Th17 cells induce ectopic lymphoid follicles in central nervous system tissue inflammation

Abstract

Ectopic lymphoid follicles are hallmarks of chronic autoimmune inflammatory diseases such as multiple sclerosis (MS), rheumatoid arthritis, Sjögren's syndrome, and myasthenia gravis. However, the effector cells and mechanisms that induce their development are unknown. Here we showed that in experimental autoimmune encephalomyelitis (EAE), the animal model of MS, Th17 cells specifically induced ectopic lymphoid follicles in the central nervous system (CNS). Development of ectopic lymphoid follicles was partly dependent on the cytokine interleukin 17 (IL-17) and on the cell surface molecule Podoplanin (Pdp), which was expressed on Th17 cells, but not on other effector T cell subsets. Pdp was also crucial for the development of secondary lymphoid structures: Pdp-deficient mice lacked peripheral lymph nodes and had a defect in forming normal lymphoid follicles and germinal centers in spleen and lymph node remnants. Thus, Th17 cells are uniquely endowed to induce tissue inflammation, characterized by ectopic lymphoid follicles within the target organ.

Figure 1. Recipients of MOG-Specific Th17 Cells Develop Ectopic Lymphoid Follicle-like Structures in the CNS

(A) Hematoxylin and eosin staining of a paraffin section of the anterior spinal cord of a Th17 cell recipient shows the anterior spinal columns (left and right upper corners of the field) with large aggregates of dark-blue lymphocytes within the subarachnoid space surrounding the anterior spinal artery (arrow) (left). Silver stain for reticulin reveals dark brown fibers indicating structural organization of the follicle (right, arrowheads). Note that the concentric pattern of reticulin surrounding the artery in the subarachnoid space differs from the smaller branching pattern of capillaries in the parenchyma. (B) Cryosection of the CNS stained for B cells (B220, blue), T cells (CD4, green), and collagen (red). Scale bars represent 50 μm. (C) Staining for T cells (CD3, green), B cells (B220, blue), and collagen (red) shows several large B cell clusters surrounded by T cells and collagen-positive fibers in the subarachnoid space. Scale bar represents 100 μm. All pictures are representative of three or more independent experiments.

Figure 2. Th17 Cells Develop a Tfh Cell-like Phenotype in the CNS

Infiltrating cells were isolated from the CNS of Th17 cell recipients at the peak of disease, i.e., mice had a score of 2.5–4 for 3–7 days. Cells were directly stained and analyzed by flow cytometry. (A) The infiltrating cells were tested for CD4 and CD19 expression. About 30% of the infiltrating cells were T cells (range 28%–53%) and about 30% were B cells (range 5%–32%). (B) The infiltrating cells were tested for expression of Vα3.2, which is part of the transgenic TCR expressed on transferred T cells. The vast majority of CD4⁺ T cells infiltrating the CNS of Th17 cell recipients are transferred cells expressing Vα3.2 (range 80%–99%). (C) CNS T cells (left) and B cells (right) were analyzed for expression of GL7 by flow cytometry (range 15%–36% for T cells, 9%–17% for B cells). (D) CNS-infiltrating T cells were analyzed for expression of CXCR5 (range 9%–90%), ICOS (range 40%–95%), Bcl6 (range 13%–27%), and CCR6 (range 5%–31%) by flow cytometry. Flow cytometry data shown are representative of at least six individual mice from two or three independent in vivo experiments.

Figure 3. Formation of Ectopic Lymphoid Follicle-like Structures in the CNS Depends Partly on IL-17 but Not IL-21

(A) WT, Il17ra^−/−, and Il21r^−/− recipients of different MOG-specific T cell subsets were monitored for disease development. 30–40 days after transfer, the CNS of the recipients was harvested and analyzed for the incidence (left) and number (right) of eLFs. Only recipients that developed disease and survived are represented in the graphs. For the quantification (right), only mice with ten or more eLFs in the CNS were considered positive and included in the analysis. Error bars represent SEM. Graphs show combined data from two to four independent in vivo experiments per column. (B and C) Th17 cells were adoptively transferred into WT and Il17ra^−/− recipients (B) or into WT and Il21r^−/− recipients (C) and mice were monitored for the development of disease. Whether disease courses were significantly different between the groups was analyzed by linear regression analysis (B, bottom, **p < 0.0001, and C, bottom, *p < 0.01). Graphs in (B) show combined data from two independent experiments (three independent experiments were performed), and graphs in (C) are representative of three independent experiments. Error bars represent SEM. Further information can be found in Table S1.

Figure 4. Podoplanin Is Specifically Expressed on Th17 Cells In Vitro

(A) Sorted naive CD4⁺ T cells were differentiated in vitro into different T cells subsets, and after 4 days expression of Pdp was determined by flow cytometry. (B) Sorted naive T cells from WT or IL-17-GFP reporter mice were differentiated in vitro with IL-6+TGF-β. After 4 days, expression of Pdp and IL-17-GFP was analyzed by flow cytometry. (C) After restimulation of in-vitro-differentiated Th1, Th2, and Th17 cells with anti-CD3 and anti-CD28, mRNA levels of Pdp were measured at the indicated time points. Data shown are representative of at least three independent experiments.

Figure 5. Pdp Is Expressed on Th17 Cells In Vivo and Is Important for Formation of the Ectopic Lymphoid Follicle-like Structures in Th17 Cell Recipients

(A–C) WT and IL-17-GFP reporter mice were immunized with 100 μg MOG in CFA. (A) On day 8 after immunization, splenocytes were harvested and cultured in the presence of MOG. After 4 days T cells were stimulated with PMA and ionomycin in the presence of monensin for 4 hr and analyzed for expression of IL-17-GFP and Pdp by flow cytometry. Data shown are representative of nine mice per group from two independent in vivo experiments. (B) At the peak of disease, T cells were sorted from the CNS, spleen, and LNs and mRNA levels of IL-17 and Pdp were determined by quantitative PCR. Graphs are representative of two independent in vivo experiments. (C) At the onset of disease, infiltrating cells were isolated from meninges and CNS parenchyma and expression of Pdp and IL-17-GFP on CD4⁺ T cells was determined by flow cytometry directly ex vivo. Data shown are representative of four mice per group from two independent experiments. (D) Th17+IL-23 cells were transferred into WT recipients treated with 100 μg polyclonal anti-Pdp on day 0, 2, 4, and 7 after transfer. Control animals were treated with goat IgG or PBS. Mice were monitored daily for development of EAE, and 30–40 days after transfer, the CNS of recipients was harvested for histological analysis. Bar graphs show the time of disease onset (left), the mean maximum disease score (middle), and the number of eLFs per recipient (right) for the anti-Pdp-treated mice versus control mice. Graphs show combined data from two independent experiments. Error bars represent SEM. The difference between the groups was tested for significance by an unpaired Student’s t test with Welch’s correction (*p = 0.0478) (right). Further information can be found in Table S2.

Figure 6. Pdp-Deficient Mice Have a Defect in the Formation of Secondary Lymphoid Structures

(A) Hematoxylin and eosin staining of paraffin sections of a LN harvested from a Pdpn^+/+ mouse (left), a LN remnant harvested from a Pdpn^−/− mouse (top right), and a rare normal-sized LN harvested from a Pdpn^−/− mouse (bottom right). Lymphocytes stain dark blue and extravasated red blood cells stain red (arrows, top right). (B) Hematoxylin and eosin staining of a paraffin section of PP harvested from Pdpn^+/+ mice (left) and Pdpn^−/− mice (right). Top panels show whole PP, and bottom panels show one follicle of the PP. (C) Cryosections from LNs, LN remnants, and spleens of 12-month-old Pdpn^−/− mice and their Pdpn^+/+ littermates were analyzed for GC structure via T cell-specific (CD3), B cell-specific (B220), and FDC-specific stains (αCR1, clone 8C12), as well as the GC marker GL7 and the structural marker collagen. Data shown are representative of three independent experiments. Scale bars in (A) (top) and (B) (bottom) represent 50 μm. Scale bars in (A) (bottom) and (B) (top) represent 100 μm. Scale bar in (C) represents 200 μm.