Immunogenicity of Ly5 (CD45)-antigens hampers long-term engraftment following minimal conditioning in a murine bone marrow transplantation model

Targeted hematopoietic stem cell depletion through SCF-blockade

BACKGROUND

Hematopoietic stem cell transplantation (HSCT) is a curative treatment for many diverse blood and immune diseases. However, HSCT regimens currently commonly utilize genotoxic chemotherapy and/or total body irradiation (TBI) conditioning which causes significant morbidity and mortality through inducing broad tissue damage triggering infections, graft vs. host disease, infertility, and secondary cancers. We previously demonstrated that targeted monoclonal antibody (mAb)-based HSC depletion with anti(α)-CD117 mAbs could be an effective alternative conditioning approach for HSCT without toxicity in severe combined immunodeficiency (SCID) mouse models, which has prompted parallel clinical αCD117 mAbs to be developed and tested as conditioning agents in clinical trials starting with treatment of patients with SCID. Subsequent efforts have built upon this work to develop various combination approaches, though none are optimal and how any of these mAbs fully function is unknown.

METHODS

To improve efficacy of mAb-based conditioning as a stand-alone conditioning approach for all HSCT settings, it is critical to understand the mechanistic action of αCD117 mAbs on HSCs. Here, we compare the antagonistic properties of αCD117 mAb clones including ACK2, 2B8, and 3C11 as well as ACK2 fragments in vitro and in vivo in both SCID and wildtype (WT) mouse models. Further, to augment efficacy, combination regimens were also explored.

RESULTS

We confirm that only ACK2 inhibits SCF binding fully and prevents HSC proliferation in vitro. Further, we verify that this corresponds to HSC depletion in vivo and donor engraftment post HSCT in SCID mice. We also show that SCF-blocking αCD117 mAb fragment derivatives retain similar HSC depletion capacity with enhanced engraftment post HSCT in SCID settings, but only full αCD117 mAb ACK2 in combination with αCD47 mAb enables enhanced donor HSC engraftment in WT settings, highlighting that the Fc region is not required for single-agent efficacy in SCID settings but is required in immunocompetent settings. This combination was the only non-genotoxic conditioning approach that enabled robust donor engraftment post HSCT in WT mice.

CONCLUSION

These findings shed new insights into the mechanism of αCD117 mAb-mediated HSC depletion. Further, they highlight multiple approaches for efficacy in SCID settings and optimal combinations for WT settings. This work is likely to aid in the development of clinical non-genotoxic HSCT conditioning approaches that could benefit millions of people world-wide.

Ex vivo expansion potential of murine hematopoietic stem cells is a rare property only partially predicted by phenotype

The scarcity of hematopoietic stem cells (HSCs) restricts their use in both clinical settings and experimental research. Here, we examined a recently developed method for expanding rigorously purified murine HSCs ex vivo. After 3 weeks of culture, only 0.1% of cells exhibited the input HSC phenotype, but these accounted for almost all functional long-term HSC activity. Input HSCs displayed varying potential for ex vivo self-renewal, with alternative outcomes revealed by single-cell multimodal RNA and ATAC sequencing profiling. While most HSC progeny offered only transient in vivo reconstitution, these cells efficiently rescued mice from lethal myeloablation. The amplification of functional HSC activity allowed for long-term multilineage engraftment in unconditioned hosts that associated with a return of HSCs to quiescence. Thereby, our findings identify several key considerations for ex vivo HSC expansion, with major implications also for assessment of normal HSC activity.

CD45.1/CD45.2 Congenic Markers Induce a Selective Bias for CD8+ T Cells during Adoptive Lymphocyte Reconstitution in Lymphocytopenia Mice

CD45.1/CD45.2 congenic markers have been used to track hematopoietic lineage differentiation following hematopoietic stem and progenitor cell (HSPC) transplantation. However, several studies suggest that a bias exists in CD45.1 versus CD45.2 hematopoietic cell reconstitution from HSPCs. Meanwhile, no definitive comparison has been reported for mature immune cells as to whether the CD45.1/CD45.2 disparity can skew the immune cell response. In this study, using lymphocytopenia Rag1-/- CD45.2 mice as hosts, we assessed the reconstitution potential of CD45.1 versus CD45.2 lymphocytes following adoptive transfer of mature T and B cells. We have found a selective bias for CD8+ T cells in that CD45.1 cells showed significantly higher reconstitution compared with CD45.2 cells, whereas CD4+ T cells and CD19+ B cells showed equivalent reconstitution. These results suggest that CD45.1/CD45.2 markers may induce an alloreactive response or a survival bias specific to CD8+ T cells, and they therefore call for caution for using them as congenic markers in immunologic models.

Acute T-Cell-Driven Inflammation Requires the Endoglycosidase Heparanase-1 from Multiple Cell Types

It has been accepted for decades that T lymphocytes and metastasising tumour cells traverse basement membranes (BM) by deploying a battery of degradative enzymes, particularly proteases. However, since many redundant proteases can solubilise BM it has been difficult to prove that proteases aid cell migration, particularly in vivo. Recent studies also suggest that other mechanisms allow BM passage of cells. To resolve this issue we exploited heparanase-1 (HPSE-1), the only endoglycosidase in mammals that digests heparan sulfate (HS), a major constituent of BM. Initially we examined the effect of HPSE-1 deficiency on a well-characterised adoptive transfer model of T-cell-mediated inflammation. We found that total elimination of HPSE-1 from this system resulted in a drastic reduction in tissue injury and loss of target HS. Subsequent studies showed that the source of HPSE-1 in the transferred T cells was predominantly activated CD4⁺ T cells. Based on bone marrow chimeras, two cellular sources of HPSE-1 were identified in T cell recipients, one being haematopoiesis dependent and the other radiation resistant. Collectively our findings unequivocally demonstrate that an acute T-cell-initiated inflammatory response is HPSE-1 dependent and is reliant on HPSE-1 from at least three different cell types.

Hematopoiesis Remains Permissive to Bone Marrow Transplantation After Expansion of Progenitors and Resumption of Blood Cell Production

The immense regenerative power of hematopoietic tissue stems from the activation of the immature stem cells and the progenitor cells. After partial damage, hematopoiesis is reconstituted through a period of intense regeneration when blood cell production originates from erythro-myeloid progenitors in the virtual absence of stem cells. Since the damaged hematopoiesis can also be reconstituted from transplanted hematopoietic cells, we asked whether this also leads to the transient state when activated progenitors initially execute blood cell production. We first showed that the early reconstitution of hematopoiesis from transplanted cells gives rise to extended populations of developmentally advanced but altered progenitor cells, similar to those previously identified in the bone marrow regenerating from endogenous cells. We then identified the cells that give rise to these progenitors after transplantation as LSK CD48^– cells. In the submyeloablative irradiated host mice, the transplanted LSK CD48^– cells preferably colonized the spleen. Unlike the endogenous hematopoiesis reconstituting cells, the transplanted whole bone marrow cells and sorted LSK CD48^– cells had greater potential to differentiate to B-lymphopoiesis. Separate transplantation of the CD150^– and CD150⁺ subsets of LSK CD48^– cells suggested that CD150^– cells had a greater preference to B-lymphopoiesis than CD150⁺ cells. In the intensively regenerating hematopoiesis, the CD71/Sca-1 plot of immature murine hematopoietic cells revealed that the expanded populations of altered myeloid progenitors were highly variable in the different places of hematopoietic tissues. This high variability is likely caused by the heterogeneity of the hematopoiesis supporting stroma. Lastly, we demonstrate that during the period when active hematopoiesis resumes from transplanted cells, the hematopoietic tissues still remain highly permissive for further engraftment of transplanted cells, particularly the stem cells. Thus, these results provide a rationale for the transplantation of the hematopoietic stem cells in successive doses that could be used to boost the transplantation outcome.

Hypoxia-induced miR-210 modulates the inflammatory response and fibrosis upon acute ischemia

Hypoxia-induced miR-210 is a crucial component of the tissue response to ischemia, stimulating angiogenesis and improving tissue regeneration. Previous analysis of miR-210 impact on the transcriptome in a mouse model of hindlimb ischemia showed that miR-210 regulated not only vascular regeneration functions, but also inflammation. To investigate this event, doxycycline-inducible miR-210 transgenic mice (Tg-210) and anti-miR-210 LNA-oligonucleotides were used. It was found that global miR-210 expression decreased inflammatory cells density and macrophages accumulation in the ischemic tissue. To dissect the underpinning cell mechanisms, Tg-210 mice were used in bone marrow (BM) transplantation experiments and chimeric mice underwent hindlimb ischemia. MiR-210 overexpression in the ischemic tissue was sufficient to increase capillary density and tissue repair, and to reduce inflammation in the presence of Wt-BM infiltrating cells. Conversely, when Tg-210-BM cells migrated in a Wt ischemic tissue, dysfunctional angiogenesis, inflammation, and impaired tissue repair, accompanied by fibrosis were observed. The fibrotic regions were positive for α-SMA, Vimentin, and Collagen V fibrotic markers and for phospho-Smad3, highlighting the activation of TGF-β1 pathway. Identification of Tg-210 cells by in situ hybridization showed that BM-derived cells contributed directly to fibrotic areas, where macrophages co-expressing fibrotic markers were observed. Cell cultures of Tg-210 BM-derived macrophages exhibited a pro-fibrotic phenotype and were enriched with myofibroblast-like cells, which expressed canonical fibrosis markers. Interestingly, inhibitors of TGF-β type-1-receptor completely abrogated this pro-fibrotic phenotype. In conclusion, a context-dependent regulation by miR-210 of the inflammatory response was identified. miR-210 expression in infiltrating macrophages is associated to improved angiogenesis and tissue repair when the ischemic recipient tissue also expresses high levels of miR-210. Conversely, when infiltrating an ischemic tissue with mismatched miR-210 levels, macrophages expressing high miR-210 levels display a pro-fibrotic phenotype, leading to impaired tissue repair, fibrosis, and dysfunctional angiogenesis.

Allogeneic bone marrow transplant in the absence of cytoreductive conditioning rescues mice with β-thalassemia major

β-thalassemia is a group of inherited blood disorders that result in defects in β-globin chain production. Cooley anemia (CA), or β-thalassemia major, is the most severe form of the disease and occurs when an individual has mutations in both copies of the adult β-globin gene. Patients with CA fail to make adult hemoglobin, exhibit ineffective erythropoiesis, experience severe anemia, and are transfusion dependent for life. Currently, allogeneic bone marrow transplantation (BMT) is the only cure; however, few patients have suitable donors for this procedure, which has significant morbidity and mortality. In this study, a novel humanized murine model of CA is rescued from lethal anemia by allogeneic BMT in the absence of cytoreductive conditioning. A single intravenous postnatal injection of allogeneic bone marrow results in stable, mixed hematopoietic chimerism. Five months after transplantation, donor cells accounted for approximately 90% of circulating erythrocytes and up to 15% of hematopoietic stem and progenitor cells. Transplanted mice are transfusion independent, have marked improvement of hematological indices, exhibit no growth retardation or signs of graft-versus-host disease, and are fertile. This study describes a method for the consistent engraftment of allogeneic donor hematopoietic cells that rescues a humanized mouse model of CA from lethal anemia, all in the absence of toxic cytoreductive conditioning.

A sex-specific reconstitution bias in the competitive CD45.1/CD45.2 congenic bone marrow transplant model

Allelic variants of the pan-haematopoietic cell marker CD45, identified as CD45.1 and CD45.2, have been established as a marker system to track haematopoietic cells following congenic mouse bone marrow transplants. Despite the frequent use of this model for studying the impact of genetic modifications on relative differentiation potential, it is now evident that a bias exists in CD45.1 versus CD45.2 cell reconstitution. While this bias has been demonstrated by reduced reconstitution potential in B cells of CD45.1 origin, differences in the development of other lymphocytes, as well as the impact of sex on this bias, remain uncertain. We performed bone marrow transplants with wild-type CD45.1 and CD45.2 donor cells, and characterised haematopoietic cell reconstitution in dual-expressing CD45.1/2 host mice. We report an increase in CD45.2 reconstitution in the bone marrow that persists in the spleen, thymus and blood. Through the use of CD45.1/2 hosts, we demonstrate the intrinsic bias towards CD45.2 reconstitution is independent of an immunogenic response to the CD45.1 epitope. Furthermore, we identify a sex-specific difference in reconstitution efficiencies, with female mice exhibiting a greater bias towards CD45.2 reconstitution than males. This work sheds new light on the limitations of the CD45.1/CD45.2 congenic system for tracking lymphocyte development.

Numerous niches for hematopoietic stem cells remain empty during homeostasis

Hematopoietic stem cells (HSCs) reside in and are maintained by special microenvironments, termed niches. It is assumed that the HSC niche space remains occupied by endogenous cells and that myelosuppressive conditioning is required to achieve high levels of HSC engraftment. We herein demonstrate that upon the transplantation of very large numbers of purified HSCs into normal mice not exposed to myeloablation, donor HSCs engrafted in niches distant from filled HSC niches without replacing host HSCs and subsequently proliferated and generated hematopoietic progenitors, leading to marked increases in the overall HSC numbers in bone marrow. Additionally, stem cell factor that is produced by CXC chemokine ligand 12-abundant reticular cells is involved in HSC engraftment. In contrast, host granulocyte/macrophage progenitors (GMPs) were replaced by the progeny of transplanted donor HSCs, and overall GMP numbers remained unchanged. Thus, inconsistent with the classical concept, numerous empty HSC niches are available for engraftment and proliferation in bone marrow.

Competition between T cells maintains clonal dominance during memory inflation induced by MCMV

Both human cytomegalovirus (HCMV) and murine cytomegalovirus (MCMV) establish persistent infections that induce the accumulation of virus-specific T cells over time in a process called memory inflation. It has been proposed that T cells expressing T-cell receptors (TCRs) with high affinity for HCMV-derived peptides are preferentially selected after acute HCMV infection. To test this in the murine model, small numbers of OT-I transgenic T cells, which express a TCR with high affinity for the SIINFEKL peptide, were transferred into congenic mice and recipients were challenged with recombinant MCMV expressing SIINFEKL. OT-I T cells were selectively enriched during the first 3 weeks of infection. Similarly, in the absence of OT-I T cells, the functional avidity of SIINFEKL-specific T cells increased from early to late times postinfection. However, even when exceedingly small numbers of OT-I T cells were transferred, their inflation limited the inflation of host-derived T cells specific for SIINFEKL. Importantly, subtle minor histocompatibility differences led to late rejection of the transferred OT-I T cells in some mice, which allowed host-derived T cells to inflate substantially. Thus, T cells with a high functional avidity are selected shortly after MCMV infection and continuously sustain their clonal dominance in a competitive manner.

Mixed chimerism through donor bone marrow transplantation: a tolerogenic cell therapy for application in organ transplantation

PURPOSE OF REVIEW

Organ transplantation is the state-of-the-art treatment for end-stage organ failure; however, long-term graft survival is still unsatisfactory. Despite improved immunosuppressive drug therapy, patients are faced with substantial side effects and the risk of chronic rejection with subsequent graft loss. The transplantation of donor bone marrow for the induction of mixed chimerism has been recognized to induce donor-specific tolerance a long time ago, but safety concerns regarding toxicities of current bone marrow transplantation (BMT) protocols impede widespread application.

RECENT FINDINGS

Recent studies in nonhuman primates and kidney transplant patients have demonstrated successful induction of allograft tolerance even though--in contrast to murine models--only transient chimerism was achieved. Progress toward the development of nontoxic murine BMT protocols revealed that Treg therapy is a potent therapeutic adjunct eliminating the need for cytotoxic recipient conditioning. Furthermore, new insight into the mechanisms underlying tolerization of CD4 and CD8 T cells in mixed chimeras has been gained and has identified possible difficulties impeding clinical translation.

SUMMARY

This review will address the recent advances in murine models as well as findings from the first clinical trials for the induction of tolerance through mixed chimerism. Both the potential for more widespread clinical application and the remaining hurdles and challenges of this tolerance approach will be discussed.

All hematopoietic stem cells engraft in submyeloablatively irradiated mice

Significant controversy exists regarding the impact of hematopoietic stroma damage by irradiation on the efficiency of engraftment of intravenously transplanted stem cells. It was previously demonstrated that in normal syngenic mice, all intravenously transplanted donor stem cells, present in the bone marrow, compete equally with those of the host. In this study, we comprehensively compared the blood cell production derived from transplanted donor stem cells with that from the host stem cells surviving various doses of submyeloablative irradiation. We compared the partial chimerism resulting from transplantation with theoretical estimates that assumed transplantation efficiencies ranging from 100% to 20%. The highest level of consensus between the experimental and the theoretical results was 100% for homing and engraftment (ie, the utilization of all transplanted stem cells). These results point to a very potent mechanism through which intravenously administered hematopoietic stem cells are captured from circulation, engraft in the hematopoietic tissue, and contribute to blood cell production in irradiated recipients. The damage done to hematopoietic stroma and to the trabecular bone by submyeloablative doses of ionizing radiation does not negatively affect the homing and engraftment mechanisms of intravenously transplanted hematopoietic progenitor and stem cells.

An acute negative bystander effect of γ-irradiated recipients on transplanted hematopoietic stem cells

Ultimate success of hematopoietic stem cell transplantation (HSCT) depends not only on donor HSCs themselves but also on the host environment. Total body irradiation is a component in various host conditioning regimens for HSCT. It is known that ionizing radiation exerts "bystander effects" on nontargeted cells and that HSCs transplanted into irradiated recipients undergo proliferative exhaustion. However, whether irradiated recipients pose a proliferation-independent bystander effect on transplanted HSCs is unclear. In this study, we found that irradiated mouse recipients significantly impaired the long-term repopulating ability of transplanted mouse HSCs shortly (∼ 17 hours) after exposure to irradiated hosts and before the cells began to divide. There was an increase of acute cell death associated with accelerated proliferation of the bystander hematopoietic cells. This effect was marked by dramatic down-regulation of c-Kit, apparently because of elevated reactive oxygen species. Administration of an antioxidant chemical, N-acetylcysteine, or ectopically overexpressing a reactive oxygen species scavenging enzyme, catalase, improved the function of transplanted HSCs in irradiated hosts. Together, this study provides evidence for an acute negative, yet proliferation-independent, bystander effect of irradiated recipients on transplanted HSCs, thereby having implications for HSCT in both experimental and clinical scenarios in which total body irradiation is involved.

Allogeneic T cells impair engraftment and hematopoiesis after stem cell transplantation

Because of the perception that depleting hematopoietic grafts of T cells will result in poorer immune recovery and in increased risk of graft rejection, pure hematopoietic stem cells (HSC), which avoid the potentially lethal complication of graft-versus-host disease (GVHD), have not been used for allogeneic hematopoietic cell transplantation (HCT) in humans. Ideal grafts should contain HSC plus mature cells that confer only the benefits of protection from pathogens and suppression of malignancies. This goal requires better understanding of the effects of each blood cell type and its interactions during engraftment and immune regeneration. Here, we studied hematopoietic reconstitution post-HCT, comparing grafts of purified HSC with grafts supplemented with T cells in a minor histocompatibility antigen (mHA)-mismatched mouse model. Cell counts, composition, and chimerism of blood and lymphoid organs were evaluated and followed intensively through the first month, and then subsequently for up to 1 yr. Throughout this period, recipients of pure HSC demonstrated superior total cell recovery and lymphoid reconstitution compared with recipients of T cell-containing grafts. In the latter, rapid expansion of T cells occurred, and suppression of hematopoiesis derived from donor HSC was observed. Our findings demonstrate that even early post-HCT, T cells retard donor HSC engraftment and immune recovery. These observations contradict the postulation that mature donor T cells provide important transient immunity and facilitate HSC engraftment.

Identification and in vivo analysis of murine hematopoietic stem cells

Hematopoietic stem cells (HSCs) can self-renew and give rise to all the cells of the blood and the immune system. As they differentiate, HSCs progressively lose their self-renewal capacity and generate lineage-restricted multipotential progenitor cells that in turn give rise to mature cells. The development of rigorous quantitative in vivo assays for HSC activity combined with multicolor flow cytometry and high-speed sorting have resulted in the phenotypic definition of HSCs to virtual purity. Here, we describe the isolation and identification of HSCs by flow cytometry and the use of competitive repopulation to assess HSC number and function.

Congenic interval of CD45/Ly-5 congenic mice contains multiple genes that may influence hematopoietic stem cell engraftment

The B6.SJL-Ptprc(d)Pep3(b)/BoyJ (B6.SJL) congenic mouse strain, a valuable and widely used tool in murine bone marrow transplantation studies, has long been considered equivalent to the parental C57B/L6 (B6) strain with the exception of a small congenic interval on chromosome 1 harboring an alternative CD45/Ly-5 alloantigen (Ly-5.1). In this study we compared functional properties of stem and stromal cells between the strains, and delineated the boundary of the B6.SJL congenic interval. We identified a 25% reduction in homing efficiency, 3.8-fold reduction in transplantable long-term hematopoietic stem cells (LT-HSCs), a 5-fold reduction in LT-HSCs capable of 24-hour homing, and a cell-intrinsic engraftment defect of 30% to 50% in B6.SJL-derived bone marrow cells relative to B6-derived cells. These functional differences were independent of stem cell number, cycling, or apoptosis. Genotypic analysis revealed a 42.1-mbp congenic interval in B6.SJL including 306 genes, and at least 124 genetic polymorphisms. Moreover, expression profiling revealed 288 genes differentially expressed between nonhematopoietic stromal cells of the 2 strains. These results indicate that polymorphisms between the B6 and SJL genotype within the B6.SJL congenic interval influence HSC engraftment and result in transcriptional variation within bone marrow stroma.

Niche recycling through division-independent egress of hematopoietic stem cells

Hematopoietic stem cells (HSCs) are thought to reside in discrete niches through stable adhesion, yet previous studies have suggested that host HSCs can be replaced by transplanted donor HSCs, even in the absence of cytoreductive conditioning. To explain this apparent paradox, we calculated, through cell surface phenotyping and transplantation of unfractionated blood, that approximately 1-5% of the total pool of HSCs enters into the circulation each day. Bromodeoxyuridine (BrdU) feeding experiments demonstrated that HSCs in the peripheral blood incorporate BrdU at the same rate as do HSCs in the bone marrow, suggesting that egress from the bone marrow to the blood can occur without cell division and can leave behind vacant HSC niches. Consistent with this, repetitive daily transplantations of small numbers of HSCs administered as new niches became available over the course of 7 d led to significantly higher levels of engraftment than did large, single-bolus transplantations of the same total number of HSCs. These data provide insight as to how HSC replacement can occur despite the residence of endogenous HSCs in niches, and suggest therapeutic interventions that capitalize upon physiological HSC egress.

Minor histocompatibility antigens on transfused leukoreduced units of red blood cells induce bone marrow transplant rejection in a mouse model

When successful, human leukocyte antigen (HLA)-matched bone marrow transplantation with reduced-intensity conditioning is a cure for several nonmalignant hematologic disorders that require chronic transfusion, such as sickle cell disease and aplastic anemia. However, there are unusually high bone marrow transplant (BMT) rejection rates in these patients. Rejection correlates with the number of transfusions before bone marrow transplantation, and it has been hypothesized that preimmunization to antigens on transfused blood may prime BMT rejection. Using a novel mouse model of red blood cell (RBC) transfusion and major histocompatibility complex-matched bone marrow transplantation, we report that transfusion of RBC products induced BMT rejection across minor histocompatibility antigen (mHA) barriers. It has been proposed that contaminating leukocytes are responsible for transfusion-induced BMT rejection; however, filter leukoreduction did not prevent rejection in the current studies. Moreover, we generated a novel transgenic mouse with RBC-specific expression of a model mHA and demonstrated that transfusion of RBCs induced a CD8(+) T-cell response. Together, these data suggest that mHAs on RBCs themselves are capable of inducing BMT rejection. Cellular immunization to mHAs is neither monitored nor managed by current transfusion medicine practice; however, the current data suggest that mHAs on RBCs may represent an unappreciated and significant consequence of RBC transfusion.

Transfusion of minor histocompatibility antigen-mismatched platelets induces rejection of bone marrow transplants in mice

Bone marrow transplantation (BMT) represents a cure for nonmalignant hematological disorders. However, compared with the stringent conditioning regimens used when performing BMT to treat hematological malignancies, the reduced intensity conditioning regimen used in the context of nonmalignant hematological disorders leads to substantially higher rates of BMT rejection, presumably due to an intact immune system. The relevant patient population typically receives transfusion support, often including platelets, and the frequency of BMT rejection correlates with the frequency of transfusion. Here, we demonstrate that immunity to transfused platelets contributes to subsequent BMT rejection in mice, even when the BMT donor and recipient are MHC matched. We used MHC-matched bone marrow because, although immunity to transfused platelets is best characterized in relation to HLA-specific antibodies, such antibodies are unlikely to play a role in clinical BMT rejection that is HLA matched. However, bone marrow is not matched in the clinic for minor histocompatibility antigens, such as those carried by platelets, and we report that transfusion of minor histocompatibility antigen-mismatched platelets induced subsequent BMT rejection. These findings indicate previously unappreciated sequelae of immunity to platelets in the context of transplantation and suggest that strategies to account for minor histocompatibility mismatching may help to reduce the chance of BMT rejection in human patients.

Conditional deletion of STAT5 in adult mouse hematopoietic stem cells causes loss of quiescence and permits efficient nonablative stem cell replacement

Currently, there is a major need in hematopoietic stem cell (HSC) transplantation to develop reduced-intensity regimens that do not cause DNA damage and associated toxicities and that allow a wider range of patients to receive therapy. Cytokine receptor signals through c-Kit and c-Mpl can modulate HSC quiescence and engraftment, but the intracellular signals and transcription factors that mediate these effects during transplantation have not been defined. Here we show that loss of one allele of signal transducer and activator of transcription 5 (STAT5) in nonablated adult mutant mice permitted engraftment with wild-type HSC. Conditional deletion of STAT5 using Mx1-Cre caused maximal reduction in STAT5 mRNA (> 97%) and rapidly decreased quiescence-associated c-Mpl downstream targets (Tie-2, p57), increased HSC cycling, and gradually reduced survival and depleted the long-term HSC pool. Host deletion of STAT5 was persistent and permitted efficient donor long-term HSC engraftment in primary and secondary hosts in the absence of ablative conditioning. Overall, these studies establish proof of principle for targeting of STAT5 as novel transplantation conditioning and demonstrate, for the first time, that STAT5, a mitogenic factor in most cell types, including hematopoietic progenitors, is a key transcriptional regulator that maintains quiescence of HSC during steady-state hematopoiesis.

Polyclonal T-cell reconstitution of X-SCID recipients after in utero transplantation of lymphoid-primed multipotent progenitors

Although successful in utero hematopoietic cell transplantation (IUHCT) of X-linked severe combined immune deficiency (X-SCID) with enriched stem and progenitor cells was achieved more than a decade ago, it remains applied only in rare cases. Although this in part reflects that postnatal transplantations have overall given good results, there are no direct comparisons between IUHCT and postnatal transplantations of X-SCID. The proposed tolerance of the fetal immune system to foreign human leukocyte antigen early in gestation, a main rationale behind IUHCT, has recently been challenged by evidence for a considerable immune barrier against in utero transplanted allogeneic bone marrow cells. Consequently, there is need for further exploring the application of purified stem and progenitor cells to overcome this barrier also in IUHCT. Herein, we demonstrate in a congenic setting that recently identified lymphoid-primed multipotent progenitors are superior to hematopoietic stem cells in providing rapid lymphoid reconstitution after IUHCT of X-SCID recipients, and sustain in the long-term B cells, polyclonal T cells, as well as short-lived B-cell progenitors and thymic T-cell precursors. We further provide evidence for IUHCT of hematopoietic stem cells giving superior B- and T-cell reconstitution in fetal X-SCID recipients compared with neonatal and adolescent recipients.

Differential susceptibility of C57BL/6NCr and B6.Cg-Ptprca mice to commensal bacteria after whole body irradiation in translational bone marrow transplant studies

Background

The mouse is an important and widely utilized animal model for bone marrow transplant (BMT) translational studies. Here, we document the course of an unexpected increase in mortality of congenic mice that underwent BMT.

Methods

Thirty five BMTs were analyzed for survival differences utilizing the Log Rank test. Affected animals were evaluated by physical examination, necropsy, histopathology, serology for antibodies to infectious disease, and bacterial cultures.

Results

Severe bacteremia was identified as the main cause of death. Gastrointestinal (GI) damage was observed in histopathology. The bacteremia was most likely caused by the translocation of bacteria from the GI tract and immunosuppression caused by the myeloablative irradiation. Variability in groups of animals affected was caused by increased levels of gamma and X-ray radiation and the differing sensitivity of the two nearly genetically identical mouse strains used in the studies.

Conclusion

Our retrospective analysis of thirty five murine BMTs performed in three different laboratories, identified C57BL/6NCr (Ly5.1) as being more radiation sensitive than B6.Cg-Ptprc^a/NCr (Ly5.2). This is the first report documenting a measurable difference in radiation sensitivity and its effects between an inbred strain of mice and its congenic counterpart eventually succumbing to sepsis after BMT.

Mobilization as a preparative regimen for hematopoietic stem cell transplantation

Current myeloablative conditioning regimens for hematopoietic stem cell (HSC) transplantation are associated with significant morbidity and mortality. Thus, alternative strategies to promote engraftment of infused HSCs with increased safety warrant investigation. Using parabiotic mice, we determined that, after mobilization with AMD3100 (a CXCR4 antagonist), HSCs exited from marrow, transited blood, and engrafted in open niches in partner marrow. We then hypothesized that mobilization before transplantation might vacate niches and improve HSC engraftment. When PeP3(b) mice were treated with AMD3100 at 2 hours before the transplantation of 4 x 10(7) marrow cells, donor cell engraftment was higher (4.6% +/- 1.1%) than in control animals (no AMD3100; 1.0% +/- 0.24%, P < .001). When mice received weekly injections of AMD3100 on 3 consecutive weeks and marrow cells were transplanted 2 hours after each mobilization, donor cell engraftment further increased (9.1% +/- 1.7%, P = .001). In contrast, in similar experiments with Balb/cByJ mice that mobilize poorly, there was no difference between the donor cell engraftment of AMD3100-treated and control recipients. These results indicate that the number of available niches regulates the number of HSCs. In addition, mobilization with AMD3100 may provide a safer preparative approach for HSC transplantation in genetic and other nonmalignant disorders.

Co-receptor and co-stimulation blockade for mixed chimerism and tolerance without myelosuppressive conditioning

BACKGROUND

A major challenge in the application of marrow transplantation as a route to immunological tolerance of a transplanted organ is to achieve hematopoietic stem cell (HSC) engraftment with minimal myelosuppressive treatments.

RESULTS

We here describe a combined antibody protocol which can achieve long-term engraftment with clinically relevant doses of MHC-mismatched bone marrow, without the need for myelosuppressive drugs. Although not universally applicable in all strains, we achieved reliable engraftment in permissive strains with a two-stage strategy: involving first, treatment with anti-CD8 and anti-CD4 in advance of transplantation; and second, treatment with antibodies targeting CD4, CD8 and CD40L (CD154) at the time of marrow transplantation. Long-term mixed chimerism through co-receptor and co-stimulation blockade facilitated tolerance to donor-type skin grafts, without any evidence of donor-antigen driven regulatory T cells.

CONCLUSION

We conclude that antibodies targeting co-receptor and co-stimulatory molecules synergise to enable mixed hematopoietic chimerism and central tolerance, showing that neither cytoreductive conditioning nor 'megadoses' of donor bone marrow are required for donor HSC to engraft in permissive strains.

Induction of Mixed Chimerism through Transplantation of CD45-Congenic Mobilized Peripheral Blood Stem Cells after Nonmyeloablative Irradiation

Clinical translation of the mixed-chimerism approach for inducing transplantation tolerance would be facilitated if mobilized peripheral blood stem cells (mPBSCs) could be used instead of bone marrow cells (BMCs). Because the use of mPBSCs for this purpose has not been investigated in nonmyeloablative murine protocols, we explored the engraftment potential of mPBSCs in a CD45-congenic model as a first step. After 2, 1.5, or 1 Gy of total body irradiation, CD45.1 B6 hosts received unseparated granulocyte colony-stimulating factor-mobilized CD45.2 B6 PBSCs or unseparated CD45.2 B6 BMCs. The same total cell numbers, or aliquots of mPBSCs and BMCs containing similar numbers of c-kit+ cells, were transplanted both with and without a short course of rapamycin-based immunosuppression (IS). Transplantation of mPBSCs induced long-term multilineage macrochimerism, but chimerism levels were significantly lower than among recipients of the same number of BMCs. Transplanting aliquots containing similar numbers of c-kit+ cells reduced the difference between mPBSCs and BMCs, but lower levels of chimerism were nonetheless observed in mPBSC recipients. Chimerism levels correlated more closely with the number of transplanted progenitor cells as determined by colony-forming unit assays. IS did not affect chimerism levels, indicating that the donor CD45 isoform or other minor disparities do not pose a major barrier to engraftment. Our findings indicate that under nonmyeloablative conditions, progenitor cells contained in mPBSCs have an engraftment capacity similar to progenitor cells from BMCs, allowing induction of lasting mixed chimerism with moderate cell numbers. On a cell-per-cell basis, unseparated BMCs have some advantages that may be minimized if the number of progenitor cells is equalized. These results are expected to facilitate the development of mPBSC-based allogeneic tolerance protocols.

Purified hematopoietic stem cell engraftment of rare niches corrects severe lymphoid deficiencies without host conditioning

In the absence of irradiation or other cytoreductive conditioning, endogenous hematopoietic stem cells (HSCs) are thought to fill the unique niches within the bone marrow that allow maintenance of full hematopoietic potential and thus prevent productive engraftment of transplanted donor HSCs. By transplantation of purified exogenous HSCs into unconditioned congenic histocompatible strains of mice, we show that ∼0.1–1.0% of these HSC niches are available for engraftment at any given point and find no evidence that endogenous HSCs can be displaced from the niches they occupy. We demonstrate that productive engraftment of HSCs within these empty niches is inhibited by host CD4⁺ T cells that recognize very subtle minor histocompatibility differences. Strikingly, transplantation of purified HSCs into a panel of severe combined immunodeficient (SCID) mice leads to a rapid and complete rescue of lymphoid deficiencies through engraftment of these very rare niches and expansion of donor lymphoid progenitors. We further demonstrate that transient antibody-mediated depletion of CD4⁺ T cells allows short-term HSC engraftment and regeneration of B cells in a mouse model of B(-) non-SCID. These experiments provide a general mechanism by which transplanted HSCs can correct hematopoietic deficiencies without any host conditioning or with only highly specific and transient lymphoablation.

Differing mechanisms of early and late B cell hyporesponsiveness induced by mixed chimerism

Mixed hematopoietic chimerism induced via nonmyeloablative bone marrow transplantation (BMT) leads to unresponsiveness of anti-Gal alpha1,3Gal beta1,4G1cNAc (Gal) natural antibody (NAb)-producing cells in alpha1,3-galactosyltransferase deficient (GalT(-/-)) mice. We analyzed the mechanisms of anti-Gal-producing B cell unresponsiveness induced by Gal(+/+) BMT. C57BL/6 (B6) GalT(-/-) mice received 3Gy whole-body irradiation and BMT from B6-CD45 congenic mice. BMT led to marked reductions in serum anti-Gal IgM levels and in the numbers of splenic anti-Gal-producing cells by 2 weeks post-BMT. B cells with anti-Gal Ig receptors were present in the spleens of 2-week but not 12-week chimeras. In vitro studies and adoptive transfer studies using B6 GalT(-/-)B cell-deficient recipients showed that B cell hyporesponsiveness to Gal at 2 weeks, but not 12 weeks, depended on persistent Gal antigen. Thus, pre-existing B-1 cells are anergic when there is continuous exposure to Gal, whereas long-term unresponsiveness does not require persistent antigen, implicating clonal deletion and/or receptor editing. These results have implications for the potential use of mixed hematopioetic chimerism as an approach to performing organ transplantation in recipients with pre-existing anti-donor IgM antibodies.

Short-term immunosuppression facilitates induction of mixed chimerism and tolerance after bone marrow transplantation without cytoreductive conditioning

BACKGROUND

Induction of mixed chimerism and tolerance usually requires cytoreduction or transplantation of high numbers of bone marrow cells (BMC). However, such protocols have only a suboptimal success rate and, more importantly, equivalent numbers of BMC cannot be routinely obtained in the clinical setting. The authors therefore evaluated whether a short-course of immunosuppression (IS) given in addition to co-stimulation blockade would facilitate chimerism induction and allow reduction of the minimally required number of BMC without cytoreduction.

METHODS

B6 mice received 200, 100, or 50 x 10 unseparated BMC from Balb/c donors plus an anti-CD40L monoclonal antibody (mAb) and CTLA4Ig (without irradiation or cytotoxic drugs). Some groups were treated additionally with IS (rapamycin, methylprednisolone, and mycophenolate mofetil for 4 weeks after bone marrow transplantation), donor-specific transfusion (DST), or anti-OX40L mAb, as indicated.

RESULTS

IS led to long-term multilineage chimerism in 9 of 10 mice receiving 200 x 10 BMC (without IS, 1 of 4; P<0.05), in all mice (n=10) receiving 100 x 10 (without IS, 6 of 9; P<0.05), and notably in 9 of 10 mice treated with 50 x 10 BMC (without IS, 4 of 10; P<0.05). With transient IS, donor skin grafts were accepted longer than 170 days in 9 of 10 mice receiving 200 x 10 (without IS, 0 of 5 mice; P<0.05), all mice receiving 100 x 10 (without IS, 6 of 9; P<0.05), and 6 of 11 mice receiving 50 x 10 BMC (without IS, 4 of 10). The use of DST or anti-OX40L mAb had no beneficial effect.

CONCLUSIONS

Transient IS significantly improves rates of chimerism and donor skin graft survival, and allows lasting mixed chimerism after transplantation of only 50 x 10 BMC. Thus, IS might help in the further development of noncytoreductive chimerism protocols.

CD45 congenic bone marrow transplantation: evidence for T cell-mediated immunity

CD45 congenic mice have been used to study stem cell engraftment in the absence of alloreactivity. Recently, impaired engraftment was reported in this model and attributed to weak immune reactivity. We have confirmed that there is indeed low-level reactivity mediated by CD8(+) cells and alpha beta-TCR(+) T cells. B6 (CD45.2) recipients were conditioned with total body irradiation (TBI) and transplanted with increasing doses of B6 (CD45.1) bone marrow cells (BMCs). Although chimerism was present at 1 month in all recipients, durable engraftment did not occur with <150 cGy of TBI, emphasizing the importance of long-term follow-up in evaluating nonmyeloablative conditioning approaches. A single dose of cyclophosphamide on day 2 also significantly enhanced engraftment. When B6 TCR beta(-/-), TCR delta(-/-), or TCR beta(-/-)/delta(-/-) (CD45.2) mice were transplanted with CD45.1 bone marrow, significantly enhanced engraftment occurred in recipients lacking alpha beta-TCR(+) T cells (p < .00005). Similarly, removal of alpha beta-TCR(+) host T cells in wild-type recipients resulted in enhanced engraftment. Engraftment was also significantly increased in CD8(-/-) and CD4(-/-)/8(-/-) recipients (p < .0005). Taken together, these results demonstrate that alpha beta-TCR(+) and CD8(+) T cells play a critical role in regulating engraftment of CD45 congenic marrow and suggest that these cells are the main effector cells in low-level alloreactivity to the CD45 disparity.

Recruitment of marrow-derived endothelial cells to experimental choroidal neovascularization by local expression of vascular endothelial growth factor

PURPOSE

The question of whether adult animals maintain a reservoir of endothelial progenitor cells (EPCs) in the bone marrow that is involved in neovascularization is under investigation. The following study was undertaken to examine the potential contribution of EPCs to the development of choroidal neovascularization (CNV) in adult mice and to examine the role of local expression of vascular endothelial growth factor (VEGF) in this process.

METHODS

Lethally irradiated, adult female nude mice were engrafted with whole bone marrow isolated from male transgenic mice expressing LacZ driven by the endothelial specific Tie-2 promoter. Two months, following bone marrow reconstitution, confirmed by quantitative Taqman PCR, an E1-deleted adenoviral vector expressing vascular endothelial growth factor (165) (Ad.VEGF(165)) was injected subretinally to induce CNV, confirmed by collagen IV immunohistochemistry. Bone marrow-derived endothelial cells were detected using either X-gal staining or Y chromosome in situ hybridization. Y chromosome positive cells within the CNV were confirmed to be endothelial cells by lectin staining.

RESULTS

Subretinal Ad.VEGF(165) was capable of inducing CNV. Four-week old lesions were found to contain LacZ expressing cells within the CNV in bone marrow transplanted animals but not in negative control animals. Eighteen percent of all Y chromosome positive cells within the CNV were found to be lectin positive while 27% of all endothelial cells within the CNV were Y chromosome positive.

CONCLUSION

Engrafted bone marrow-derived EPCs were shown to differentiate into endothelial cells at the site of subretinal VEGF-induced CNV in mice. These results suggest that EPCs contribute to the formation of neovascularization and that subretinal expression of VEGF might play an important role in recruitment of these cells to the site of CNV.

Murine marrow cellularity and the concept of stem cell competition: geographic and quantitative determinants in stem cell biology

In unperturbed mice, the marrow cell numbers correlate with the stem cell numbers. High levels of long-term marrow engraftment are obtained with infusion of high levels of marrow cells in untreated mice. To address the issue of stem cell competition vs 'opening space', knowledge of total murine marrow cellularity and distribution of stem and progenitor cells are necessary. We determined these parameters in different mouse strains. Total cellularity in BALB/c mice was 530+/-20 million cells; stable from 8 weeks to 1 year of age. C57BL/6J mice had 466+/-48 million marrow cells. Using these data, theoretical models of infused marrow (40 million cells) replacing or adding to host marrow give chimerism values of 7.5 and 7.0%, respectively; the observed 8-week engraftment of 40 million male BALB/c marrow cells into female hosts (72 mice) gave a value of 6.91+/-0.4%. This indicates that syngeneic engraftment is determined by stem cell competition. Our studies demonstrate that most marrow cells, progenitors and engraftable stem cells are in the spine. There was increased concentration of progenitors in the spine. Total marrow harvest for stem cell purification and other experimental purposes was both mouse and cost efficient with over a four-fold decrease in animal use and a financial saving.

Effects of fluorescent and nonfluorescent tracing methods on lymphocyte migration in vivo

BACKGROUND

The use of fluorescent dyes to monitor in vivo cellular migration and proliferation has greatly expanded, but little is known about their potential influence on cell migration.

METHODS

Adoptive transfer studies of lymphocytes labeled with various dyes were performed, and their in vivo homing was compared with that of coinjected unlabeled control cells. In addition, in vitro migration and binding studies were performed to analyze the various steps of transmigration separately.

RESULTS

These data showed that the intracellular fluorescent dyes calcein acetoxymethyl ester, 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethyl ester, 5-chloromethylfluorescein diacetate, 5-(and-6)-carboxyfluorescein diacetate, succinimidyl ester, and fluorescein isothiocyanate affect in vivo homing of especially B lymphocytes to lymphoid organs, without any direct effect on in vitro chemotactic or adhesive activity. The only label that did not affect migration was the extracellular and nonfluorescent molecule biotin, provided that the labeling was performed at room temperature. Interestingly, by using the highly versatile congenic Ly5.1-Ly5.2 system, we also demonstrated intrinsic differences in lymphocyte migration based on allelic differences.

CONCLUSIONS

Our data showed that fluorescent labeling of lymphocytes has a severe effect on their homing capacity in vivo. Labeling of cells with biotin appeared to be a good alternative for this purpose; however, if direct fluorescence is required, the negative effects on cell migration should be considered.

Local irradiation enhances congenic donor pluripotent hematopoietic stem cell engraftment similarly in irradiated and nonirradiated sites

Long-term multilineage chimerism is achieved in CD45 congenic mice receiving high bone marrow doses with or without mediastinal irradiation (MI). Increased donor chimerism results in MI-treated compared with nonirradiated animals, suggesting that MI makes "space" for engraftment of donor pluripotent hematopoietic stem cells (PHSCs). We have now examined whether space is systemic or whether increased engraftment of donor marrow in locally irradiated mice is confined to the irradiated bones. While increased donor chimerism was observed in irradiated bones compared with nonirradiated bones of MI-treated animals 4 weeks following bone marrow transplantation (BMT), these differences were minimal by 40 weeks. MI-treated chimeras contained more adoptively transferable donor PHSCs in the marrow of both irradiated and distant bones compared with non-MI-treated chimeras. Similar proportions of donor PHSCs were present in irradiated and nonirradiated bones of locally irradiated mice at both 4 and 40 weeks. Irradiated bones contained more donor short-term repopulating cells than distant bones at 4 weeks, but not 40 weeks, after BMT. Our study suggests that local proliferation of donor PHSCs in mice receiving local irradiation rapidly leads to a systemic increase in donor PHSC engraftment.

Labeling of hematopoietic stem and progenitor cells in novel activatable EGFP reporter mice

Conditional activation and inactivation of genes using the Cre/loxP recombination system is a powerful tool for the analysis of gene function and for tracking cell fate. Here we report a novel silent EGFP reporter mouse line generated by enhancer trap technology using embryonic stem (ES) cells. Following transfection with the silent EGFP reporter construct, positive ES cell clones were treated with Cre recombinase. These "activated clones" were then further selected on the basis of ubiquitous EGFP expression during in vitro differentiation. The parental "silent" clones were then used for generating mice. Upon Cre-mediated activation in ovo tissues tested from these mice express EGFP. Long-term, strong and sustainable expression of EGFP is observed in most myeloid and lymphoid cells. As shown by in vivo transplantation assays, the majority of hematopoietic stem cells (HSCs) and spleen colony-forming units (CFU-S) reside within the EGFP positive fraction. Most in vitro colony-forming units (CFU-Cs) isolated from bone marrow also express EGFP. Thus, these reporter mice are useful for the analysis of Cre-mediated recombination in HSCs and hematopoietic progenitor cells. This, in combination with the high accessibility of the loxP sites, makes these mice a valuable tool for testing cell/tissue-specific Cre-expressing mice. .

Nonmyeloablative conditioning is sufficient to allow engraftment of EGFP-expressing bone marrow and subsequent acceptance of EGFP-transgenic skin grafts in mice

Immunologic reactions against gene therapy products may prove to be a frequent problem in clinical gene therapy protocols. Enhanced green fluorescence protein (EGFP) is commonly used as a marker in gene transfer protocols, and immune responses against EGFP-expressing cells have been documented. The present study was designed to investigate the effect of a pharmacologic, nonmyeloablative, conditioning regimen on the development of EGFP+ donor/recipient mixed bone marrow chimerism and ensuing tolerance to EGFP-expressing transplants. To this end, C57BL/6J (B6) mice were treated with soluble formulations of either busulfan (Busulfex) or the closely related compound treosulfan, followed by transplantation of bone marrow cells from EGFP-transgenic (B6-EGFP.Tg) donor mice. Such conditioning regimens resulted in long-term persistence of donor EGFP+ cells among various hematopoietic lineages from blood, bone marrow, and thymus. Stable hematopoietic chimeras transplanted at 10 to 17 weeks after bone marrow transplantation (BMT) with B6-EGFP.Tg skin grafts all accepted their transplants, whereas non-EGFP chimeric B6 control animals were able to mount rejection of the EGFP+ B6 skin grafts. Control third-party grafts from major histocompatibility complex (MHC)-mismatched mice were rejected within 20 days, indicating that acceptance of EGFP-expressing skin grafts was the result of specific immune tolerance induction by the transplantation of EGFP-transgenic bone marrow. Long-term tolerance to EGFP in chimeric recipients was confirmed by the absence of anti-EGFP-reactive T cells and antibodies. These results broaden the therapeutic potential for using hematopoietic molecular chimerism in nonmyeloablated recipients as a means of preventing rejection of genetically modified cells.

Donor chimerism and stem cell function in a murine congenic transplantation model after low-dose radiation conditioning: effects of a retroviral-mediated gene transfer protocol and implications for gene therapy

OBJECTIVE

We investigated low-dose radiation conditioning for the transplantation of retrovirus-transduced cells in a C57Bl6/J murine model.

MATERIALS AND METHODS

The effect of low-dose radiation on stem cell function was investigated using a competitive repopulation assay. Stem cell function of marrow cells that underwent a retroviral-mediated gene transfer (RMGT) protocol was examined by this assay, and donor chimerism of these cells when transplanted into 160-cGy conditioned syngeneic hosts was compared to fresh marrow.

RESULTS

Irradiation with 300 or 160 cGy substantially decreased stem cell function as measured by competitive repopulation. Animals conditioned with 160 cGy and transplanted with 20 x 10(6) fresh marrow cells permitted donor cell engraftment of 53.6% +/- 11.4% 6 months after transplant compared to 100% donor cell engraftment after 1100 cGy irradiation. Lymphoid and myeloid engraftment did not significantly differ from total engraftment in submyeloablated hosts. When transplanted into lethally irradiated hosts, the competitive repopulating activity of marrow treated with a single dose of 5-fluorouracil followed by ex vivo culture according to a standard RMGT protocol was equal to 5-fluorouracil-only treated marrow. However, cells treated with 5-fluorouracil or 5-fluorouracil plus ex vivo culture for RMGT repopulated less well than fresh marrow cells in 160 cGy conditioned hosts.

CONCLUSIONS

Low-dose irradiation decreases host stem cell function, allowing engraftment of both fresh and RMGT protocol-treated marrow, although the engraftment of 5-fluorouracil-treated cells was reduced at least two-fold, and 5-fluorouracil plus RMGT protocol-treated cells at least three-fold, compared to fresh marrow. Modification of current RMGT protocols may be important for optimizing engraftment under these conditions.

Selective growth advantage of wild-type lymphocytes in X-linked SCID recipients

The cytokine receptor common gamma chain (gamma c) plays a pivotal role in multiple interleukin signaling, and gamma c gene mutations cause an X-linked form of SCID (X-SCID). Recently, gamma c gene transfer into the autologous X-SCID BM achieved appreciable lymphocyte reconstitution, contrasting with the limited success in previous gene therapy trials targeting hematopoietic stem cells. To understand the mechanisms underlying this success, we examined the repopulating potential of the wild-type (WT) BM cells using an X-SCID mouse model. Limited numbers of WT cells were infused into non-ablated WT and X-SCID hosts. Whereas no appreciable engraftment was observed in WT recipients, donor-derived lymphocytes repopulated well in X-SCID, reaching 37% (10(6)cells given) and 53% (10(7) cells given) of the normal control value 5 months post BMT. A lineage analysis showed a predominance of the donor-derived lymphocytes (CD4(+) T, CD8(+) T, B and NK cells) in X-SCID while the donor-derived granulocytes and monocytes engrafted poorly. These results showed a selective advantage of WT cells in X-SCID, and that the advantage was restricted to lymphocytes. In human gene therapy for X-SCID, an analogous growth advantage would greatly enhance the repopulation of lymphocytes derived from a very small number of gamma c gene-supplemented precursors.

Growth factor treatment prior to low-dose total body irradiation increases donor cell engraftment after bone marrow transplantation in mice

Low-toxicity conditioning regimens prior to bone marrow transplantation (BMT) are widely explored. We developed a new protocol using hematopoietic growth factors prior to low-dose total body irradiation (TBI) in recipients of autologous transplants to establish high levels of long-term donor cell engraftment. We hypothesized that treatment of recipient mice with growth factors would selectively deplete stem cells, resulting in successful long-term donor cell engraftment after transplantation. Recipient mice were treated for 1 or 7 days with growth factors (stem cell factor [SCF] plus interleukin 11 [IL-11], SCF plus Flt-3 ligand [FL], or granulocyte colony-stimulating factor [G-CSF]) prior to low-dose TBI (4 Gy). Donor cell chimerism was measured after transplantation of congenic bone marrow cells. High levels of donor cell engraftment were observed in recipients pretreated for 7 days with SCF plus IL-11 or SCF plus FL. Although 1-day pretreatments with these cytokines initially resulted in reduced donor cell engraftment, a continuous increase in time was observed, finally resulting in highly significantly increased levels of donor cell contribution. In contrast, G-CSF treatment showed no beneficial effects on long-term engraftment. In vitro stem cell assays demonstrated the effect of cytokine treatment on stem cell numbers. Donor cell engraftment and number of remaining recipient stem cells after TBI were strongly inversely correlated, except for groups treated for 1 day with SCF plus IL-11 or SCF plus FL. We conclude that long-term donor cell engraftment can be strongly augmented by treatment of recipient mice prior to low-dose TBI with hematopoietic growth factors that act on primitive cells.

IFN-gamma production by intrinsic renal cells and bone marrow-derived cells is required for full expression of crescentic glomerulonephritis in mice

The contribution of IFN-gamma from bone marrow (BM) and non-BM-derived cells to glomerular and cutaneous delayed-type hypersensitivity (DTH) was studied in mice. Chimeric IFN-gamma mice (IFN-gamma(+/+) BM chimera), in which IFN-gamma production was restricted to BM-derived cells, were created by transplanting normal C57BL/6 (wild-type (WT)) BM into irradiated IFN-gamma-deficient mice. BM IFN-gamma-deficient chimeric mice (IFN-gamma(-/-) BM chimera) were created by transplanting WT mice with IFN-gamma-deficient BM. WT and sham chimeric mice (WT mice transplanted with WT BM) developed crescentic glomerulonephritis (GN) with features of DTH (including glomerular T cell and macrophage infiltration) in response to an Ag planted in their glomeruli and skin DTH following subdermal Ag challenge. IFN-gamma-deficient mice showed significant protection from crescentic GN and reduced cutaneous DTH. IFN-gamma(+/+) BM chimeric and IFN-gamma(-/-) BM chimeric mice showed similar attenuation of crescentic GN as IFN-gamma-deficient mice, whereas cutaneous DTH was reduced only in IFN-gamma(-/-) BM chimeras. In crescentic GN, IFN-gamma was expressed by tubular cells and occasional glomerular cells and was colocalized with infiltrating CD8(+) T cells, but not with CD4(+) T cells or macrophages. Renal MHC class II expression was reduced in IFN-gamma(+/+) BM chimeric mice and was more severely reduced in IFN-gamma-deficient mice and IFN-gamma(-/-) BM chimeric mice. These studies show that IFN-gamma expression by both BM-derived cells and intrinsic renal cells is required for the development of crescentic GN, but IFN-gamma production by resident cells is not essential for the development of cutaneous DTH.

In vivo persistence of retrovirally transduced murine long-term repopulating cells is not limited by expression of foreign gene products in the fully or minimally myeloablated setting

Many nonmalignant hematologic disorders could potentially be treated by genetic correction of as few as 5-10% of target lineage cells. However, immune system clearance of cells expressing gene products perceived as foreign could be limiting. There is evidence that tolerance to foreign proteins can result when myeloablative conditioning is used, but this limits the overall applicability of such techniques. Therefore, we sought to evaluate the engraftment of hematopoietic stem cells carrying a foreign transgene after low-dose irradiation by comparing in vivo survival of murine long-term repopulating cells (LTRC) transduced with either a retroviral vector expressing the bacterial neomycin phosphotransferase gene (neo) or a vector containing neo gene sequences but modified to prevent protein expression (nonexpression). First, marrow cells from congenic donors were transduced with either vector and transplanted into recipients treated with standard dose irradiation of 800 rads. High-level engraftment and gene marking resulted, without differences in the marking levels or pattern of persistence of the cells between cells transduced with either vector. Low-dose irradiation at 100 rads was tested using higher cell doses. Marking levels as high as 10% overall were obtained, again with no differences between mice receiving cells transduced with the neo versus the nonexpression vectors. To investigate a potentially more immunogenic protein, marrow cells were transduced with a vector containing the green fluorescent protein (GFP) gene, and their persistence was studied in recipient mice receiving 100 rads. Stable GFP expression in 5-10% of circulating cells was observed long term. We conclude that even with very low dose conditioning, engraftment by genetically modified LTRC cells at clinically significant levels can be achieved without evidence for clearance of cells known to be expressing immunogenic proteins.