Characterization of hepatic macrophages and evaluation of inflammatory response in heme oxygenase-1 deficient mice exposed to scAAV9 vectors

Adeno-associated viral (AAV) vectors are characterised by low immunogenicity, although humoral and cellular responses may be triggered upon infection. Following systemic administration high levels of vector particles accumulate within the liver. Kupffer cells (KCs) are liver resident macrophages and an important part of the liver innate immune system. Decreased functional activity of KCs can contribute to exaggerated inflammatory response upon antigen exposure. Heme oxygenase-1 (HO-1) deficiency is associated with considerably reduced numbers of KCs. In this study we aimed to investigate the inflammatory responses in liver and to characterise two populations of hepatic macrophages in adult wild type (WT) and HO-1 knockout (KO) mice following systemic administration of one or two doses (separated by 3 months) of self-complementary (sc)AAV9 vectors. At steady state, the livers of HO-1 KO mice contained significantly higher numbers of monocyte-derived macrophages (MDMs), but significantly less KCs than their WT littermates. Three days after re-administration of scAAV9 we observed increased mRNA level of monocyte chemoattractant protein-1 (Mcp-1) in the livers of both WT and HO-1 KO mice, but the protein level and the macrophage infiltration were not affected. Three days after the 1(st) and 3 days after the 2(nd) vector dose the numbers of AAV genomes in the liver were comparable between both genotypes indicating similar transduction efficiency, but the percentage of transgene-expressing MDMs and KCs was higher in WT than in HO-1 KO mice. In the primary culture, KCs were able to internalize AAV9 particles without induction of TLR9-mediated immune responses, but no transgene expression was observed. In conclusion, in vivo and in vitro cultured KCs have different susceptibility to scAAV9 vectors. Regardless of the presence or absence of HO-1 and initial numbers of KCs in the liver, scAAV9 exhibits a low potential to stimulate inflammatory response at the analysed time points.