In vitro and in vivo prevention of human CD8+ CTL-mediated xenocytotoxicity by pig c-FLIP expression in porcine endothelial cells

Pancreatic cancer immunotherapy using a tumor lysate vaccine, engineered to express α-gal epitopes, targets pancreatic cancer stem cells

Pancreatic cancer is a lethal disease that remains one of the most resistant to traditional therapies. Immunotherapy in pancreatic cancer induces the recruitment and activation of T cells that recognize tumor-associated antigens (TAAs); thus, the mechanism differs from that of chemotherapy and radiotherapy. The goal of cancer immunotherapy is to elicit immune responses against autologous tumors, and especially to induce multiple T cell clones against a variety of TAAs. In the present study, we prepared a polyvalent tumor lysate vaccine engineered to express the α-gal epitopes, Galα1-3Galβ1-4 GlcNAc-R (i.e., α-gal tumor lysate), from primary tumors. The vaccine elicited strong antibody production against multiple TAAs in pancreatic cancer cells and induced activation of multiple tumor-specific T cells in α1,3-galactosyltransferase (α1,3GT) knockout (KO) mice. The tumor lysate vaccine exhibited a similar effect on pancreatic cancer stem cells (CSCs) with the CD44+CD24+ phenotype. Furthermore, in vivo experiments using NOD/SCID mice, inoculated with splenocytes from KO mice vaccinated with the α-gal tumor lysate and injected with pancreatic cancer cells, showed successful induction of a marked immune response that resulted in suppression of tumorigenesis and significant improvement in overall survival. In contrast, inoculation of lymphocytes from KO mice vaccinated with control tumor lysate vaccine had no effects on tumor growth and survival. The results of both in vitro and in vivo experiments emphasize the efficiency of tumor lysate vaccines expressing α-gal epitopes in targeting all pancreatic cancer cells, including differentiated cancer cells and pancreatic CSCs. The α-gal tumor lysate vaccine could be the basis for a novel therapeutic approach in human clinical trials.

Increased immunogenicity of tumor-associated antigen, mucin 1, engineered to express alpha-gal epitopes: a novel approach to immunotherapy in pancreatic cancer

Mucin 1 (MUC1), a bound mucin glycoprotein, is overexpressed and aberrantly glycosylated in >80% of human ductal pancreatic carcinoma. Evidence suggests that MUC1 can be used as a tumor marker and is a potential target for immunotherapy of pancreatic cancer. However, vaccination with MUC1 peptides fails to stimulate the immune response against cancer cells because immunity toward tumor-associated antigens (TAA), including MUC1, in cancer patients is relatively weak, and the presentation of these TAAs to the immune system is poor due to their low immunogenicity. We investigated whether vaccination with immunogenetically enhanced MUC1 (by expressing alpha-gal epitopes; Galalpha1-3Galbeta1-4GlcNAc-R) can elicit effective antibody production for MUC1 itself as well as certain TAAs derived from pancreatic cancer cells and induced tumor-specific T-cell responses. We also used alpha1,3galactosyltransferase (alpha1,3GT) knockout mice that were preimmunized with pig kidney and transplanted with B16F10 melanoma cells transfected with MUC1 expression vector. Vaccination of these mice with alpha-gal MUC1 resulted in marked inhibition of tumor growth and significant improvement of overall survival time compared with mice vaccinated with MUC1 alone (P = 0.003). Furthermore, vaccination with pancreatic cancer cells expressing alpha-gal epitopes induced immune responses against not only differentiated cancer cells but also cancer stem cells. The results suggested that vaccination using cells engineered to express alpha-gal epitopes is a novel strategy for treatment of pancreatic cancer.

Adenovirus-mediated gene expression of the human c-FLIP(L) gene protects pig islets against human CD8(+) cytotoxic T lymphocyte-mediated cytotoxicity

Cell-mediated immunity, especially of human CD8+ cytotoxic T lymphocytes (CTLs) is believed to have an important role in the long-term survival of pig islet xenografts. Protection against human CD8+ CTL cytotoxicity may reduce the direct damage to pig islets and enable long-term xenograft survival in pig-to-human islet xenotransplantation. We have previously reported that c-FLIP(S/L) genes, which are potent inhibitors of death receptor-mediated proapoptotic signals through binding competition with caspase-8 for recruitment to the Fas-associated via death domain (FADD), markedly suppress human CD8+ CTL-mediated xenocytotoxicity. In addition, the cytoprotective effects of c-FLIP(L) seem to be significantly stronger than those of c-FLIP(S). Accordingly, in the present study, expression of c-FLIP(L) was induced in intact pig islets by adenoviral transduction. Consequently, the cytoprotective capacity of the transgene in pig islets was examined in in vitro and in vivo exposure to human CD8+ CTLs. Cells from untransduced islets or mock islets were sensitive to CD8+ CTL-mediated lysis (59.3% +/- 15.9% and 64.0% +/- 8.9% cytotoxicity, respectively). In contrast, cells from pig islets transduced with the c-FLIP(L) gene were markedly protected from lysis (30.5% +/- 3.5%). Furthermore, prolonged xenograft survival was elicited from pig islets transduced with this molecule as assessed using an islet transplant model using the rat kidney capsule. Thus, these data indicate that intact pig islets can be transduced to express c-FLIP(L) with adenovirus. Pig islets expressing c-FLIP(L) are significantly resistant to human CTL killing and further exhibit beneficial effects to prolong xenograft survival.

Intracellular and extracellular remodeling effectively prevents human CD8(+)cytotoxic T lymphocyte-mediated xenocytotoxicity by coexpression of membrane-bound human FasL and pig c-FLIP(L) in pig endothelial cells

Human CD8(+) cytotoxic T lymphocyte (CTL)-mediated cytotoxicity, which participates in xenograft rejection, is mediated mainly by the Fas/FasL apoptotic pathway. We previously developed methods to inhibit human CTL xenocytotoxicity by extracellular remodeling using overexpression of membrane-bound human FasL on pig xenograft cells, and by intracellular blockade of death receptor-mediated apoptotic signals, such as the Fas/FasL pathway using the pig c-FLIP(L) molecule. To investigate the cooperative effects of both membrane-bound FasL and pig c-FLIP(L), we cotransfected both genes into pig endothelial cells (PEC). The double remodeling with these molecules effectively prevented CD8(+) CTL killing. Although double transfectants and single high transfectants of either membrane-bound FasL or c-FLIP(L) gene displayed similar inhibition of CTL cytotoxicity, the expression levels of these 2 molecules in double transfectants were almost half the expression levels of single transfectants. Furthermore, to show in vivo prolongation of xenograft survival, we transplanted PEC transfectants under the rat kidney capsule. Prolonged survival was displayed by PEC double transfectant xenografts whereas those from either parental PEC or MOCK (vehicle control) were completely rejected by day 5 posttransplantation. These data suggested that intracellular and extracellular remodeling by coexpression of membrane-bound FasL and pig c-FLIP(L) in xenograft cells may prevent an innate cellular response to xenografts. The gene compatibility of these molecules to generate transgenic pigs may be sufficient to create a window of opportunity to facilitate long-term xenograft survival.