Glomerular mesangial cells (MC), like most cell types secrete hyaluronan (HA), which attached to the cell surface via CD44, is the backbone of a hydrophilic gel matrix around these cells. Reduced extracellular matrix thickness and viscosity result from HA cleavage during inflammation. HA fragments were reported to trigger innate immunity via Toll-like receptor-(TLR-) 2 and/or TLR4 in immune cells. We questioned whether HA fragments also regulate the immunostimulatory capacity of smooth muscle cell-like MC. LPS (TLR4-ligand) and PAM3CysSK4 (TLR2-ligand) induced IL-6 secretion in MC; highly purified endotoxin-free HA 3000？Da up to 50？μg/mL did not. Bovine-testis-hyaluronidase from was used to digest MC-HA into HA fragments of different size directly in the cell culture. Resultant HA fragments did not activate TLR4-deficient MC, while TLR2-deficient MC responded to LPS-contamination of hyaluronidase, not to produced HA fragments. Hyaluronidase increased the stimulatory effect of TLR2-/-3/-5 ligands on their TLR-receptors in TLR4-deficient MC, excluding any effect by LPS-contamination. Supplemented heparin suppressed every stimulatory effect in a dose-dependent manner. We conclude that the glycosaminoglycan HA creates a pericellular jelly barrier, which covers surface receptors like the TLRs. Barrier-thickness and viscosity balanced by HA-synthesis and degradation and the amount of HA-receptors on the cell surface regulate innate immunity via the accessibility of the receptors. 1. Background In 1934, Meyer and Palmer were the first to describe hyaluronan (HA, formerly called hyaluronic acid) as an isolate from the vitreous humor [1]. Since then, HA has been extensively examined in terms of its physical, chemical, and biological properties [2, 3]. HA occurs ubiquitously throughout the body, serving as the backbone of hydrate coat surrounding cells [4, 5], when attached to the cell surface. The main receptor on the cell surface is CD44 [6]. HA of high-molecular weight ( ？Da[lton]) can bind water in a relation 1？:？1000 [2]. Degradation of HA reduces the binding capacity for water and turns the aqueous gel into a fluid of low viscosity [7]. HA occurs in viscous fluids such as the synovial fluid, which is the rationale for its topical use in diseases of the joint [8]. High-molecular weight HA is also used topically in aesthetic medicine [9] while, a preparation for systemic application has been used in horses and greyhounds [10, 11]. Another aspect of HA is its immunologic effect. Termeer et al. identified HA as an agonist for Toll-like receptor-(TLR-) 4 [12].
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