Harnessing dendritic cells for the induction of transplantation tolerance:

HLA DR Genome Editing with TALENs in Human iPSCs Produced Immune-Tolerant Dendritic Cells

Although human induced pluripotent stem cells (iPSCs) can serve as a universal cell source for regenerative medicine, the use of iPSCs in clinical applications is limited by prohibitive costs and prolonged generation time. Moreover, allogeneic iPSC transplantation requires preclusion of mismatches between the donor and recipient human leukocyte antigen (HLA). We, therefore, generated universally compatible immune nonresponsive human iPSCs by gene editing. Transcription activator-like effector nucleases (TALENs) were designed for selective elimination of HLA DR expression. The engineered nucleases completely disrupted the expression of HLA DR on human dermal fibroblast cells (HDF) that did not express HLA DR even after stimulation with IFN-γ. Teratomas formed by HLA DR knockout iPSCs did not express HLA DR, and dendritic cells differentiated from HLA DR knockout iPSCs reduced CD4⁺ T cell activation. These engineered iPSCs might provide a novel translational approach to treat multiple recipients from a limited number of cell donors.

Co-Stimulation-Impaired Bone Marrow-Derived Dendritic Cells Prevent Dextran Sodium Sulfate-Induced Colitis in Mice

Dendritic cells (DC) are important in the onset and severity of inflammatory bowel disease (IBD). Tolerogenic DC induce T-cells to become therapeutic Foxp3+ regulatory T-cells (Tregs). We therefore asked if experimental IBD could be prevented by administration of bone marrow-derived DC generated under conventional GM-CSF/IL-4 conditions but in the presence of a mixture of antisense DNA oligonucleotides targeting the primary transcripts of CD40, CD80, and CD86. These cell products (which we call AS-ODN BM-DC) have demonstrated tolerogenic activity in preventing type 1 diabetes and preserving beta cell mass in new-onset type 1 diabetes in the NOD mouse strain, in earlier studies. In addition to measuring efficacy in prevention of experimental IBD, we also sought to identify possible mechanism(s) of action. Weight, behavior, stool frequency, and character were observed daily for 7–10 days in experimental colitis in mice exposed to dextran sodium sulfate (DSS) following injection of the AS-ODN BM-DC. After euthanasia, the colons were processed for histology while spleen and mesenteric lymph nodes (MLNs) were made into single cells to measure Foxp3+ Treg as well as IL-10+ regulatory B-cell (Breg) population frequency by flow cytometry. AS-ODN BM-DC prevented DSS-induced colitis development. Recipients of these cells exhibited significant increases in Foxp3+ Treg and IL-10+ Breg in MLN and spleen. Histological examination of colon sections of colitis-free mice remained largely architecturally physiologic and mostly free of leukocyte infiltration when compared with DSS-treated animals. Although DSS colitis is mainly an innate immunity-driven condition, our study adds to the growing body of evidence showing that Foxp3+ Treg and IL-10 Bregs can suppress a mainly innate-driven inflammation. The already-established safety of human DC generated from monocytic progenitors in the presence of the mixture of antisense DNA targeting the primary transcripts of CD40, CD80, and CD86 in humans offers the potential to adapt them for clinical IBD therapy.

Immunosuppression and monitoring of rejection in hand transplantation

Advances in vascularized composite allotransplantation over the last decade have achieved significant milestones in basic science and translational research, as well as clinically with highly encouraging functional and immunologic outcomes. However, certain immunologic challenges remain. In particular, although tolerance has been induced to nearly all components of a hand allograft in experimental models, the skin component may still be subject to acute rejection episodes. Currently, conventional immunosuppressive protocols have been successful at preventing allograft loss; however, they have not prevented episodes of acute skin rejection. Furthermore, the profound side effect profile of the life-long, high-dose, multidrug immunosuppression regimen that is necessary to maintain a viable graft alters the risk to benefit ratio of this non-life-saving procedure. Therefore, there must be a concerted effort in the scientific community to develop novel protocols to either minimize immunosuppression or to induce tolerance to the allograft to promote the widespread application of this life-changing procedure.

Tolerogenic Donor-Derived Dendritic Cells Risk Sensitization In Vivo owing to Processing and Presentation by Recipient APCs

Modification of allogeneic dendritic cells (DCs) through drug treatment results in DCs with in vitro hallmarks of tolerogenicity. Despite these observations, using murine MHC-mismatched skin and heart transplant models, donor-derived drug-modified DCs not only failed to induce tolerance but also accelerated graft rejection. The latter was inhibited by injecting the recipient with anti-CD8 Ab, which removed both CD8(+) T cells and CD8(+) DCs. The discrepancy between in vitro and in vivo data could be explained, partly, by the presentation of drug-modified donor DC MHC alloantigens by recipient APCs and activation of recipient T cells with indirect allospecificity, leading to the induction of alloantibodies. Furthermore, allogeneic MHC molecules expressed by drug-treated DCs were rapidly processed and presented in peptide form by recipient APCs in vivo within hours of DC injection. Using TCR-transgenic T cells, Ag presentation of injected OVA-pulsed DCs was detectable for ≤ 3 d, whereas indirect presentation of MHC alloantigen by recipient APCs led to activation of T cells within 14 h and was partially inhibited by reducing the numbers of CD8(+) DCs in vivo. In support of this observation when mice lacking CD8(+) DCs were pretreated with drug-modified DCs prior to transplantation, skin graft rejection kinetics were similar to those in non-DC-treated controls. Of interest, when the same mice were treated with anti-CD40L blockade plus drug-modified DCs, skin graft survival was prolonged, suggesting endogenous DCs were responsible for T cell priming. Altogether, these findings highlight the risks and limitations of negative vaccination using alloantigen-bearing "tolerogenic" DCs.

Achievements and challenges in composite tissue allotransplantation

Overall, more than 60 hand/forearm/arm transplantations and 16 face transplantations have been performed in the past 12 years. In the European experience summarized here, three grafts have been lost in response to a vascular thrombosis (n = 1), rejection and incompliance with immunosuppression (n = 1) and death (n = 1). The overall functional and esthetic outcome is very satisfactory, but serious side effects and complications related to immunosuppression are challenges hindering progress in this field. The high levels of immunosuppression, skin rejection, nerve regeneration, donor legislation and the acceptance level need to be addressed to promote growth of this promising new field in transplantation and reconstructive surgery.

Dendritic cells as a tool to induce transplantation tolerance: obstacles and opportunities

Dendritic cells are the key component to regulate and coordinate adaptive immune responses, including tolerance. This overview will briefly summarize different strategies to generate tolerogenic dendritic cell and the in vivo use of these cells in experimental transplantation models. We discuss some obstacles and possible solutions including alternative strategies for the use of negative vaccination in the context of organ transplantation.

Adoptive transfer of FTY720-treated immature BMDCs significantly prolonged cardiac allograft survival

A sphingosine 1 phosphate receptor modulator, FTY720, has been used to alleviate symptoms in allotransplantation and autoimmune disease models with impressive efficacy, while it only achieved moderate success in clinical trials. Infusion of immature bone marrow-derived dendritic cell (BMDC) progenitors before transplantation could induce donor specific tolerance. In this study, we investigated the possibility of using FTY720-DCs (FTY720-treated immature BMDCs) to prevent severe alloimmune response. Our results indicate that FTY720-DCs could markedly prolong graft survival compared with Ctrl-DCs (nonconditioned immature BMDCs) as manifested by reduced inflammatory infiltration into the graft. IFN-γ production by CD4(+) and CD8(+) T cells were significantly reduced, while FoxP3(+) regulatory T cells among CD4(+) T cells were upregulated. Although FTY720 seldom altered the phenotype or the phagocytosis of BMDCs in vitro, it severely hampered their capability to trigger antigen-specific and allogeneic T-cell response. When splenic T cells were co-cultured with FTY720-DCs, the proportion of regulatory T cells increased, accompanied by elevated IL-10 production. Consistently, infusion of FTY720-DCs could preferentially promote Treg proliferation and upregulate PD-1 expression on conventional T cells in allogeneic mature BMDC priming experiment. These results suggest that infusion of FTY720-DCs before cardiac transplantation could significantly prolong functional graft survival by acting as a balancer of alloimmune response.

Differentiation of dendritic cells from human embryonic stem cells

Improving our understanding of the interactions between human dendritic cells (DCs) and T cells may contribute to the development of therapeutic strategies for a variety of immune-mediated disorders. The possibility of using DCs themselves as tools to manipulate immune responses opens even greater therapeutic avenues. Current methods of generating human DCs are both inadequate and susceptible to high levels of variability between individuals. DCs differentiated from human embryonic stem cells (hESCs) could provide a more reliable, consistent solution. DCs have now successfully been differentiated from hESCs and more recently this has been repeated using protocols that avoid the inclusion of animal products, an important modification for clinical use. We have developed a novel method for the generation of DCs from hESCs in the absence of animal products that does not necessitate a separate embryoid body (EB) generation step. The technique involves the use of four growth factors and their successive removal from culture, resulting in accumulation of DCs with phenotypic, morphological, and immunostimulatory properties comparable to those of classical human monocyte-derived DCs. In addition to the application of hESC-derived DCs in basic research and novel approaches to cancer immunotherapy, they may also play a central role in the field of regenerative medicine. Tolerogenic DCs differentiated from hESCs may be used to persuade the immune system of the recipients of cell replacement therapy to tolerate allogeneic tissues differentiated from the same hESC line. Such an approach may help to address the immunological barriers that threaten to derail the clinical application of hESCs.

A nonredundant role for canonical NF-κB in human myeloid dendritic cell development and function

The plastic role of dendritic cells (DCs) in the regulation of immune responses has made them interesting targets for immunotherapy, but also for pathogens or tumors to evade immunity. Functional alterations of DCs are often ascribed to manipulation of canonical NF-κB activity. However, though this pathway has been linked to murine myeloid DC biology, a detailed analysis of its importance in human myeloid DC differentiation, survival, maturation, and function is lacking. The myeloid DC subsets include interstitial DCs and Langerhans cells. In this study, we investigated the role of canonical NF-κB in human myeloid DCs generated from monocytes (monocyte-derived DCs [mo-DCs]) or CD34(+) progenitors (CD34-derived myeloid DCs [CD34-mDCs]). Inhibition of NF-κB activation during and after mo-DC, CD34-interstitial DC, or CD34-Langerhans cell differentiation resulted in apoptosis induction associated with caspase 3 activation and loss of mitochondrial transmembrane potential. Besides regulating survival, canonical NF-κB activity was required for the acquisition of a DC phenotype. Despite phenotypic differences, however, Ag uptake, costimulatory molecule and CCR7 expression, as well as T cell stimulatory capacity of cells generated under NF-κB inhibition were comparable to control DCs, indicating that canonical NF-κB activity during differentiation is redundant for the development of functional APCs. However, both mo-DC and CD34-mDC functionality were reduced by NF-κB inhibition during activation. In conclusion, canonical NF-κB activity is essential for the development and function of mo-DCs as well as CD34-mDCs. Insight into the role of this pathway may help in understanding how pathogens and tumors escape immunity and aid in developing novel treatment strategies aiming to interfere with human immune responses.

Tolerogenicity is not an absolute property of a dendritic cell

Pharmacological modulation is known to temper the immune capacity of DC, enhancing the notion that modulated Ag-bearing DC might be used therapeutically to induce tolerance. We have investigated phenotypic features shared by such DC, and queried their potential to tolerize in different settings. Immature, IL-10, TGF-beta and 1alpha,25-dihydroxyvitamin D(3)-modulated BMDC all induced tolerance to male skin in female TCR transgenic A1.RAG mice, and the modulated DC also tolerized after exposure to the TLR4-ligand LPS. Transcript profiling revealed that this was achieved despite retaining much of the normal LPS-maturation response. No shared tolerance-associated transcripts could be identified. Equivalent BMDC could not tolerize in Marilyn TCR-transgenic mice. Simultaneous presentation of both A1.RAG and Marilyn peptide-Ag (Dby-H2E(k) and Dby-H2A(b)) on immature (C57BL/6JxCBA/Ca) F1 BMDC also only achieved tolerance in A1.RAG mice. Both strains registered Ag, but Foxp3(+) Treg were only induced in A1.RAG mice. In contrast, Marilyn T cells showed greater proliferation and an inflammatory bias, in response to Ag presented by immature F1 BMDC in vitro. In summary, while pharmacological agents can skew DC to reinforce their immature tolerogenic phenotype, the outcome of presentation is ultimately an integrated response including T-cell-intrinsic components that can over-ride for immune activation.

Enemy at the gates: dendritic cells and immunity to mucosal pathogens

Dendritic cells (DC) are diverse and specialized hematopoietic cells serving as an essential bridge between innate and adaptive immunity. DC exist in all lymphoid and nonlymphoid organs and are amongst the first responders to infection at epithelial surfaces including mucosal tissues. DC of the lung, gut, and vaginal mucosa display different phenotypes and functions reflecting each unique tissue and, in contrast to their counterparts in spleen and lymph nodes, are constantly exposed to both harmful and benign factors of their environments. Mucosal DC recognize and respond to pathogens through engagement of pattern recognition receptors, and activated DC migrate to draining lymph nodes to induce adaptive immune responses. The specialized function of DC aids in the induction of immunity and pathogen control at the mucosa. Such specialization includes the potent antiviral interferon response of plasmacytoid DC to viral nucleic acids, the ability of mucosal DC to capture organisms in the gut lumen, the capacity of DC to cross-present antigen from other infected cells, and the ability of mucosal DC to initiate IgA class switching in B cells. DC plasticity is also critical in the immune response to mucosal pathogens, as DC can respond to the microenvironment and sense pathogen-induced stress in neighboring epithelial cells. Finally, DC interact with each other through crosstalk to promote antigen presentation and T-cell immunity. Together, these processes condition mucosal DC for the induction of a tailored immune response to pathogens.