430. Antibodies to Recombinant Human α-L-Iduronidase Enhance Uptake into Macrophages in Murine Mucopolysaccharidosis Type I

Abstract

Mucopolysaccharidosis I (MPS I) is an inherited, and progressive lysosomal storage disease caused by lack or low level of α-L-iduronidase (IDU), altering the catabolism of glycosaminoglycans (GAG) in lysosomes. Some clinical improvement in patients has been observed by enzyme replacement therapy with recombinant human IDU (rhIDU), but studies suggested that anti-rhIDU antibodies reduce the effectiveness of treatment. Most MPS I treated patients develop a humoral immune response against rhIDU. To determine how anti-rhIDU antibodies alter the tissue and cellular distribution of rhIDU in vivo, we sensitized MPS I mice to rhIDU by administering rhIDU via tail-vein injection from 4 to 16 weeks of age. Mice that developed anti-rhIDU antibodies showed a shift in rhIDU uptake preferentially towards tissues with high reticuloendothelial (RE) content (liver, spleen, thymus) vs. low RE content (lung, kidney, heart, brain). Furthermore, in mice with anti-rhIDU antibodies, rhIDU was mainly localized in Kupffer cells, with relatively less available for hepatocytes. In order to further understand how the humoral immune response against rhIDU alters its distribution, we comparedin vitro uptake of rhIDU in the absence or presence of antibodies against rhIDU in MPS I fibroblasts and macrophages. Bone marrow monocytes were harvested from MPS I mice and derived into macrophages. MPS I macrophages were incubated with rhIDU with or without immunized rabbit serum, pre-immune serum, murine Fc receptor blocking agent and mannose 6-phosphate. We evaluated two types of immunized rabbit serum: one that completely abolished rhIDU uptake into human MPS I fibroblasts (inhibiting serum) and one that only partially (30%) inhibited rhIDU uptake into human fibroblasts (partially-inhibiting serum). The uptake of rhIDU in the presence of serum containing rhIDU antibodies was enhanced into MPS I mouse macrophages when both inhibiting and partially-inhibiting serum was applied. Uptake of rhIDU per cell was higher in macrophages when immune serum than was uptake of rhIDU into human fibroblasts normalized per cell. Treatment with mannose 6-phosphate or Fc receptor block partially inhibited the uptake in macrophages. Our study demonstrated that even a low-level humoral immune response against rhIDU, which only partially inhibits rhIDU uptake into fibroblasts in vitro, nevertheless might alter its tissue distribution in vivo. At the cellular level, antibody-positive mice show reduced rhIDU distribution in hepatocytes, while distribution to tissue macrophages is maintained. In vitro, immunized serum reduced rhIDU uptake into human fibroblasts but increased uptake into murine macrophages. Our findings imply that the altered tissue distribution of rhIDU caused by anti-rhIDU antibodies is partly due to reduced uptake into fibroblasts and partly due to enhanced uptake into tissue macrophages. These results imply that functional immune assays of rhIDU uptake in vitro into fibroblasts may not completely predict the impact of the humoral immune response against enzyme replacement therapy.