Rat nigral xenografts survive in the brain of MHC class II-, but not class I-deficient mice

Monoclonal antibody-mediated immunosuppression enables long-term survival of transplanted human neural stem cells in mouse brain

BACKGROUND

As the field of stem cell therapy advances, it is important to develop reliable methods to overcome host immune responses in animal models. This ensures survival of transplanted human stem cell grafts and enables predictive efficacy testing. Immunosuppressive drugs derived from clinical protocols are frequently used but are often inconsistent and associated with toxic side effects. Here, using a molecular imaging approach, we show that immunosuppression targeting costimulatory molecules CD4 and CD40L enables robust survival of human xenografts in mouse brain, as compared to conventional tacrolimus and mycophenolate mofetil.

METHODS

Human neural stem cells were modified to express green fluorescent protein and firefly luciferase. Cells were implanted in the fimbria fornix of the hippocampus and viability assessed by non-invasive bioluminescent imaging. Cell survival was assessed using traditional pharmacologic immunosuppression as compared to monoclonal antibodies directed against CD4 and CD40L. This paradigm was also implemented in a transgenic Alzheimer's disease mouse model.

RESULTS

Graft rejection occurs within 7 days in non-immunosuppressed mice and within 14 days in mice on a traditional regimen. The addition of dual monoclonal antibody immunosuppression extends graft survival past 7 weeks (p < .001) on initial studies. We confirm dual monoclonal antibody treatment is superior to either antibody alone (p < .001). Finally, we demonstrate robust xenograft survival at multiple cell doses up to 6 months in both C57BL/6J mice and a transgenic Alzheimer's disease model (p < .001). The dual monoclonal antibody protocol demonstrated no significant adverse effects, as determined by complete blood counts and toxicity screen.

CONCLUSIONS

This study demonstrates an effective immunosuppression protocol for preclinical testing of stem cell therapies. A transition towards antibody-based strategies may be advantageous by enabling stem cell survival in preclinical studies that could inform future clinical trials.

Concise Review: Innate and Adaptive Immune Recognition of Allogeneic and Xenogeneic Cell Transplants in the Central Nervous System

Over the last 30 years, numerous allogeneic and xenogeneic cell grafts have been transplanted into the central nervous system (CNS) of mice and men in an attempt to cure neurological diseases. In the early studies, human or porcine embryonic neural cells were grafted in the striatum of animals or patients in an attempt to replace lost neurons. Although the immune-privileged status of the brain as a recipient organ was widely accepted, it rapidly became evident that CNS-grafted allogeneic and xenogeneic cells could be recognized and rejected by the immune system, resulting in poor neural graft survival and limited functional recovery. Since then, the CNS transplantation field has witnessed a sharp rise in the number of studies in which allogeneic and xenogeneic neural or mesenchymal stem cells (NSCs or MSCs, respectively) are transplanted, predominantly aiming at providing trophic stimulation and promoting endogenous repair of the brain. Interestingly, in many recent NSC and MSC-based publications functional improvement was used as the principal measure to evaluate the success of cell transplantation, while the fate of transplanted cells remained largely unreported. In this review, we first attempt to understand why primary neural cell isolates were largely substituted for NSCs and MSCs in cell grafting studies. Next, we review the current knowledge on the immune mechanisms involved in the recognition and rejection of allogeneic and xenogeneic cellular grafts in the CNS. Finally, we propose strategies to reduce graft immunogenicity and to improve graft survival in order to design improved cell-based CNS therapies. Stem Cells Translational Medicine 2017;6:1434-1441.

Cell therapy for Parkinson's disease: Functional role of the host immune response on survival and differentiation of dopaminergic neuroblasts

Parkinson's disease (PD) is a neurodegenerative disorder, whose cardinal pathology is the loss of dopaminergic neurons in the substantia nigra. Current treatments for PD have side effects in the long term and do not halt disease progression or regenerate dopaminergic cell loss. Attempts to compensate neuronal cell loss by transplantation of dopamine-producing cells started more than 30 years ago, leading to several clinical trials. These trials showed safety and variable efficacy among patients. In addition to variability in efficacy, several patients developed graft-induced dyskinesia. Nevertheless, they have provided a proof of concept that motor symptoms could be improved by cell transplantation. Cell transplantation in the brain presents several immunological challenges. The adaptive immune response should be abolished to avoid graft rejection by the host. In addition, the innate immune response will always be present after transplanting cells into the brain. Remarkably, the innate immune response can have dramatic effects on the survival, differentiation and proliferation of the transplanted cells, but has been hardly investigated. In this review, we analyze data on the functional effects of signals from the innate immune system on dopaminergic differentiation, survival and proliferation. Then, we discussed efforts on cell transplantation in animal models and PD patients, highlighting the immune response and the immunomodulatory treatment strategies performed. The analysis of the available data lead us to conclude that the modulation of the innate immune response after transplantation can increase the success of future clinical trials in PD by enhancing cell differentiation and survival. This article is part of a Special Issue entitled SI: PSC and the brain.

IgG response to intracerebral xenotransplantation: specificity and role in the rejection of porcine neurons

Xenogenic fetal neuroblasts are considered as a potential source of transplantable cells for the treatment of neurodegenerative diseases, but immunological barriers limit their use in the clinic. While considerable work has been performed to decipher the role of the cellular immune response in the rejection of intracerebral xenotransplants, there is much still to learn about the humoral reaction. To this end, the IgG response to the transplantation of fetal porcine neural cells (PNC) into the rat brain was analyzed. Rat sera did not contain preformed antibodies against PNC, but elicited anti-porcine IgG was clearly detected in the host blood once the graft was rejected. Only the IgG1 and IgG2a subclasses were up-regulated, suggesting a T-helper 2 immune response. The main target of these elicited IgG antibodies was porcine neurons, as determined by double labeling in vitro and in vivo. Complement and anti-porcine IgG were present in the rejecting grafts, suggesting an active role of the host humoral response in graft rejection. This hypothesis was confirmed by the prolonged survival of fetal porcine neurons in the striatum of immunoglobulin-deficient rats. These data suggest that the prolonged survival of intracerebral xenotransplants relies on the control of both cell-mediated and humoral immune responses.

The immunological challenges of cell transplantation for the treatment of Parkinson's disease

Dopaminergic cell transplantation is an experimental therapy for Parkinson's disease (PD). It has many potential theoretical advantages over current treatment strategies such as providing continuous local dopaminergic replenishment, eliminating motor fluctuations and medication-induced dyskinesias, slowing down disease progression or even reversing disease pathology in the host. Recent studies also show that dopaminergic cell transplants provide long-term neuromodulation in the basal ganglia that simulates the combined effects of oral dopaminergic therapy and surgical therapies like deep brain stimulation, the contemporary therapeutic approach to advanced PD. However, dopaminergic cell transplantation in PD as not been optimized and current experimental techniques have many drawbacks. In published experiments to date of attempted dopaminergic grafting in PD, the major challenges are unacceptable graft-induced dyskinesias or failure of such grafts to exceed the benefits afforded by sham surgery. A deleterious host immune response to the transplant has been implicated as a major putative cause for these adverse outcomes. This article focuses on recent advances in understanding the immunology of the transplantation in PD and possible methods to overcome adverse events such that we could translate cell replacement strategies into viable clinical treatments in the future.

Neural stem/progenitor cells transplanted to the hypoglossal nucleus integrates with the host CNS in adult rats and promotes motor neuron survival

Transplantation of neural stem cells and the mobilization of endogenous neuronal precursors in the adult brain have been proposed as therapeutic strategies for central nervous system disorders and injuries. The aim of the present study was to investigate the possible survival and integration of grafted neural progenitor cells (NPCs) from the subventricular zone (SVZ) in a hypoglossal nerve avulsion model with substantial neuronal loss. Adult neural progenitor cells (NPCs) from the subventricular zone (SVZ) were cultured from inbred transgenic eGFP Lewis rats and transplanted to the hypoglossal nucleus of inbred Lewis rat from the same family but that were not carrying the eGFP strain after avulsion of the hypoglossal nerve. Grafted cells survived in the host more than 3 months and differentiated into neurons [βIII tubulin (Tuj-1 staining)] with fine axon- and dendrite-like processes as well as astrocytes (GFAP) and oligodendrocytes (O4) with typical morphology. Staining for synaptic structures (synaptophysin and bassoon) indicated integration of differentiated cells from the graft with the host CNS. Furthermore, transplantation of NPCs increased the number of surviving motoneurons in the hypoglossal nucleus after nerve avulsion that, if untreated, result in substantial neuronal death. The NPCs used in this study expressed VEGF in vitro as well as in vivo following transplantation that may mediate the rescue effect of the axotomized motoneurons.

Effect of carbamylated erythropoietin on major histocompatibility complex expression and neural differentiation of human neural stem cells

The expression of major histocompatibility complex (MHC) on human neural stem cells (hNSCs) is tightly related to the fate of these cells in transplantation, therefore strategies to relieve rejection and promote graft survival are necessary to be applied. This study investigated the effect of carbamylated erythropoietin (CEPO) on MHC expression and differentiation of hNSCs with or without IFN-gamma incubation. Results showed that low levels of MHC molecules were expressed on hNSCs and increased by IFN-gamma. CEPO enhanced MHC-I antigens in both proliferative and differentiated hNSCs, but decreased MHC-II antigens in differentiated hNSCs and those cells exposed to IFN-gamma. Furthermore, CEPO promoted neural differentiation of hNSCs and outgrowth of neurites. Western blot analysis revealed activation of Stat3, Stat5 and Akt during these processes. These results suggest that CEPO may have immunoregulatory function in hNSCs besides its neuroprotection.

Effect of inflammatory cytokines on major histocompatibility complex expression and differentiation of human neural stem/progenitor cells

To develop transplantation of neural stem/progenitor cells (NSPCs) as a successful treatment of neurodegenerative disorders, the possible induction of an inflammatory response following implantation needs to be taken into consideration. Inflammatory cytokines can upregulate major histocompatibility complex (MHC) expression on transplanted cells, thereby rendering them more susceptible to graft rejection. Furthermore, cytokines also have a profound effect on cell differentiation, migration, and proliferation, which can greatly affect the outcome of transplantation. Here we studied the effect of three inflammatory cytokines, interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha), and interleukin-6 (IL-6), from three different species (human, monkey, rat) on expression of MHC molecules and differentiation of two human NSPC lines derived from striatum and hippocampus. Human and monkey IFN-gamma strongly upregulate MHC expression in both NSPC lines in a dose-dependent manner, whereas rat IFN-gamma has an effect on MHC expression only in hippocampal cells. Furthermore, TNF-alpha, but not IL-6, upregulates MHC expression in both NSPC lines. Differentiation of NSPCs in the presence of cytokines showed that IFN-gamma increased the neuronal yield threefold in striatal NSPC cultures and increased the number of oligodendrocytes twofold in hippocampal NSPC cultures. Addition of TNF-alpha enhanced gliogenesis in both cell lines, whereas IL-6 stimulated neurogenesis. Human NSPC lines' response to cytokines is therefore species specific and also dependent on the NSPCs' region of origin. The successful translation of different cell lines from animal models to clinical trials could be substantially influenced by the species-specific regulation of MHC and differentiation as reported here. Disclosure of potential conflicts of interest is found at the end of this article.

Allorecognition of human neural stem cells by peripheral blood lymphocytes despite low expression of MHC molecules: role of TGF-beta in modulating proliferation

Neural stem cells (NSCs) transplantation has been proposed as a means of restoring damaged brain tissue, a possibility rendered more likely by reports of low NSCs immunogenicity in various experimental models because of low expression of MHC class I and II as well as co-stimulatory molecules. We investigated the immunogenicity of a human NSC line grown in normal culture conditions and in the presence of pro-inflammatory cytokines IFN-gamma and tumor necrosis factor alpha by one-way mixed lymphocyte reaction (MLR) experiments with peripheral blood lymphocytes from eight HLA-incompatible donors. NSCs stimulated lymphocyte proliferation in almost all donors tested, with stimulation indices in the range of the low-end distribution curve of MLR between donors. The healthy subject that gave negative MLR results was the best compatible donor with respect to NSC haplotype. Since we observed low MLR responses overall, we studied if NSCs might exert any immunomodulatory activity. We detected transcription and release of the immunomodulatory molecule transforming growth factor beta (TGF-beta)-1; moreover, the addition of TGF-beta1 in MLR experiments down-regulated proliferative responses. To further confirm the immunological potential of human NSCs, we studied xenogeneic recognition of NSCs by immunocompetent cells derived from C57BL/6 mice, showing that NSCs can elicit an allo(xeno) response ex vivo. Our data indicate that NSCs have low but not negligible immunogenic potential that is sufficient to activate peripheral lymphocytes. Secretion of TGF-beta1 might balance the immunogenicity of NSCs. Nevertheless, the possibility that allo-NSCs grafting might induce in the long term an immune activation, thus vanishing their therapeutical effect, should not be overlooked and deserves further investigation.

Sustained survival of xenografted human neural stem/progenitor cells in experimental brain trauma despite discontinuation of immunosuppression

Neural stem cells have emerged as a promising therapeutic tool in CNS disease and injuries. In the clinical setting, cultured human neural stem/progenitor cells (hNSC) are an attractive possibility for transplantation to the damaged brain. However, transplantation of hNSC requires toxic immunosuppressive treatment to avoid rejection. The aim of the current study was to evaluate if shortening the duration of immunosuppression by cyclosporin A would affect hNSC survival and differentiation after transplantation to the site of a focal brain injury in the rat. hNSC were xenografted to the hippocampus and the medial limit of an experimentally induced cortical contusion. The animals received immunosuppression for either 6 or 3 weeks or no immunosuppression. The status of the grafted human cells was analysed by immunohistochemistry. No statistically significant differences were observed between the two immunosuppressed groups regarding graft survival, migration or proliferation at 6 weeks post-transplantation. In contrast, the graft survival was extremely poor in the non-immunosuppressed group. Furthermore, the expression of the differentiation markers nestin, neuronal nuclei (NeuN) and glial fibrillary acidic protein (GFAP) in the transplanted cells did not differ significantly between the two immunosuppressed groups. Moreover, a fourth group of eight animals that were immunosuppressed for 3 weeks were allowed to survive for 6 months. Five of these rats demonstrated robust graft survival in the hippocampus and scattered cells in the cortex. This study demonstrates the importance of immunosuppression but also the possibility of shortening immunosuppression without impacting on the phenotype of the grafted hNSC.

Increased capacity for axonal outgrowth using xenogenic tissue in vitro and in a rodent model of Parkinson's disease

BACKGROUND

It has previously been proposed that grafted neurons may have the potential for more extensive axonal outgrowth in a xenogenic environment, and may thus possess a primary advantage over allografts in central nervous system repair and circuit reconstruction. In order to directly address this issue, fibre outgrowth from primary dopaminergic neurons was examined both in vitro and in vivo in an allogenic or xenogenic environment. A combination of species was used to circumvent problems relating to different gestational/developmental periods for such cells.

METHODS

In the in vitro experiments, axon length was measured over 2 to 14 days in cultures derived from either rat or mouse ventral mesencephalon (VM) tissue co-cultured onto either a monolayer of the rat Neu7 inhibitory cell line or fetal rodent cortical tissue. In the in vivo experiments, fetal rat or mouse VM tissue was transplanted into the striatum or substantia nigra of 6-hydroxydopamine-lesioned athymic rats. Amphetamine-induced rotations were observed for 3 months post-transplantation and the degree of graft-mediated neurite outgrowth was analyzed.

RESULTS

Embryonic VM manifested a greater capacity for neurite formation and outgrowth on xenogenic tissue, which was shown to be significant using co-cultures of cortical cells. The transplantation study showed that xenograft-derived fibres had a greater capacity for extensive fibre projection compared with those originating from allografts.

CONCLUSION

Results from the present study provide evidence for the hypothesis that xenografts are less responsive to inhibitory molecules present in the adult host environment and as such can project over great distances. Thus neural xenotransplantation may have the potential for more complete circuit reconstruction within the damaged host brain than equivalent allografted tissue.

Compartmentalization of TCR repertoire alteration during rejection of an intrabrain xenograft

Xenograft rejections of embryonic pig neural cells implanted into the adult rat striatum occurs within 3-4 weeks, following a dramatic T cell infiltration. Little is known about the cross-talk between the brain and peripheral lymphoid tissues which results in this recruitment and lymphocyte homing. To better characterize the dynamics of the T cell response against xenogeneic neural cells implanted into the brain parenchyma, we used both qualitative and quantitative methods to follow the alterations of the CDR3 length distribution (CDR3-LD) of the TCR (T cell receptor) beta chain in the transplanted striatum and compared this response to that observed in the deep cervical lymph nodes, spleen, and blood. Data showed that the T cell repertoire diversity was highly altered in the recipient brain during xenograft rejection. Comparison of the alterations of the CDR3-LD between several animals revealed a single public alteration in the Vbeta20 family, and many private alterations of the CDR3-LD which differed from one infiltrated brain to another. Alterations of the T cell repertoire were also observed in lymphocytes homed into the deep cervical lymph nodes. However, they differed from the alterations detected in the infiltrated brains. Conversely, no significant alteration of the CDR3-LD was detected in the spleen or in the blood. These data suggest that the deep cervical lymph nodes play an active role in the process of xenograft recognition or/and rejection. However, they also indicate that the fate of T cells homed in the brain and deep cervical lymph nodes differs.

MHC expression after human neural stem cell transplantation to brain contused rats

Human neural stem cells survive and improve motor function after transplantation to the contused brain. However, the transplants might be rejected and that depends on the graft immunogenicity, the host immunological status and the immunosuppression strategy. We transplanted human neural stem cells to rats with brain contusion and analyzed the donor and host MHC antigen expression and the effect of a short-term immunosuppression with cyclosporine. In vitro human neural stem cells expressed only MHC-II antigens. This expression was down-regulated 6 weeks after transplantation. The host response was characterized by an increased MHC-II expression which was down-regulated by a longer term of immunosuppression. These findings are novel and necessary in order to understand the immunogenicity of human neural stem cell grafts.