CD301b/MGL2+ Mononuclear Phagocytes Orchestrate Autoimmune Cardiac Valve Inflammation and Fibrosis.

BACKGROUND: Valvular heart disease is common and affects the mitral valve (MV) most frequently. Despite the prevalence of MV disease (MVD), the cellular and molecular pathways that initiate and perpetuate it are not well understood. METHODS: K/B.g7 T-cell receptor transgenic mice spontaneously develop systemic autoantibody-associated autoimmunity, leading to fully penetrant fibroinflammatory MVD and arthritis. We used multiparameter flow cytometry, intracellular cytokine staining, and immunofluorescent staining to characterize the cells in inflamed K/B.g7 MVs. We used genetic approaches to study the contribution of mononuclear phagocytes (MNPs) to MVD in this model. Specifically, we generated K/B.g7 mice in which either CX3CR1 or CD301b/macrophage galactose N-acetylgalactosamine-specific lectin 2 (MGL2)-expressing MNPs were ablated. Using K/B.g7 mice expressing Cx3Cr1-Cre, we conditionally deleted critical inflammatory molecules from MNPs, including the Fc-receptor signal-transducing tyrosine kinase Syk and the cell adhesion molecule very late antigen-4. We performed complementary studies using monoclonal antibodies to block key inflammatory molecules. We generated bone marrow chimeric mice to define the origin of the inflammatory cells present in the MV and to determine which valve cells respond to the proinflammatory cytokine tumor necrosis factor (TNF). Finally, we examined specimens from patients with rheumatic heart disease to correlate our findings to human pathology. RESULTS: MNPs comprised the vast majority of MV-infiltrating cells; these MNPs expressed CX3CR1 and CD301b/MGL2. Analogous cells were present in human rheumatic heart disease valves. K/B.g7 mice lacking CX3CR1 or in which CD301b/MGL2-expressing MNPs were ablated were protected from MVD. The valve-infiltrating CD301b/MGL2+ MNPs expressed tissue-reparative molecules including arginase-1 and resistin-like molecule α. These MNPs also expressed the proinflammatory cytokines TNF and interleukin-6, and antibody blockade of these cytokines prevented MVD. Deleting Syk from CX3CR1-expressing MNPs reduced their TNF and interleukin-6 production and also prevented MVD. TNF acted through TNF receptor-1 expressed on valve-resident cells to increase the expression of vascular cell adhesion molecule-1. Conditionally deleting the vascular cell adhesion molecule-1 ligand very late antigen-4 from CX3CR1-expressing MNPs prevented MVD. CONCLUSIONS: CD301b/MGL2+ MNPs are key drivers of autoimmune MVD in K/B.g7 mice and are also present in human rheumatic heart disease. We define key inflammatory molecules that drive MVD in this model, including Syk, TNF, interleukin-6, very late antigen-4, and vascular cell adhesion molecule-1.

Brief Report: Arthritis in KRN T Cell Receptor-Transgenic Mice Does Not Require Interleukin-17 or Th17 Cells.

OBJECTIVE: Th17 cells and interleukin-17 (IL-17) cytokine family members are implicated in the pathogenesis of many rheumatic diseases. Most studies in mouse models of inflammatory arthritis have demonstrated a key role for the proinflammatory cytokine IL-17A and its receptor, the IL-17 receptor (IL-17R) A/C heterodimer. The aim of this study was to use a rigorous genetic approach to evaluate the contribution of Th17 cells and IL-17 in the autoantibody-dependent KRN T cell receptor-transgenic mouse model of arthritis. METHODS: We bred KRN mice expressing the major histocompatibility complex class II molecule A(g7) (referred to as K/B/g7 mice) and genetically lacking the related cytokines IL-17A and IL-17F or their critical receptor subunit, IL-17RA. Using bone marrow transplantation, we generated mice in which hematopoietic cells from K/B/g7 donor mice lacked the key Th17-differentiating transcription factor, retinoic acid receptor-related orphan nuclear receptor γt (Rorγt). RESULTS: K/B/g7 mice lacking both IL-17A and IL-17F produced normal titers of pathogenic autoantibodies, and arthritis developed in a typical manner. Similarly, neither IL-17RA nor Rorγt expression by hematopoietic cells was required for disease development in this model. CONCLUSION: Despite prior reports suggesting that Th17 cells and IL-17A are crucially involved in the pathogenesis of arthritis in K/BxN mice, the results presented here provide genetic evidence that IL-17A and IL-17F, IL-17RA, and Rorγt expression by hematopoietic cells are dispensable for normal arthritis progression in the K/B/g7 mouse model system. We discuss potential explanations for the discrepancies between these 2 highly similar model systems. These findings plus those in other mouse models of arthritis provide insight regarding why therapeutic biologic agents targeting the Th17/IL-17 axis are beneficial in some human rheumatic diseases but not others.