CP127374 Here we demonstrate that LMW HA can increase production of IFN��. This effect is not via the ability of LMW HA to activate TLR2 (as has been previously described) but rather by TLR4 activation. Furthermore, unlike other LMW HA signaling, LMW HA-induced TLR4 activation depends on TRIF and not MyD88. In a study on the effects of HA on neutrophils, Leu et al. observed that apoptosis of neutrophils is decreased in TLR4 null mice [41]. They demonstrated that was due to IFN�� mediated TRAIL-TRAILR interactions, as the addition of IFN�� led to an increase in TRAIL/TRAILR in inflammatory neutrophils. Administration of intratracheal HA fragments (2000 ug/ml) was found to result in an increase in IFN�� in whole lung neutrophils, an effect that was mitigated in the TLR4-/- mice.
Our studies now provide a biochemical mechanism for these observations. Our data demonstrate that LMW HA directly induces IFN�� production and that this occurs via a newly defined TLR4-TRIF-TBK1-IRF3 pathway. Thus, tissue damage causing fragmentation of matrix hyaluronan generating local high concentrations of HA fragment-induced IFN��may prime the innate response for a potential viral infection (Figure 6), expanding the range of the ��danger signal�� properties of LMW HA. The induction of IFN�� by HA, an endogenous danger signal, raises the possibility that the anti-viral effects of the interferons may be triggered early in injury, perhaps priming the immune system to launch a full anti-viral program. Alternatively, the production of IFN�� may be intended to modify the pro-inflammatory effects of HA.
Although the type I interferons are best known for their effects in viral infection, they also have the ability to inhibit inflammatory responses [6]. For example, in the eye, IFN�� appears to be important in suppressing inflammatory responses; IFN�� is produced in substantial amounts by retinal pigmented epithelial cells, and eliminates production of T cell chemoattractant CXCL9 in response to TNF��/ IFNg/IL-1�� [42]. Moreover, in a murine model, the increase in type I interferon ten days after influenza infection led to significantly decreased neutrophilic responses to subsequent bacterial pneumonia and increased mortality [43]. Therefore, HA induced activation of IFN�� via TLR4-TRIF-TBK1 may also act as a potential brake for innate inflammatory responses.
Taken together, a model emerges whereby immediate tissue damage can lead to HA fragment-induced IFN�� that primes the innate response for a potential viral infection (Figure 6). Alternatively, persistent tissue damage leading to the accumulation of HA fragments may in fact serve to down regulate certain inflammatory responses. Thus this novel downstream effect of HA expands the AV-951 role of this endogenous danger signal, and opens avenues for further investigation. In addition, the redundancy of inflammatory pathways triggered by LMW HA may also add to the robustness of an inflammatory response.