Supranormal enzymatic activity allows storage removal in the MPS I–affected tissues. (A) Morphologic analysis of 1-μm sections, toluidine blue stained, from different organs of treated and control animals, as indicated. Representative images show the lysosomal distention in the indicated tissues in MPS I mice (no distention is present in WT animals). Kidney: arrows and arrowheads identify lysosomal storage, which is evident in the tubules (arrows), in the glomeruli (arrowheads; * marks the glomeruli), and in interstitial fibroblasts; liver: arrows highlight storage within hepatocytes, and storage is also present within Kupffer cells; spleen: pathologic storages (arrows) are predominant in the red pulp; heart: pathologic storage is abundant in the endothelium and in myocytes (arrows); frontal cortex: pathologic storage is present within neurons (arrows) and in endothelial cells (v = vessels); subjective score in MPS I: 3-4 in the different examined tissues. Magnification ×100 and ×200. Residual storage is present in all tissues of mice treated with HCTs. Two representative animals (HCT [a] and HCT [b], both having donor-cell engraftment > 70%, as assessed by quantification of donor GFP⁺ cells in peripheral blood by cytofluorimetric analysis) are shown, demonstrating a different grade of storage (subjective score HCT [a]: 1-2 and HCT [b]: 2-3 in the different examined tissues). Magnification ×100 and ×200. A strong reduction of storage is evident in all the examined tissues from a representative GT mouse having a VCN of 5 in the BM (subjective score: 0-1 in the different examined tissues). Magnification ×100. (B) Lysosomal distention/cell engulfment was scored (see “Methods” for details) in the liver, spleen, heart, kidney, and cortex of treated and control mice. Mean ± SEM are shown; n = 4 representative mice analyzed per group (≥ 3 representative images per mouse). *P < .05; **P < .01; ***P < .001 with 1-way ANOVA. (C-D) GAGs were quantified in the urine (C) and in the tissues (D, as indicated below x-axis; the liver, spleen, and kidney were chosen as representative tissues) of treated and control mice. Mean ± SEM are shown; n ≥ 4 representative mice analyzed per group. *P < .05; **P < .01; ***P < .001 with 1-way ANOVA. (E) Western blot for heparin cofactor II–thrombin (HCII-T) complex was performed on the serum of MPS I, GT, and WT mice. Glyceraldehyde 3-phosphate dehydrogenase (GAPDH; bottom image) was used as an internal control.