Serglycin (SG) is a proteoglycan expressed by hematopoietic cells and is constitutively secreted by multiple myeloma (MM) cells. SG participates in the regulation of various inflammatory events. We found that SG secreted by human MM cell lines inhibits both the classical and lectin pathways of complement, without influencing alternative pathway activity. The inhibitory effect of SG is due to direct interactions with C1q and mannosebinding lectin (MBL). C1q-binding is mediated through the glycosaminoglycan moieties of SG, whereas MBL binds additionally to SG protein core. Interactions between SG and C1q as well as MBL are diminished in the presence of chondroitin sulfate type E. In addition, we localized the SG-binding site to the collagen-like stalk of C1q. Interactions between SG and C1q as well as MBL are ionic in character and only the interaction with MBL was found to be partially dependent on the presence of calcium. We found the serum levels of SG to be elevated in patients with MM compared to healthy controls. Moreover, we found that SG expressed from myeloma plasma cells protects these cells from complement activation induced by treatment with anti-thymocyte immunoglobulins. This might protect myeloma cells during immunotherapy and promote survival of malignant cells.Keywords: Complement . Multiple myeloma . Serglycin Introduction Serglycin (SG) is a proteoglycan with a 17 kDa protein moiety containing a characteristic domain rich in serine/glycine repeats, which serves as the attachment site for glycosaminoglycan (GAG) chains [1]. SG is mainly expressed by cells of hematopoietic origin and is a key intracellular proteoglycan [2,3]. SG can carry eight GAG chains, which depending on cell-type, can be either chondroitin sulfate (CS), dermatan sulfate (DS), heparan sulfate (HS), and heparin [3,4]. SG is constitutively secreted from monocytes [5] and macrophages [6] into the extracellular matrix and the secretion increases upon activation of these cells. The Eur. J. Immunol. 2011. 41: 437-449 DOI 10.1002 Innate immunity 437 biological function of SG is not fully elucidated, but the results obtained from studies with SG-knockout mice suggest a role for SG in the delivery of proteins into secretory granules and/or directing the secretion of these molecules [2]. It appears that SG interacts with a plethora of molecules, playing a role in the delivery of these components into secretory vesicles and further into their targets via its highly negatively charged GAG chains.Other studies have demonstrated that SG plays a key role in the storage of mast cell proteases and granzyme B and the maturation of secretory granules of mast cells and cytotoxic T-cells [7,8]. Furthermore, SG plays a crucial role in cytotoxic T-cells, as part of a multimolecular complex with perforin and granzyme B, in the delivery of the latter to the target cells during cytotoxic cell-granule-mediated cell death [9]. SG is constitutively secreted but also present on the cell surface of myeloma plasma cells and affects bone mineralization [1...