|transplantology and blood groups|
Interaction of natural blood group anti-A and anti-B antibodies of recipient with the corresponding antigens of transplanted material often hinders or makes difficult allotransplantation of organs and tissues because antigens A and B are present not only on erythrocytes but on many other organs. A practically convenient way to overcome this obstacle is affinity removal of the corresponding antibodies. We have developed adsorbents on the base of Sepharose FF and synthetic trisaccharides A and B attached to the matrix via flexible hydrophilic polymer. Using the type B adsorbent hemosorption procedure was performed in 1997 for the group 0 patient to whom B group heart (incompatible by antigen system ABH) had been transplanted. As this patient had own ‘natural’ anti-B antibodies and additional immune process was started due to transplantation of alien organ, lethal case was possible as the consequence of immune rejection. After 14 immunosorption procedures both antibodies were removed and immune cells stopped to synthesize them, i.e. so-called accommodation took place. Thanks for timely immunosorption the patient survived [ ]. In two years analogous situation was encountered during lung transplantation, this also being documented recently [ ].
Similar problem of acute rejection due to the pre-existing natural anti-carbohydrate antibodies is important for xenotransplantation. The most important xenoantigen is Galα1-3Gal. Removal or neutralisation of these antibodies is necessary for successful transplantation of animal organs to humans. To remove specifically the antibodies we have developed an affinity adsorbent analogous to those described above. Also polymeric, oligomeric, and dendrimeric derivatives of oligosaccharides were used for neutralization of antibodies. It was shown that just disaccharide was the principal epitope, whereas more complex oligosaccharides bound only slightly better. Polyvalency of saccharide is of much greater importance; polyacrylamide derivative of the disaccharide neutralized efficiently both G and M class antibodies, whereas low molecular oligosaccharides (from di- to penta-) even in high concentrations were incapable to neutralize completely cytotoxic action of class M antibodies. We have synthesized several affinity adsorbents containing xenodisacharide or more complex oligosaccharides as active component [ ]. Such adsorbents were successfully used for xenoantigen removal during pig heart transplantation to baboons.
It has been found that a part of human antibodies to xenodisaccharide is crypted [ ] and that both subclasses of xenoantigens, IgG and IgM, take part in complement-dependent cytotoxicity process.
A convenient test-system for the study of αGal-transferase, enzyme which synthesizes xenoantigen, has been developed and enzyme specificity has been studied using this system [ ].
Cytoprotection. Complement mediated damage of – mainly endothelial – cells plays an important role in a number of different pathophysiological processes, among which are acute vascular rejections of allo- and xenografts, ischemia/reperfusion injury (I/R injury), severe sepsis or septic shock, and arteriosclerosis. One example from the clinical practice is organ transplantation, in which no satisfactory therapeutic options are available to prevent and/or treat the I/R injury of the graft, nor for the treatment of acute vascular rejection reactions due to preformed antibodies. In both cases complement-mediated damage to mainly endothelial cells plays a major role. In the case of HLA-immunized recipients or in ABO-incompatible- and xenotransplantation, specific antibodies bind to the graft endothelium and are responsible for activation of the complement system and subsequently the endothelial cells themselves. In I/R injury the ischemia during storage of the graft leads to changes on the endothelium that render it susceptible to complement- and coagulation-mediated damage upon reperfusion. The use of soluble complement inhibitors/endothelial cell protectants might therefore be a valid therapeutical approach for both clinical problems. There is a need for cost effective substances that selectively inhibit cellular, in particular complement-mediated endothelial cell damage mediated by the patient’s natural immunity, such as complement system, without having systemic effects on blood coagulation.
The discovered [ ] non-glycosylated polyacrylamide conjugates (sTyr-PAA) bearing highly dense sulfated tyrosine or tyramine are site-specific blockers of complement activation and P-selectin mediated leukocyte adhesion in vitro and in vivo, being even more active in vitro than fucoidan or low molecular weight dextran sulfate – the most potent known inhibitors up to now. They interfere much less with blood coagulation than the latter substances. They protect endothelial cells from complement-mediated cytotoxicity. Endothelial cell protection by sTyr-PAA is probably accomplished by functional replacement of the natural heparan sulfate proteoglycans that are shed from the cell surface upon damage-induced activation.