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The complement system is a central part of the innate immune system, and is an essential part in recognizing and clearing non/altered-self surfaces in the body. This thesis comprises of projects in which the initiation of the alternative pathway (AP) of complement in the fluid phase as well on various artificial and lipid surfaces has been studied. We have also synthesized and evaluated polymer-lipids as liposome coatings to suppress innate immune activation with focus on complement regulation.

In paper I we investigated how “C3b-like” C3(H₂O) is in regards to form an initial fluid phase AP C3 convertase. Even though C3(H₂O) could form a C3 convertase, it was much slower in comparison to the convertase generated by C3b. 

In paper II the contact activation of C3 on various artificial and lipid surfaces as a potential targeted AP activation pathway was explored. C3 bound selectively to lipid surfaces with negatively charged phospholipids and cholesterol, activated platelets and apoptotic cells. Thus, AP was initiated without prior proteolytic cleavage of C3 nor by preformed C3(H₂O) on specific surfaces in a selective manner.

In paper III and IV, synthetic phosphatidylcholine inspired polymer-lipids consisting of poly(2-methacryloyloxyethyl phosphorylcholine)-conjugated lipids (PMPC-lipids) with different degrees of MPC polymerization were synthesized. The protein adsorption, with focus on complement proteins onto the PMPC-lipids were evaluated, indicating that PMPC-lipids with a longer polymer chain are better to suppress protein adsorption. 

In paper V fragmented heparin-conjugated (fHep) lipids were investigated for their potential ability to recruit complement regulators to a lipid bilayer surface for complement regulation. This study indicated that fHep-liposomes could recruit the main fluid phase regulator of the AP, factor H, as well as the coagulation regulator antithrombin from human plasma. 

To conclude, the results from this thesis indicates that C3(H₂O) in the fluid phase is a poor initiator of the AP, however contact activated C3 could be targeting activation pathway for the AP. We could also successfully synthesize PMPC-lipids and fHep-lipids for protein suppression and potential complement regulation on coated liposomes.