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Cross-Species Chemoattraction and Xenograft Failure: Do Neutrophils Play a Role?

2004; Wolters Kluwer; Volume: 78; Issue: 12 Linguagem: Inglês

10.1097/01.tp.0000147340.23034.18

1534-6080

Timothy Devos, Mark Waer, An Billiau,

Transplantation: Methods and Outcomes

Polymorphonuclear granulocytes (PMN, neutrophils) are well known to contribute to the pathogenesis of many congenital disorders (e.g., CR3 deficiency, chronic granulomatous disease, myeloperoxidase deficiency) and acquired disease states (e.g., inflammatory bowel disease, glomerulonephritis, immune vasculitis, rheumatoid arthritis, neutrophil dermatoses, sepsis). They play a key role in mediating ischemia-reperfusion injury following solid organ allotransplantation, and more recent evidence suggests that, following early migration into the allograft, neutrophils are capable of amplifying T-cell mediated allograft rejection (1). In xenotransplantation, the major hurdle of hyperacute rejection (HAR) has led research efforts to focus on the role of natural xenoantibodies and complement activation, while the neutrophil was considered to play a secondary role and was therefore given little attention. Meanwhile, it seems that experimental prevention of HAR becomes feasible thanks to the genetic modification of donor animals (DAF/CD55 transgenic pigs, α-Gal knockout pigs and mice) and, consequently, the mechanism of acute vascular rejection (AVR) and the cellular aspects of xenograft rejection now receive full attention. AVR is characterized by a dense cellular infiltrate, which is dominated not only by natural killer (NK) cells and macrophages, but also by PMN. It is therefore plausible to assume that neutrophils take an active part in the rejection process and this hypothesis is gaining increasing interest. Vercellotti et al. (2) showed that the adhesion of human neutrophils to porcine endothelial cells (PECs) was promoted by human natural xenoantibodies and complement, and, particularly, that endothelial deposition of iC3b was critically involved in the adhesion process. Ehrnfelt et al. (3) showed that human PMNs adhere to PECs, provided these are activated by human natural xenoantibodies. These studies indicate that the interaction between neutrophils and xenoendothelium requires preactivation of endothelial cells by complement and/or natural xenoantibodies and, consequently, that the PMN plays a secondary role in HAR. In contrast, others have provided evidence that human PMN have the inherent capacity to directly interact with xenogeneic endothelial cells. In contrast to Ehrnfelt et al. (3), Sheikh et al. (4) demonstrated that, even in the absence of natural xenoantibodies, human neutrophils adhere more avidly to naïve porcine endothelial cells than to naïve human endothelial cells. In this issue of Transplantation, Cardozo et al. (5) confirm this observation and, moreover, they show that a soluble factor, produced by naïve porcine endothelium, mediates chemotaxis of human PMN. The authors of the above-mentioned studies recognize that considerable difficulty lies in imitating the physiological rheological conditions in the in vitro settings they are using. Variability in the experimental set-up may therefore account for the dissonance in results, and appropriate in vivo studies will be able to circumvent this problem. The work by Cardozo et al. (5) is of considerable interest. It had been previously shown that porcine endothelial cells, activated by natural xenoantibodies, produce IL-8 and PAF (3), but the capacity of naïve PECs to mediate chemotaxis of human neutrophils in a xenoantibody-independent context is a novel and remarkable finding. It will be extremely important to identify the soluble chemotactic factor that is secreted by the nonactivated PECs, and, most importantly, to investigate the relevance of this finding in vivo. Secondly, this study illustrates how adhesion of neutrophils to PECs affects neutrophil function. By the use of transwell migration assays, the authors show that PMN-EC interactions can lead to biologically relevant phenomena such as respiratory burst, diapedesis, and chemotaxis. The importance of this study lies in the fact that it draws the attention to the PMN as a potential destroyer of xenografts. This may be particularly relevant to the recent data on the fate of α-Gal knockout pig organs following transplantation in primates. Using conventional immunosuppression, these xenografts survived for several months without HAR or the classical form of AVR. Eventually, graft failure did occur, but this appeared to be due to a process of microvascular thrombosis (6). It would be interesting to know whether the findings of Cardozo et al. (5) are reproduced when human PMN encounter porcine endothelial cells from α-Gal knockout pigs, and whether the PMN contributes to the process of eventual failure of this type of graft in vivo. Should direct adhesion of human PMN to the xenoendothelium play a significant role in eliciting neutrophil-mediated damage to xenografts, it may represent a target for novel therapeutic solutions. Blockade of neutrophil adhesion and their migration through the xenogeneic endothelium would be an attractive strategy and merits further study. With this respect, VCAM-1 seems to be a crucial adhesion molecule, as its expression on porcine endothelial cells is known to be upregulated by the binding of human neutrophils, NK cells, as well as anti-α-Gal antibodies. Antiporcine VCAM-1 monoclonal antibodies (MoAb) have been shown to impede the migration of human monocytes and natural killer cells across stimulated PECs (7). Alternatively, anti-PECAM-1 MoAb has been shown to block the transendothelial migration of human neutrophils through human aortic ECs in vitro, but has not been studied in the xenogeneic context. Finally, it would be interesting to know whether, apart from PMN, other cell types of the human innate system are susceptible to the chemotactic effect described by Cardozo et al. (5). Human monocytes and NK cells are known to activate porcine endothelium; however, the existence of a chemotactic influence of porcine endothelium on these cells has not been addressed. Having overcome the major obstacle of HAR, previously “unseen” xenotransplant barriers are being exposed. Porcine von Willebrand Factor, unlike its equivalent in primates, exhibits high-affinity binding to the GPIb(alpha) receptor on primate platelets, and this interaction was recently shown to play a key role in pulmonary xenograft failure (8). Similarly, Cardozo et al. (5) deliver evidence that a “constitutive” affinity between certain cellular and/or soluble components of discordant species may represent important obstacles to the success of xenotransplantation.