No abstract

Inhalation of silica crystals causes inflammation in the alveolar space. Prolonged silica exposure can lead to the development of silicosis, an irreversible, fibrotic pulmonary disease. The mechanisms by which silica and other crystals activate immune cells are not well understood. Here, we demonstrate that silica and aluminum salt crystals activate the NALP3 inflammasome. NALP3 activation requires crystal phagocytosis and crystal uptake leads to lysosomal damage and rupture. Sterile lysosomal damage is also sufficient to induce NALP3 activation and inhibition of phagosomal acidification or cathepsin B impairs NALP3 activation. These results indicate that the NALP3 inflammasome can sense lysosomal damage induced by various means as an endogenous danger signal.

When a cell dies in vivo the event does not go unnoticed. The host has evolved mechanisms to detect the death of cells and rapidly investigate the nature of their demise. If cell death is a result of natural causes, that is, it is part of normal physiological processes, then there is little threat to the organism. In this situation, little else is done other than removing the corpse. However, if cells have died as the consequence of some violence or disease, then both defence and repair mechanisms are mobilized. The importance of this process to host defence and disease pathogenesis has only been appreciated relatively recently. This article will review our current knowledge of these processes.The immune system was recognized in ancient times and rediscovered by Jenner and Pasteur based on its ability to confer protection upon repeat exposure to a pathogen. Through subsequent studies by Von Behring and others it rapidly became apparent that the immune system had the potential to respond not only to whole microorganisms, but to virtually any molecule that was "foreign" to the host. However, whereas injection of such molecules would often provoke a robust immune response, this did not invariably occur. Immunization protocols improved in the 1920s with the discovery by Ramon and Glenny of immunostimulatory molecules (adjuvants [G]) that could boost immune responses to co-administered antigens. Adjuvants were typically of microbial origin and became widely used to promote the effectiveness of immunizations. In the 1960s, Dresser showed that a foreign protein when highly purified would only elicit an immune response if it was admixed with a microbial adjuvant 1 . Injected by itself, the antigen not only failed to elicit immunity but actually induced a state of tolerance 2 . However, the significance of these observations was not well appreciated and adjuvants remained one of those things that everyone used because they were part of standard operating procedures.In 1989 Janeway put these empirical observations into a conceptual framework 3 (Fig. 1). He proposed that the immune system did not respond to all foreign antigens but only to those that are potentially associated with infection. The underlying idea here was that the immune system evolved to protect organisms against microorganisms and this discrimination between infectious versus noninfectious antigens focused defences on real threats rather than innocuous situations.At this time, it was already recognized that in order to stimulate T cell responses, antigens had to first be acquired and presented on MHC molecules of an antigen presenting cell (APC). Moreover, it was further known that APCs also provided additional costimulatory signals necessary to activate T cells. Janeway incorporated these principles into his model (Fig. 1). He postulated that the discrimination between infectious and noninfectious non-self molecules was made by the APCs of the innate immune system through receptors that would recognize pathogen-associated molecular patterns (PAMP...

Dying cells stimulate inflammation, and this response is thought to contribute to the pathogenesis of many diseases. Very little has been known, however, about how cell death triggers inflammation. We found here that the acute neutrophilic inflammatory response to cell injury requires the signaling protein myeloid differentiation primary response gene 88 (Myd88). Analysis of the contribution of Myd88-dependent receptors to this response revealed only a minor reduction in mice doubly deficient in Toll-like receptor 2 (Tlr2) and Tlr4 and normal responses in mice lacking Tlr1, Tlr3, Tlr6, Tlr7, Tlr9, Tlr11 or the interleukin-18 receptor (IL-18R). However, mice lacking IL-1R showed a markedly reduced neutrophilic inflammatory response to dead cells and tissue injury in vivo as well as greatly decreased collateral damage from inflammation. This inflammatory response required IL-1alpha, and IL-1R function was required on non-bone-marrow-derived cells. Notably, the acute monocyte response to cell death, which is thought to be important for tissue repair, was much less dependent on the IL-1R-Myd88 pathway. Also, this pathway was not required for the neutrophil response to a microbial stimulus. These findings suggest that inhibiting the IL-1R-Myd88 pathway in vivo could block the damage from acute inflammation that occurs in response to sterile cell death, and do so in a way that might not compromise tissue repair or host defense against pathogens.

The acute inflammatory response is a double-edged sword. On the one hand it plays a key role in initial host defense particularly against many infections. On the other hand its aim is imprecise and as a consequence, when it is drawn into battle, it can cause collateral damage in tissues. In situations where the inciting stimulus is sterile, the cost-benefit ratio may be high; because of this, sterile inflammation underlies the pathogenesis of a number of diseases. While there have been major advances in our understanding of how microbes trigger inflammation, much less has been learned about this process in sterile situations. This review focuses on a subset of the many sterile stimuli that can induce inflammation – specifically dead cells and a variety of irritant particles, including crystals, minerals, and protein aggregates. Although this subset of stimuli is structurally very diverse and might appear to be unrelated, there is accumulating evidence that the innate immune system may recognize them in similar ways and stimulate the sterile inflammatory response via common pathways. Here we review established and emerging data about these responses.