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

Background -Valvular heart disease (VHD) is common and affects the mitral valve (MV) most frequently. Despite the prevalence of mitral valve disease (MVD), the cellular and molecular pathways that initiate and perpetuate it are not well understood. Methods -K/B.g7 T cell receptor (TCR) transgenic mice spontaneously develop systemic autoantibody-associated autoimmunity, leading to fully-penetrant fibro-inflammatory 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/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 (VLA-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 pro-inflammatory cytokine TNF. Finally, we examined specimens from patients with rheumatic heart disease (RHD) 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 RHD 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 (Arg-1) and resistin-like molecule alpha (RELM-α). These MNPs also expressed the pro-inflammatory cytokines TNF and IL-6, and antibody-blockade of these cytokines prevented MVD. Deleting Syk from CX3CR1-expressing MNPs reduced their TNF and IL-6 production and also prevented MVD. TNF acted through TNFR1 expressed on valve-resident cells to increase expression of vascular cell adhesion molecule-1 (VCAM-1). Conditionally deleting the VCAM-1 ligand VLA-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 RHD. We define key inflammatory molecules that drive MVD in this model, including Syk, TNF, IL-6, VLA-4, and VCAM-1.