Generation of Non-Alloreactive Antiviral Central Memory CD8 Human Veto T Cells for Cell Therapy

Previous studies in mice demonstrated that CD8 T cells exhibit marked veto activity enhancing engraftment in several models for T cell-depleted bone marrow (TDBM) allografting. To reduce the risk of graft-versus-host disease (GVHD) associated with allogeneic CD8 veto T cells, these studies made use of naive CD8 T cells stimulated against third-party stimulators under cytokine deprivation and subsequent expansion in the presence of IL-15. More recently, it was shown that mouse CD8 veto T cells can be generated by stimulating CD8 memory T cells from ovalbumin immunized mice under cytokine deprivation, using ovalbumin as a third-party antigen. These cells also exhibited substantial enhancement of BM allografting without GVHD. In this study, we tested the hypothesis that stimulation and expansion of human CD8 memory T cells under IL-15 and IL-7 deprivation during the early phase of activation against recall viral antigens can lead to substantial loss of alloreactive T clones while retaining marked veto activity. Memory CD8 T cells were enriched by removal of CD45RA+, CD4+, and CD56+ cells from peripheral blood of cytomegalovirus (CMV)- and Epstein-Barr virus (EBV)-positive donors. In parallel, CD14+ monocytes were isolated; differentiated into mature dendritic cells (mDCs); pulsed with a library of CMV, EBV, adenovirus, and BK virus peptides; and irradiated. The CD8 T cell-enriched fraction was then cultured with the pulsed mDCs in the presence of IL-21 for 3 days, after which IL-15 and IL-7 were added. After 12 days of culture, the cells were tested by limiting dilution analysis for the frequency of alloreactive T cell clones and their veto activity. In preclinical runs using GMP reagents, we established that within 12 days of culture, a large number of highly homogenous CD8 T cells, predominantly expressing a central memory phenotype, could be harvested. These cells exhibited marked veto activity in vitro and >3-log depletion of alloreactivity. Based on these preclinical data, a phase 1-2 clinical trial was initiated to test the safety and efficacy of these antiviral CD8 central memory veto cells in the context of nonmyeloablative (NMA) T cell-depleted haploidentical hematopoietic stem cell transplantation (HSCT). In 2 validation runs and 11 clinical runs using GMP reagents, >1 × 1010 cells were generated from a single leukapheresis in 12 out of 13 experiments. At the end of 12 days of culture, there were 97 ± 2.5% CD3+CD8+ T cells, of which 84 ± 9.0% (range, 71.5% to 95.1%) exhibited the CD45RO+CD62L+ CM phenotype. Antiviral activity tested by intracellular expression of INF-γ and TNF-α and showed an average of 38.8 ± 19.6% positive cells on 6 hours of stimulation against the viral peptide mixture. Our results demonstrate a novel approach for depleting alloreactive T cell clones from preparations of antiviral CD8 veto cells. Based on these results, a phase 1-2 clinical trial is currently in progress to test the safety and efficacy of these veto cells in the context of NMA haploidentical T cell-depleted HSCT. Studies testing the hypothesis that these non-alloreactive CD8 T cells could potentially offer a platform for off-the-shelf veto chimeric antigen receptor T cell therapy in allogenic recipients, are warranted.

Lung Injury Repair by Transplantation of Adult Lung Cells Following Preconditioning of Recipient Mice

Repair of injured lungs represents a longstanding therapeutic challenge. We recently demonstrated that human and mouse embryonic lung tissue from the canalicular stage of development are enriched with lung progenitors, and that a single cell suspension of canalicular lungs can be used for transplantation, provided that lung progenitor niches in the recipient mice are vacated by strategies similar to those used in bone marrow transplantation. Considering the ethical limitations associated with the use of fetal cells, we investigated here whether adult lungs could offer an alternative source of lung progenitors for transplantation. We show that intravenous infusion of a single cell suspension of adult mouse lungs from GFP+ donors, following conditioning of recipient mice with naphthalene and subsequent sublethal irradiation, led to marked colonization of the recipient lungs, at 6-8 weeks post-transplant, with donor derived structures including epithelial, endothelial, and mesenchymal cells. Epithelial cells within these donor-derived colonies expressed markers of functionally distinct lung cell types, and lung function, which is significantly compromised in mice treated with naphthalene and radiation, was found to be corrected following transplantation. Dose response analysis suggests that the frequency of patch forming cells in adult lungs was about threefold lower compared to that found in E16 fetal lungs. However, as adult lungs are much larger, the total number of patch forming cells that can be collected from this source is significantly greater. Our study provides proof of concept for lung regeneration by adult lung cells after preconditioning to vacate the pulmonary niche. Stem Cells Translational Medicine 2018;7:68-77.

Tolerance induction in presensitized bone marrow recipients by veto CTLs: effective deletion of host anti-donor memory effector cells

Veto cells have been defined as cells capable of inducing apoptosis of effector CD8 cells recognizing their disparate MHC Ags. Tolerance induced by donor-type veto cells is desirable, because it is restricted to depletion of anti-donor clones without depletion of other immune specificities. It has been shown that anti-third party CTLs exhibit marked veto activity with reduced capacity to induce graft-vs-host disease, when tested on naive effector cells. However, presensitized T cells could play an important role in graft rejection, and therefore, their sensitivity to veto cells could be critical to the implementation of the latter cells in bone marrow transplantation. To address this question, we compared naive and presensitized TCR transgenic effector CD8 T cells, bearing a TCR against H-2(d). Both cell types exhibited similar predisposition to killing by veto CTLs in vitro, and this killing was dependent in both cell types on Fas-FasL signaling as shown by using Fas-deficient CD8 T cells from (lprx2c) F(1) mice. When tested in a stringent mouse model, in which bone marrow rejection is mediated by adoptively transferred host type T cells into lethally irradiated recipients, veto CTLs were equally effective in overcoming rejection of naive or presensitized host T cells.

Hematopoietic stem cell transplantation across major genetic barriers: tolerance induction by megadose CD34 cells and other veto cells

Studies in mice and humans demonstrate that transplantation of hematopoietic progenitors in numbers larger than commonly used overcomes major genetic barriers. In vitro studies suggest that veto cells, within the population of hematopoietic progenitors, facilitate this favorable outcome. Tolerance induction can be further enhanced by other veto cells. Perhaps the most potent veto cell is the CD8(+) CTL. However, this cell is also associated with marked GVHD, which can be separated from the veto activity by generating anti-third party CTLs under IL-2 deprivation.

Overcoming T cell-mediated rejection of bone marrow allografts by T-regulatory cells: synergism with veto cells and rapamycin

Recently, we have shown that anti-third-party cytotoxic T lymphocytes (CTLs) depleted of alloreactivity against the host are endowed with marked veto activity and can facilitate bone marrow (BM) allografting without graft-versus-host disease. We also demonstrated synergism between rapamycin (RAPA) and the veto cells. CD4(+)CD25(+) T-regulatory (Treg) cells are suppressor cells that can enhance alloengraftment. We investigated whether donor Tregs would be synergistic with veto CTLs and RAPA in augmenting alloengraftment or, conversely, would suppress veto CTL effects. Lethally irradiated C3H mice were transplanted at day 2 after irradiation with Balb-nude BM. Graft rejection was induced by purified host-type T cells infused 1 day prior to BMT. The addition of Tregs led to moderate enhancement of engraftment. RAPA at different doses was synergistic with Tregs. The addition of veto CTLs to Tregs enabled reducing the effective RAPA dose fourfold. Combining all three agents was necessary to overcome rejection at low-dose RAPA. Chimerism analysis at 5 to 9 months revealed a significant presence of host-type cells coexisting with the predominant donor T cells, suggesting that tolerance had been attained. The synergistic effects between Tregs, veto CTLs, and RAPA offer an attractive approach for facilitating alloengraftment.

Tolerance induction by third-party "off-the-shelf" CD4+CD25+ Treg cells

OBJECTIVE

Recent reports have shown that donor or host CD4(+)CD25(+) Treg cells can be used to control GVHD or graft rejection following allogeneic BMT in mice. In the present study we investigated the potential of third-party Treg cells compared to donor-type cells to facilitate BM allografting.

METHODS

Graft rejection is assessed in a mouse model of T cell-mediated BM allograft rejection. Lethally irradiated C3H mice are transplanted at day 2 after irradiation with T cell-depleted Balb/Nude BM. Graft rejection is induced by purified host-type T cells infused one day prior to BMT. Cells tested for their facilitating activity are added to the T cell-depleted BM allograft.

RESULTS

Naïve or ex vivo-expanded third-party Treg cells can effectively enhance engraftment of T cell-depleted BM allografts, exhibiting reactivity in vitro and in vivo similar to that found for donor-type Treg cells.

CONCLUSION

The use of third-party Treg cells in contrast to donor-type cells could allow advanced preparation of a large bank of Treg cells (off-the-shelf), with all the appropriate quality controls required for cell therapy.

VEGF-induced adult neovascularization: recruitment, retention, and role of accessory cells

Adult neovascularization relies on the recruitment of circulating cells, but their angiogenic roles and recruitment mechanisms are unclear. We show that the endothelial growth factor VEGF is sufficient for organ homing of circulating mononuclear myeloid cells and is required for their perivascular positioning and retention. Recruited bone marrow-derived circulating cells (RBCCs) summoned by VEGF serve a function distinct from endothelial progenitor cells. Retention of RBCCs in close proximity to angiogenic vessels is mediated by SDF1, a chemokine induced by VEGF in activated perivascular myofibroblasts. RBCCs enhance in situ proliferation of endothelial cells via secreting proangiogenic activities distinct from locally induced activities. Precluding RBCCs strongly attenuated the proangiogenic response to VEGF and addition of purified RBCCs enhanced angiogenesis in excision wounds. Together, the data suggest a model for VEGF-programmed adult neovascularization highlighting the essential paracrine role of recruited myeloid cells and a role for SDF1 in their perivascular retention.

Large-Scale Preparation of Human Anti—Third-Party Veto Cytotoxic T Lymphocytes Depleted of Graft-Versus-Host Reactivity: A New Source for Graft Facilitating Cells in Bone Marrow Transplantation

Induction of donor type chimerism in mildly prepared hosts without graft-versus-host disease (GvHD) is a most desirable goal in bone morrow transplantation. We have recently demonstrated in a mouse model that donor veto cytotoxic T lymphocytes (CTLs) can facilitate the induction of donor type chimerism in sublethally irradiated recipients without causing GvHD if they are effectively depleted of alloreactivity against host cells by means of stimulation against a third party. We extend this approach to human cells, by preparing CTLs in two major steps: primary culture in the absence of interleukin 2, leading to death by neglect of antihost clones, and addition of interleukin 2 and subsequent dilution of antihost clones as a consequence of the expansion of the anti-third-party clones. CTLs prepared in this way specifically suppress host cytotoxic T cells directed against antigens of the donor, but not against fourth-party antigens, as demonstrated in a standard (51)Cr release assay. We conclude that human anti-third-party CTLs afford a new source of veto cells that are depleted of potential graft-versus-host-reactive clones. The cells generated by this approach could potentially be used to facilitate engraftment of allogeneic hematopoietic stem cells.

Tolerance induction by veto CTLs in the TCR transgenic 2C mouse model. I. Relative reactivity of different veto cells

Several bone marrow cells and lymphocyte subpopulations, known as veto cells, were shown to induce transplantation tolerance across major histocompatibility Ags. Due to the low frequency of the effector T cells against which the veto cells inhibitory activity is aimed, the fate of the effector cells was traditionally followed indirectly by functional limiting dilution assays, which are cumbersome and depend on numerous parameters. In the present study the fate of the effector T cells was monitored directly by FACS, using TCR transgenic mouse CD8(+) T cells in which the transgene is directed against H-2(d) (the 2C model). This assay is validated by demonstrating the potency, selectivity, radiation sensitivity, and contact dependency of anti-third-party CTLs previously demonstrated by the limiting dilution assay. In contrast to veto CTLs, nonactivated CD8(+) T cells lack veto activity. Comparison by FACS in the 2C model revealed a hierarchy of veto cells, in the order of veto CTLs activated NK cells, activated CD4(+) T cells, and activated B cells. The latter cells as well as nonactivated CD4(+) or NK cells were shown to be completely devoid of veto activity.

Tolerance induction by veto CTLs in the TCR transgenic 2C mouse model. II. Deletion of effector cells by Fas-Fas ligand apoptosis

The direct assay of veto CTLs in the 2C mouse model enables monitoring, by FACS, the fate of the TCR transgenic effector CD8(+) T cells, the transgene of which can be stained with clonotypic Ab 1B2. After the addition of veto cells, CD8(+)1B2(+) effector cells increasingly express annexin V, and maximal apoptosis is attained 72 h after initiation of MLR. This veto activity can be partially blocked by anti-CD8 Abs directed against the allele expressed by the veto CTLs, but not by the effector cells. When effector CD8(+) T cells were from 2C mice, which lack Fas expression ((2CX lpr)F(2)), deletion of effector cells was not exhibited by veto cells. The protein levels of the apoptosis inhibitors FLIP and Bcl2 in purified CD8(+)1B2(+) effector cells at different time points after MLR showed an initial up-regulation of these inhibitors, with marked reduction of FLIP, but not of Bcl2, by 48 h after initiation of culture. Taken together, these results are in accordance with a Fas-FasL-based mechanism in which prolonged binding between the effector cell and the veto cell might be required to allow FLIP to be down-regulated. Such prolonged interaction might be afforded through the interaction of CD8 molecules on the veto cell with the alpha3 domain of H2 class 1 on the effector cell.

Crossing the HLA barriers

Studies in mice and humans demonstrate that transplantation of hematopoietic progenitors in numbers larger than commonly used ("megadose" transplants) overcomes major genetic barriers. In vitro studies suggest that veto cells, within the population of hematopoietic progenitors, facilitate this favorable outcome. Thus, when purified CD34+ cells were added to bulk mixed-lymphocyte reactions (MLRs), they suppressed CTLs against donor's stimulators but not against stimulators from a third party. This tolerizing activity depends on cell contact and can be blocked by the caspase inhibitor BD-FMK, suggesting that the effector host T cells are deleted by apoptosis upon interaction with the CD34+ cells. Early myeloid CD33+ cells generated by short-term ex vivo expansion of CD34+ cells also exhibit veto activity, and these cells can be grown in large numbers. Tolerance induction can be further enhanced by other veto cells. Perhaps the most potent veto cell is the CD8+ CTL. However, this cell is also associated with marked graft-versus-host disease (GVHD). GVHD can be separated from the veto activity by generating anti-third party CTLs under IL2 deprivation. Under such selective pressure, only the stimulated clones which make IL2 can survive, while anti-host clones die. In vivo studies show that such anti-third party veto CTLs can be used safely for tolerance induction without GVHD.

Immune regulatory activity of CD34+ progenitor cells: evidence for a deletion-based mechanism mediated by TNF-alpha

Previous studies suggest that cells within the CD34(+) hematopoietic stem cell compartment are endowed with immune regulatory activity. Furthermore, it is possible to expand the human regulatory cells upon short-term culture of purified CD34+ cells with an early-acting cytokine cocktail. We now show that addition of anti-CD28, anti-CD2, interleukin-2 (IL-2), anti-IL-10, or IL-12 to the bulk mixed lymphocyte reaction (MLR) cannot reverse the inhibitory activity of the CD34+ cells, ruling out anergy-based mechanisms or mechanisms involving Th1-Th2 skewing. Furthermore, phenotyping of cells present after addition of CD34+ cells to the bulk MLR ruled out potential induction of plasmacytoid dendritic precursors, known to be endowed with regulatory activity. In contrast, the inhibitory activity of CD34+ cells could be reversed by adding the caspase inhibitor BD-FMK to the bulk MLR, indicating a deletion-based mechanism. The deletion can be inhibited by anti-tumor necrosis factor-alpha (anti-TNF-alpha) and not by anti-transforming growth factor-beta (anti-TGF-beta), suggesting a potential role for TNF-alpha in the regulatory activity of CD34+ cells.

Anti-third-party veto CTLs overcome rejection of hematopoietic allografts: synergism with rapamycin and BM cell dose

Several bone marrow cells and lymphocyte subpopulations, known as "veto cells," were shown to induce transplantation tolerance across major histocompatibility antigens. Some of the most potent veto cells are of T-cell origin, and in particular a very strong veto activity was documented for cytotoxic T-lymphocyte (CTL) lines or clones. However, these cells also possess marked graft-versus-host (GVH) reactivity. In the present study we evaluated a new approach to deplete CTLs of antihost clones by stimulating the donor T cells against third-party stimulators in the absence of exogenous interleukin 2 (IL-2). We demonstrate that such CTLs are depleted of GVH reactivity while maintaining marked veto activity in vitro. Furthermore, marked synergism was exhibited between the veto CTLs and rapamycin when tested in a murine model, which measures T-cell-mediated bone marrow allograft rejection, or in sublethally irradiated allogeneic hosts. Our results suggest that engraftment of early progenitors could be enhanced by using host-nonreactive anti-third-party CTLs, in conjunction with nonmyeloablative rapamycin-based conditioning protocols, thereby significantly reducing the toxicity of allogeneic transplantation.

Hematopoietic stem cell transplantation across major genetic barriers: tolerance induction by megadose CD34 cells and other veto cells

Studies in mice and humans demonstrate that transplantation of hematopoietic progenitors in numbers larger than commonly used ("megadose" transplants) overcomes major genetic barriers. In vitro studies suggest that veto cells, within the population of hematopoietic progenitors, facilitate this favorable outcome. Thus, when purified CD34(+) cells were added to bulk mixed-lymphocyte reactions (MLRs) they suppressed CTLs against the donor's stimulators, but not against stimulators from a third party. This tolerizing activity depends on cell contact and can be blocked by the caspase inhibitor BD-FMK, suggesting that the effector host T cells are deleted by apoptosis upon interaction with the CD34(+) cells. Early myeloid CD33(+) cells generated by short-term ex vivo expansion of CD34(+) cells also exhibit veto activity, and these cells can be grown in large numbers. Tolerance induction can be further enhanced by other veto cells. Perhaps the most potent veto cell is the CD8+ CTL. However, this cell is also associated with marked GVHD (graft-versus-host disease. GVHD can be separated from the veto activity by generating anti-third party CTLs under IL2 deprivation. Under such selective pressure only the stimulated clones which make IL2 can survive, while anti-host clones die. In vivo studies show that such anti-third party veto CTLs can be used safely for tolerance induction without GVHD.

Anti-third party CD8+ CTLs as potent veto cells: coexpression of CD8 and FasL is a prerequisite

Several bone marrow cells and lymphocyte subpopulations, known as "veto cells," were shown to induce transplantation tolerance across major histocompatibility antigens. Recently, it has been suggested that anti-third party CTLs depleted of alloreactivity are endowed with marked veto activity and therefore might potentially facilitate bone marrow allografting without graft versus host disease (GVHD). The veto mechanism is still obscure. While early studies emphasized the role of CD8-mediated apoptosis, more recent evidence indicates a role for Fas-FasL. In the present study we show, by using blocking anti-CD8 antibody, by generating CTLs from FasL or perforin mutated mice, and by gene transfer of FasL, that the veto activity of anti-third party CD8+ CTLs is dependent upon the simultaneous expression of both CD8 and FasL.

Induction of donor-type chimerism and transplantation tolerance across major histocompatibility barriers in sublethally irradiated mice by Sca-1(+)Lin(-) bone marrow progenitor cells: synergism with non-alloreactive (host x donor)F(1) T cells

Induction of transplantation tolerance by means of bone marrow (BM) transplantation could become a reality if it was possible to achieve engraftment of hematopoietic stem cells under nonlethal preparatory cytoreduction of the recipient. To that end, BM facilitating cells, veto cells, or other tolerance-inducing cells, have been extensively studied. In the present study, we show that BM cells within the Sca-1(+)Lin(-) cell fraction, previously shown to be enriched for early hematopoietic progenitors, are capable of reducing specifically antidonor CTL-p frequency in vitro and in vivo, and of inducing split chimerism in sublethally 7-Gy-irradiated recipient mice across major histocompatibility complex barriers. The immune tolerance induced by the Sca-1(+)Lin(-) cells was also associated with specific tolerance toward donor-type skin grafts. The minimal number of cells required to overcome the host immunity remaining after 7 Gy total body irradiation is very large and, therefore, it may be very difficult to harvest sufficient cells for patients. This challenge was further addressed in our study by demonstrating that non-alloreactive (host x donor)F(1) T cells, previously shown to enhance T-cell-depleted BM allografts in lethally irradiated mice, synergize with Sca-1(+)Lin(-) cells in their capacity to overcome the major transplantation barrier presented by the sublethal mouse model.

The role of megadose CD34+ progenitor cells in the treatment of leukemia patients without a matched donor and in tolerance induction for organ transplantation

Throughout the 1980s, transplantation of unmodified (T cell-replete) bone marrow from full haplotype incompatible family donors was associated with an unsuccessful outcome because of graft failure and severe graft-versus-host disease (GVHD), at times affecting up to 90% of recipients. Although extensive T cell depletion of donor bone marrow was successful in preventing GVHD in children with severe combined immunodeficiency disease (SCID), results were disappointing in leukemic patients because the benefit of preventing GVHD was offset by graft failure. Resistance to engraftment appears to be mediated by host-derived cytotoxic T-lymphocyte precursors that survive supralethal conditioning. In the present paper, we review data that show that these genetic histocompatibility barriers can be overcome in stringent mouse models, employing lethally as well as sublethally irradiated recipients, by two major approaches that are synergistic to each other: escalation of hematopoietic progenitor cell dose and the use of nonalloreactive T cells. The former approach is already being successfully implemented in the treatment of leukemic patients.

Megadose of T cell-depleted bone marrow overcomes MHC barriers in sublethally irradiated mice

Graft-versus-host disease (GVHD) is uniformly lethal in recipients of HLA-mismatched marrow. In patients with severe combined immunodeficiency disease, this major obstacle can be overcome by rigorous T-cell depletion before transplantation. In leukaemia patients, however, the benefit of preventing GVHD is offset by graft rejection or graft failure. Very recently, this problem was overcome by supplementing T cell-depleted bone marrow transplants with megadoses of peripheral blood stem cells collected by leukapheresis after mobilization of the donor stem cells with granulocyte colony-stimulating factor (G-CSF). In the present study, we further demonstrate in a mouse model (C57BL/6-->C3H/Hej) that escalation of bone marrow doses by four- to fivefold leads to full donor-type chimerism in sublethally irradiated (6.5 Gy) recipients. Thus, the new source of G-CSF mobilized human haematopoietic stem cells may enable extending the use of mismatched bone marrow transplants to patients with non-malignant diseases for whom supralethal conditioning is not a prerequisite.