Slit2 May Underlie Divergent Induction by Thyrotropin of IL-23 and IL-12 in Human Fibrocytes

Abstract

IL-23 and IL-12, two structurally related heterodimeric cytokines sharing a common subunit, divergently promote Th cell development and expansion. Both cytokines have been implicated in the pathogenesis of thyroid-associated ophthalmopathy (TAO), an autoimmune component of Graves disease. In TAO, CD34+ fibrocytes, putatively derived from bone marrow, can be identified in the orbit. There they masquerade as CD34+ orbital fibroblasts (OF) (CD34+ OF) and cohabitate with CD34− OF in a mixed fibroblast population (GD-OF). Slit2, a neural axon repellent, is expressed and released by CD34− OF and dampens the inflammatory phenotype of fibrocytes and CD34+ OF. In this study we report that thyrotropin (TSH) and the pathogenic, GD-specific monoclonal autoantibody, M22, robustly induce IL-23 in human fibrocytes; however, IL-12 expression is essentially undetectable in these cells under basal conditions or following TSH-stimulation. In contrast, IL-12 is considerably more inducible in GD-OF, cells failing to express IL-23. This divergent expression and induction of cytokines appears to result from cell type–specific regulation of both gene transcription and mRNA stabilities. It appears that the JNK pathway activity divergently attenuates IL-23p19 expression while enhancing that of IL-12p35. The shift from IL-23p19 expression in fibrocytes to that of IL-12p35 in their derivative CD34+ OF results from the actions of Slit2. Thus, Slit2 might represent a molecular determinant of balance between IL-23 and IL-12 expression, potentially governing immune responses in TAO.

Teprotumumab as a Novel Therapy for Thyroid-Associated Ophthalmopathy

“…This receptor was subsequently found to be functional and could mediate effects of both TSH and thyroid-stimulating autoantibodies ( 7 , 14 ). Activation of several genes, including those encoding cytokines implicated in the pathogenesis of TAO, has been reported ( 19 , 33 – 35 ). Fibrocytes are trafficked to sites of tissue injury through several chemokine networks, most notably the CXCL-12/CXCR4 pathway ( 36 ) which is under the control of TSHR signaling.…”

Mechanisms That Underly T Cell Immunity in Graves’ Orbitopathy

“…They infiltrate into GO orbital connective tissues and more likely produce IFN-γ instead of IL-17A ( 31 ). TSH and M22 robustly induced gene and protein expression of IL-23 in GO fibrocytes, but not IL-12, which was significantly produced by GO OFs under the same conditions ( 34 ). However, pure CD34 + OFs preferentially expressed Il23p19 , while their homologous CD34 - OFs greatly expressed Il12p35 ( 34 ).…”

“…TSH and M22 robustly induced gene and protein expression of IL-23 in GO fibrocytes, but not IL-12, which was significantly produced by GO OFs under the same conditions ( 34 ). However, pure CD34 + OFs preferentially expressed Il23p19 , while their homologous CD34 - OFs greatly expressed Il12p35 ( 34 ). The distinct roles of CD34 + and CD34 - OFs reflect the potential shift from a non-pathogenic to pathogenic state of circulating Th17 cells into orbit-infiltrating Th17 cells, which is consistent with the TSHR signaling that drives the specific cytokine milieu by CD34 + fibrocytes that masquerade as CD34 + OFs within orbital connective tissues.…”

Cytokines as Targets of Novel Therapies for Graves’ Ophthalmopathy

“…The axon guidance glycoprotein Slit2 is produced by CD34− OF, and it inhibits fibrocyte differentiation and modulates their characteristic gene expression profile. Slit2 upregulates IL-12 expression and it attenuates that of IL-23, which are both involved in GD and GO (83). CD34+ fibrocytes obtained from peripheral blood mononuclear cells (PBMCs) produce low basal levels of HA, very few of which were affected by bovine thyrotropin (bTSH).…”