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Complement activation is a relevant driver in the pathomechanisms of vasculitis. The involved proteins in the interaction between endothelia, complement, and platelets in these conditions are only partially understood. Thrombospondin-1 (TSP-1), found in platelet α-granules and released from activated endothelial cells, interacts with factor H (FH) and vWF. However, to our knowledge, direct regulatory interaction with the complement cascade has not yet been described. Our study shows that TSP-1 is a potent, FH-independent inhibitor of the alternative complement pathway. TSP-1 binds to complement proteins and inhibits cleavage of C3 and C5 and the formation of the membrane attack complex. We validated complement-regulatory function in blood samples from patients with primary complement defects. The physiological relevance of TSP-1 was demonstrated in patients with antineutrophil cytoplasmic antibody-associated vasculitis (AAV) by significantly enhanced TSP-1 staining in glomerular lesions and increased complement activity and NETosis after TSP-1 deficiency in an in vitro and in vivo model of AAV. The complement-inhibiting function of TSP-1 represents an important mechanism in the interaction of endothelia and complement. In particular, the interplay between released TSP-1 and the complement system locally, especially on surfaces, influences the balance between complement activation and inhibition and may be relevant in various vascular diseases.

Complement-associated disorders are caused by the dysregulation and disbalance of the complement system, especially excessive activation. Most drugs that target the complement system are designed to inhibit the complement pathway at either the proximal or terminal levels. The use of a natural complement regulator such as factor H (FH) could provide a superior treatment option by restoring balance to an overactive complement system. We recently reported the moss-based production of an analog of human FH with an optimized glycan profile (CPV-104), which showed in vitro and in vivo characteristics comparable to its human counterpart. Here, we follow up our previous work, focusing in more detail on the time course and long-term efficacy of CPV-104 treatment in FH-deficient ( FH –/– ) mice. The analysis of long-term treatment effects following multiple injections of human FH into mice was previously hindered by the immune response, so we developed a protocol for the sustained depletion of CD20 + B-cells and CD4 + T-cells, preventing antibody formation without influencing the C3G phenotype. Using this dual-depletion method, we were able to complete dosing interval experiments in FH –/– mice, administering up to three injections of CPV-104 at different intervals. Repeated CPV-104 administration was able to lastingly resolve C3 deposits, offering additional rationale for the clinical testing of CPV-104 in human C3G patients. Moreover, our novel dual-depletion method has the potential for adaptation to different mouse models, allowing the testing of multiple doses of other therapeutic proteins.

Complement activation is a relevant driver in the pathomechanisms of vasculitis. The involved proteins in the interaction between endothelia, complement and platelets in these conditions are only partially understood. Thrombospondin-1 (TSP-1), found in platelet α-granules and released from activated endothelial cells, interacts with factor H (FH) and von Willebrand factor (vWF). However, direct regulatory interaction with the complement cascade has not yet been described. We could show that TSP-1 is a potent, FH-independent inhibitor of the alternative complement pathway. TSP-1 binds to complement proteins, inhibits cleavage of C3 and C5 and the formation of the membrane attack complex. Complement-regulatory function is validated in blood samples from patients with primary complement defects. Physiological relevance of TSP-1 is demonstrated in ANCA-vasculitis patients by significantly enhanced TSP-1 staining in glomerular lesions and increased complement activity and NETosis following TSP-1 deficiency in an ANCA-vasculitis model. The newly described complement-inhibiting function of TSP-1 represents an important mechanism in the interaction of endothelia, complement and platelets. In particular, the interplay between released TSP-1, vWF and the complement system locally, especially on surfaces, influences the balance between complement activation and inhibition and may be relevant in various vascular diseases.