Complement heparin

Protamine. Protamine, whose use to reverse heparin anticoagulation has increased over the last two decades, has also been incriminated. Reactions may involve a number of mechanisms including IgE, IgG and complement. The incidence of anaphylactic reactions is estimated at 0.19% (retrospective studies) and 0.69% (prospective studies), respectively [27]. 

RE Edens, RJ Linhardt, JM Weiler. Heparin, not just an anticoagulant anymore six and one-half decades of studies on the ability of heparin to regulate complement activation. Compl Prof 1 96-120, 1993. 

Apart from its potent antiproliferative activity, tetrasaccharide 15 was effective in blocking human complement in vitro and inhibited the release of heparan sulfate from cardiac microvascular endothelial cells. To overcome hyperacute rejection, the tetrasaccharide has been investigated in a guinea pig to rat cardiac xenotransplantation model and significantly prolonged the survival of heart recipients when compared to control and heparin treated groups [46]. 

When given rapidly, protamine causes hypotension due to a decrease in vascular resistance, possibly linked to the release of nitric oxide from endothelium. Flypotension can be minimised by slow administration over 10-15 minutes. Protamine does not affect myocardial contractility. In some patients, systemic hypotension occurs in conjunction with pulmonary hypertension and, in severe cases, right ventricular failure. The mechanism is activation of the complement pathways by the heparin-protamine complex leading to release of thromboxane A2, which mediates pulmonary vasoconstriction. Unlike in anaphylaxis, plasma histamine concentrations are not increased. When this syndrome develops protamine administration should be stopped, and some have recommended giving heparin in an attempt to reduce the size of the heparin-protamine complex. 

Futhan stabilized complement activation in EDTA plasma for extended periods of time at 4°C, as measured by RIA. Background levels of C3a and C4a in stabilized plasma from whole blood drawn into Futhan+EDTA and stored at 4°C for up to 48 hours showed only minimal ex vivo activation (Table I). EDTA effectively inhibited ex vivo generation of C3a, whereas there was an ongoing production of C4a. Heparin also appeared to keep the C3a, but not the C4a, at a low level. Heparin + EDTA was no better in preventing ex vivo C3a generation than EDTA or heparin alone, but yielded a marked improvement in the stabilization of C4a. Indeed, C4a levels remained almost as low with this combination as with Futhan. 

To make nanoparticles able to escape complement activation, Passirani et alJ" proposed to coat nanospheres with heparin. This compound, which is a polysaccharide, is a physiological inhibitor of complement activation in vivo. Heparin-coated poly(methylmetha-crylate) nanoparticles were prepared by emulsion polymerization. In the method, the radical polymerization of methylmethacrylate was initiated by heparin according to an original method involving cerium ions and allowing heparin to covalently attach to poly(methyl-methacrylate). 

Passirani, C. Barratt, G. Devissaguet, J.P. Labarre, D. Interactions of nanoparticles bearing heparin or dextran covalently bound to poly(methylmethacrylate) with the complement system. Life Sci. 1998, 62, 775-785. 

Studies have shown that heparin is able to exert several different biological activities. In addition to the one of interest for this review, an anticoagulant effect, heparin has the ability to increase plasma lipoprotein activity, both inhibit and stimulate the alternate pathway complement activation, and enhance the release of collagenase from bone in tissue culture (S17). It has also been associated with thrombocytopenia. Several recent reviews on heparin are available (C13, T7, T8). 

This makes the blood cells antigenic and results in the production of IgG antibodies against them. The antibodies so produced activate the complement system. Complement destroys the blood cells. This can lead to haemolytic anaemia if red blood cells are involved and can occur in response to penicillin. If white blood cells are the target of the reaction then the result is agranulocytosis this can occur for example with carbimazole (used to treat an overactive thyroid gland, see Chapter 6). Thrombocytopenia can be the result if the cells involved are platelets this can happen in response to heparin (used in thrombotic disorders, see Chapter 4). 

Panasewicz found that heparin prevented shock from transfusion of heterogenous blood in cats and rabbits (outdated human blood Group B). It also prevented the heterohaemolysis in vivo and in vitro but not the heteroagglutination in vitro, and he found this was due mainly to the inactivation of complement. With the specific blood group reactions , the intensity of the iso-agglutinin reaction is decreased by pure heparin, thrombophob, thrombocid and Elheparin in 20 per cent albumin. All have the same effect on the anti-D reaction in sodium chloride but in 20 per cent albumin with AB serum, heparin, liquemin, thrombophob, and heparin vitrum cause an increase in the reaction, thrombocid causes a decrease and Elheparin has no effect. 

The other bioactive polysaccharide that seemed interesting to be used to produce surface-modified nanoparticles reducing their recognition by the host defense was heparin. Heparin is used as a drug for its anticoagulation properties. Additionally, it is an inhibitor of the complement activation phenomenon [116-118]. It was demonstrated that heparin-coated nanoparticles did not activate the complement system [19, 31, 32] and remained in the blood stream for a longer time compared with nanoparticles, which do not show heparin on the nanoparticle surface [89], Other polysaccharides extracted from mushrooms were found to inhibit the activation process of the complement. They could be alternative polysaccharides to produce nanoparticles with a reduced capacity to activate the complement, such as heparin [119],... 

Kazatchkine M D, Fearon D T, Silbert J E, et al. (1979). Surface-associated heparin inhibits zymosan-induced activation of the human alternative complement pathway by augmenting the regulatory action of the control proteins on particle-bound C3b. /. Exp. Med. 150 1202-1215. 

Antithrombin III (AT3), a protein (432 aa, Mr 58 kDa) acting as inhibitor of thrombin and all active proteases of the blood clotting system ( serpins) except Factor Vila by binding to them in 1 1 complex in similar manner as BPTI binds to trypsin. The presence of heparin enhances the inhibitory activity of antithrombin by several hundredfold. In antithrombin, Arg is the reactive center residue that provides a specific cleavage site for thrombin [T. Halkier, Mechanisms in Blood Coagulation, Fibrinolysis and the Complement System, Cambridge University Press, 1991). 

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