Serum amyloid A (SAA) is a multifunctional acute-phase protein whose natural role seems to be participation in many physiologic and pathological processes. Prolonged increased SAA level in a number of chronic inflammatory and neoplastic diseases gives rise to reactive systemic amyloid A amyloidosis, where the N-terminal 76-amino acid residue-long segment of SAA is deposited as amyloid fibrils. Recently, a specific interaction between SAA and the ubiquitous inhibitor of cysteine proteases--human cystatin C (hCC)--has been described. Here, we report further evidence corroborating this interaction, and the identification of the SAA and hCC binding sites in the SAA-hCC complex, using a combination of selective proteolytic excision and high-resolution mass spectrometry. The shortest binding site in the SAA sequence was determined as SAA(86-104), whereas the binding site in hCC sequence was identified as hCC(96-102). Binding specificities of both interacting sequences were ascertained by affinity experiments (ELISA) and by registration of mass spectrum of SAA-hCC complex.

Human cystatin C (hCC), like many other amyloidogenic proteins, dimerizes and possibly makes aggregates by subdomain swapping. Inhibition of the process should suppress the fibrillogenesis leading to a specific amyloidosis (hereditary cystatin C amyloid angiopathy, HCCAA). It has been reported that exogenous agents like monoclonal antibodies against cystatin C are able to suppress formation of cystatin C dimers and presumably control the neurodegenerative disease. We have studied in detail two monoclonal antibodies (mAbs) representing very different aggregation inhibitory potency, Cyst10 and Cyst28, to find binding sites in hCC sequence responsible for the immunocomplex formation and pave the way for possible immunotherapy of HCCAA. We used the epitope extraction/excision mass spectrometry approach with the use of different enzymes complemented by affinity studies with synthetic hCC fragments as a basic technique for epitope identification. The results were analyzed in the context of hCC structure allowing us to discuss the binding sites for both antibodies. Epitopic sequences for clone Cyst28 which is a highly potent dimerization inhibitor were found in N-terminus, loop 1 and 2 (L1, L2) and fragments of β2 and β3 strands. The crucial difference between conformational epitope sequences found for both mAbs seems to be the lack of interactions with hCC via N-terminus and the loop 1 in the case of mAb Cyst10. Presumably the interactions of mAbs with hCC via L1 and β sheet fragments make the hCC structure rigid and unable to undergo the swapping process.

Human cystatin C (hCC) is a small cysteine protease inhibitor whose oligomerization by propagated domain swapping is linked to certain neurological disorders. One of the ways to prevent hCC dimerization and fibrillogenesis is to enable its interaction with a proper antibody. Herein, the sites of interaction of hCC with dimer-preventing mouse monoclonal anti-hCC antibodies Cyst28 are studied and compared with the binding sites found for mAb Cyst10 that has almost no effect on hCC dimerization. In addition, hCC epitopes in complexes with native polyclonal antibodies extracted from human serum were studied. The results obtained with hydrogen–deuterium exchange mass spectrometry (HDX MS) were compared with the previous findings made using the excision/extraction MS approach. The main results from the two complementary MS-based approaches are found to be in agreement with each other, with some differences being attributed to the specificity of each method. The findings of the current studies may be important for future design of hCC dimerization inhibitors.Electronic supplementary materialThe online version of this article (doi:10.1007/s00726-016-2316-y) contains supplementary material, which is available to authorized users.

Hereditary cystatin C amyloid angiopathy (HCCAA) is a severe neurodegenerative disorder related to the point mutation in cystatin C gene resulting in human cystatin C (hCC) L68Q variant. One of the potential immunotherapeutic approaches to HCCAA treatment is based on naturally occurring antibodies against cystatin C. A recent growing interest in autoantibodies, especially in the context of neurodegenerative diseases, emerges from their potential use as valuable diagnostic markers and for controlling protein aggregation. In this work, we present characteristics of natural anti-hCC antibodies isolated from the IgG fraction of human serum by affinity chromatography. The electrophoresis (1-D and 2-D) results demonstrated that the isolated NAbs are a polyclonal mixture, but their electrophoretic properties did not allow to classify the new autoantibodies to any particular type of IgG. The Fc-glycan status of the studied autoantibodies was assessed using mass spectrometry analysis. For the isolated NAbs, the epitopic fragments in hCC sequence were identified by MS-assisted proteolytic excision of the immune complex and compared with the ones predicted theoretically. The knowledge of hCC fragments binding to NAbs and other ligands may contribute to the search for new diagnostic methods for amyloidosis of different types and the search for their treatment.

Amyloid formation is associated with a number of neurodegenerative diseases that affect the independence and quality of life of aging populations. One of rather atypical, occurring at a young age amyloidosis is hereditary cystatin C amyloid angiopathy (HCCAA) related to aggregation of L68Q variant of human cystatin C (hCC). Human cystatin C plays a very important role in many aspects of human health; however, its amyloidogenic properties manifested in HCCAA present a real, lethal threat to some populations and any work on factors that can affect possible influencing hCC aggregation is not to overestimate. It was proved that interaction of hCC with monoclonal antibodies suppresses significantly hCC dimerization process. Therefore, immunotherapy seems to be the right approach toward possible HCCAA treatment. In this work, the hCC fragment encompassing residue 60-70 (in 2 variants: linear peptide and multiple antigenic peptide) was used as an immunogen in rabbit immunization. As a result, specific anti-hCC antibodies were found in both rabbit sera. Surprisingly, rabbit antibodies were obtained after immunization with only a short peptide. The obtained antibodies were characterized, and their influence on the aggregation propensity of the hCC molecules was evaluated. The antibodies turned out not to have any significant influence on the cystatin C dimerization process. Nevertheless, we hope that antibodies elicited in rabbits by other hCC fragments could lead to elaboration of effective treatment against HCCAA.