Molecular cloning and functional characterization of the pig homologue of integrin-associated protein (IAP/CD47)

Expression of recipient CD47 on rat insulinoma cell xenografts prevents macrophage-mediated rejection through SIRPα inhibitory signaling in mice

We have previously proven that the interspecies incompatibility of CD47 is responsible for in vitro phagocytosis of xenogeneic cells by host macrophages. Utilizing an in vivo model in the present study, we investigated whether genetically engineered expression of mouse CD47 in rat insulinoma cells (INS-1E) could inhibit macrophage-mediated xenograft rejection. INS-1E cells transfected with the pRc/CMV-mouse CD47 vector (mCD47-INS-1E) induced SIRPα-tyrosine phosphorylation in mouse macrophages in vitro, whereas cells transfected with the control vector (cont-INS-1E) did not. When these cells were injected into the peritoneal cavity of streptozotocin-induced diabetic Rag2^−/−γ chain ^−/− mice, which lack T, B, and NK cells, the expression of mouse CD47 on the INS-1E cells markedly reduced the susceptibility of these cells to phagocytosis by macrophages. Moreover, these mice became normoglycemic after receiving mCD47-INS-1E, whereas the mice that received cont-INS-1E failed to achieve normoglycemia. Furthermore, injection of an anti-mouse SIRPα blocking monoclonal antibody into the mouse recipients of mCD47-INS-1E cells prevented achievement of normoglycemia. These results demonstrate that interspecies incompatibility of CD47 significantly contributes to in vivo rejection of xenogeneic cells by macrophages. Thus, genetic induction of the expression of recipient CD47 on xenogeneic donor cells could provide inhibitory signals to recipient macrophages via SIPRα; this constitutes a novel approach for preventing macrophage-mediated xenograft rejection.

Transcellular migration of neutrophil granulocytes through the blood-cerebrospinal fluid barrier after infection with Streptococcus suis

BACKGROUND

A critical point during the course of bacterial meningitis is the excessive influx of polymorphnuclear neutrophils (PMNs) from the blood into the brain. Both paracellular and transcellular routes of leukocyte transmigration through the blood-brain barrier have been described in CNS diseases so far. Thus, we investigated the mechanism of PMN transmigration through the blood-CSF barrier under inflammatory conditions.

METHODS

In an "inverted" Transwell culture model of the blood-CSF barrier, the zoonotic agent Streptococcus suis (S. suis) was used to stimulate porcine choroid plexus epithelial cells (PCPECs) specifically from the physiologically relevant basolateral side. Barrier function was analyzed by measuring TEER and TR-dextran-flux, and tight junction morphology was investigated by immunofluorescence. Route and mechanism of PMN transmigration were determined by immunofluorescence, electron microscopy and FACS analysis. Quantitative real time-PCR was used to determine expression levels of ICAM-1 and VCAM-1.

RESULTS

Here, we show that the transmigration of PMNs through PCPECs was significantly higher after stimulation with TNFα or infection with S. suis strain 10 compared to its non-encapsulated mutant. Barrier function was not significantly affected by PMN migration alone, but in combination with S. suis infection. Tight junction and cytoskeletal actin reorganisation were also observed after stimulation with S. suis or TNFα. Most strikingly, PMNs preferentially migrated across PCPECs via the transcellular route. Extensive sequential analyses of the PMN transmigration process with Apotome(®)-imaging and electron microscopy revealed that paracellular migrating PMNs stop just before tight junctions. Interestingly, PMNs subsequently appeared to proceed by transcellular migration via funnel-like structures developing from the apical membrane. It is noteworthy that some PMNs contained bacteria during the transmigration process. Flow cytometric and transmigration inhibition studies with integrin-specific antibodies showed that PMN traversal is dependent on CD11b/CD18. Analysis of cell adhesion molecules in PCPECs revealed a significant increase of ICAM-1 and VCAM-1 expression after TNFα and S. suis stimulation.

CONCLUSION

Our data underline the relevance of the blood-CSF barrier as a gate for leukocyte entry into the CNS and suggest a novel transcellular migration step during the pathogenesis of bacterial meningitis.

Gene silencing of CD47 and antibody ligation of thrombospondin-1 enhance ischemic tissue survival in a porcine model: implications for human disease

BACKGROUND

Insufficient tissue perfusion underlies many acute and chronic diseases. Tissue perfusion in turn requires adequate blood flow, determined in large part by the relative state of relaxation or constriction of arterial vessels. Nitric oxide (NO) produced by vascular cells modulates blood flow and tissue perfusion by relaxing and dilating arteries. Recently, we reported that the secreted protein thrombospondin-1 (TSP1), through its cell surface receptor CD47, limits the ability of NO to relax and dilate blood vessels and thus decreases tissue perfusion. In the present study, we tested the hypothesis that blocking TSP1-CD47 signaling increases ischemic tissue survival in random cutaneous porcine flaps.

METHODS

Random cutaneous flaps 2 x 10 cm2 were raised in white hairless Yucatan miniature pigs and were treated with a monoclonal antibody to TSP1, an antisense morpholino oligonucleotide to CD47 or control agents and tissue survival assessed. Primary vascular smooth muscle cell cultured from Yucatan pigs were also treated with the same agents +/- and an NO donor (DEA/NO) and cGMP quantified.

RESULTS

Antibody blockade of TSP1 or morpholino suppression of CD47 dramatically enhanced survival of random tissue flaps. These responses correlated with increased blood vessel patency and tissue blood flow on vessel injection studies. NO-stimulated cGMP flux in Yucatan vascular smooth muscle cell was abrogated after antibody or morpholino treatment.

CONCLUSION

Antibody ligation of TSP1 or antisense morpholino knock down of CD47 greatly increased tissue survival to ischemia. Given the similarity between porcine and human soft tissues these results suggest significant therapeutic potential for people.

Liver xenografts for the treatment of acute liver failure: clinical and experimental experience and remaining immunologic barriers

A critical element restricting the application of liver transplantation is the shortage of human deceased donor organs. Xenotransplantation using pig organs might be a solution to this shortage. Although the problems that still require resolution include the immunologic barrier, the potential risk of transferring infectious agents with the transplanted organ, and uncertainty about whether the transplanted organ will function satisfactorily in the human environment, recent progress in the genetic manipulation of pigs has led to the prospect that clinical xenografting, at least as a bridge to allotransplantation, may be possible in the foreseeable future. Experience with clinical auxiliary and orthotopic liver xenotransplantation and experimental liver xenotransplantation in nonhuman primate and other large animal models is reviewed, and the remaining immunologic problems are discussed. Evidence suggests that, in patients with hepatic failure, the pig liver may be less susceptible to antibody-mediated injury than other pig organs, such as the heart or kidney. Pig Kupffer cells and other macrophages will recognize and phagocytose primate red blood cells, but this problem should be overcome by pretransplant depletion of macrophages from the organ-source pig. From the evidence currently available, it does not seem unduly optimistic to anticipate that a liver from an alpha1,3-galactosyltransferase gene-knockout pig would survive at least long enough to function as a successful bridge to allotransplantation.

Role for CD47-SIRPalpha signaling in xenograft rejection by macrophages

We have previously proven that human macrophages can phagocytose porcine cells even in the absence of Ab or complement opsonization, indicating that macrophages present a pivotal immunological obstacle to xenotransplantation. A recent report indicates that the signal regulatory protein (SIRP)alpha is a critical immune inhibitory receptor on macrophages, and its interaction with CD47, a ligand for SIRPalpha, prevents autologous phagocytosis. Considering the limited compatibility (73%) in amino acid sequences between pig and human CD47, we hypothesized that the interspecies incompatibility of CD47 may contribute to the rejection of xenogeneic cells by macrophages. In the present study, we have demonstrated that porcine CD47 does not induce SIRPalpha tyrosine phosphorylation in human macrophage-like cell line, and soluble human CD47-Fc fusion protein inhibits the phagocytic activity of human macrophages toward porcine cells. In addition, we have verified that manipulation of porcine cells for expression of human CD47 radically reduces the susceptibility of the cells to phagocytosis by human macrophages. These results indicate that the interspecies incompatibility of CD47 significantly contributes to the rejection of xenogeneic cells by macrophages. Genetic induction of human CD47 on porcine cells could provide inhibitory signaling to SIRPalpha on human macrophages, providing a novel approach to preventing macrophage-mediated xenograft rejection.

Attenuation of phagocytosis of xenogeneic cells by manipulating CD47

Signal regulatory protein alpha (SIRPalpha) is a critical immune inhibitory receptor on macrophages, and its interaction with CD47, a ligand for SIRPalpha, prevents autologous phagocytosis. We hypothesized that interspecies incompatibility of CD47 may contribute to the rejection of xenogeneic cells by macrophages. Here, we show that pig CD47 does not interact with mouse SIPRalpha. Similar to CD47-/- mouse cells, porcine red blood cells (RBCs) failed to induce SIRPalpha tyrosine phosphorylation in mouse macrophages. Blocking SIRPalpha with antimouse SIRPalpha mAb (P84) significantly enhanced the phagocytosis of CD47+/+ mouse cells, but did not affect the engulfment of porcine or CD47-/- mouse cells by mouse macrophages. CD47-deficient mice, whose macrophages do not phagocytose CD47-/- mouse cells, showed markedly delayed clearance of porcine RBCs compared with wild-type mouse recipients. Furthermore, mouse CD47 expression on porcine cells markedly reduced their phagocytosis by mouse macrophages both in vitro and in vivo. These results indicate that interspecies incompatibility of CD47 contributes significantly to phagocytosis of xenogeneic cells by macrophages and suggest that genetic manipulation of donor CD47 to improve its interaction with the recipient SIRPalpha may provide a novel approach to prevent phagocyte-mediated xenograft rejection.

Species- and cell type-specific interactions between CD47 and human SIRPalpha

CD47 on red blood cells (RBCs) reportedly signals "self" by binding SIRPalpha on phagocytes, at least in mice. Such interactions across and within species, from mouse to human, are not yet clear and neither is the relation to cell adhesion. Using human SIRPalpha1 as a probe, antibody-inhibitable binding to CD47 was found only with human and pig RBCs (not mouse, rat, or cow). In addition, CD47-mediated adhesion of human and pig RBCs to SIRPalpha1 surfaces resists sustained forces in centrifugation (as confirmed by atomic force microscopy) but only at SIRPalpha-coating densities far above those measurable on human neutrophils, monocytes, and THP-1 macrophages. While interactions strengthen with deglycosylation of SIRPalpha1, low copy numbers explain the absence of RBC adhesion to phagocytes under physiologic conditions and imply that the interaction being studied is not responsible for red cell clearance in humans. Evidence of clustering nonetheless suggests mechanisms of avidity enhancement. Finally, using the same CD47 antibodies and soluble SIRPalpha1, bone marrow-derived mesenchymal stem cells were assayed and found to display CD47 but not bind SIRPalpha1 significantly. The results thus demonstrate that SIRPalpha-CD47 interactions, which reportedly define self, exhibit cell type specificity and limited cross-species reactivity.

Antibody- and complement-independent phagocytotic and cytolytic activities of human macrophages toward porcine cells

BACKGROUND

It has been speculated that host macrophages contribute to rapid clearance of transplanted xenogeneic cells. To address such a possibility, phagocytotic and cytolytic activities of human macrophages toward xenogeneic porcine cells were evaluated in vitro in the absence of antibodies and complement factors.

METHODS

Human peripheral monocyte-derived macrophages (P-macrophages) and reticulo-endothelial macrophages (RE-macrophages) were obtained from volunteers' peripheral blood and from the perfusion effluents of liver allografts for transplantation, respectively. 5-(and 6-) carboxyfluorescein diacetate succinimidyl ester (CFSE)-labeled human autologous red blood cells (auto-RBCs), ABO-incompatible RBCs (incom-RBCs) and xenogeneic porcine RBCs (xeno-RBCs) were incubated with the human macrophages; subsequently, the macrophages that had phagocytosed the RBCs could be identified as CFSE positive cells by FCM analyses and confocal microscopy. Cytolytic activity was quantified by calculating levels of lactate dehydrogenase in each culture supernatant.

RESULTS

Human RE-macrophages spontaneously phagocytosed and had a remarkable cytolytic activity toward xeno-RBCs, but not toward auto-RBCs or incom-RBCs. Elimination of alpha-galactosyl xenoantigen (alpha-Gal) epitopes on xeno-RBCs did not prevent phagocytotic or cytolytic activity of RE-macrophages.

CONCLUSIONS

These findings indicate phagocytotic and cytolitic activities of human macrophages toward porcine cells are initiated by a factor other than alpha-Gal in a mechanism independent of antibody/complement opsonization.

Alpha-Pal/NRF-1 regulates the promoter of the human integrin-associated protein/CD47 gene

Integrin-associated protein (IAP or CD47) is expressed in a variety of tissues, including the nervous system and immune system. To understand how cells control the expression of the IAP gene, we cloned the 5'-proximal region of the human IAP gene and investigated IAP promoter activity by transient transfection. RT-PCR confirmed the expression of IAP transcripts in human neuroblastoma IMR-32 and hepatoma HepG2 cells. Deletion analysis identified a core promoter of the human IAP gene located between nucleotide positions -232 and -12 relative to the translation initiation codon in these two cell lines. Site-directed mutagenesis and gel electrophoretic mobility shift assay identified a alpha-Pal/NRF-1 binding element within the IAP core promoter. Supershift assays using the alpha-Pal/NRF-1 antiserum confirmed the binding of this transcription factor on the alpha-Pal/NRF-1 site. Overexpression of the DNA binding domain of alpha-Pal/NRF-1 in cells enhanced DNA-alpha-Pal/NRF-1 binding in vitro. Furthermore, overexpression of full-length alpha-Pal/NRF-1 significantly enhanced IAP promoter activity while overexpression of dominant-negative mutant reduced promoter activity both in the cultured human cell lines and primary mouse cortical cells. These results revealed that alpha-Pal/NRF-1 is an essential transcription factor in the regulation of human IAP gene expression.