Tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, and nitric oxide (NO) may play a role in lipopolysaccharide (LPS)-induced cardiac depression. Toll-like receptor-4 (TLR-4) mediates the cytokine response to LPS in immune cells. TLR-4 also is expressed in human and murine myocardial tissue. Therefore, the hypothesis that LPS induces proinflammatory cytokines in the heart via TLR-4 was tested. C3H/HeJ (TLR-4 deficient) and C3HeB/FeJ mice were studied. LPS induced a robust increase in myocardial TNF-alpha and IL-1beta mRNA in C3HeB/FeJ mice. The response in C3H/HeJ mice was blunted and delayed. Myocardial TNF-alpha and IL-1beta protein levels were higher in C3HeB/FeJ mice, as were inducible NO synthase protein and NO production. Activation of myocardial NF-kappaB was observed within 30 min in C3HeB/FeJ mice but not in C3H/HeJ mice. These findings suggest that myocardial TLR-4 is involved in signaling cytokine production within the heart during endotoxic shock.

The innate antiviral response is mediated, at least in part, by Toll-like receptors (TLRs). TLR3 signaling is activated in response to viral infection, and the absence of TLR3 in mice significantly increases mortality after infection with enteroviruses that cause myocarditis and/or dilated cardiomyopathy. We screened TLR3 in patients diagnosed with enteroviral myocarditis/cardiomyopathy and identified a rare variant in one patient as well as a significantly increased occurrence of a common polymorphism compared with controls. Expression of either variant resulted in significantly reduced TLR3-mediated signaling after stimulation with synthetic double-stranded RNA. Furthermore, Coxsackievirus B3 infection of cell lines expressing mutated TLR3 abrogated activation of the type I interferon pathway, leading to increased viral replication. TLR3-mediated type I interferon signaling required cellular autophagy and was suppressed by 3-methyladenine and bafilomycin A1, by inhibitors of lysosomal proteolysis, and by reduced expression of Beclin 1, Atg5, or microtubule-associated protein 1 light chain 3␤ (MAP1LC3␤). However, TLR3-mediated signaling was restored upon exogenous expression of Beclin 1 or a variant MAP1LC3␤ fusion protein refractory to RNA interference. These data suggest that individuals harboring these variants may have a blunted innate immune response to enteroviral infection, leading to reduced viral clearance and an increased risk of cardiac pathology.Virus-induced myocarditis is an important cause of morbidity and mortality (1, 2). The enteroviruses have been considered the most common etiologic agents; however, other viruses, such as the adenoviruses, have also been implicated (3). The advent of molecular hybridization and PCR techniques has directly demonstrated infection of the myocardium with these viruses and has provided evidence of a viral etiology for dilated cardiomyopathy (DCM) 5 (1, 3). These findings further support the hypothesis that DCM is in some cases a long term sequela of acute or chronic myocarditis (4, 5), either due to the pathogenic effect of persistent viral replication or to an ongoing autoimmune process secondary to the viral infection.The recognition of viruses or viral particles by the host triggers the activation of an innate immune response that is characterized by the production of mediators such as tumor necrosis factor-␣, interleukins, interferons, and nitric oxide, all of which are toxic to replicating viruses (6). This initial antiviral response by the host is now known to be mediated at least in part by Toll-like receptors (TLRs) (5, 6). It has been shown that double-stranded (ds) RNA is the primary ligand for TLR3 (7). Although most TLR signaling pathways utilize the adaptor molecule MyD88 (myeloid differentiation primary response gene 88), TLR3 signals through the adaptor molecule called Toll-like receptor adaptor molecule 1 (Ticam1) or Toll/Interleukin 1 receptor domain-containing adapter-inducing interferon ␤ (TRIF) (8, 9). Activation of this pathway triggers produ...

The precise molecular mechanisms responsible for sepsis-induced myocardial dysfunction remain undefined. Toll-like receptor-4 (TLR-4) engages lipopolysaccharide (LPS) and activates signaling pathways leading to the expression of proinflammatory cytokines implicated in myocardial dysfunction. We determined whether TLR-4 was necessary for LPS-induced myocardial dysfunction in vivo. The effects of LPS on left ventricular (LV) function were studied in mice with defective TLR-4 signaling (C3H/HeJ, TLR-4 deficient) and wild-type mice (C3HeB/FeJ). Mice (n = 5/group) were injected with LPS or diluent, and LV function was examined by using two-dimensional echocardiography and conductance catheters. LPS significantly decreased all indexes of LV function in wild-type mice when compared with controls; LV function was not depressed in the LPS-treated TLR-4-deficient mice relative to controls. LPS increased myocardial nitric oxide synthase-2 expression and cGMP only in wild-type mice. This study suggests that TLR-4 mediates the LV dysfunction that occurs in LPS-induced shock. Therefore, TLR-4 might be a therapeutic target for attenuating the effects of LPS on the heart.