Nrf2 is essential for cholesterol crystal-induced inflammasome activation and exacerbation of atherosclerosis.

Oxidative stress and inflammation--two components of the natural host response to injury--constitute important etiologic factors in atherogenesis. The pro-inflammatory cytokine interleukin (IL)-1 significantly enhances atherosclerosis, however, the molecular mechanisms of IL-1 induction within the artery wall remain poorly understood. Here we have identified the oxidative stress-responsive transcription factor NF-E2-related 2 (Nrf2) as an essential positive regulator of inflammasome activation and IL-1-mediated vascular inflammation. We show that cholesterol crystals, which accumulate in atherosclerotic plaques, represent an endogenous danger signal that activates Nrf2 and the NLRP3 inflammasome. The resulting vigorous IL-1 response critically depended on expression of Nrf2, and Nrf2-deficient apolipoprotein E (Apoe)-/- mice were highly protected against diet-induced atherogenesis. Importantly, therapeutic neutralization of IL-1α and IL-1ß reduced atherosclerosis in Nrf2+/- Apoe-/- but not in Nrf2-/- Apoe-/- mice, suggesting that the pro-atherogenic effect of Nrf2-signaling was primarily mediated by its permissive role in IL-1 production. Our studies demonstrate a role for Nrf2 in inflammasome activation, and identify cholesterol crystals as disease-relevant triggers of the NLRP3 inflammasome and potent pro-atherogenic cytokine responses. These findings suggest a common pathway through which oxidative stress and metabolic danger signals converge and mutually perpetuate the chronic vascular inflammation that drives atherosclerosis.

IL-21R on T cells is critical for sustained functionality and control of chronic viral infection.

Chronic viral infection is often associated with the dysfunction of virus-specific T cells. Our studies using Il21r-deficient (Il21r-/-) mice now suggest that interleukin-21 (IL-21) is critical for the long-term maintenance and functionality of CD8+ T cells and the control of chronic lymphocytic choriomeningitis virus infection in mice. Cell-autonomous IL-21 receptor (IL-21R)-dependent signaling by CD8+ T cells was required for sustained cell proliferation and cytokine production during chronic infection. Il21r-/- mice showed normal CD8+ T cell expansion, effector function, memory homeostasis, and recall responses during acute and after resolved infection with several other nonpersistent viruses. These data suggest that IL-21R signaling is required for the maintenance of polyfunctional T cells during chronic viral infections and have implications for understanding the immune response to other persisting antigens, such as tumors.

The kinase activity of Rip2 determines its stability and consequently Nod1- and Nod2-mediated immune responses.

Rip2 (RICK, CARD3) has been identified as a key effector molecule downstream of the pattern recognition receptors, Nod1 and Nod2; however, its mechanism of action remains to be elucidated. In particular, it is unclear whether its kinase activity is required for signaling or for maintaining protein stability. We have investigated the expression level of different retrovirally expressed kinase-dead Rip2 mutants and the role of Rip2 kinase activity in the signaling events that follow Nod1 and Nod2 stimulation. We show that in primary cells expressing kinase-inactive Rip2, protein levels were severely compromised, and stability could not be reconstituted by the addition of a phospho-mimetic mutation in its autophosphorylation site. Consequently, inflammatory cytokine production in response to Nod1 and Nod2 ligands was abrogated both in vitro and in vivo in the absence of Rip2 kinase activity. Our results highlight the central role that Rip2 kinase activity plays in conferring stability to the protein and thus in the preservation of Nod1- and Nod2-mediated innate immune responses.

Dyslipidemia inhibits Toll-like receptor-induced activation of CD8alpha-negative dendritic cells and protective Th1 type immunity.

Environmental factors, including diet, play a central role in influencing the balance of normal immune homeostasis; however, many of the cellular mechanisms maintaining this balance remain to be elucidated. Using mouse models of genetic and high-fat/cholesterol diet-induced dyslipidemia, we examined the influence of dyslipidemia on T cell and dendritic cell (DC) responses in vivo and in vitro. We show that dyslipidemia inhibited Toll-like receptor (TLR)-induced production of proinflammatory cytokines, including interleukin (IL)-12, IL-6, and tumor necrosis factor-alpha, as well as up-regulation of costimulatory molecules by CD8alpha(-) DCs, but not by CD8alpha(+) DCs, in vivo. Decreased DC activation profoundly influenced T helper (Th) cell responses, leading to impaired Th1 and enhanced Th2 responses. As a consequence of this immune modulation, host resistance to Leishmania major was compromised. We found that oxidized low-density lipoprotein (oxLDL) was the key active component responsible for this effect, as it could directly uncouple TLR-mediated signaling on CD8alpha(-) myeloid DCs and inhibit NF-kappaB nuclear translocation. These results show that a dyslipidemic microenvironment can directly interfere with DC responses to pathogen-derived signals and skew the development of T cell-mediated immunity.