Human Serum Amyloid A has Cytokine‐Like Properties

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Abstract
Human serum amyloid A (SAA) proteins are a group of 12–14 kDa apolipoproteins found predominantly in the high-density lipoprotein (HDL) fraction of plasma. Several functions have been proposed for SAA, but its primary physiological function remains elusive. In this report, we used the monocytic cell line THP-1 to investigate whether recombinant SAA1 (rSAA) or the HDL–rSAA protein complex can affect the capacity of these cells to produce inflammatory cytokines in vitro. Incubation of rSAA, plasma HDL (which contains ≤ 30 μg/ml of SAA) or HDL–rSAA complex with THP-1 cells induced synthesis of IL-1β, IL-1ra and sTNFR-II protein and mRNA. The induction of cytokine synthesis was not due to endotoxin contamination since the effect was abrogated by protein denaturation. The rSAA and HDL–rSAA complex did not induce detectable levels of IL-6 or TNFα protein or mRNA. In contrast 10 μg/ml LPS stimulated secretion of the inflammatory cytokines, IL-1β, IL-6 and TNFα, as well as IL-1ra and sTNFR-II from THP-1 cells. We confirmed that rSAA has chemoattractant properties in vivo, by subcutaneous injections into mice and examined the histology of the injection site at 72 h, however, the HDL–rSAA complex has a substantially reduced effect.

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