Human Serum Amyloid A has Cytokine-Like Properties

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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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