C5b-8 step lysis of swine endothelial cells by human complement and functional feature of transfected CD59

Complement inhibition in a xenogeneic model of interactions between human whole blood and porcine endothelium

Xenotransplantation (xeno-Tx) is considered as an alternative solution to overcome the shortage of human donor organs. However, the success of xeno-Tx is hindered by immune reactions against xenogeneic cells (e. g. of porcine origin). More specifically, activation of innate immune mechanisms such as complement and triggering of the coagulation cascade occur shortly after xeno-Tx, and adhesion of human leukocytes to porcine endothelium is another early critical step mediating the immune attack. To investigate the therapeutic potential of complement inhibition in the context of xenogeneic interactions, we have employed a whole-blood model in the present study. Incubation of human blood with porcine endothelial cells (PAECs) led to activation of complement and coagulation as well as to increased leukocyte adhesion. The observed responses can be attributed to the pig-to-human xenogeneicity, since the presence of human endothelium induced a minor cellular and plasmatic inflammatory response. Importantly, complement inhibition using a potent complement C3 inhibitor, compstatin analogue Cp40, abrogated the adhesion of leukocytes and, more specifically, the attachment of neutrophils to porcine endothelium. Moreover, Cp40 inhibited the activation of PAECs and leukocytes, since the levels of the adhesion molecules E-selectin, ICAM-1, ICAM-2, and VCAM-1 on PAECs and the surface expression of integrin CD11b on neutrophils were significantly decreased. Along the same line, inhibition of CD11b resulted in decreased leukocyte adhesion. Taken together, our findings provide a better understanding of the mechanisms regulating the acute innate immune complications in the context of xeno-Tx and could pave the way for complement-targeting therapeutic interventions.

Effect of tandem forms of DAF(CD55) on complement-mediated xenogeneic cell lysis

BACKGROUND

It is difficult to produce a transgenic animal with high expression of decay-accelerating factor (CD55: DAF) or other molecules. The purpose of this study was to assess the effect of tandem forms of DAF on a xenogeneic cell membrane against human complement.

METHODS

cDNAs of the delta-Short Consensus Repeat (SCR) 1-DAF, the double-DAF, the triple-DAF, and the tetra-DAF with a FLAG-tag were established. Chinese hamster ovary (CHO) cell lines and a pig endothelial cell (PEC) line expressing these molecules were established. The amelioration of complement-mediated lysis by the transfectant molecules on these cells was examined. The CHO cell transfectants were also incubated with normal human serum, and the amount of C3 deposited was determined by FACS analysis.

RESULTS

Stable CHO cells and PEC transfectants, in which each molecule was clearly expressed, and Western blots showed that each band corresponded to the expected molecular weight. The extent of amelioration of complement-mediated lysis by these four molecules was then examined. A clear tendency was found, as follows: The higher the tandem number of DAF, the greater was the effect on cytotoxicity. Additional experiments focusing on triple-DAF and tetra-DAF did not indicate any significant difference in complement-mediated lysis. Consistent with the complement-regulatory ability, the inhibitory effect of the deposition of C3 fragments by these molecules was closely related to the degree of amelioration.

CONCLUSION

These data indicate that tandem DAF, especially a triple-DAF, is a very effective form for protecting against complement activation.

Regulation of complement-mediated swine endothelial cell lysis by a surface-bound form of human C4b binding protein

BACKGROUND

Human C4b-binding protein (C4bp) functions as a cofactor for factor I in the degradation of C4b and C3b and, in addition, accelerates the rate of decay of the C4b2a complex.

METHODS

In this study, we constructed a surface-bound form of human C4b-binding protein (C4bp-PI) consisting of a short consensus repeat 1-8 of the alpha-chain of C4bp and a glycosyl phosphatidylinositol (GPI) of the decay-accelerating factor (CD55) and established stable swine endothelial cell (SEC) lines expressing C4bp-PI by transfection of cDNA. Amelioration of complement-mediated lysis by the transfectant molecules was tested as an in vitro hyperacute rejection model of swine to human discordant xenograft, using the lactate dehydrogenase assay.

RESULTS

Flow cytometric profiles of the stable SEC lines with C4bp-PI showed a high level of expression of this molecule. The cell lysate of the SEC line with C4bp-PI showed strong cofactor activity in not only C4b but also C3b, whereas the activity of plasma C4bp to bind to C3 was very weak. Approximately 150 x 10(4) molecules of C4bp-PI per SEC blocked human complement-mediated cell lysis by approximately 75%.

CONCLUSIONS

The results suggest that the surface-bound form of C4bp will be very useful in clinical xenotransplantation.

Significant downregulation of the major swine xenoantigen by N-acetylglucosaminyltransferase III gene transfection

Introduction of the beta-D-mannoside beta-1,4-N-acetylglucosaminyltransferase III (GnT-III) gene into swine endothelial cells (SEC) reduced their susceptibility to normal human serum (NHS) in complement-mediated cell lysis and also suppressed the antigenicity to human natural antibodies as evidenced by flow cytometric analysis, as well as Griffonia simplicifolia 1 isolectin (1B4 lectin) binding to the Gal alpha1-3 Gal beta 1-4 GlcNAc-R (the alpha-galactosyl epitope). Western blot analysis indicated that proteins smaller than 66 kDa had diminished reactivity to NHS and 1B4 lectin. GnT-III, a key enzyme involved in branch formation of N-linked sugars, was found to downregulate the expression of xenoantigen, suggesting that this approach may be of value in clinical xenotransplantation in the future.