Antigen-driven T cell-macrophage interactions mediate the interface between innate and adaptive immunity in histidyl-tRNA synthetase-induced myositis

IntroductionPrevious work in humans has demonstrated that both innate and adaptive immune signaling pathways contribute to the pathogenesis of idiopathic inflammatory myopathy (IIM), a systemic autoimmune disease targeting muscle as well as extra-muscular organs. To better define interactive signaling networks in IIM, we characterized the cellular phenotype and transcriptomic profiles of muscle-infiltrating cells in our established murine model of histidyl-tRNA synthetase (HRS)-induced myositis.MethodsMyositis was induced in wild type (WT) and various congenic/mutant strains of C57BL/6 mice through intramuscular immunization with recombinant HRS. Histopathological, immunohistochemical, flow cytometric, and transcriptomic assessments were used to characterize the functional relationship between muscle-infiltrating cell populations in these strains lacking different components of innate and/or adaptive immune signaling.ResultsRAG1 KO mice developed markedly reduced muscle inflammation relative to WT mice, demonstrating a key requirement for T cells in driving HRS-induced myositis. While the reduction of mononuclear cell infiltrates in CD4-Cre.MyD88fl/fl conditional knockout mice and OT-II TCR transgenic mice highlighted roles for both innate and TCR-mediated/adaptive immune signaling in T cells, diminished inflammation in Lyz2-Cre.MyD88fl/fl conditional knockout mice underscored the importance of macrophage/myeloid cell populations in supporting T cell infiltration. Single cell RNA sequencing-based clustering of muscle-infiltrating subpopulations and associated pathway analyses showed that perturbations of T cell signaling/function alter the distribution and phenotype of macrophages, fibroblasts, and other non-lymphoid cell populations contributing to HRS-induced myositis.DiscussionOverall, HRS-induced myositis reflects the complex interplay between multiple cell types that collectively drive a TH1-predominant, pro-inflammatory tissue phenotype requiring antigen-mediated activation of both MyD88- and TCR-dependent T cell signaling pathways

Structural and Thermodynamic Approach to Peptide Immunogenicity

In the conventional paradigm of humoral immunity, B cells recognize their cognate antigen target in its native form. However, it is well known that relatively unstable peptides bearing only partial structural resemblance to the native protein can trigger antibodies recognizing higher-order structures found in the native protein. On the basis of sound thermodynamic principles, this work reveals that stability of immunogenic proteinlike motifs is a critical parameter rationalizing the diverse humoral immune responses induced by different linear peptide epitopes. In this paradigm, peptides with a minimal amount of stability (ΔGX<0 kcal/mol) around a proteinlike motif (X) are capable of inducing antibodies with similar affinity for both peptide and native protein, more weakly stable peptides (ΔGX>0 kcal/mol) trigger antibodies recognizing full protein but not peptide, and unstable peptides (ΔGX>8 kcal/mol) fail to generate antibodies against either peptide or protein. Immunization experiments involving peptides derived from the autoantigen histidyl-tRNA synthetase verify that selected peptides with varying relative stabilities predicted by molecular dynamics simulations induce antibody responses consistent with this theory. Collectively, these studies provide insight pertinent to the structural basis of immunogenicity and, at the same time, validate this form of thermodynamic and molecular modeling as an approach to probe the development/evolution of humoral immune responses

The role of Jo-1 in the immunopathogenesis of polymyositis: current hypotheses

Polymyositis represents an autoimmune disease in which T cells mediate destruction of muscle cells. Although the precise trigger(s) for this process remain unknown, distinct clinical subsets exist that are characterized by antibodies directed against specific nuclear and cytoplasmic antigens including Jo-1 (histidyl-transfer RNA synthetase). Coupled with a range of genetic and histomorphologic data, the stereotypical serologic response suggests that antigen-specific T cells directed against Jo-1 can promote T cell-mediated cytolysis of muscle cells as well as anti-Jo-1 antibody formation in selected patients with polymyositis. Beyond a previously developed animal model that has demonstrated the capacity of Jo-1 to promote humoral and cell-mediated immune responses leading to myositis, recent studies have revealed the existence of Jo-1-specific T cells in the peripheral blood of patients with Jo-1 antibody-positive polymyositis. Even more striking, investigators have discovered that Jo-1 can serve as a chemokine for immature dendritic cells and T lymphocytes. Collectively, these findings suggest a mechanism by which Jo-1 can bridge the innate and adaptive immune responses, leading to the breakdown of tolerance and autoimmune destruction of muscle

Role of Jo-1 in the Immunopathogenesis of the Anti-synthetase Syndrome

Histidyl-tRNA synthetase (HRS = Jo-1) represents a key autoantibody target in the anti-synthetase syndrome that is marked by myositis as well as extra-muscular organ complications including interstitial lung disease (ILD). Over the last 25 years, a wealth of clinical, epidemiological, genetic, and experimental data have collectively supported a role for Jo-1 in mediating deleterious cell-mediated, adaptive immune responses contributing to the disease phenotype of the anti-synthetase syndrome. Complementing these studies, more recent work suggests that unique, non-enzymatic functional properties of Jo-1 also endow this antigen with the capacity to activate components of the innate immune system, particularly cell surface as well as endosomal Toll-like receptors and their downstream signaling pathways. Combining these facets of Jo-1-mediated immunity now supports a more integrated model of disease pathogenesis that should lead to improved therapeutic targeting in the anti-synthetase syndrome and related subsets of idiopathic inflammatory myopathy

Animal models of inflammatory myopathy

The idiopathic inflammatory myopathies (IIMs) represent a heterogeneous group of disorders characterized by mononuclear cell infiltration of muscle and varying degrees of muscle dysfunction. To better understand the pathogenesis of these diseases, investigators have devised a number of infectious, genetic, and antigen-induced animal models that replicate different aspects of muscle involvement. Although the underlying heterogeneity of disorders encompassed by IIM precludes development of a single unifying model, several recently developed experimental systems have provided tremendous insight regarding the contributions of both immune- and non-immune-mediated disease pathways in various subsets of IIM. In turn, by elucidating the pathogenic roles of such disparate factors as endoplasmic reticulum stress and innate immune signaling, these models have established the foundation for more novel, targeted therapeutic intervention

Pulmonary complications of inflammatory myopathy

Pulmonary manifestations contribute significantly to the morbidity and mortality of the idiopathic inflammatory myopathies, ranging from intrinsic lung disease to secondary complications that include aspiration pneumonia, opportunistic infection, congestive heart failure, and hypoventilation. Newer classification schemes for interstitial lung disease have permitted closer correlation between histologic subtype and clinical outcome, while diagnostic techniques such as bronchoalveolar lavage have begun to define the cellular elements responsible for immune-mediated pulmonary dysfunction. Investigators have identified several serum markers correlating with inflammatory disease activity in the lung that should enhance noninvasive monitoring of therapeutic responses to newer regimens involving agents such as cyclosporine and tacrolimus. Taken together, these advances have contributed to better understanding of the immunopathogenesis of myositis-associated interstitial lung disease that should ultimately translate into more effective treatment

Interstitial lung disease in rheumatoid arthritis

Rheumatoid arthritis (RA) is the most common systemic autoimmune disease in the United States, affecting 1% to 2% of the adult population. Although joints and synovium are the primary targets in this disorder, extra-articular manifestations involving the lungs can lead to significant morbidity and excess mortality. Among the various pulmonary complications that occur in RA, interstitial lung disease (ILD) is the most damaging, with effects ranging from subclinical inflammation/scarring to end-stage pulmonary fibrosis. New insights during the past several years have underscored the epidemiologic impact of clinically/functionally significant RA-associated ILD (RA-ILD) and have begun to identify factors contributing to the pathogenesis of this potentially devastating complication of RA. Despite these advancements, the complexity of RA-ILD and the lack of reliable predictors for disease progression highlight the need for improved biomarker development. Establishing such detailed molecular signatures will ultimately guide the application and timing of therapeutic agents that include immunomodulators as well as newly studied antifibrotic agents

Animal models in myositis

The etiology of the idiopathic inflammatory myopathies remains elusive. Delineation of pathogenic mechanisms in humans is hindered by the heterogeneity of different patient populations as well as the complexity and chronicity of the disease. Therefore, appropriate animal models are required to help clarify the immunopathogenesis of these disorders and to explore promising new therapies. The purpose of this review is to discuss recently published animal models in myositis, with a particular focus on idiopathic inflammatory myopathy. Over the last few years, there has been considerable progress in the development of animal models for polymyositis and inclusion body myositis, but reports focusing on dermatomyositis have been limited. Although some of these systems are entirely novel, others have elucidated pathogenic mechanisms of existing models. Several new animal models of myositis have emerged over the last few years that have revealed new insights regarding the pathophysiology of idiopathic inflammatory myopathy and that should set the foundation for development of new, more effective therapies against this often intractable disease

Citrullinated Autoantigen Targets as Markers of Extra-Articular Disease in Rheumatoid Arthritis

Citrullination represents an increasingly recognized posttranslational modification stemming from underlying physiological stressors that dysregulate intracellular calcium flux. Although this enzymatic process mediated by various isoforms of peptidylarginine deiminase (PAD) is fairly ubiquitous (occurring in normal as well as pathological states), the immune response to citrullinated proteins is heavily influenced by underlying HLA status and therefore highly associated with rheumatoid arthritis (RA) (Szodoray et al. 2010). Given that the humoral immune responses to citrullinated proteins may serve as an immunological “fingerprint” in RA, the question is whether delineating targets of anti-citrullinated protein antibodies (ACPAs) can provide insight regarding the site where immune tolerance is bypassed/broken or clarify the underlying pathophysiology of articular and extra-articular manifestations in this systemic autoimmune disease—even if the relative contribution of protein deimination versus citrulline-targeted immunity remains unresolved. In fact, extensive investigation over the last 10–15 years has yielded an expanded repertoire of ACPA specificities potentially linked with defined extra-articular manifestations, such as premature atherosclerosis, myocardial dysfunction, and interstitial lung disease (ILD), which negatively impact clinical outcome. Fueling these discoveries, novel approaches for identifying citrullinated autoantigen/autoantibody combinations have supported the search for additional biomarkers of extra-articular involvement that should further elucidate the immunobiology of relevant systemic disease pathways in RA