An unexpected role for uric acid as an inducer of T helper 2 cell immunity to inhaled antigens and inflammatory mediator of allergic asthma.

Although deposition of uric acid (UA) crystals is known as the cause of gout, it is unclear whether UA plays a role in other inflammatory diseases. We here have shown that UA is released in the airways of allergen-challenged asthmatic patients and mice, where it was necessary for mounting T helper 2 (Th2) cell immunity, airway eosinophilia, and bronchial hyperreactivity to inhaled harmless proteins and clinically relevant house dust mite allergen. Conversely, administration of UA crystals together with protein antigen was sufficient to promote Th2 cell immunity and features of asthma. The adjuvant effects of UA did not require the inflammasome (Nlrp3, Pycard) or the interleukin-1 (Myd88, IL-1r) axis. UA crystals promoted Th2 cell immunity by activating dendritic cells through spleen tyrosine kinase and PI3-kinase δ signaling. These findings provide further molecular insight into Th2 cell development and identify UA as an essential initiator and amplifier of allergic inflammation.

Activation of MDA5 requires higher-order RNA structures generated during virus infection.

Recognition of virus presence via RIG-I (retinoic acid inducible gene I) and/or MDA5 (melanoma differentiation-associated protein 5) initiates a signaling cascade that culminates in transcription of innate response genes such as those encoding the alpha/beta interferon (IFN-alpha/beta) cytokines. It is generally assumed that MDA5 is activated by long molecules of double-stranded RNA (dsRNA) produced by annealing of complementary RNAs generated during viral infection. Here, we used an antibody to dsRNA to show that the presence of immunoreactivity in virus-infected cells does indeed correlate with the ability of RNA extracted from these cells to activate MDA5. Furthermore, RNA from cells infected with encephalomyocarditis virus or with vaccinia virus and precipitated with the anti-dsRNA antibody can bind to MDA5 and induce MDA5-dependent IFN-alpha/beta production upon transfection into indicator cells. However, a prominent band of dsRNA apparent in cells infected with either virus does not stimulate IFN-alpha/beta production. Instead, stimulatory activity resides in higher-order structured RNA that contains single-stranded RNA and dsRNA. These results suggest that MDA5 activation requires an RNA web rather than simply long molecules of dsRNA.