Deficiency in mouse hyaluronidase 2: a new mechanism of chronic thrombotic microangiopathy.

Hyaluronan is a major component of the extracellular matrix and glycocalyx. Its main somatic degrading enzymes are the hyaluronidases 1 and 2, none of which is active in the bloodstream. We generated hyaluronidase 2 deficient mice. They suffer from mild chronic anemia and thrombocytopenia, in parallel with a 10-fold increase in plasma hyaluronan concentration. The current study explores the mechanism of these hematological anomalies. The decreased erythrocyte and platelet counts were assigned to peripheral consumption. The erythrocyte half-life was reduced from 25 to 8 days without signs of premature aging. Hyaluronidase 2 deficient platelets were functional. Major intrinsic defects in erythrocyte membrane or stability, as well as detrimental effects of high hyaluronan levels on erythrocytes, were ruled out in vitro. Normal erythrocytes transfused into hyaluronidase 2 deficient mice were quickly destroyed but neither splenectomy nor anti-C5 administration prevented from chronic hemolysis. Schistocytes were present in hyaluronidase 2 deficient smears at a level of 1 to 6%, while virtually absent in control mice. Hyaluronidase 2 deficient mice had increased markers of endothelial damage and microvascular fibrin deposition, without renal failure, accumulation of ultra-large multimers of von Willebrand factor, deficiency of A Disintegrin And Metalloproteinase with ThromboSpondin type 1 motifs, member 13 (ADAMTS13), or hypertension. There was no sign of structural damage in hepatic or splenic sinusoids, or in any other microvessels. We conclude that hyaluronidase 2 deficiency induces chronic thrombotic microangiopathy with hemolytic anemia in mice. The link between this uncommon condition and hyaluronidase 2 remains to be explored in man.