Inflammasomes continue to generate interest in an increasing number of disciplines owing to their unique ability to integrate a myriad of signals from pathogen- and damage-associated molecular patterns into a proinflammatory response. This potent caspase-1–dependent process is capable of activating the innate immune system, initiating pyroptosis (an inflammatory form of programmed cell death), and shaping adaptive immunity. The NLRP3 inflammasome is the most thoroughly studied of the inflammasome complexes that have been described thus far, perhaps owing to its disparate assortment of agonists. This review will highlight our current understanding of the mechanisms of both priming and activation of the NLRP3 inflammasome.

Objective: Arthritis and valvular carditis co-exist in several human rheumatic diseases, including systemic lupus erythematosus, rheumatic fever, and rheumatoid arthritis. K/BxN T-cell receptor transgenic mice develop spontaneous, autoantibody-associated arthritis and valvular carditis. The common Fc receptor gamma signaling chain (FcRγ) is required for carditis in K/BxN mice. FcRγ pairs with numerous receptors in a variety of cells. Here we sought to identify the FcRγ-associated receptors and FcγR-expressing cells that mediate valvular carditis in this model. Methods: We bred K/BxN mice lacking the genes encoding the activating Fc gamma receptors (FcγRI, III, and IV) and assessed for valvular carditis. We similarly assessed complement component C3-deficient K/BxN mice. Immunohistochemistry, bone marrow transplantation, and macrophage depletion were used to define the key FcRγ-expressing cell type. Results: Genetic deficiency of only one of the activating FcγRs did not prevent carditis, whereas deficiency of all three activating FcγRs did. Further analysis demonstrated that FcγRIII and FcγRIV were the key drivers of valve inflammation; FcγRI was dispensable. C3 was not required. FcRγ expression by radioresistant cells was critical for valvular carditis to develop, and further analysis pointed to macrophages as the key candidate FcγR-expressing effectors of carditis. Conclusion: FcγRIII and FcγRIV acted redundantly to promote valvular carditis in the K/BxN mouse model of systemic autoantibody-associated arthritis. Macrophage depletion reduced the severity of valve inflammation. These findings suggest that pathogenic autoantibodies engage FcγRs on macrophages to drive valvular carditis and provide new insight into the pathogenesis of cardiovascular inflammation in the setting of autoantibody-associated chronic inflammatory diseases.

IntroductionThe effector functions of immunoglobulin G (IgG) are mediated by interaction of its Fc region with Fc receptors (FcγRs) and/or the complement system. The three main pathways of complement activation converge at C3. However, C3-independent pathways can activate C5 and other downstream complement components during IgG-initiated inflammatory responses. These C3-independent pathways of C5 activation are triggered by activating FcγRs in some systems or can be activated by factors of the coagulation cascade such as thrombin. Here we studied the interplay of C3, C5, and activating FcγRs in a model of spontaneous autoantibody-driven arthritis.MethodsWe utilized the K/BxN TCR transgenic mouse model of arthritis. We bred K/BxN mice bearing targeted or naturally-occurring mutations in one or more of the genes encoding complement components C3, C5, and FcRγ, the cytoplasmic signaling chain shared by the activating FcγRs. We measured arthritis development, the production of arthritogenic autoantibodies, T cell activation status and cytokine synthesis. In addition, we treated mice with anti-C5 monoclonal antibodies or with the thrombin inhibitor argatroban.ResultsWe have previously shown that genetic deficiency of C5 protects K/BxN mice from the development of arthritis. We found here that C3-deficient K/BxN mice developed arthritis equivalent in severity to C3-sufficient animals. Arthritis also developed normally in K/BxN mice lacking both C3 and FcRγ, but could be ameliorated in these animals by treatment with anti-C5 monoclonal antibody or by treatment with argatroban. Production of arthritogenic autoantibodies, T cell activation, and T cell cytokine production were not affected by the absence of C3, C5, and/or FcRγ.ConclusionsIn K/BxN mice, C5-dependent autoantibody-driven arthritis can occur in the genetic absence of both complement C3 and activating FcγRs. Our findings suggest that in this setting, thrombin activates C5 to provoke arthritis.

Immunoglobulin G (IgG) immune complexes have been shown to modify immune responses driven by antigen presenting cells in either a pro- or anti-inflammatory direction depending upon the context of stimulation. However, the ability of immune complexes to modulate the inflammasome-dependent innate immune response is unknown. Here we show that IgG immune complexes suppress IL-1α and IL-1β secretion through inhibition of inflammasome activation. The mechanism by which this inhibition occurs is via immune complex ligation of activating Fcγ receptors (FcγR), resulting in prevention of both activation and assembly of the inflammasome complex in response to NLRP3, NLRC4, or AIM2 agonists. In vivo, administration of antigen in the form of an immune complex during priming of the immune response inhibited resultant adaptive immune responses in a NLRP3 dependent model of allergic airway disease. Our data reveal an unexpected mechanism regulating CD4+ T cell differentiation, whereby immune complexes suppress inflammasome activation and the generation of IL-1α and IL-1β from antigen presenting cells, which are critical for the antigen-driven differentiation of CD4+ T cells.

Summary Allelic exclusion of antigen receptor loci is a fundamental mechanism of immunological self tolerance. Incomplete allelic exclusion leads to dual T cell receptor (TCR) expression and can allow developing autoreactive αβ T lymphocytes to escape clonal deletion. Because allelic exclusion at the TCRβ locus is more stringent than at the TCRα locus, dual TCRβ expression has not been considered a likely contributor to autoimmunity. We show here that incomplete TCRβ allelic exclusion permits developing thymocytes bearing the autoreactive, transgene-encoded KRN TCR to be positively selected more efficiently, thereby accelerating the onset of spontaneous autoimmune arthritis. Our findings highlight dual TCRβ expression as a mechanism that can enhance the maturation of autoreactive pathogenic T cells and lead to more rapid development of autoimmune disease.

The immunopathogenic mechanisms mediating inflammation in multi-organ autoimmune diseases may vary between the different target tissues. We utilized the K/BxN TCR transgenic mouse model to investigate the contribution of CD4+ T cells and β2 integrins in the pathogenesis of autoimmune arthritis and endocarditis. Depletion of CD4+ T cells following the onset of arthritis specifically prevented the development of cardiac valve inflammation. Genetic absence of β2 integrins had no effect on the severity of arthritis and unexpectedly increased the extent of cardiovascular pathology. The exaggerated cardiac phenotype of the β2 integrin-deficient K/BxN mice was accompanied by immune hyperactivation and was linked to a defect in regulatory T cells. These findings are consistent with a model in which the development of arthritis in K/BxN mice relies primarily on autoantibodies, whereas endocarditis depends on an additional contribution of effector T cells. Furthermore, strategies targeting β2 integrins for the treatment of systemic autoimmune conditions need to consider not only the role of these molecules in leukocyte recruitment to sites of inflammation, but also their impact on the regulation of immunological tolerance.

The class A macrophage scavenger receptor Msr1 (SR-A, CD204) has been reported to participate in the maintenance of immunological tolerance. We investigated the role of Msr1 in a mouse model of autoantibody-dependent arthritis. Genetic deficiency of Msr1 in K/BxN TCR transgenic mice decreased the incidence and severity of arthritis, due to decreased autoantibody production. Despite normal initial activation of autoreactive CD4+ T cells, potentially autoreactive B cells in Msr1−/− K/BxN mice retained a naïve phenotype and did not expand. This was not due to an intrinsic B cell defect. Rather, we found that macrophages lacking Msr1 were inefficient at taking up the key autoantigen glucose-6-phosphate isomerase (GPI) and that Msr1-deficient mice had elevated serum concentrations of GPI. Arthritis developed normally when bone marrow from Msr1−/− K/BxN mice was transplanted into hosts whose macrophages did express Msr1. Thus, Msr1 can regulate the concentration of a soluble autoantigen. In this model, the absence of Msr1 led to higher levels of soluble autoantigen and protected mice from developing pathogenic autoantibodies, likely due to altered cognate interactions of autoreactive T and B cells with impaired differentiation of follicular helper T cells.

The NLRP3 inflammasome plays a critical role in regulating inflammatory and cell death pathways in response to a diverse array of stimuli. Activation of the NLRP3 inflammasome results in activation of the cysteine protease caspase-1 and the subsequent processing and secretion of the pro-inflammatory cytokines IL-1β and IL-18. In this issue of the European Journal of Immunology, Licandro and colleagues [Eur. J. Immunol. 2013. 43: DOI 10.1002/eji.201242918] show that the NLRP3 inflammasome contributes to oxidative DNA damage. In addition, activation of the NLRP3 inflammasome modulates a number of pathways involved in DNA damage repair, cell cycle, and apoptosis; suggesting a novel role for the NLRP3 inflammasome in DNA damage responses following cellular stress.