Transgenic expression of human signal regulatory protein alpha in Rag2^−/−γ_c^−/−mice improves engraftment of human hematopoietic cells in humanized mice

Abstract

Transplantation of human hematopoietic stem cells into severely immunocompromised newborn mice allows the development of a human hematopoietic and immune system in vivo. NOD/scid/γ c −/− (NSG) and BALB/c Rag2 −/− γ c −/− mice are the most commonly used mouse strains for this purpose and a number of studies have demonstrated the high value of these model systems in areas spanning from basic to translational research. However, limited cross-reactivity of many murine cytokines on human cells and residual host immune function against the xenogeneic grafts results in defective development and maintenance of human cells in vivo. Whereas NSG mice have higher levels of absolute human engraftment than similar mice on a BALB/c background, they have a shorter lifespan and NOD ES cells are unsuitable for the complex genetic engineering that is required to improve human hematopoiesis and immune responses by transgenesis or knockin of human genes. We have generated mice that faithfully express a transgene of human signal regulatory protein alpha (SIRPa), a receptor that negatively regulates phagocytosis, in Rag2 −/− γ c −/− mice on a mixed 129/BALB/c background, which can easily be genetically engineered. These mice allow significantly increased engraftment and maintenance of human hematopoietic cells reaching levels comparable to NSG mice. Furthermore, we found improved functionality of the human immune system in these mice. In summary, hSIRPa-transgenic Rag2 −/− γ c −/− mice represent a unique mouse strain supporting high levels of human cell engraftment, which can easily be genetically manipulated.

Engineering of a functional bone organ through endochondral ossification

“…In fact, the capacity of the ectopic marrow to attract and retain rare circulating HSCs advocates its function as a "stem cell trap," ultimately providing a tool to investigate the physiological factors and mechanisms underlying the still debated concept of the HSC niche. Furthermore, on the basis of the concept of mutual influences between the BM microenvironment and leukemia cells (27,28), the possibility of establishing a human origin BM microenvironment in humanized hematopoietic murine models (29)(30)(31) has a large relevance to investigate the development and progression of blood cell malignancies. Last but not least, the generation of an extramedullary bone organ, accessible to advanced imaging techniques (e.g., intravital confocal microscopy) (14), opens the perspective to investigate and exploit processes underlying the efficient expansion of HSCs for therapeutic purposes (13).…”

Cancer stem cell definitions and terminology: the devil is in the details

“…Continued genetic modifications of candidate host mice will probably enable at least some of the species-specific needs to be addressed in the future. These include the possibility of creating transgenic mice that will express species-specific human cytokines, or the use of humanized mice (mice transplanted with human haematopoietic or local tissue components) to create more human-type microenvironments -as has proved useful for certain types of normal human cells [53][54][55][56][57] . Orthotopic injections of CSCs into various target organs may also prove useful, and mimicry of natural tumour immunosurveillance mechanisms may be achievable using injections of specific immune effector cells (TABLE 2).…”

Polymorphic Sirpa is the genetic determinant for NOD-based mouse lines to achieve efficient human cell engraftment

“…24 This in vitro finding is also applicable to the in vivo setting, as shown by another study in which a human SIRPA BAC transgene introduced into Rag2 null Il2rg null mice on a mixed 129; BALB/c background significantly improved the efficiency of human hematopoietic engraftment. 29 SIRPA is a transmembrane protein that contains 3 Ig-like domains within the extracellular region. It is expressed in macrophages, myeloid cells, and neurons, and interacts with its ligand CD47 through its respective IgV-like domains, where the NOD strain has specific polymorphism.…”

“…44 Furthermore, a recent study has shown that the enforced expression of human SIRPA by a human BAC transgene enables the 129;Balb/c.Rag1 null Il2rg null mouse to engraft human cells as efficiently as the NSG mouse. 29 Therefore, in xenograft models, the degree of SIRPA-CD47 interaction decided by Sirpa polymorphism is one of the most critical factors to achieve efficient human cell engraftment. Further study is required to understand how the different binding affinity between these mouse polymorphic SIRPAs and human CD47 is translated into cytoplasmic signaling that leads to respective efficiency for xenotransplantation capabilities.…”