Perlecan: a major IL‐2‐binding proteoglycan in murine spleen

Abstract

Although interleukin-2 (IL-2) is typically considered a soluble cytokine, our laboratory has shown that the availability of IL-2 in lymphoid tissues is regulated, in part, by an association with heparan sulfate glycosaminoglycan. Heparan sulfate is usually found in proteoglycan form, in which the heparan sulfate chains are covalently linked to a specific core protein. We now show that perlecan is one of the major IL-2-binding heparan sulfate proteoglycans in murine spleen. IL-2 binds perlecan via heparan sulfate chains, as enzymatic removal of heparan sulfate from splenic perlecan abolishes its ability to bind IL-2. Furthermore, we demonstrate that perlecan-bound IL-2 supports the proliferation of an IL-2-dependent cell line. Identification of perlecan as a major heparan sulfate proteoglycan that binds IL-2 has implications for both the localization and regulation of IL-2 in vivo. Immunology and Cell Biology (2008) 86, 192-199; doi:10.1038/sj.icb.7100128; published online 27 November 2007 Keywords: extracellular matrix; heparan sulfate; interleukin-2; murine; spleen The availability of many cytokines and chemokines are regulated not only by their production, but also by their association with components of the extracellular matrix. 1,2 Heparan sulfate (HS) is one of the primary matrix components implicated in these associations. Our laboratory has previously demonstrated that interleukin-2 (IL-2) is retained in the extracellular matrix of lymphoid tissues by association with HS. 3 In vivo, HS chains are usually found covalently linked to a core protein. The core protein bearing HS chains with IL-2-binding activity is unknown. In the current study, we identify perlecan as a major splenic HS proteoglycan (HSPG) that binds IL-2. Identification of perlecan as an IL-2-binding HSPG has implications regarding the localization, function and regulation of IL-2 in vivo.HS is a glycosaminoglycan (GAG) composed of linear, repeating disaccharides containing uronic acid (UA) and N-acetyl-D-glucosamine. 4 The variability of HS oligosaccharides is due, in part, to modifications of the sugar residues, including isomerization, N-and O-sulfation and N-acetylation. Such modifications allow for at least 32 potential disaccharide units comprising an HS chain. 5 HS oligosaccharides are most often found covalently linked to a specific core protein, forming a protein-HS complex referred to as a HSPG. The cell-and tissue-specific expression of proteoglycan core proteins often determines when and where HS chains are synthesized and expressed. 4 Most studies assessing the involvement of HS in biological systems have concentrated on the role of HS in blood vessels, cartilage and connective tissues. 6 In contrast, the involvement of HS in the modulation of immunity only recently received attention. 7 HS has been implicated in altering T-cell responses by activation of antigenpresenting cells. [8][9][10] In the thymus, HS has been shown to influence T-cell development. 11 The capacity to bind immunomodulatory factors accounts for many of t...

Interleukin‐2 is present in human blood vessels and released in biologically active form by heparanase

“…Furthermore, this IL-2 is liberated by heparinase digestion. Release of IL-2 by heparinase is consistent with previous results from our laboratory demonstrating that IL-2 associates with the GAG chains, rather than the protein core, of HSPGs 3,4 .…”

“…Therefore, even though IL-2 is apparently widely distributed along both major vessels and smaller, intraparenchymal vessels 3,4,22 its release by heparanase would be tightly controlled by the above mechanisms. Conversely, dysregulation of heparanase contributes to the metastatic capabilities of many cancers.…”

“…A polyclonal rabbit anti-perlecan antibody, which recognizes perlecan isolated from Engelbreth Holm Swarm tumor matrix, was the kind gift of Dr. Gerardo Castillo (paid consultant, ProteoTech, Seattle, WA). 4 Two distinct polyclonal anti-heparanase antibodies were used. One rabbit polyclonal was the kind gift of Dr. Jeffrey Platt (University of Michigan, AnnArbor, MI) 21 .…”

“…In vivo , heparan sulfate is usually found in proteoglycan form, in which the HS GAG chains are covalently linked to a specific core protein. We recently demonstrated that perlecan is the major proteoglycan that binds IL-2 in murine spleens 4 . Since perlecan is one of the main heparan sulfate proteoglycans (HSPGs) found in blood vessels, we asked whether IL-2 is retained in human vessels.…”

Heparan Sulfate: A Ubiquitous Glycosaminoglycan with Multiple Roles in Immunity

“…Importantly, HSPG also facilitate cytokine localization in specific niches, forming depots where they can be made available to target cells. For instance, perlecan binds IL-2, sequestering it from the circulation and subsequently depositing the cytokine in the marginal zone and red pulp of the murine spleen to modulate murine T cells homeostasis (50). HSPGs also facilitate the storing of IL-2 within the vascular smooth muscle wall where cytokine availability is regulated through heparanase-mediated ECM degradation (51).…”