Page 20 - Haematologica June

Editorials
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graft. This was the goal and the reports accross the differ- ent generation on immunodeficient recipient. Indeed pre- vious comparisons of different models with increased immunodeficiency, NOD/SCID mice (non-obese diabet- ic/severe combined immunodeficiency mice), NOD/SCID beta2 (β2-microglobulin-deficient NOD-scid mice), NSG (NOD-Scid-IL-2Rgcnull) mice, showed that the degree of immunodeficiency clearly matters but is not the key to explain xenograft failure of some samples. Fewer AML cells are necessary to initiate the graft and a better engraftment is reached using more immunodefi- cient recipients as compared with first generation NOD/SCID mice.5-7 However, changing the permissive- ness of the recipient maintains the sample stratification in terms of engraftment potential. This means that samples with the potential to engraft better remain better engrafters and the poorer remain poorer. (In the context of this Editorial, non-engrafters remained mostly non- engrafters using different strains of immunodeficient mice.7) Thus, further increasing the immunodeficiency of the recipient would actually jeopardize the recipient via- bility without improving the overall engraftment rate.
Independently of the recipient used, it was shown that the xenograft potential of AML samples was linked to intrinsic properties of the cells injected. Engraftment fail- ure is related to good prognosis AML and, inversely, xenograft potential is a poor prognosis marker.8,9 Paczulla et al.10 and our own team recently showed that increasing the incubation period from 10 to 30 weeks allows some successful leukemic xenografts of good prognosis-related samples incapable of engrafting NSG mice during a con- ventional 10-12 week period. Actually, these samples have a lower frequency of stem-progenitor cells associat- ed with a lower expansion capacity ex vivo compared to poor prognosis-related samples’ cells efficiently engraft- ing NSG mice.11 These data suggest that the non-engrafter samples might just have a slower progression, and that the recipient residual immunity is not an insurmountable obstacle for these samples.
Eventually, beyond the recipient immunity or the graft- ed cells defect, the last explanation for xenograft failure is the lack of a human specific microenvironment support for some categories of leukemia samples. In the last decade, one strategy approaching the issue from two dif- ferent directions was adopted to try to improve the com- patibility of the animal for human cells. The approach had been to humanize the murine recipient either genet- ically (by forcing the expression of human cytokines) or by injecting the mice with cellular components of the human bone marrow microenvironment (BMME). Different immunodeficient mouse strains expressing var- ious human cytokines have been generated over the last decade and are reviewed by Theocharides et al.12 Among them, the NSG-S mice used by Krevvata et al.2 is an engi- neered strain, with knock-in for human stem cell factor (SCF), granulocyte-macrophage colony-stimulating factor (GM-CSF), and IL-3 in the background of the NSG recip- ient.13 Alternatively, humanizing recipient BMME is achieved by injection of stromal cells of the human BMME, such as mesenchymal stem cells (MSC), or endothelial cells, or osteoblast progenitors.14-16 For differ- ent reasons, intravenous (IV) or intra-bone marrow bolus
of stromal cells is less and less used for the benefit of humanized ectopic ossicle approaches, subcutaneously implanted by surgery with either matrigel scaffold, sponges, or ceramic seeded with human stromal cells. These methods have certainly improved the situation since they recently resulted in successful engraftment of samples previously defined as non-engrafters, including good pronosis AML.15,16 However two disadvantages were reported for these models. First, these ectopic leukemic grafts of good prognosis-related samples were reported to not invade recipient bone marrows, thus actu- ally limiting the size of the human leukemic population in this particular case.15,16 Secondly, some protocols are quite demanding to handle in routine lab practice, such as the pre-treatment of the ectopic niche with parathyroid hor- mone to favor the osteoblastic differentiation of the MSC prior to the introduction of the leukemic cells.15 Thus, direct IV injection will probably remain the most com- mon in vivo protocol to explore the LIC compartment, and increasing the chances of successful engraftment in this setting is of particular interest.
Krevvata et al. show on a large cohort (n=77) of AML patients that 82% of AML samples engraft NSG-S recipi- ent versus 50% in the NSG strain.2 Sixty-seven percent of non-engrafter AML in the NSG strain become engrafters in the NSG-S strain during a conventional incubation period. This was also true for good prognosis inv16 AML, which are core binding factor (CBF) mutated AML known to repeatedly fail xenograft procedure. NSG-S also pres- ents the advantages of faster engraftment and a leukemic burden present in the peripheral blood similar to that of patients, allowing simple blood sampling for longitudinal monitoring. However, the downside of the NSG-S model is the management of the leukemia progression that reduces viability of the cohorts. Xenografted with the same sample in NSG-S mice die faster than in NSG. Although the swiftness and quantity of engraftment is clearly shown in this model, further comparative tests should investigate the quality of the graft, for example, to exclude LIC alteration and exhaustion. The Authors even- tually found that 18% of the samples remain non- engrafters in NSG-S, providing opportunities for further investigation into graft failures. Engrafting good progno- sis AML samples, including CBF-AML, is a big step for- ward, opening up opportunities for previously impossible investigation, such as identifying the phenotype of their LICs, or analyzing their in situ behavior in the endos- toeum by intravital microscopy, or the possibility of com- parison of clonal architecture and clonal evolution in vivo with poor prognosis AML samples. This model could also allow the in vivo comparison of drug resistance mecha- nisms of these two groups of patients on the condition of first determining whether NSG-S mice can support an induction regimen, as Farge et al. have recently shown for NSG mice.17
The second part of the study of Krevvata et al. reports negative results but is nonetheless equally important. The Authors have performed a deep analysis of low- and high-risk MDS sample engraftment in NSG-S mice with or without MSCs co-injection. MSCs from different ori- gins were tested: healthy donor-derived MSC (normal), allogeneic patient-derived MSC (allo), or patient-derived
haematologica | 2018; 103(6)