The same systemic autoimmune disease provokes arthritis and endocarditis via distinct mechanisms

Abstract

The immune mechanisms that provoke concomitant inflammation of synovial joints and cardiac valves in disorders such as rheumatic fever and systemic lupus erythematosus remain poorly defined. Here, we report the discovery of spontaneous endocarditis-in addition to their well-studied autoimmune arthritis-in K/BxN T cell receptor (TCR) transgenic mice. The same adaptive immune system elements were required for initiation of arthritis and endocarditis, and both diseases were dependent on autoantibodies. In contrast, the participation of key innate immune system molecules and perhaps T cells as effectors of inflammation differed between the 2 target tissues. Arthritis in K/BxN TCR transgenic mice depended primarily on complement C5 and not FcRgamma-using receptors; conversely, endocarditis depended essentially on FcRgamma receptors and not C5. Elucidating how a single systemic autoimmune disease engages distinct immune effector pathways to damage different target tissues is essential for optimizing the treatment of such disorders.

Fig. 1.

Histological characterization of mitral valve inflammation in K/BxN mice. (A) Long-axis sections through the hearts of 8-week-old nonarthritic BxN mice (Left) and arthritic K/BxN mice (Right) show inflammation of the mitral valve in K/BxN mice. Hematoxylin and eosin (H&E); original magnifications are indicated. (B and C) Immunofluorescent staining of similar sections demonstrates deposition of (B) complement C3 (green) and (C) Ig IgG (red) in the inflamed mitral valve of K/BxN mice; the sections are counterstained with DAPI to detect nuclei (blue). (Objective: ×10.) (D) Sections of 2 different K/BxN hearts are shown, demonstrating characteristic nodular collections of amorphous cells similar to Aschoff bodies or nodules (Left). (Objective: ×40.) Some cells have characteristic “caterpillar”-shaped nuclei (arrows) and “owl's-eye” nuclei (arrowhead) typical of Anitschkow cells (Right). (Objective: ×100.) Though the origin of these cells is debated, the structures are well-described characteristics of rheumatic carditis in humans. (E) A mildly inflamed mitral valve from a K/BxN mouse heart is shown in the upper left (H&E). Serial sections from this same specimen were stained with biotinylated antibodies recognizing the indicated antigens (detected with red staining) and with DAPI to detect nuclei (blue). Vβ6 is the TCR β-chain encoded by the KRN transgene. The isotype control antibody displayed is rat IgG2a. (Objective: ×10.)

Fig. 2.

Adaptive immune requirements for endocarditis in K/BxN mice. (A) The KRN TCR transgene and the MHC molecule I-Ag7 are required for the development of arthritis and endocarditis. KRN/B6.g7 indicates KRN/B6 mice with congenically expressed A^g7. Numbers represent the incidence of arthritis and endocarditis. A valve was considered to have endocarditis if an inflammatory valve infiltrate was present. (B and C) B cells and CD40 are required for endocarditis. Mitral valves from (B) B-cell-sufficient μMT+/− and B cell-deficient μMT−/− K/BxNmice and from (C) CD40-sufficient and CD40-deficient K/BxN mice. The incidence of mitral valve inflammation in mice of each genotype is indicated. (H&E, objective: ×40.)

Fig. 3.

Differential contributions of C5 and FcRγ to arthritis and endocarditis in K/BxN mice. (A) Arthritis development is impaired in C5-deficient (circles, n = 23) relative to C5-sufficient K/BxN mice (squares, n = 26) as measured by arthritis score and change in ankle thickness as described (29). Error bars, SEM; *, P < 0.05, **, P < 0.001, ***, P < 0.0001. (B and C) Mitral valve inflammation in K/BxN mice is not affected by C5 deficiency as measured by maximum mitral thickness (B) and shown by histology (C). (D) Arthritis scores are equivalent, and only a mild decrement in ankle thickness occurs in FcRγ-deficient (triangles, n = 8) relative to FcRγ-sufficient (squares, n = 10) KRN+ A^g7+ C57BL/6 mice. Error bars, SEM; NS = not significant, *, P < 0.05. (E and F) Mitral valve inflammation is significantly reduced in FcRγ-deficient relative to FcRγ-sufficient mice as measured by maximum mitral thickness (E) and shown by histology (F). For (B) and (E), each point represents one mouse; P values were calculated using Student's t test. (H&E, objective: ×20.)

Fig. 4.

Cell transfer systems to provoke arthritis and endocarditis. (A) Arthritis scores and ankle thickness measurements following transfer of bone marrow from K/BxN mice into irradiated B6 Rag-deficient (diamonds) or BxN Rag-deficient (triangles) mice (mean, SEM). For the B6-Rag recipients, the mice were killed due to wasting disease 6 weeks following transplantation. (B) Arthritis scores and ankle thickness measurements following transfer of K/B splenocytes (circles) or K/BxN splenocytes (squares) into BxN TCRα−/− mice (mean, SEM). (C) The maximal thickness of the mitral valves was determined for each mouse at the time of sacrifice. Endocarditis was defined as in Fig. 2A. (D) Anti-GPI IgG titers were determined by ELISA for each mouse at the time of sacrifice (6–8 weeks following splenocyte or bone marrow cell transfer, sera were diluted 1:1,000). For (C) and (D), closed symbols indicate mice with endocarditis, whereas open symbols indicate mice without endocarditis; for one mouse (open symbol with “x” in D), serum was available but histologic evaluation of the heart was not. Bars indicate mean values; P values for comparisons between the groups are provided (Student's t test).