A unique amyloidogenic apolipoprotein serum amyloid A (apoSAA) isoform expressed by the amyloid resistant CE/J mouse strain exhibits higher affinity for macrophages than apoSAA1 and apoSAA2 expressed by amyloid susceptible CBA/J mice

CBA/J and other inbred strains of mice that express the amyloidogenic apolipoprotein serum amyloid A (apoSAA) apoSAA₂, together with apoSAA₁, are susceptible to amyloid A (AA) amyloidosis, whereas CE/J mice that express a single unique isoform, apoSAA_CEJ, are resistant. Studies indicate that CBA/J×CE/J hybrid mice that express apoSAA₂ in the presence of apoSAA_CEJ are protected from amyloidogenesis. To define a mechanism by which expression of apoSAA_CEJ may protect from AA formation in the presence of apoSAA₂, binding of recombinant apoSAA (r-apoSAA) isoforms, validated by N-terminal sequencing, to a murine macrophage cell line was investigated. Maximal specific binding occurred after incubation of radiolabeled apoSAA with IC-21 macrophages (1×10⁵ cells/ml) for 30 min at 4°C. The binding of ¹²⁵I-r-apoSAA₁, ¹²⁵I-r-apoSAA₂ and ¹²⁵I-r-apoSAA_CEJ was specific and saturable, with an affinity (K_d) of about 2.8, 3.2 and 1.3 nM, respectively, and approximately 2–4×10⁶ sites per cell. Competitive binding experiments indicate apoSAA_CEJ binds with higher affinity to macrophages than does either apoSAA₁ or apoSAA₂. We suggest that greater cellular affinity of apoSAA_CEJ compared to apoSAA₂ may contribute to protection from AA amyloid in certain CBA/J×CE/J hybrid mice by interfering with interaction of apoSAA₂ by macrophages and hence either membrane associated or intracellular degradation.