Binding of human serum amyloid P component (hSAP) to human neutrophils

Abstract

Human serum amyloid P component (hSAP) and human C-reactive protein (hCRP) are normal serum constituents related to the pentraxin family of plasma proteins. hSAP has morphological and immunochemical identity and extensive sequence similarity to the amyloid P (AP) component found in normal tissues and particularly in amyloid deposits. hCRP and its proteolytic products have been previously shown to bind and to interact with various types of human leukocytes. Binding-displacement experiments with '251-labeled hSAP and hCRP show that both proteins have specific highaffinity binding sites on normal human polymorphonuclear leukocytes (PMN) and each can compete efficiently with the binding of the other. Scatchard analysis of hSAP-displacement curves reveals a heterogeneous population of hSAP-binding sites existing on the PMN cells, among them about 300000 low-affinity binding sites with K,% X M and about 30000 high-affinity binding sites with Kd 5 SXlO-*M. hAP was found to be degraded by enzymes from human neutrophils to yield a mixture of low-molecular-mass peptides, similarly to the case of CRP reported previously. The binding of hSAP can be efficiently inhibited by this peptide mixture. The results suggest that both hCRP and hSAP, together with related peptides, may participate in vivo in an unknown mechanism of regulation of human neutrophils.Serum amyloid P component (SAP) and C-reactive protein (CRP) belong to an evolutionary conserved family of human proteins, known as pentraxins [1-41. Human SAP (hSAP) is continuously present in human plasma in significant amounts (30-50 mg/l) and shows minor changes during the acute-phase response. In contrast, hCRP plasma levels in healthy individuals are less than 1 pg/ml and rise up to 1000-fold during inflammation [2-41. hSAP and hCRP share 60% sequence similarity [5]. Both proteins are arranged as planar pentamers, each consisting of five identical subunits. hCRP circulates in plasma as a single non-glycosylated pentamer (monomeric unit about 23 kDa) in coexistence with higher oligomers, whereas hSAP appears exclusively as a pair of glycosylated pentamers (monomeric unit about 25 kDa) oriented face-to-face as a decameric complex [2, 3, 6, 71. The aggregated tissue form of serum amyloid P, referred to as amyloid P (AP), is associated with nearly all types of amyloid fibrils (8-lo]. A number of features are shared by both hCRP and MAP, including, appearance in various sites of the body associated with tissue damage ; binding to nuclear constituents of disrupted cells and certain organic phos- -3, 8-15]. Extensive data have been accumulated concerning interactions of hCRP with human phagocytes, such as neutrophils [16-191, monocytes and macrophages [20-221. Initial receptor-mediated binding of hCRP to neutrophils is followed by its internalization and/or its proteolytic degradation by membrane and lysosomal proteases [23]. Proteolysis was found to yield a specific set of low-molecular-mass peptides (LMMP), some of which were shown to play a role in hCRPmediated modulation o...

Serum Amyloid P Component Binds to Fcγ Receptors and Opsonizes Particles for Phagocytosis

“…SAP levels vary from ϳ20 -50 g/ml in normal individuals, with males having slightly higher levels than females (34). The binding of SAP to human PMN was previously demonstrated by Landsmann et al (13). These investigators reported that SAP bound to human PMN with high affinity and that CRP and SAP competed for binding.…”

“…The existence of a receptor for SAP on mouse macrophages and human polymorphonuclear leukocytes (PMN) has been previously reported (12,13). However, the nature of the receptor and its function were unknown.…”

Binding of human serum amyloid P componentto L‐selectin

“…We found previously that circulating Lselectin plasma concentrations drop by almost 50% in infants undergoing open heart surgery employing cardiopulmonary bypass [33], which may be due to trapping of soluble L-selectin by SAP absorbed onto the surface of plastic tubings. L-Selectin molecules are clustered at the tip of leukocyte microvilli [34] and are therefore candidates to explain the binding of SAP observed in leukocytes [35]. Indeed, in a fashion analogous to GlyCAM-1 [14], SAP elicited rapid activation of b2 integrins on human lymphocytes, as assessed by expression of the mAb 24-recognized neoepitope.…”

Serum Amyloid P Component Bound to Gram-Negative Bacteria Prevents Lipopolysaccharide-Mediated Classical Pathway Complement Activation

“…It has been shown that SAP can interact with phagocytes via specific receptors (19,29). Furthermore, the ability of substrate-bound SAP to activate C3b and C3bi receptors of monocytes has been described (29).…”