D. Bharadwaj et al., Serum amyloid P component binds to Fc gamma receptors and opsonizes particles for phagocytosis, J IMMUNOL, 166(11), 2001, pp. 6735-6741
Serum amyloid P component (SAP) is a member of the pentraxin family of prot
eins. These proteins are characterized by cyclic pentameric structure, calc
ium-dependent ligand binding, and frequent regulation as acute-phase serum
proteins. SAP is the serum precursor of the P component of amyloid. It bind
s to a broad group of molecules, including autoantigens, through a pattern
recognition binding site. The related pentraxin, C-reactive protein (CRP),
is a strong acute-phase reactant in man and an opsonin. We previously deter
mined that the binding of CRP to leukocytes occurs through Fe receptors for
IgG (Fc gammaR). We now report that SAP also binds to Fc gammaR and opsoni
zes particles for phagocytosis by human polymorphonucIear leukocytes (PMN).
Specific, saturable binding of SAP to FcRI, FcRIIa, and Fc gamma RIIIb exp
ressed on transfected COS cells was detected using SAP-biotin and PE-strept
avidin. Zymosan was used to test the functional consequences of SAP and CRP
binding to Fc gammaR. Both SAP and CRP bound to zymosan and enhanced its u
ptake by PMN. This enhanced phagocytosis was abrogated by treatment of PMN
with wortmannin, a phosphatidylinositol-3 kinase inhibitor, or with piceata
nnol, a Syk inhibitor, consistent with uptake through FcR. Treatment of PMN
with phosphatidylinositol-specific phospholipase C to remove Fc gamma RIII
b also decreased phagocytosis of SAP-opsonized zymosan, but not CRP-opsoniz
ed zymosan. These results suggest that SAP may function in host defense. In
addition, as SAP binds to chromatin, a major immunogen in systemic lupus e
rythematosus, it may provide a clearance mechanism for this Ag through Fc g
ammaR bearing cells. The Journal of Immunology, 2001.