Therapeutic approaches for transplantation

Therapeutic implications of NK cell regulation of allogeneic CD8 T cell-mediated immune responses stimulated through the direct pathway of antigen presentation in transplantation

Natural killer (NK) cells are a population of innate type I lymphoid cells essential for early anti-viral responses and are known to modulate the course of humoral and cellular-mediated T cell responses. We assessed the role of NK cells in allogeneic CD8 T cell-mediated responses in an immunocompetent mouse model across an MHC class I histocompatibility barrier to determine its impact in therapeutic clinical interventions with polyclonal or monoclonal antibodies (mAbs) targeting lymphoid cells in transplantation. The administration of an NK cell depleting antibody to either CD8 T cell replete or CD8 T cell-depleted naïve C57BL/6 immunocompetent mice accelerated graft rejection. This accelerated rejection response was associated with an in vivo increased cytotoxic activity of CD8 T cells against bm1 allogeneic hematopoietic cells and bm1 skin allografts. These findings show that NK cells were implicated in the control host anti-donor cytotoxic responses, likely by competing for common cell growth factors in both CD8 T cell replete and CD8 T cell-depleted mice, the latter reconstituting in response to lymphopenia. Our data calls for precaution in solid organ transplantation under tolerogenic protocols involving extensive depletion of lymphocytes. These pharmacological biologics with depleting properties over NK cells may accelerate graft rejection and promote aggressive CD8 T cell cytotoxic alloresponses refractory to current immunosuppression.

Mechanisms of Tolerance Induction by Hematopoietic Chimerism: The Immune Perspective

Hematopoietic chimerism is one of the effective approaches to induce tolerance to donor‐derived tissue and organ grafts without administration of life‐long immunosuppressive therapy. Although experimental efforts to develop such regimens have been ongoing for decades, substantial cumulative toxicity of combined hematopoietic and tissue transplants precludes wide clinical implementation. Tolerance is an active immunological process that includes both peripheral and central mechanisms of mutual education of coresident donor and host immune systems. The major stages include sequential suppression of early alloreactivity, establishment of hematopoietic chimerism and suppressor cells that sustain the state of tolerance, with significant mechanistic and temporal overlap along the tolerization process. Efforts to devise less toxic transplant strategies by reduction of preparatory conditioning focus on modulation rather than deletion of residual host immunity and early reinstitution of regulatory subsets at the central and peripheral levels. Stem Cells Translational Medicine2017;6:700–712

Adoptive transfer of donor corneal antigen-specific regulatory T cells can prolong mice corneal grafts survival

PURPOSE

To explore the effects of adoptive transferring T regulatory cells (T reg cells) stimulated by donor corneal antigen (Ag) to prevent corneal transplantation immune rejection in mice.

METHODS

C57BL/6 mice were used as donors and BALB/c mice as recipients. Corneal Ag was harvested by homogenization and centrifugation. Bone marrow dendritic cells (DCs) from BALB/c mice were cultured with stimulation of granulocyte-macrophage colony-stimulating factor and interleukin-4 for 5 days. Then, donor corneal Ag was added to obtain donor corneal Ag-loaded DCs. The DCs were used to stimulate CD4+CD25+ and CD4+CD25+ T cells from the recipient to yield Ag-stimulated T reg cells. Penetrating keratoplasty was performed in the mice. The recipients were randomly divided into 3 groups receiving 0.1 mL of phosphate-buffered saline, 1 × 10(6) naive T reg cells, and Ag-stimulated T reg cells, respectively, given by retroorbital injection at the end of surgery. The allografts were observed, and histopathological examination was performed 15 days after surgery.

RESULTS

: The corneal Ag mainly comprised 2 proteins with molecular weight 54 and 42 kD, respectively. Corneal Ag-loaded DCs expressed higher levels of CD11c, CD80, and CD86 than bone marrow precursor cells. Both CD4CD25 and CD4+CD25+ T cells showed vigorous proliferative responses to corneal Ag-loaded DCs. Mean survival time of the mice corneal allografts in phosphate-buffered saline, naive T reg cells, and Ag-stimulated groups was 8.1 ± 1.1, 14.3 ± 2.0, and 23.3 ± 2.6 days, respectively (P < 0.01 among groups). Histopathological examination revealed less inflammatory cells infiltration in Ag-stimulated than in naive mice.

CONCLUSIONS

: Adoptive transfer of donor corneal Ag-specific T reg cells prolonged survival time of corneal allografts in our mouse model, which might suggest a useful approach to cellular immunotherapy for corneal transplantation immune rejection.

Complement and cellular cytotoxicity in antibody therapy of cancer

The effective and practical use of mAbs in cancer therapy became a reality with the development of the chimeric anti-CD20 mAb, rituximab. Several additional mAbs have since been approved for clinical use. Despite these successes, the mechanisms by which mAbs mediate antitumor activity are still unclear. Preclinical studies indicate complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC) both can contribute to mAb-induced tumor cell lysis. However, evidence related to the relative clinical importance of each mechanism, and whether they are synergistic or antagonistic, is conflicting. New ways to enhance both CDC and ADCC are being developed in attempt to develop a more effective anticancer mAb. Continued research on the mechanisms of mAb therapy will be necessary if we are to take optimal advantage of the current mAbs and develop more effective mAbs in the future.

Undifferentiated Murine Embryonic Stem Cells Cannot Induce Portal Tolerance but May Possess Immune Privilege Secondary to Reduced Major Histocompatibility Complex Antigen Expression

Induction of donor-specific tolerance using embryonic stem (ES) cells followed by transplantation of ES cell-derived tissues from the same allogeneic strain could theoretically engender successful transplantation without immunosuppression. We sought to induce tolerance using bona fide murine ES cells in immunocompetent mice. ES cells were evaluated for the expression of markers restricted to undifferentiated cells [stage-specific embryonic antigen-1 (SSEA-1) and OCT-4] and the ability to form teratomas in immunodeficient mice. BALB/cByJ mice underwent intraportal inoculation with YC5-EYFP ES cells (129 strain; R1-derived) or saline followed by transplantation with 129X1/SvJ, CBA/J, or BALB/cByJ nonvascularized, neonatal cardiac grafts. Mice were sacrificed at graft failure and underwent histologic evaluation of transplanted grafts and lymphoid organs. ES cells and early differentiated progeny underwent real time (RT)-PCR and fluorescence-activated cell sorting (FACS) analysis to detect major histocompatibility complex (MHC) gene transcription and antigen expression. ES cells expressed markers restricted to undifferentiated cells while maintaining the ability to form teratomas in immunodeficient mice. No prolongation of allograft survival or evidence of lymphoid chimerism was observed in immunocompetent recipient mice despite hepatic teratoma formation. MHC class I, class II, and nonclassical antigens were undetectable on ES cells and early differentiated progeny despite the presence of mRNA transcripts. Class I expression was strongly upregulated upon exposure to gamma-interferon. Intraportal inoculation with murine ES cells does not produce lymphoid chimerism or induce donor-specific unresponsiveness to neonatal cardiac grafts in unmanipulated immunocompetent hosts. However, specific differentiated cell types such as ES cellderived dendritic cells, or alternate routes of ES cell administration, may be effective. ES cells appear to have immune privilege, allowing them to form teratomas in immunocompetent mice.

In vitro–expanded donor alloantigen–specific CD4+CD25+ regulatory T cells promote experimental transplantation tolerance

CD4+CD25+ regulatory T (Treg) cells play a critical role in the induction and maintenance of peripheral immune tolerance. In experimental transplantation models in which tolerance was induced, donor-specific Treg cells could be identified that were capable of transferring the tolerant state to naive animals. Furthermore, these cells appeared to have indirect allospecificity for donor antigens. Here we show that in vivo alloresponses can be regulated by donor alloantigen-specific Treg cells selected and expanded in vitro. Using autologous dendritic cells pulsed with an allopeptide from H2-Kb, we generated and expanded T-cell lines from purified Treg cells of CBA mice (H2k). Compared with fresh Treg cells, the cell lines maintained their characteristic phenotype, suppressive function, and homing capacities in vivo. When cotransferred with naive CD4+CD25− effector T cells after thymectomy and T-cell depletion in CBA mice that received CBK (H2k+Kb) skin grafts, the expanded Treg cells preferentially accumulated in the graft-draining lymph nodes and within the graft while preventing CBK but not third-party B10.A (H2k+Dd) skin graft rejection. In wild-type CBA, these donor-specific Treg cells significantly delayed CBK skin graft rejection without any other immunosuppression. Taken together, these data suggest that in vitro–generated tailored Treg cells could be considered a therapeutic tool to promote donor-specific transplant tolerance.

Mesenchymal Stem Cell Content of Human Vertebral Bone Marrow

Mesenchymal stem cells (MSCs) are capable of down-regulating alloimmune responses and promoting the engraftment of hematopoietic stem cells. MSCs may therefore be suitable for improving donor-specific tolerance induction in solid-organ transplantation. Cells from cadaveric vertebral bone marrow (V-BM), aspirated iliac crest-BM, and peripheral blood progenitor cells were compared. Cells were characterized by flow cytometry and colony assays. MSCs generated from V-BM were assayed for differentiation capacity and immunomodulatory function. A median 5.7 x 10(8) nucleated cells (NCs) were recovered per vertebral body. The mesenchymal progenitor, colony-forming unit-fibroblast, frequency in V-BM (11.6/10(5) NC, range: 6.0-20.0) was considerably higher than in iliac crest-BM (1.4/10(5) NC, range: 0.4-2.6) and peripheral blood progenitor cells (not detectable). MSC generated from V-BM had the typical MSC phenotype (CD105(pos)CD73(pos)CD45(neg)CD34(neg)), displayed multilineage differentiation potential, and suppressed alloreactivity in mixed lymphocyte reactions. V-BM may be an excellent source for MSC cotransplantation approaches.

Targeted bone marrow radioablation with 153Samarium-lexidronam promotes allogeneic hematopoietic chimerism and donor-specific immunologic hyporesponsiveness

BACKGROUND

Transplantation tolerance, defined as acceptance of a graft by an otherwise fully immunocompetent host, has been an elusive goal. Although robust tolerance has been achieved by the induction of stable hematopoietic chimerism after bone marrow transplantation, lethal or sublethal radiation conditioning used to induce long-term chimerism precludes its clinical use. We studied whether targeted delivery of radiation to bone marrow could allow for bone marrow cell (BMC) engraftment, chimerism, and donor-specific tolerance in the absence of the side effects associated with external irradiation.

METHODS

We administered a radioactive bone-seeking compound (Samarium-Lexidronam, Quadramet, Berlex Laboratories, Wayne, NJ) together with transient T-cell costimulatory blockade to recipient mice. Allogeneic BMCs were given 7 or 14 days after preconditioning. Costimulatory blockade was obtained by the use of an anti-CD154 antibody for 4 weeks. Chimerism was assessed by flow cytometry. Mice then received donor-specific and third-party skin grafts. Graft survival was analyzed with mechanisms of donor-specific hyporesponsiveness.

RESULTS

High levels of stable chimerism across an allogeneic barrier were achieved in mice by a single administration of Samarium-Lexidronam, transient T-cell costimulatory blockade, and BMC transplantation. A large percentage of chimeric animals retained donor-derived skin grafts for more than 120 days without requiring additional immunosuppression, suggesting that harsh cytotoxic preconditioning is not necessary to achieve stable chimerism and donor specific hyporesponsiveness. Analysis of the T-cell repertoire in chimeras indicates T-cell deletional mechanisms.

CONCLUSIONS

These data broaden the potential use of BMC transplantation for tolerance induction and argue for its potential in treating autoimmune diseases.

Heterologous immunity: an overlooked barrier to tolerance

In less than 50 years the field of organ transplantation has transitioned from an experimental concept to clinical commonplace. Notwithstanding the dramatic improvements in patient and allograft outcomes, chronic rejection and the complications from life-long immunosuppressive therapy remain significant problems. The induction of transplantation tolerance, indefinite allograft acceptance independent of chronic immunosuppressive therapy, remains the ultimate objective in transplantation. Many strategies have achieved tolerance to transplanted tissue in rodents; however, few, if any, have shown equal efficacy when tested in non-human primate transplant models or human patients. A critical distinction between specific pathogen-free mice and primates or human patients is the exposure of the latter to environmental pathogens and the resultant-acquired immune history. Recent data has shown that virally induced, alloreactive immune responses can provide a potent barrier to tolerance. In this review, we discuss one of the most robust methods for tolerance, the induction of hematopoietic chimerism as well as the influence of viral infections on the alloimmune response.

Amelioration of graft versus host disease by galectin-1

Graft versus host disease is a significant cause of morbidity and mortality following allogeneic hematopoietic stem cell transplantation. Galectin-1, a mammalian lectin that modulates T cell function and apoptosis, has been shown to be immunomodulatory in animal models of autoimmune disease. We investigated the efficacy of galectin-1 in a murine model of graft versus host disease and found that 68% of galectin-1-treated mice survived, compared to 3% of vehicle-treated mice. Galectin-1-treated animals also had reduced inflammatory infiltrates in tissues compared to animals treated with vehicle alone. Galectin-1 did not affect engraftment of donor hematopoietic cells. However, galectin-1-treated animals demonstrated increased cellularity in bone marrow and spleen with increased numbers of splenic B cells and CD4 T cells compared to those animals treated with vehicle alone. Galectin-1 treatment also significantly improved reconstitution of normal splenic architecture following transplant. Production of type I cytokines interleukin-2 (IL-2) and interferon-gamma was reduced in splenocytes derived from galectin-1-treated transplanted mice when compared to animals treated with vehicle alone, while production of the type II cytokines, IL-4 and IL-10, was similar between the two groups of animals. Although splenocytes from galectin-1-treated transplanted animals responded to both third party antigens and leukemic challenge, host alloreactivity was significantly reduced when compared to cells from vehicle-treated animals. These results demonstrate that galectin-1 therapy is capable of increasing survival and suppressing the graft versus host immune response without compromising engraftment or immune reconstitution following allogeneic hematopoietic stem cell transplant.

Reprogramming immune responses: enabling cellular therapies and regenerative medicine

Recent advances in cellular therapies have led to the emergence of a multidisciplinary scientific approach to developing therapeutics for a wide variety of diseases and genetic disorders. Although most cell-based therapies currently consist of heterogeneous cell populations, it is anticipated that the standard of care will eventually be well-characterized stem cell lines that can be modified to meet the individual needs of the patient. Many challenges have to be overcome, however, before such "designer cells" can become a clinical reality. One of the major hurdles will be to prevent immune rejection of the therapeutic cells. A patient's immune system may react to genetically modified or allogeneic cells as foreign, leading to their destruction. We propose that specific reprogramming of the immune system to accept cellular therapies can be accomplished by establishing hematopoietic chimerism. Successful engraftment of hematopoietic stem cells (HSCs), which have the same origin as those cells intended for therapeutic use, should lead to a re-education of the immune system so that the donor cells are recognized as self and will not be rejected. Developing safe, nontoxic protocols for reprogramming the immune system is critical to the success of this approach. Two major requirements exist for achieving stable HSC engraftment: (A) depletion or displacement of host stem cells, and (B) adequate immune suppression. Available data indicate that an agent such as busulfan is effective in depleting stem cells and that immune suppression can be accomplished with monoclonal antibodies that specifically target immune-reactive cells in the periphery.

Value of F-18 fluorodeoxyglucose positron emission tomography for predicting the clinical outcome of patients with aggressive lymphoma prior to and after autologous stem-cell transplantation

STUDY OBJECTIVES

To determine and compare the values of positron emission tomography (PET) with F-18 fluorodeoxyglucose (FDG) and CT for predicting clinical outcome of patients with aggressive lymphoma undergoing salvage cytoreductive chemotherapy followed by high-dose chemotherapy and autologous stem-cell transplantation (ASCT).

PATIENTS AND METHODS

Forty-three patients with lymphoma who underwent ASCT with FDG-PET evaluation were studied. Group 1 (n = 20) patients (6 patients with Hodgkin disease [HD], and 14 patients with non-Hodgkin lymphoma [NHL]) underwent PET 2 to 5 weeks after initiation of salvage chemotherapy, prior to ASCT. Group 2 (n = 23) patients (6 patients with HD, and 17 patients with NHL) underwent PET within a median interval of 2.4 months (range, 2 to 6 months) after ASCT.

MEASUREMENTS AND RESULTS

Study end points were complete remission, relapse, or death. In group 1, 8 of 20 patients (40%) were disease free after a median follow-up of 13.3 months; 12 patients relapsed or died. PET findings were true-negative in 7 of 8 patients and true-positive in 11 of 12 patients who relapsed after ASCT. In group 2, 9 of 23 patients (39%) were disease free after a median follow-up of 16.5-months; 14 patients relapsed. PET findings were true-negative in 8 of 9 patients and true-positive in 13 of 14 patients who relapsed. Positive and negative predictive values of PET were 92% and 88% (group 1) and 93% and 89% (group 2), respectively. Predictive accuracy values of PET were 90% and 91% for group 1 and group 2, respectively, vs 58% and 67% for CT (p < 0.05).

CONCLUSIONS

PET findings but not CT results were strongly correlated with disease-free survival (p < 0.01). Our results show that FDG-PET can be used to predict the post-ASCT outcome of lymphoma patients with high accuracy.

Converting nonhuman primate dendritic cells into potent antigen-specific cellular immunosuppressants by genetic modification

T cell depletion plus donor bone marrow cell (BMC) infusion induces long-term kidney allograft survival in a limited number of rhesus macaque recipients. Therefore, there is a need to enhance the tolerogenic activity of donor BMCs. The tolerogenic effect of donor BMCs is ascribed to a veto activity, mediated by a CD8+ subset that upregulates immunoregulatory effector molecules, transforming growth factor-beta1 (TGF-beta), and FasL, after interaction with donor-reactive cytotoxic T lymphocyte precursors (CTLp), leading to clonal inactivation/deletion of donor-reactive CTLp. Of note, the receptors for TGF-beta1- and FasL-induced signal transduction are upregulated in activated T cells. Since mature dendritic cells (DCs) are exceptionally efficient activators of T cells, we postulated that mature DCs modified to overexpress TGF-beta1 and FasL might exert potent veto (i.e., inactivating/deleting) activity independent of CD8 expression. A fusion protein comprising antihuman CD40 single-chain antibody and soluble coxsackie-adenovirus receptor enabled high-efficiency transduction of rhesus monocyte-derived DCs (Rh MDDCs) by recombinant adenovirus (Ad). Mature Rh MDDCs transduced with Ad encoding active TGF-beta1 retained a mature phenotype yet exhibited potent alloantigen-specific cellular immunosuppression. Such modified MDDCs have the potential to promote tolerance induction to allografts in vivo.

Identification of regulatory T cells in tolerated allografts

Induction of transplantation tolerance with certain therapeutic nondepleting monoclonal antibodies can lead to a robust state of peripheral "dominant" tolerance. Regulatory CD4+ T cells, which mediate this form of "dominant" tolerance, can be isolated from spleens of tolerant animals. To determine whether there were any extra-lymphoid sites that might harbor regulatory T cells we sought their presence in tolerated skin allografts and in normal skin. When tolerated skin grafts are retransplanted onto T cell-depleted hosts, graft-infiltrating T cells exit the graft and recolonize the new host. These colonizing T cells can be shown to contain members with regulatory function, as they can prevent nontolerant lymphocytes from rejecting fresh skin allografts, without hindrance of rejection of third party skin. Our results suggest that T cell suppression of graft rejection is an active process that operates beyond secondary lymphoid tissue, and involves the persistent presence of regulatory T cells at the site of the tolerated transplant.

Alefacept, an immunomodulatory recombinant LFA-3/IgG1 fusion protein, induces CD16 signaling and CD2/CD16-dependent apoptosis of CD2(+) cells

Alefacept, an immunomodulatory recombinant fusion protein composed of the first extracellular domain of LFA-3 fused to the human IgG1 hinge, C(H)2, and C(H)3 domains, has recently been shown in phase II and III clinical trials to safely reduce disease expression in patients with chronic plaque psoriasis. Alefacept modulates the function of and selectively induces apoptosis of CD2(+) human memory-effector T cells in vivo. We have sought to gain further understanding of the mechanisms of action that influence the biological activity of alefacept and may contribute to its efficacy and patient responsiveness. Specifically evaluated is the ability of alefacept to activate intracellular signals mediated via CD2 and/or Fc gamma RIII (CD16). Experimentation using isoforms of alefacept engineered to have amino acid substitutions in the IgG1 C(H)2 domain that impact Fc gamma R binding indicate that alefacept mediates cognate interactions between cells expressing human CD2 and CD16 to activate cells, e.g., increase extracellular signal-regulated kinase phosphorylation, up-regulate cell surface expression of the activation marker CD25, and induce release of granzyme B. In the systems used, this signaling is shown to require binding to CD2 and CD16 and be mediated through CD16, but not CD2. Experimentation using human CD2-transgenic mice and isoforms of alefacept confirmed the requirement for Fc gamma R binding for detection of the pharmacological effects of alefacept in vivo. Thus alefacept acts as an effector molecule, mediating cognate interactions to activate Fc gamma R(+) cells (e.g., NK cells) to induce apoptosis of sensitive CD2(+) target cells.

Genetic therapies and xenotransplantation

The number of patients in need of an organ transplant is increasing, while the number of satisfactory sources of organs has declined in many countries [101]. The resulting shortage of human organs has spurred an urgent effort to investigate alternative therapies, including the use of animal organs, tissues and cells (i.e., xenotransplantation). Advances in genetic engineering have provided essential tools for the development of practical solutions to human disease. The area of xenotransplantation is no exception. In fact, the use of genetic therapies is especially attractive in the transplant setting as it offers an opportunity to manipulate the donor tissue rather than the recipient. This review will describe the obstacles in the clinical application of xenotransplantation and how genetic engineering might be used to address them.

Stem cells: hype and reality

This update discusses what is known regarding embryonic and adult tissue-derived pluripotent stem cells, including the mechanisms underlying self-renewal without senescence, differentiation in multiple cell types both in vitro and in vivo, and future potential clinical uses of such stem cells. In Section I, Dr. Lansdorp reviews the structure and function of telomerase, the enzyme that restores telomeric ends of chromosomes upon cell division, highly present in embryonic stem cells but not adult stem cells. He discusses the structure and function of telomerase and signaling pathways activated by the enzyme, with special emphasis on normal and leukemic hematopoietic stem cells. In Section II, Dr. Pera reviews the present understanding of mammalian pluripotent embryonic stem cells. He discusses the concept of pluripotentiality in its embryonic context, derivation of stem cells from embryonic or fetal tissue, the basic properties of the stem cells, and methods to produce specific types of differentiated cell from stem cells. He examines the potential applications of stem cells in research and medicine and some of the barriers that must be crossed to achieve these goals. In Section III, Dr. Verfaillie reviews the present understanding of pluripotency of adult stem cells. She discusses the concept of stem cell plasticity, a term used to describe the greater potency described by several investigators of adult tissue-derived stem cells, critically reviews the published studies demonstrating stem cell plasticity, and possible mechanisms underlying such plasticity, and examines the possible role of pluripotent adult stem cells in research and medicine.