to treat cardiac diseases, it is unclear how and to which extent transplanted MSCs exert their beneficial effects. To address these questions, pre-clinical MSC administrations are needed for which pigs appear to be the species of choice. This requires the use of porcine cells to prevent immune rejection. However, it is currently unknown to what extent porcine MSCs (pMSCs) resemble human MSCs (hMSCs). Aim of this study was to compare MSC from porcine bone marrow (BM) with human cells for phenotype, multi-lineage differentiation potential, immune-modulatory capacity and the effect on cardiac function after transplantation in a mouse model of myocardial infarction. Flow cytometric analysis revealed that pMSC expressed surface antigens also found on hMSC, including CD90, MSCA-1 (TNAP/W8B2 antigen), CD44, CD29 and SLA class I. Clonogenic outgrowth was significantly enriched following selection of CD271+ cells from BM of human and pig (129 ± 29 and 1961 ± 485 fold, respectively). hMSC and pMSC differentiated comparably into the adipogenic, osteogenic or chondrogenic lineages, although pMSC formed fat much faster than hMSC. Immuno-modulation, an important feature of hMSC, was clearly demonstrated for pMSC when co-cultured with porcine peripheral blood cells stimulated with PMA and pIL-2. Finally, pMSC transplantation after myocardial infarction attenuated adverse remodelling to a similar extent as hMSC when compared to control saline injection. These findings demonstrate that pMSCs have comparable characteristics and functionality with hMSCs, making reliable extrapolation of pre-clinical pMSC studies into a clinical setting very well possible.

An atlas of bloodstream-accessible bone marrow proteins for site-directed therapy of acute myeloid leukemia.

The concept of arming antibodies with bioactive payloads for a site-specific therapy of cancer has gained considerable interest in recent years. However, a successful antibody-based targeting approach critically relies on the availability of a tumor-associated target that is not only preferentially expressed in the tumor tissue but is also easily accessible for antibody therapeutics coming from the bloodstream. Here, we perfused the vasculature of healthy and acute myeloid leukemia (AML)-bearing rats with a reactive ester derivative of biotin and subsequently quantified the biotinylated proteins to identify AML-associated bone marrow (BM) antigens accessible from the bloodstream. In total, >1400 proteins were identified. Overall, 181 proteins were >100-fold overexpressed in AML as compared with normal BM. Eleven of the most differentially expressed proteins were further validated by immunohistochemistry and confocal microscopic analyses, including novel antigens highly expressed in AML cells (for example, adaptor-related protein complex 3 ß2) and in the leukemia-modified extracellular matrix (ECM) (for example, collagen-VI-α-1). The presented atlas of targetable AML-associated BM proteins provides a valuable basis for the development of monoclonal antibodies that could be used as carriers for a site-specific pharmacodelivery of cytotoxic drugs, cytokines or radionuclides to the BM in AML.

Efficacy of high-dose cyclophosphamide in combination with total-body irradiation in the treatment of acute myelocytic leukemia: studies in a relevant rat model.

The efficacy of current clinical leukemia treatment with high-dose cyclophosphamide, supralethal total-body irradiation (TBI), and bone marrow transplantation was evaluated in a rat model for human acute myelocytic leukemia. In rats, both at an early stage of disease and in an advanced stage after remission-induction with 1-beta-D-arabinofuranosylcytosine, treatment with cyclophosphamide (100 mg/kg i.p.) followed by either acute, unfractionated TBI (900 centigrays) or fractionated TBI (200 centigrays for seven doses; 450 centigrays for three doses) achieved cure in 90 to 100% and 75% of the animals, respectively. The remaining rats died from treatment-related toxicity despite isologous bone marrow transplantation. Applying the cyclophosphamide-TBI treatment regimens in advanced stage leukemia (tumor load, 5 X 10(9) cells) resulted in death from leukemia relapse in the majority of rats (71%). From the increase in life span after treatment, it was deduced that a 9-log leukemic cell kill was achieved at the most. There was no significant difference between the regimens using fractionated or unfractionated TBI. Toxicity-related deaths occurred mainly in the TBI group receiving 450 centigrays for three doses (38%). In another approach, (repeated) low-dose cyclophosphamide was given subsequent to high-dose cyclophosphamide-TBI treatment applied in advanced stage leukemia. This proved to be effective in eradicating residual leukemia in 80 to 90% of the rats without destroying the bone marrow graft. In general, the outcome of the various treatment regimens was predictable through accurate information on the tumor load at various stages of disease. The major obstacle in extrapolating the present experimental results to clinical practice is the lack of similar quantitative data in human leukemia.

Acetyldinaline: a new oral cytostatic drug with impressive differential activity against leukemic cells and normal stem cells--preclinical studies in a relevant rat model for human acute myelocytic leukemia.

Acetyldinaline [CI-994; GOE 5549; PD 123 654; 4-acetylamino-N-(2'-aminophenyl)-benzamide] is the acetylated derivative form of the original compound Dinaline (GOE 1734; PD 104 208). The efficacy and toxicity of Acetyldinaline for remission-induction treatment of leukemia were evaluated and compared with those observed in previous studies of Dinaline in the Brown Norway acute myelocytic leukemia, as a preclinical model for human acute myelocytic leukemia. There were three treatment groups. Leukemic animals received either 1 or 2 courses of 5 daily p.o. administrations of Acetyldinaline with a "full dose" of 23.7 mg/kg once daily (first group), a twice daily "half dose" of 11.85 mg/kg with an interval of 8 h (second group), or a "half dose" of 11.85 mg/kg once daily (third group). The drug-free interval between the 2 courses was 2 or 9 days. With repeated daily p.o. administrations of 23.7 mg/kg either in a single daily dose or a split daily dose of 2 x 11.85 mg/kg for 1 course, at least an 8-log leukemic cell kill was achieved. In contrast, with these treatment schedules, less than a 1-log cell kill of normal pluripotent hemopoietic stem cells (CFU-S) in the femoral bone marrow was found. Split daily dose treatment was more effective resulting in 37.5% cures, while no cures were observed with the single daily treatment for one course. Treatment with single daily dose of 23.7 mg/kg or a split daily dose of 2 x 11.85 mg/kg for 2 courses, with either a 2- or 9-day interval in between, resulted in lethal toxicity in most of rats. This result was comparable with that previously observed after equimolar doses of Dinaline (20 mg/kg). The half-dose once daily treatment with Acetyldinaline (11.85 mg/kg) for 1 or 2 cycles resulted in about a 4.5 or > 8-log leukemic cell kill, respectively. Toxic side effects, i.e., damage to the gastro-intestinal tract and hemorrhages in the lungs, were more pronounced with full dose either in the single or the split daily dose regimen. No significant toxicity was observed at the half-dose treatment once daily. In conclusion, the impressive differential activity against leukemic cells and normal stem cells observed in this relevant rat model for human acute myelocytic leukemia warrants the introduction of this compound in clinical phase I/II studies.

Modeling BCR-ABL and MLL-AF9 leukemia in a human bone marrow-like scaffold-based xenograft model.

Although NOD-SCID IL2Rγ-/- (NSG) xenograft mice are currently the most frequently used model to study human leukemia in vivo, the absence of a human niche severely hampers faithful recapitulation of the disease. We used NSG mice in which ceramic scaffolds seeded with human mesenchymal stromal cells were implanted to generate a human bone marrow (huBM-sc)-like niche. We observed that, in contrast to the murine bone marrow (mBM) niche, the expression of BCR-ABL or MLL-AF9 was sufficient to induce both primary acute myeloid leukemia (AML) and acute lymphocytic leukemia (ALL). Stemness was preserved within the human niches as demonstrated by serial transplantation assays. Efficient engraftment of AML MLL-AF9 and blast-crisis chronic myeloid leukemia patient cells was also observed, whereby the immature blast-like phenotype was maintained in the huBM-sc niche but to a much lesser extent in mBM niches. We compared transcriptomes of leukemias derived from mBM niches versus leukemias from huBM-like scaffold-based niches, which revealed striking differences in the expression of genes associated with hypoxia, mitochondria and metabolism. Finally, we utilized the huBM-sc MLL-AF9 B-ALL model to evaluate the efficacy of the I-BET151 inhibitor in vivo. In conclusion, we have established human niche models in which the myeloid and lymphoid features of BCR-ABL+ and MLL-AF9+ leukemias can be studied in detail.

Cryopreservation of autologous marrow grafts in acute leukemia: survival of in vivo clonogenic leukemic cells and normal hemopoietic stem cells.

The survival of pluripotent hemopoietic stem cells and in vivo clonogenic leukemic cells after cryopreservation was determined in a rat model for human acute myelocytic leukemia (BNML). These stem cell populations can be selectively quantified with modified spleen colony assays (day 8 and day 12 CFU-S; LCFU-S). It appeared that the most primitive rat hemopoietic stem cell (day 12 CFU-S) was significantly less vulnerable to the freezing and thawing procedure as compared with the clonogenic leukemic cell (30% and 1.4% survival, respectively; p = 0.0026). Survival of the day 8 CFU-S population fell between those percentages (8.6%). In view of autologous bone marrow transplantation (ABMT), an attempt was made to extrapolate these and previously reported BNML rat data to man. Taking into account that a) only 1% of the clonogenic leukemic cells survive cryopreservation; b) the fraction of clonogenic leukemic cells in man is approximately 0.001; c) leukemic cells reinfused with the autologous marrow graft may lodge at sites unfavourable for growth; and d) supralethal high-dose chemoradiotherapy significantly hampers the regrowth of leukemia, it becomes rather unlikely that leukemic cells in the autologous marrow graft significantly contribute to a leukemia relapse after ABMT. Therefore, residual leukemia in the host surviving high-dose chemoradiotherapy is the most crucial factor as regards the final outcome of ABMT in acute leukemia.

Dinaline: a new oral drug against leukemia? Preclinical studies in a relevant rat model for human acute myelocytic leukemia (BNML).

The efficacy and toxicity of Dinaline (GOE 1734; PD 104 208; NSC 328786; 4-amino-N-(2'-aminophenyl)benzamide) was evaluated in the Brown Norway acute myelocytic leukemia, which is generally accepted as a relevant preclinical model for human acute myelocytic leukemia. Upon repeated daily oral administration at least an 8 log leukemic cell kill was achieved with only less than a 1 log kill for normal pluripotent hemopoietic stem cells. Daily split-dose treatment even proved to be more effective and resulted in 40-50% cures. However, toxicity was also more pronounced in particular in regard to the gastrointestinal tract. So far, the mode of action of Dinaline is unknown, but its striking therapeutic index warrants further clinical investigation.

The contribution of residual leukemic cells in the graft to leukemia relapse after autologous bone marrow transplantation: mathematical considerations.

A hypothetical model for estimating the probability of leukemia development, supported by experimental evidence, provides a basis on which the conclusion can be drawn that residual leukemic cells in the graft will not contribute significantly to the occurrence of a leukemia relapse after autologous bone marrow transplantation.

The BN acute myelocytic leukemia (BNML) (a rat model for studying human acute myelocytic leukemia (AML)).

Even if animal models have many properties in common with the human disease, as is the case for the BNML and human AML, they have their limitations with respect to the extrapolation to the clinical situation. This also holds for the BNML; thus, conclusions should only be drawn with great caution. Nevertheless, the studies in the BNML model have added considerably to the understanding of various processes that occur during the development of leukemia, e.g., the interaction of leukemic cells and normal hemopoietic stem cells in relation to the microenvironment. The methodology developed in the BNML model allows the quantification of the relative effectiveness of any given treatment with regard to the antileukemic activity compared with the toxicity for normal host tissues. Furthermore, the cell kinetic studies performed in the BNML as a consequence of timed sequential chemotherapy has been helpful in designing an approach to take advantage of this phenomenon in the treatment of acute leukemia. The comparison of the various treatment modalities, employed for the conditioning prior to bone marrow transplantation, made it possible to determine the relative effectiveness of the various approaches. The fractionation of total body irradiation for conditioning purposes was supposed to have a negligible effect with regard to a reduced antileukemic effect. Detailed studies that were conducted in the BNML model did not confirm this hypothesis indicating that (hyper-)fraction of TBI results in a reduced antileukemic effect. The in vitro purging studies in the BNML aimed at the elimination of residual leukemic cells in autologous bone marrow transplantation contributed to the introduction of this method in clinical practice. However, extended studies in the BNML model also indicated that the contribution of the residual leukemia cell in the patient contributed to a much greater extend to the recurrence of leukemia then did the residual cells in the autologous marrow graft. A major contribution of the BNML was achieved in the study of the area of so-called "minimal residual disease" (MRD). A number of so-far unknown aspects of relapsing leukemia could be identified and studied. A new concept of discriminating locally relapsing leukemia and a delayed occurrence of generalized spreading of leukemia formed the basis for the explanation of the observed heterogeneity in the distribution of leukemic cells during the remission and the subsequent relapse phase. In conclusion, it is obvious that proper comparison of the human disease as well as the counterpart in the animal model requires a detailed knowledge of both.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of variables determining the engraftment potential of human acute myeloid leukemia in the immunodeficient NOD/SCID human chimera model.

Among a variety of immunodeficient mouse strains the non-obese diabetic (NOD)/LtSz scid/scid strain appears to be most useful in allowing the engraftment of human AML. However, the large variability in ability to engraft and the levels of engraftment reached have not been explained. To address these issues we have investigated the NOD/SCID repopulating ability of 27 newly diagnosed AML samples. Patients were selected for the absence of internal tandem duplications in the Flt3 gene as we previously reported this mutation to be associated with an enhanced engraftment potential in this model. We observed that secondary AML (n = 6) had a significantly increased level of engraftment when compared to primary AML (n = 21, median levels 73.3% for secondary AML vs 8.94% for primary AML, P = 0.01). Within the primary AML, a significantly higher engraftment was observed in the FAB class M0 than in FAB classes M2, M4 and M5. Within primary AML, samples of patients who failed to respond to the initial therapy gave rise to a higher level of engraftment than samples of patients who did respond to therapy. A similar observation of an increased engraftment correlating with a poorer patient prognosis could be made when applying cytogenetic risk stratification. However, within the primary AML the most important clinical parameter correlating with the level of engraftment appeared to be the patient's WBC count at diagnosis (P = 0.0000). Covariate analysis with the WBC count as a covariate could also fully explain the differences observed in the cytogenetic risk groups, or on the basis of the initial therapy response. Although large differences could be observed, the ability to engraft the NOD/SCID mice was not linked to either the autonomous or cytokine-induced proliferation in vitro. As the leukemic cobblestone area-forming cell frequencies also revealed no correlation with repopulation in the NOD/SCID model, we consider it very likely that the level of engraftment reflects the in vivo proliferative ability of the AML samples assayed rather than the number of leukemia-initiating cells infused into the NOD/SCID mice. Phenotypic analysis based on the expression of CD33, CD34 and CD38 before and after passage in NOD/SCID showed that in 10 out of 16 samples investigated phenotypes were different. In summary, in addition to the Flt3 internal tandem duplications we have identified a series of clinical parameters that determine the NOD/SCID repopulating ability of AML samples, whilst our data strongly suggest that AML in NOD/SCID does not reflect the leukemic process in the patient.

In vitro resistance of the brown Norway rat acute myelocytic leukemia (BNML) to lymphokine-activated killer activity.

In in vivo allogeneic bone marrow transplantation studies with the Brown Norway (BN) rat as recipient and the WAG/Rij rat as allogeneic donor a significant graft-versus-leukemia (GVL) effect is observed. Studies were performed to investigate whether lymphokine-activated killer (LAK) cells play a role in this GVL effect. Splenocytes from WAG/Rij and BN rats were activated in vitro by recombinant human interleukin-2 (rhIL-2) for 5-6 days. The cytolytic activity of these LAK cells was tested on four rat solid tumor cell lines, i.e. an ureter carcinoma, a rhabdomyosarcoma, and two lung tumors, and on leukemic cells derived from the BN rat acute myelocytic leukemia (BNML) and the WAG/Rij acute lymphocytic leukemia (L4415). The panel of target cells also included the murine cell lines P815 and YAC. Both WAG/Rij and BN LAK cells were not capable of lysing the leukemic cells in contrast to significant cytolytic activity on the rat solid tumor cell lines and P815 and YAC. BNML cells showed to be resistant to lysis by human NK cells. Phenotypical analysis of the rat LAK population revealed a decrease in the CD4/CD8 ratio compared to the unstimulated splenocyte population. Rat LAK cells displayed no antibody-dependent cellular cytotoxicity (ADCC) on the leukemic cells, whereas IL-2-stimulated human peripheral blood cells showed moderate ADCC activity on the leukemic cells. To investigate whether cytokines play a role in lysis of leukemic target cells, graded numbers of LAK cells and leukemic cells were co-cultivated for seven days in an agar-based colony culture system. This resulted in moderate suppression of leukemic colony formation. From the current in vitro studies it appears that the graft-versus-leukemia observed in in vivo allogeneic bone marrow transplantation studies is probably not due to a direct leukemic cell kill by LAK cells.

Monitoring of leukemia growth in a rat model using a highly sensitive assay for the detection of LacZ marked leukemic cells.

A very sensitive assay for the detection of LacZ marked cells of an in vitro growing subline of the brown Norway rat myelocytic leukemia (BNML) model was developed. By combining cytochemical X-gal staining with D-galactose mediated suppression of endogenous background beta-galactose activity, a detection sensitivity of one leukemic cell per 10(8) normal bone marrow cells could be achieved. A detailed analysis of the in vivo growth pattern and kinetics of this cell line is presented. Also, it is shown that after cyclophosphamide treatment of leukemic rats no leukemic colonies are formed in an agar-colony assay, whereas the leukemic cells remain detectable in the bone marrow for a considerable time period. Eventually, however, all leukemic cells disappear from the marrow. These findings are discussed in the light of prolonged detection of rare leukemic cells in patients in continuing remission.

Human acute myeloid leukemia cells with internal tandem duplications in the Flt3 gene show reduced proliferative ability in stroma supported long-term cultures.

Recently, in-frame internal tandem duplications have been reported within the regions coding for the juxtamembrane through the first tyrosine kinase domain of the Flt3 gene. These duplications have been reported to lead to autophosphorylation of the receptor. In this study we investigated the effect of such mutations in the Flt3 gene on the in vitro proliferation of human acute myeloid leukemia cells. The mutations were detected in 10 out of 59 AML bone marrow samples analyzed and were not restricted to a specific FAB class or cytogenetic aberration. PCR analysis of those samples showed all mutations to be present in exon 11 of the gene. Whilst samples without a mutation of the Flt3 gene showed an increased cell production in response to either IL-3 and G-CSF or IL-6, SCF, TPO and Flt3L in long-term stroma supported cultures, mutant samples failed to do so. As we could not find a relationship between the absence of a response and either FAB class or cytogenetic aberrations, we interpret these results as an indication that the internal tandem duplications in the Flt3 gene are the prime cause of this unresponsiveness. Although our study does not explain the mechanism by which these mutations cause this unresponsiveness it does suggest that AML cells need a wild-type Flt3 for optimal in vitro proliferation.

In vivo development of an acetyldinaline resistant subline of the BN rat acute myelocytic leukemia (BNML).

The cytostatic drug acetyldinaline [ACD, CI-994, 4-acetylamine-N-(2-aminophenyl)-benzamide] shows an extreme antileukemic effect in the Brown Norway (BN) rate model for acute myelocytic leukemia (BNML) with only minor toxicity for normal pluripotent hemopoietic stem cells. So far, the mode of action is unknown. A resistant subline (BNML/ACD-R) was developed in vivo in the BNML model. Leukemic rats received repeated oral administrations of ACD. When the leukemia relapsed after initial remission-induction with ACD, the cells were transferred to new recipients which were again treated. In total, the animals received 247 oral administrations of ACD (33 x 2 mg/kg per day and 214 x 5 mg/kg per day) before full resistance was reached. The cell line was transferred 17 times in total. Treatment of the final resistant cell line with therapeutically highly active doses of 23.7 mg/kg per day and 11.85 mg/kg per day ACD for 5 days, that resulted in an increase of life span (ILS) of 57 and 18 days, respectively, when applied to the sensitive parent BNML line (BNML/S), resulted in only 10 and 3 days ILS, respectively. These results indicate that a significant degree of resistance has been achieved, which can be overcome partially by increasing the dose of ACD. Whether the development of a resistant subpopulation of the BNML is a result of acquired resistance or whether a naturally resistant subpopulation has been selected out after prolonged treatment with ACD remains to be established. The currently available resistant subline BNML/ACD-R now offers the possibility for further studies on the mechanism of action of ACD.

Efficacy of acetyldinaline for treatment of minimal residual disease (MRD): preclinical studies in the BNML rat model for human acute myelocytic leukemia.

The efficacy of acetyldinaline [4-acetylamino-N-(2'-aminophenyl)-benzamide] for eradication of minimal residual disease (MRD), which is left after bone marrow transplantation, and the risk of a bone marrow graft being jeopardized by this treatment was studied in the Brown Norway rat acute myelocytic leukemia model (BNML). To mimic the clinical situation, MRD induction treatment was given to rats showing clinical signs of leukemia and consisted of 80 mg/kg cyclophosphamide and 7.0 Gy X-rays total body irradiation resulting in a 6-8 log leukemic cell kill leaving 10-1000 leukemic cells in the animals. Treatment was completed with a syngeneic bone marrow transplant. A high dose level (HD) treatment of 23.7 mg acetyldinaline/kg per day and a low dose level (LD) treatment of 11.85 mg/kg per day, each given orally for five consecutive days, were compared. The increase in the survival time, the cure rate, and the toxic death rate were evaluated. One 5-day course of LD treatment, started at a time interval of 10, 17, or 24 days following MRD induction, resulted in 44%, 11% or 0% cures. With two 5-day courses of LD treatment, 89%, 22%, or 0% cures were achieved. With LD treatment, maximally an 8 log leukemic cell kill was obtained and no toxicity-related deaths were observed (only less than a 1 log kill of normal hemopoietic stem cells). In contrast, a single course of HD treatment resulted in 56% of the rats (10/18) dying from intestinal tract toxicity, while from the remaining eight rats at risk for relapse, three (37%) showed a very late relapse and five were cured (63%). It was evident that the leukemic cell load at the start of the acetyldinaline treatment determined the probability of relapse. An important finding was that acetyldinaline did not interfere with bone marrow regeneration. The highly curative potential of acetyldinaline treatment in the BNML model during the phase of MRD warrants the introduction of this compound in clinical phase I/II studies.

L4415: further characterization of the rat model for human acute lymphocytic leukemia.

The biological properties of a transplantable lymphocytic leukemia, L4415 in the WAG/Rij rat, are described. The radiation-induced L4415 leukemia is characterized as a relatively slowly growing, non-immunogenic, immature T-cell leukemia which shows a reproducible growth pattern upon intravenous (i.v.) transfer. Survival time following i.v. inoculation is inversely related to the number of leukemic cells in the inoculum, which allows a quantitative estimate in terms of log leukemic cell kill of the effect of treatment. The first signs of leukemic growth are found in the bone marrow, the spleen, and the liver. Leukemic cells can be detected in the peripheral blood 13 days after inoculation. Due to replacement of normal hemopoietic tissue by leukemic cells and their number increasing exponentially thereafter, normal hemopoiesis is inhibited in the later stages of the disease as indicated by severe thrombocytopenia and anemia. Death is caused by a combination of splenic rupture, gastrointestinal and pulmonary hemorrhage, and impaired functions of heavily infiltrated organs. Hepatosplenomegaly and lymphadenopathy are prominent features at autopsy. Cyclophosphamide- and radiosensitivity of the clonogenic leukemic cells have been determined, a 2.9 log cell kill could be induced by single dose cyclophosphamide inoculation and a dosage giving a surviving fraction of 0.37 (D0) of 0.99 Gy with an extrapolation number (N) of 8.5 were calculated. Based on these data, the L4415 rat leukemia may be regarded as a relevant model for human acute lymphocytic leukemia and may thus serve to explore new treatment strategies.

In vivo targeting of leukemic cells using diphtheria toxin fused to murine GM-CSF.

We have previously demonstrated that diphtheria toxin (DT) fused to human GM-CSF effectively eliminates human long-term leukemia initiating cells in SCID mice. However, because huGM-CSF does not react with the murine GM-CSF receptor possible side-effects to nonleukemic tissues could not be analyzed in the AML/SCID model. To overcome this problem, we used murine GM-CSF fused to DT and studied the therapeutic index in the rat leukemia model BNML/LT12. In DT-mGM-CSF dose escalation experiments, severe dose-dependent toxicity to organs such as liver, kidney and lung was observed. Therefore, the antileukemic effects were evaluated with the lower doses. Daily intraperitoneal bolus injections of 75 microg/kg/day for 7 days induced a 3 log leukemic cell kill. The dose of 75 microg/kg/day had no effect on the hemopoietic progenitor cell subsets. These in vivo studies show that the DT-GM-CSF fusion protein can be used for specifically targeting leukemic cells and thus has potential as a therapeutic agent in the treatment of AML.

Treatment of acute myelocytic leukemia with interleukin-6 Pseudomonas exotoxin fusion protein in a rat leukemia model.

We studied the applicability of interleukin-6 Pseudomonas exotoxin fusion protein (IL-6PE4E) for treatment of acute myelocytic leukemia (AML). Leukemic cells from five out of 10 AML patients studied expressed IL-6 receptor (IL-6R) and proliferation in vitro was inhibited in four of these cases. The potential of this approach in vivo was tested in a pre-clinical model for AML; the Brown Norway acute myelocytic leukemia (BNML). To obtain IL-6R expression levels on BNML cells comparable to the numbers expressed on human AML, human IL-6R gene transfectants of the BNML sub-line LT12 (LT12/IL-6R) were generated. IL-6PE4E is cytotoxic in vitro to LT12/IL-6R expressing 1400 high affinity IL-6R per cell with 50% inhibition of DNA synthesis at 1 ng/ml. In vivo treatment of leukemic rats carrying LT12/IL-6R leukemia indicated that the maximal tolerated dose of IL-6PE4E was 275 +/- 25 microg/kg/day, when continuously administered for 7 days and resulted in a 90% reduction in leukemic cell load. At this dose level of IL-6PE4E no reduction of normal hemopoietic progenitors was seen in non-leukemic rats. At higher dose levels (350-1050 microg/kg/day) severe systemic toxicity was encountered. On the basis of these pre-clinical studies the feasibility of growth factor-toxins for selective in vivo targeting to AML cells is evaluated.

Retrovirus-mediated transfer and expression of marker genes in the BN rat acute myelocytic leukemia model for the study of minimal residual disease (MRD).

To study minimal residual disease (MRD) in leukemia, we transferred the Escherichia coli genes encoding beta-galactosidase (lacZ) and neomycin resistance (neo(r)) into the subline LT12 of the Brown Norway rat acute myelocytic leukemia (BNML), employing the retroviral BAG vector. In this way leukemic cells were genetically marked. Ten independent cell lines were characterized during in vitro growth as well as during two subsequent in vivo passages for expression of neo(r) for which the neomycin analogue G418 was used, and for lacZ expression for which the substrate 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-gal) was used. Out of 10 lines, four revealed permanent high expression of lacZ in all cells. In four other lines greatly varying lacZ expression between the individual cells from these lines was observed. In the remaining two lines lacZ expression was gradually lost. In contrast, neo(r) expression was gradually lost in eight out of the 10 lines, particularly rapidly during in vivo passaging. In the remaining two lines neo(r) expression was retained. The genetic modification did not alter the in vitro leukemogenicity of the cells. Long term in vivo expression of neo(r) and lacZ was followed in two selected lines up to 12 subsequent passages, i.e. one from the group of homogeneous high lacZ expression and one from the group of heterogeneous lacZ expression. In both lines lacZ expression was retained whereas neo(r) expression was rapidly lost after the third passage. The feasibility of using genetically marked leukemic cells for studies of minimal residual disease (MRD) was explored by injecting rats with leukemic cells, treating them with chemotherapy at full blown leukemia development to reduce the tumor load, mimicking the induction of a state of MRD and studying lacZ expression at relapse. LacZ expression was evident in 100% of the cells whereas neo(r) expression was lost in a considerable fraction. These results indicate that the viral vector BAG can be used to mark leukemia cells genetically although a selection of clones with the desired stability of long-term expression is required.