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

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.

Cardiotrophin-Like Cytokine Factor 1 Exhibits a Myeloid-Biased Hematopoietic-Stimulating Function

Cardiotrophin-like cytokine factor 1 (CLCF1) is secreted as a complex with the cytokine receptor-like factor 1 (CRLF1). Syndromes caused by mutations in the genes encoding CLCF1 or CRLF1 suggest an important role for CLCF1 in the development and regulation of the immune system. In mice, CLCF1 induces B-cell expansion, enhances humoral responses and triggers autoimmunity. Interestingly, inactivation of CRLF1, which impedes CLCF1 secretion, leads to a marked reduction in the number of bone marrow (BM) progenitor cells, while mice heterozygous for CLCF1 display a significant decrease in their circulating leukocytes. We therefore hypothesized that CLCF1 might be implicated in the regulation of hematopoiesis. To test this hypothesis, murine hematopoietic progenitor cells defined as Lin⁻Sca1⁺c-kit⁺ (LSK) were treated in vitro with ascending doses of CLCF1. The frequency and counts of LSK cells were significantly increased in the presence of CLCF1, which may be mediated by several CLCF1-induced soluble factors including IL-6, G-CSF, IL-1β, IL-10, and VEGF. CLCF1 administration to non-diseased C57BL/6 mice resulted in a pronounced increase in circulating myeloid cells, which was concomitant with augmented LSK and myeloid cell counts in the BM. Likewise, CLCF1 administration to mice following sub-lethal irradiation or congeneic BM transplantation (BMT) resulted in accelerated LSK recovery along with a sustained increase in BM-derived CD11b⁺ cells. Altogether, our observations establish an important and unforeseen role for CLCF1 in regulating hematopoiesis with a bias toward myeloid cell differentiation.

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.

Boosting Hematopoietic Engraftment after in Utero Transplantation through Vascular Niche Manipulation

In utero hematopoietic stem/progenitor cell transplantation (IUHSCT) has only been fully successful in the treatment of congenital immunodeficiency diseases. Using sheep as a large animal model of IUHSCT, we demonstrate that administration of CD146⁺CXCL12⁺VEGFR2⁺ or CD146⁺CXCL12⁺VEGFR2⁻ cells prior to, or in combination with, hematopoietic stem/progenitor cells (HSC), results in robust CXCL12 production within the fetal marrow environment, and significantly increases the levels of hematopoietic engraftment. While in the fetal recipient, donor-derived HSC were found to reside within the trabecular bone, the increased expression of VEGFR2 in the microvasculature of CD146⁺CXCL12⁺VEGFR2⁺ transplanted animals enhanced levels of donor-derived hematopoietic cells in circulation. These studies provide important insights into IUHSCT biology, and demonstrate the feasibility of enhancing HSC engraftment to levels that would likely be therapeutic in many candidate diseases for IUHSCT.

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After IUHSCT, HSC engraft in the trabecular bone of the metaphysis

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CD146^++(+/−) cells engraft in diaphysis and make hematopoiesis-supporting cytokines

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Donor cell-derived CXCL12 and VEGFR2 significantly increase HSC engraftment

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IUHSCT of CD146⁺CXCL12⁺VEGFR2⁺ cells prior to HSC could be curative in several diseases

Inflammatory Ly6Chi monocytes and their conversion to M2 macrophages drive atherosclerosis regression

Atherosclerosis is a chronic inflammatory disease, and developing therapies to promote its regression is an important clinical goal. We previously established that atherosclerosis regression is characterized by an overall decrease in plaque macrophages and enrichment in markers of alternatively activated M2 macrophages. We have now investigated the origin and functional requirement for M2 macrophages in regression in normolipidemic mice that received transplants of atherosclerotic aortic segments. We compared plaque regression in WT normolipidemic recipients and those deficient in chemokine receptors necessary to recruit inflammatory Ly6Chi (Ccr2-/- or Cx3cr1-/-) or patrolling Ly6Clo (Ccr5-/-) monocytes. Atherosclerotic plaques transplanted into WT or Ccr5-/- recipients showed reduced macrophage content and increased M2 markers consistent with plaque regression, whereas plaques transplanted into Ccr2-/- or Cx3cr1-/- recipients lacked this regression signature. The requirement of recipient Ly6Chi monocyte recruitment was confirmed in cell trafficking studies. Fate-mapping and single-cell RNA sequencing studies also showed that M2-like macrophages were derived from newly recruited monocytes. Furthermore, we used recipient mice deficient in STAT6 to demonstrate a requirement for this critical component of M2 polarization in atherosclerosis regression. Collectively, these results suggest that continued recruitment of Ly6Chi inflammatory monocytes and their STAT6-dependent polarization to the M2 state are required for resolution of atherosclerotic inflammation and plaque regression.

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.

mosGCTL-7, a C-Type Lectin Protein, Mediates Japanese Encephalitis Virus Infection in Mosquitoes

Japanese encephalitis virus (JEV) is an arthropod-borne flavivirus prevalent in Asia and the Western Pacific and is the leading cause of viral encephalitis. JEV is maintained in a transmission cycle between mosquitoes and vertebrate hosts, but the molecular mechanisms by which the mosquito vector participates in transmission are unclear. We investigated the expression of all C-type lectins during JEV infection in Aedes aegypti The C-type lectin mosquito galactose-specific C-type lectin 7 (mosGCTL-7) (VectorBase accession no. AAEL002524) was significantly upregulated by JEV infection and facilitated infection in vivo and in vitro mosGCTL-7 bound to the N-glycan at N154 on the JEV envelope protein. This recognition of viral N-glycan by mosGCTL-7 is required for JEV infection, and we found that this interaction was Ca²⁺ dependent. After mosGCTL-7 bound to the glycan, mosPTP-1 bound to mosGCTL-7, promoting JEV entry. The viral burden in vivo and in vitro was significantly decreased by mosPTP-1 double-stranded RNA (dsRNA) treatment, and infection was abolished by anti-mosGCTL-7 antibodies. Our results indicate that the mosGCTL-7/mosPTP-1 pathway plays a key role in JEV infection in mosquitoes. An improved understanding of the mechanisms underlying flavivirus infection in mosquitoes will provide further opportunities for developing new strategies to control viral dissemination in nature.IMPORTANCE Japanese encephalitis virus is a mosquito-borne flavivirus and is the primary cause of viral encephalitis in the Asia-Pacific region. Twenty-four countries in the WHO Southeast Asia and Western Pacific regions have endemic JEV transmission, which exposes >3 billion people to the risks of infection, although JEV primarily affects children. C-type lectins are host factors that play a role in flavivirus infection in humans, swine, and other mammals. In this study, we investigated C-type lectin functions in JEV-infected Aedes aegypti and Culex pipiens pallens mosquitoes and cultured cells. JEV infection changed the expression of almost all C-type lectins in vivo and in vitro, and mosGCTL-7 bound to the JEV envelope protein via an N-glycan at N154. Cell surface mosPTP-1 interacted with the mosGCTL-7-JEV complex to facilitate virus infection in vivo and in vitro Our findings provide further opportunities for developing new strategies to control arbovirus dissemination in nature.

Comparative sensitivity analyses of quantitative polymerase chain reaction and flow cytometry in detecting cellular microchimerism in murine tissues

Cellular microchimerism is defined as the presence of small populations of cells from one individual in another genetically distinct individual. The pivotal role of cellular microchimerism in a variety of immune settings is increasingly recognized, e.g. in context of pregnancy, transplantation and cancer. However, the detection of chimeric cells is overshadowed by technical limitations. This study aimed to overcome these limitations by testing the sensitivity and detection limit of a molecular biology approach (quantitative polymerase chain reaction, qPCR) and a cellular approach (flow cytometry) in order to identify experimentally induced cellular microchimerism in mice. Leukocytes isolated from lymph nodes or spleens of transgenic enhanced green fluorescent protein (eGFP) and CD45.1 mice respectively were used as targets to be detected as microchimeric cells among wild type (wt) or haploidentical cells. The detection limit of microchimeric cells by flow cytometry was 0.05% or lower for the respective eGFP(+) or CD45.1(+) cell subsets, which equals 48 cells or fewer per 1×10(5) wt cells. The detection limit of CD45.1(+) and CD45.2(+) cells among haploidentical CD45.1(+)2(+) cells by flow cytometry was 48 cells (0.05%) and 198 cells (0.2%), respectively. Using qPCR, a detection limit of 198 eGFP(+) cells per 1×10(5) wt cells, respective 0.2%, could be achieved. We here introduce two technical approaches to reliably detect low number of chimeric cells at a low detection limit and high sensitivity in transgenic mouse systems.

Dose response kinetics of CD8 lymphocytes from young animals transfused into old animals and challenged with influenza

Transfusion of autologous leukocytes after prolonged storage has been proposed as a means of rejuvenating the immune system of older individuals. The rationale for this approach is that age related immune decline is associated with a diminished pool of naïve T cells following atrophy of the thymus and reduction in thymic output. The presence of high levels of naïve T cells within the blood of young individuals could provide a boost to the immune system of an older “self” through a rejuvenation of the naïve T cell pool. However what remains unresolved is whether the cells could be incorporated effectively into the T cell pool of the host and whether effectors could be generated. Using CD45 congenic mice in our experiments we show that the transfusion of young donor cells into older congenic host animals leads to their successful incorporation into the peripheral T cell pool. When the recipients were challenged with influenza virus, specific effector CD8 cells were generated which were of both host and donor origin. We found no relationship between the number of responder cells of donor origin at the time of assay and the number of cells injected.

Differential representation of B cell subsets in mixed bone marrow chimera mice due to expression of allelic variants of CD45 (CD45.1/CD45.2)

The CD45 congenic marker system is a highly utilized technique to track hematopoietic cells following bone marrow transplantation (BMT), with CD45.1 and CD45.2 being efficiently distinguished by flow cytometry. During the analysis of control mixed BM chimera mice in which lethally irradiated recipients were transplanted with an equal number of BM cells from WT CD45.1 and WT CD45.2 mice, we observed an unequal reconstitution of specific B cell subsets in the bone marrow (BM), lymph node (LN) and spleen. Specifically, in the BM and LN, there was an increase in the percentage of CD45.2 mature B cells. In the spleen, an increase in the percentage of CD45.2 transitional (T) 1 and T2 cells was observed. In contrast, the percentage of splenic CD45.1 marginal zone (MZ) B cells was significantly increased. When we compared the percentage of B cell subsets in unmanipulated WT CD45.1 and WT CD45.2 mice, we found that WT CD45.2 mice had significantly more LN B cells while WT CD45.1 mice exhibited an increase in MZ B cells. These data indicate that the alteration in the ratio of CD45.1 and CD45.2 B cell subsets in mixed chimera mice is a cell-intrinsic effect. Thus whenever the CD45 congenic system is used to track two genetically distinct populations of immune cells WT chimeras must be generated to allow normalization of the experimental data to avoid the reporting of unintentionally skewed data.

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.

Development and tolerization of hyperacute rejection in a transgenic mouse graft versus host model

BACKGROUND

The hyperacute rejection mediated by preexisting antibodies is a major impediment to the success of transplants across allogeneic and xenogeneic barriers. We report a new mouse model that allows us to not only monitor the sensitization of B cells mediating the hyperacute response but also validate therapeutic strategies for tolerizing them.

MODEL

The new model system uses 5C.C7,RAG2 T-cell receptor transgenic T cells and B10.S(9R),CD3[Latin Small Letter Open E] hosts for adoptive transfer experiments.

RESULTS AND CONCLUSIONS

In the allogeneic hosts, transgenic T cells expanded briefly before being chronically deleted. Once the deletion was initiated, a second graft of donor cells was used to assess a hyperacute response. The rapid rejection of the second cohort correlated with the appearance of donor-specific antibodies in the serum. Interestingly, chronically stimulated T cells were relatively resistant to hyperacute rejection, suggesting an explanation for the slower rejection kinetics of the first cohort even as the second cohort of identical donor cells was being hyperacutely rejected. Finally, we could tolerize the potential for a hyperacute response, by pretreating recipients with a single infusion of naive donor B cells before the first T-cell transfer. This treatment not only abrogated the development of a hyperacute response but also allowed the primary graft to survive in vivo for extended periods.

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.

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.

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.

PD-1-dependent mechanisms maintain peripheral tolerance of donor-reactive CD8+ T cells to transplanted tissue

Peripheral mechanisms of self-tolerance often depend on the quiescent state of the immune system. To what degree such mechanisms can be engaged in the enhancement of allograft survival is unclear. To examine the role of the PD-1 pathway in the maintenance of graft survival following blockade of costimulatory pathways, we used a single-Ag mismatch model of graft rejection where we could track the donor-specific cells as they developed endogenously and emerged from the thymus. We found that graft-specific T cells arising under physiologic developmental conditions at low frequency were actively deleted at the time of transplantation under combined CD28/CD40L blockade. However, this deletion was incomplete, and donor-specific cells that failed to undergo deletion up-regulated expression of PD-1. Furthermore, blockade of PD-1 signaling on these cells via in vivo treatment with anti-PD-1 mAb resulted in rapid expansion of donor-specific T cells and graft loss. These results suggest that the PD-1 pathway was engaged in the continued regulation of the low-frequency graft-specific immune response and thus in maintenance of graft survival.

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.