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Biomedical subjects

Megan Sykes

Publications and source records attributed to Megan Sykes.

At least 55 records · Page 3Linked to original sources
PubMed2004

Marked prolongation of porcine renal xenograft survival in baboons through the use of alpha1,3-galactosyltransferase gene-knockout donors and the cotransplantation of vascularized thymic tissue.

The use of animal organs could potentially alleviate the critical worldwide shortage of donor organs for clinical transplantation. Because of the strong immune response to xenografts, success will probably depend upon new strategies of immune suppression and induction of tolerance. Here we report our initial results using alpha-1,3-galactosyltransferase knockout (GalT-KO) donors and a tolerance induction approach. We have achieved life-supporting pig-to-baboon renal xenograft survivals of up to 83 d with normal creatinine levels.

Animals↗
PubMed2004

T cells from presensitized donors fail to cause graft-versus-host disease in a pig-to-mouse xenotransplantation model.

BACKGROUND: The ability of T cells from pigs, the most suitable donors for clinical xenotransplantation, to induce graft-versus-host disease (GVHD) and to facilitate hematopoietic cell engraftment in highly disparate xenogeneic recipients remains unclear. In this article, the authors address these questions in a presensitized pig-to-mouse transplantation model using porcine cytokine-transgenic mice. METHODS: Swine donors were presensitized by mouse skin grafting and boosted by the injection of mouse cells after the skin graft was rejected. Porcine peripheral blood mononuclear cells (PBMC) and splenocytes were collected at various times after mouse skin grafting, and their potential to induce GVHD and to facilitate donor hematopoietic cell engraftment in conditioned murine recipients was evaluated. RESULTS: Presensitization of donor pigs resulted in marked enhancement of anti-mouse responses, as reflected in augmented anti-mouse mixed lymphocyte responses, cell-mediated cytotoxicity, and antibody production. However, injection of high numbers of PBMC and splenocytes (>1 x 10(8)) with bone marrow cells from the presensitized pigs failed to induce detectable GVHD or long-term chimerism in mice that were treated with depleting anti-T-cell and natural killer cell antibodies, cobra venom factor, medronate-liposomes, and 4 Gy of whole-body and 7 Gy of thymic irradiation. Histologic analysis revealed no mononuclear cell infiltration or GVHD-associated lesions in the liver, intestine, lungs, or skin of the mouse recipients. Furthermore, the recipient mice had no detectable T cells or anti-pig immunoglobulin G antibodies in the blood by 6 weeks after injection of porcine cells. CONCLUSION: These results demonstrate that porcine T-cell function is severely impaired in the xenogeneic murine microenvironment.

Animals↗
PubMed2004

Position paper of the Ethics Committee of the International Xenotransplantation Association.

Xenotransplantation (XTx) provides a potential solution to the shortage of human organs and tissues, and has several advantages over other possible solutions to this problem. However, a number of scientific and ethical barriers exist, and need to be addressed in order to advance the field of XTx in a manner that optimizes its potential to benefit society and minimizes its risk. Some of the most pressing ethical issues are discussed, and the position of the Ethics Committee of the International Xenotransplantation Association is presented.

Animals↗
PubMed2004

Tolerance and cancer: mechanisms of tumor evasion and strategies for breaking tolerance.

The development of malignant disease might be seen as a failure of immune surveillance. However, not all tumors are naturally immunogenic, and even among those that are immunogenic, the uncontrolled rapid growth of a tumor may sometimes out-run a robust immune response. Nevertheless, recent evidence suggests that mechanisms of tolerance that normally exist to prevent autoimmune disease may also preclude the development of an adequate antitumor response and that tumors themselves have the ability to thwart the development of effective immune responses against their antigens. A major challenge has been to develop approaches to breaking this tolerance in tumor-bearing hosts, and recent advances in our understanding of antigen presentation and tolerance have led to some promising strategies. An alternative approach is to use T cells from nontumor-bearing, allogeneic hosts in the form of lymphocyte infusions, with or without hematopoietic cell transplantation. Immunotherapy may occur in this setting via the response of nontolerant, tumor antigen-specific T cells from nontumor-bearing hosts or via the powerful destructive effect of an alloresponse directed against antigens shared by malignant cells in the recipient. Approaches to exploiting this beneficial effect without the deleterious consequence of graft-versus-host disease in allogeneic hematopoietic cell recipients are discussed.

Antigens, Neoplasm↗
PubMed2004

Mechanisms of early peripheral CD4 T-cell tolerance induction by anti-CD154 monoclonal antibody and allogeneic bone marrow transplantation: evidence for anergy and deletion but not regulatory cells.

Anti-CD154 (CD40L) monoclonal antibody (mAb) plus bone marrow transplantation (BMT) in mice receiving CD8 cell-depleting mAb leads to long-term mixed hematopoietic chimerism and systemic donor-specific tolerance through peripheral and central deletional mechanisms. However, CD4(+) T-cell tolerance is demonstrable in vitro and in vivo rapidly following BMT, before deletion of donor-reactive CD4 cells is complete, suggesting the involvement of other mechanisms. We examined these mechanisms in more detail. Spot enzyme-linked immunosorbent (ELISPOT) analysis revealed specific tolerization (within 4 to 15 days) of both T helper 1 (Th1) and Th2 cytokine responses to the donor, with no evidence for cytokine deviation. Tolerant lymphocytes did not significantly down-regulate rejection by naive donor-reactive T cells in adoptive transfer experiments. No evidence for linked suppression was obtained when skin expressing donor alloantigens in association with third-party alloantigens was grafted. T-cell receptor (TCR) transgenic mixing studies revealed that specific peripheral deletion of alloreactive CD4 T cells occurs over the first 4 weeks following BMT with anti-CD154. In contrast to models involving anti-CD154 without BMT, BMT with anti-CD154 leads to the rapid induction of anergy, followed by deletion of pre-existing donor-reactive peripheral CD4(+) T cells; the rapid deletion of these cells obviates the need for a regulatory cell population to suppress CD4 cell-mediated alloreactivity.

Animals↗
PubMed2004

Induction of human T-cell tolerance to porcine xenoantigens through mixed hematopoietic chimerism.

Xenotransplantation from pigs could provide a potential solution to the severe shortage of allogeneic donor organs. Because xenogeneic tissues are subject to vigorous immune rejection, tolerance induction is likely to be essential to the success of clinical xenotransplantation. Here we explore the possibility of inducing human T-cell tolerance to porcine xenografts through mixed chimerism. We previously showed that NOD/SCID-Tg mice expressing porcine cytokine transgenes permit the induction of durable porcine hematopoietic chimerism. In this study we achieved human T-cell development in these mice by engrafting human fetal thymus/liver tissues. In porcine hematopoietic chimeras, human thymus grafts were populated with porcine class II(high) cells in addition to human cells, and human T cells were tolerant of the porcine hematopoietic donor as measured by mixed lymphocyte reaction assay and skin grafting. This study proves the principle that porcine chimerism induces tolerance of xenoreactive human T cells.

Animals↗
PubMed2004

Tolerance in mixed chimerism - a role for regulatory cells?

The establishment of mixed hematopoietic chimerism induces life-long donor-specific organ graft tolerance while obviating the need for chronic immunosuppression. Recent advances have dramatically reduced the conditioning toxicity required to achieve mixed chimerism. We argue that the achievement of high levels of donor chimerism ensures life-long deletion of donor-reactive T cells, precluding and obviating the need for regulatory mechanisms in the maintenance of tolerance. However, in situations where high levels of donor chimerism cannot be established or sustained, control of immune responsiveness can be achieved through additional mechanisms, including regulatory T cells.

Graft Survival↗
PubMed2004

Earlier low-dose TBI or DST overcomes CD8+ T-cell-mediated alloresistance to allogeneic marrow in recipients of anti-CD40L.

Treatment with a single injection of anti-CD40L (CD154) monoclonal antibody (mAb) and fully mismatched allogeneic bone marrow transplant (BMT) allows rapid tolerization of CD4+ T cells to the donor. The addition of in vivo CD8 T-cell depletion leads to permanent mixed hematopoietic chimerism and tolerance. We now describe two approaches that obviate the requirement for CD8 T-cell depletion by rapidly tolerizing recipient CD8 T cells in addition to CD4 cells. Administration of donor-specific transfusion (DST) to mice receiving 3 Gy total body irradiation (TBI), BMT and anti-CD40L mAb on day 0 uniformly led to permanent mixed chimerism and tolerance, compared with only 40% of mice receiving similar treatment without DST. In the absence of DST, moving the timing of 3 Gy TBI to day -1 or day -2 instead of day 0 led to rapid (by 2 weeks) induction of CD8+ cell tolerance, and also permitted uniform achievement of permanent mixed chimerism and donor-specific tolerance in recipients of anti-CD40L and BMT on day 0. These nontoxic regimens overcome CD8+ and CD4+ T-cell-mediated alloresistance without requiring host T-cell depletion, permitting the induction of permanent mixed chimerism and tolerance.

Animals↗
PubMed2004

CD154 blockade for induction of mixed chimerism and prolonged renal allograft survival in nonhuman primates.

Costimulatory blockade with anti-CD154 monoclonal antibody (aCD154) prolongs allograft survival in nonhuman primates, but has not reliably induced tolerance when used alone. In the current studies, we evaluated the effect of adding CD154 blockade to a chimerism inducing nonmyeloablative regimen in primates. We observed a significant improvement of donor bone marrow (DBM) engraftment, which has been associated with a lower incidence of acute rejection and long-term survival of renal allografts without the need for previously required splenectomy. Among the long-term survivors, four never showed evidence of rejection, with the longest survival exceeding 1700 days following discontinuation of immunosuppression. Nevertheless, late chronic rejection was observed in three of eight recipients, indicating the necessity of further modifications of the regimen. Control recipients receiving no DBM or donor splenocytes in place of DBM rejected their allografts. Thus, DBM engraftment with, at least, transient mixed chimerism appears essential for induction of allograft tolerance using this conditioning regimen. Modification of the original mixed chimerism approach, by the addition of costimulatory blockade, has been shown to enhance mixed chimerism and induce renal allograft tolerance with less morbidity in nonhuman primates.

Animals↗
PubMed2004

Mouse retrovirus mediates porcine endogenous retrovirus transmission into human cells in long-term human-porcine chimeric mice.

Porcine endogenous retrovirus (PERV) is a potential pathogen in clinical xenotransplantation; transmission of PERV in vivo has been suggested in murine xenotransplantation models. We analyzed the transmission of PERV to human cells in vivo using a model in which immunodeficient NOD/SCID transgenic mice were transplanted with porcine and human lymphohematopoietic tissues. Our results demonstrate, we believe for the first time, that human and pig cells can coexist long-term (up to 25 weeks) without direct PERV infection of human cells. Despite the transplantation of porcine cells that did not produce human-tropic PERV, human cells from the chimeric mice were frequently found to contain PERV sequences. However, this transmission was due to the pseudotyping of PERV-C (a virus without human tropism) by xenotropic murine leukemia virus, rather than to de novo generation of human-tropic PERV. Thus, pseudotyping might account for the PERV transmission previously observed in mice. The absence of direct human cell infection following long-term in vivo coexistence with large numbers of porcine cells provides encouragement regarding the potential safety of using pigs that do not produce human-tropic PERV as source animals for transplantation to humans.

Amino Acid Sequence↗
PubMed2004

Mixed hematopoietic chimerism allows cure of autoimmune diabetes through allogeneic tolerance and reversal of autoimmunity.

Bone marrow transplantation from diabetes-resistant strains with complete replacement of the recipient immune system by the allogeneic donor has led to tolerance to donor islets and cure of diabetes in a mouse model of type 1 diabetes. However, the ability to tolerize host T-cells of diabetic NOD mice is unknown. We demonstrate that nonmyeloablative conditioning achieves mixed hematopoietic chimerism across major histocompatibility complex (MHC) barriers in spontaneously diabetic NOD mice. This conditioning preserves alloreactive and autoreactive diabetogenic host NOD T-cells, but when mixed chimerism was established, diabetic NOD mice accepted donor-type allogeneic islet grafts and were cured of diabetes, despite a significant recipient T-cell contribution. Furthermore, induction of mixed chimerism permitted acceptance of NOD islet grafts, demonstrating reversal of autoimmunity. Allogeneic bone marrow transplantation was critical for tolerization of diabetogenic and alloreactive host T-cells. Thus, mixed hematopoietic chimerism induces tolerance to donor islets and reverses established autoimmunity in diabetic NOD mice.

Animals↗
PubMed2003

Peritoneal cavity B cells are precursors of splenic IgM natural antibody-producing cells.

Peritoneal cavity B-1 cells are believed to produce IgM natural Abs. We have used alpha1,3-galactosyltransferase-deficient (GalT(-/-)) mice, which, like humans, produce IgM natural Abs against the carbohydrate epitope Galalpha1,3Gal (Gal), to demonstrate that peritoneal cavity B-1b cells with anti-Gal receptors produce anti-Gal IgM Abs only after LPS stimulation. Likewise, peritoneal cavity cells of GalT(-/-) and wild-type mice do not produce IgM Abs of other specificities without LPS stimulation. Development of Ab-secreting capacity is associated with loss of CD11b/CD18 (Mac-1) expression. In contrast, there are large numbers of cells producing anti-Gal and other IgM Abs in fresh splenocyte preparations from GalT(-/-) and (for non-Gal specificities) wild-type mice. These cells are Mac-1(-) but otherwise B-1b-like in their phenotype. We therefore hypothesized a pathway wherein peritoneal cavity B cells migrate into the spleen after activation in vivo and lose Mac-1 expression to become IgM Ab-producing cells. Consistent with this possibility, splenectomy reduced anti-Gal Ab production after immunization of GalT(-/-) mice with Gal-positive rabbit RBC. Furthermore, splenectomized B6 GalT(-/-), Ig micro -chain mutant ( micro (-/-)) (both Gal- and B cell-deficient) mice produced less anti-Gal IgM than nonsplenectomized controls after adoptive transfer of peritoneal cavity cells from B6 GalT(-/-) mice. When sorted GalT(-/-) Mac-1(+) peritoneal cavity B cells were adoptively transferred to B6 GalT(-/-), micro (-/-) mice, IgM Abs including anti-Gal appeared, and IgM-producing and Mac1(-) B cells were present in the spleen 5 wk after transfer. These findings demonstrate that peritoneal cavity Mac-1(+) B-1 cells are precursors of Mac-1(-) splenic IgM Ab-secreting cells.

Adoptive Transfer↗
PubMed2003

Local irradiation enhances congenic donor pluripotent hematopoietic stem cell engraftment similarly in irradiated and nonirradiated sites.

Long-term multilineage chimerism is achieved in CD45 congenic mice receiving high bone marrow doses with or without mediastinal irradiation (MI). Increased donor chimerism results in MI-treated compared with nonirradiated animals, suggesting that MI makes "space" for engraftment of donor pluripotent hematopoietic stem cells (PHSCs). We have now examined whether space is systemic or whether increased engraftment of donor marrow in locally irradiated mice is confined to the irradiated bones. While increased donor chimerism was observed in irradiated bones compared with nonirradiated bones of MI-treated animals 4 weeks following bone marrow transplantation (BMT), these differences were minimal by 40 weeks. MI-treated chimeras contained more adoptively transferable donor PHSCs in the marrow of both irradiated and distant bones compared with non-MI-treated chimeras. Similar proportions of donor PHSCs were present in irradiated and nonirradiated bones of locally irradiated mice at both 4 and 40 weeks. Irradiated bones contained more donor short-term repopulating cells than distant bones at 4 weeks, but not 40 weeks, after BMT. Our study suggests that local proliferation of donor PHSCs in mice receiving local irradiation rapidly leads to a systemic increase in donor PHSC engraftment.

Adoptive Transfer↗
PubMed2003

Graft-versus-host-reactive donor CD4 cells can induce T cell-mediated rejection of the donor marrow in mixed allogeneic chimeras prepared with nonmyeloablative conditioning.

Murine mixed hematopoietic chimerism can be achieved following nonmyeloablative conditioning with cyclophosphamide, T cell-depleting monoclonal antibodies, and thymic irradiation. Donor lymphocyte infusions (DLIs) 35 days after bone marrow transplantation (BMT) convert mixed to full donor chimerism and mediate graft-versus-lymphoma effects without graft-versus-host disease. We evaluated the role of T-cell subsets in DLIs in converting mixed to full donor chimerism in a fully major histocompatibility complex-mismatched strain combination. Whereas DLIs administered on day 35 converted 100% of mixed chimeras to full donor chimerism, conversion was less frequent when either CD4 or CD8 cells were depleted, indicating that both subsets contribute to the conversion. Surprisingly, administration of CD8-depleted DLIs led to complete loss of donor chimerism in a high proportion (54%) of recipients compared with CD4-plus CD8-depleted DLIs (15%) or CD4-depleted DLIs (0%) (P <.05). DLIs administered at early time points after BMT (eg, day 21) also precipitated rejection of donor marrow by recipient alphabeta T cells, in association with donor CD4 cell expansion and high production of interleukin 2 (IL-2), IL-4, and interferon-gamma. Thus, DLIs can paradoxically induce marrow rejection by residual host alphabeta T cells. These results have implications for the timing of and use of subset depletion of DLIs in recipients of nonmyeloablative transplants.

Animals↗