Direct Comparison of NSG and NBSGW Mice for MDS Patient-Derived Xenograft Models

Introduction: Patient-derived xenograft (PDX) models of Myelodysplastic Syndromes (MDS) based on conditioned NSG mice are the most commonly used and well-established. Recently, our group has published an extensive pre-clinical study including n=49 MDS PDX by employing such a model (Schmitt et al., Leukemia 2022). However, achieving adequate engraftment rates and sufficient numbers of PDX can still be challenging, especially for low risk MDS patient samples. Moreover, human erythropoiesis is known to be severely impaired in NSG mice due to residual macrophage activity. Thus, there is a need for further optimization of MDS PDX models. The KIT-deficient mouse strains NBSGW and NSGW41 have already been shown to provide improved engraftment of cord blood and support of human erythropoiesis compared to NSG mice. (McIntosh et al., Stem Cell Reports 2015; Rahmig et al., Stem Cell Reports 2016). Therefore, in this study, we compared NSG and NBSGW mice of both sexes regarding human MDS engraftment, lineage differentiation, clonal bone marrow (BM) composition and survival.

Methods: BM-derived CD34+ hematopoietic stem cells and autologous in-vitro-expanded mesenchymal stroma cells from n=5 MDS patients (n=3 MDS-MLD, n=2 MDS-EB-2) with varying IPSS-R scores (n=1 very low, n=1 low, n=1 intermediate, n=2 high) were intrafemorally co-transplanted into male and female busulfan-conditioned NSG and non-conditioned NBSGW mice. As controls, samples of n=2 age-matched, healthy (HY) donors were used. Human long-term engraftment (>0.01 %) was determined 12 weeks post transplantation. Endpoint evaluation was performed after 19-30 weeks. Peripheral blood (PB) and BM were analyzed for different human immunophenotypic cell populations including CD33, CD34, CD41, CD45, CD71 and CD235a. Further, human CD45+ cells from PDX BM were isolated by FACS for DNA extraction. Subsequently, extracted PDX and original patient samples were subjected to NGS panel sequencing of MDS-specific mutations.

Results: Overall, our data showed that female and male NBSGW mice enabled higher MDS engraftment rates than NSG mice. Engraftment (% human CD45) of MDS and HY samples in NBSGW females was found to be considerably higher in BM and PB than in NSG females (BM: 17.2% (0.01-71.1%) in NSG mice versus 43.7% (1.1-87.7%) in NBSGW; PB: 1.5% (0.01-6.4%) in NSG mice versus 3.5% (0.03-18.4%) in NBSGW). Moreover, NBSGW mice showed improved support of human thrombopoiesis, as well as higher levels of erythropoiesis and CD34+ cells in both MDS and HY PDX compared to NSG mice. However, survival of NSG females was significantly longer than for NBSGW females (p=0.0024), as well as for NSG (p=0.0006) and NBSGW (p<0.0001) males. Interestingly, males of both strains showed lower engraftment rates and shorter overall survival in the experiment as compared to females. Observed early deaths of males were not associated with engraftment rates. Panel sequencing results of n=4 MDS patients and their corresponding PDX showed no differences between NSG and NBSGW mice in mutational profiles and variant allele frequencies of transplanted MDS samples. Both strains were able to faithfully recapitulate the patient's clonal BM composition.

Conclusions: Female NBSGW mice are superior engrafters for primary MDS samples in comparison to female NSG mice and provide the opportunity to investigate human thrombopoiesis and erythropoiesis in-vivo. While NSG PDX were already proven to faithfully recapitulate the clonal BM composition of transplanted patient samples, we found that the same is true for NBSGW mice. However, female NSG mice were more robust in terms of survival. This has to be taken into account when designing long-term PDX studies. Finally, in our study, male mice of both strains were shown to be inferior acceptors for PDX as compared to females.

Nowak:Pharmaxis Ltd.: Current equity holder in private company, Research Funding; Celgene: Honoraria; Takeda: Honoraria; AbbVie: Other: Investigator funded clinical trial; Sumitomo Dainippon Pharma Co. Ltd: Research Funding; Apogenix: Research Funding; Affimed GmbH: Research Funding.