Stroma-derived interleukin-34 controls the development and maintenance of langerhans cells and the maintenance of microglia

Abstract

Colony stimulating factor-1 (Csf-1) receptor and its ligand Csf-1 control macrophage development, maintenance, and function. The development of both Langerhans cells (LCs) and microglia is highly dependent on Csf-1 receptor signaling but independent of Csf-1. Here we show that in both mice and humans, interleukin-34 (IL-34), an alternative ligand for Csf-1 receptor, is produced by keratinocytes in the epidermis and by neurons in the brain. Mice lacking IL-34 displayed a marked reduction of LCs and a decrease of microglia, whereas monocytes, dermal, and lymphoid tissue macrophages and DCs were unaffected. We identified IL-34 as a nonredundant cytokine for the development of LCs during embryogenesis as well as for their homeostasis in the adult skin. Whereas inflammation-induced repopulation of LCs appears to be dependent on Csf-1, once inflammation is resolved, LC survival is again IL-34-dependent. In contrast, microglia and their yolk sac precursors develop independently of IL-34 but rely on it for their maintenance in the adult brain.

Figure 1. Csf-1r Signaling Is Critical for LC Homeostasis in Adult Skin

(A–F) Plots show the percentage and total cell number (±SEM) of LCs among CD45⁺ epidermal cells in adult mice (A–C, E and F) or neonates on postnatal day (p)1, p5, and p10 (D). Graphs represent data of pooled experiments. *p < 0.05, **p < 0.01, ***p < 0.001 (Student's t test, unpaired). (A) Csf1r^−/− and control mice (Csf1r^+/− or Csf1r^+/+) (n ≥ 3). (B) Csf1^op/op and control mice (Csf1^op/+ or Csf1^+/+) (n = 5). (C) Langerin (Lang) Cre⁺ × Csf1r^fl/fl or control mice (LangCre⁺ × Csf1r^fl/+ or LangCre⁻ × Csf1r^fl/fl) (n = 4). (D) LangCre⁺ × Csf1r^fl/fl or LangCre⁻ × Csf1r^fl/fl neonates. On p1, R1 represents LC precursors (Langerin⁻) and R2 represents LCs (Langerin⁺). p2: n = 6, p5: n ≥ × p10: n ≥ 6. (E) Anti-(α)Csf-1r treated or isotype treated WT mice (n = 6). One representative of at least three individual experiments is shown. (F) Rosa26-CreER^T2+ (CreER^T2+) × Csf1r^fl/fl and CreER^T2− × Csf1r^fl/fl mice were treated with tamoxifen and the skin was analyzed 7 days post treatment (n = 2). One representative of two individual experiments is shown.

Figure 2. IL-34 Is Expressed by Keratinocytes in the Epidermis and Controls the Development of LCs

(A) Quantitative RT-PCR analysis of Il34 mRNA expression of liver, brain, lung, spleen, and skin in WT mice. Data were normalized to the expression of RNA polymerase 2 (n = 2). Shown is one representative of three individual experiments. (B) Quantitative RT-PCR analysis of Il34 and Csf1 mRNA expression of WT epidermis and dermis. Data were normalized to the expression of RNA polymerase 2 (n = 2). One representative of two individual experiments is shown. (C) X-gal staining (blue) of skin sections from Il34^LacZ/+ and Il34^+/+ mice (scale bar represents 40 mm). HF, Hair follicle. (D) Skin sections of Il34^+/+ mice and Il34^LacZ/+ mice were stained with X-gal (blue) and mAb against K14 (red) (scale bar represents 20 μm). (E) Plots show the percentage and total cell number (±SEM) of LCs (CD11b⁺MHCII⁺) among CD45⁺ epidermal cells in adult Il34^LacZ/LacZ and Il34^LacZ/+ mice (n = 5). Pooled data of several individual experiments. *p < 0.05, **p < 0.01, ***p < 0.001 (Student's t test, unpaired). (F) Images display anti-MHCII (LCs) and anti-CD3 (DETCs) staining on ear-derived epidermal sheets from adult Il34^LacZ/LacZ and Il34^LacZ/+ mice (scale bar represents 50 μm).

Figure 3. IL-34 Contributes to the Homeostasis of Adult Microglia

(A) X-gal staining (blue) of brain sections from Il34^LacZ/+ and Il34^+/+ mice (scale bar represents 200 μm). (B) Brain sections of Il34^LacZ/+ mice were stained with X-gal (blue) and NeuN (brown) (scale bar represents 50 μm). (C) Plots show the percentage and total cell number (±SEM) of microglia (CD45⁺CD11b⁺F4/80⁺) in adult Il34^LacZ/LacZ and Il34^LacZ/+ mice (n = 12). Pooled data of at least four different experiments are shown. *p < 0.05, **p < 0.01, ***p < 0.001 (Student's t test, unpaired). (D and E) Brain sections of Il34^LacZ/LacZ and Il34^LacZ/+ adult mice were stained with anti-Iba-1 (scale bar represents 200 μm). (E) Mean number of microglia (Iba-1⁺ cells) per mm² (n = 2).

Figure 4. IL-34 Controls the Development of LCs during Embryogenesis

(A and B) Quantitative RT-PCR analysis of Il34 and Csf1 mRNA expression in the developing skin (A) and the developing brain (B) at different time points during embryogenesis and in newborn (NB) and adult WT mice. Total skin is shown from E12.5–E16.5, and epidermis and dermis are shown after E17.5. Data were normalized to the expression of HPRT1 (n = 3). Shown are pooled data of two individual experiments. (C) Plots show the percentage (among CD45⁺ cells) and total cell number (±SEM) of primitive macrophages (CD11b⁺F4/80⁺) in the yolk sac at E10.5 (n ≥ 3). (D) Plots show the percentage and total cell number (±SEM) (among CD45⁺ cells) of microglia precursors (CD11b⁺F4/80⁺) in the brain at E10.5, E14.5, and E17.5 and of LC precursors (CD11b⁺F4/80⁺) in the skin (limb buds) at E10.5 and E14.5 and in the epidermis at E17.5 in Il34^LacZ/LacZ and Il34^LacZ/+ embryos (for E10.5: n ≥ 3, for E14.5: n ≥ 4 and for E17.5: n = 5). Data are pooled of 1–2 different experiments. (E) Plots show the percentage and total cell number (±SEM) of epidermal LC precursors and microglia (both CD11b⁺F4/80⁺) among CD45⁺ cells in newborn Il34^LacZ/LacZ and Il34^LacZ/+ mice (LC precursors: n = 2, microglia: n = 6). One representative of three individual experiments is shown. (C–E) *p < 0.05, **p < 0.01, ***p < 0.001 (Student's t test, unpaired).

Figure 5. IL-34 Is Not Critical for the Repopulation of LCs in Inflammation but for Their Maintenance

(A and B) Il34^LacZ/LacZ and Il34^LacZ/+ mice were exposed to UV light. (A) Plots show the percentage and total cell number (±SEM) of monocytes (CD11b⁺Ly6C⁺) among ear skin cell suspensions (CD45⁺ Ly6G⁻) and of LCs (Epcam⁺Langerin⁺) among CD45⁺ epidermal cells 7 days (d) after UV treatment or in untreated control mice (n = 4 for UV-treated groups and n = 3 for untreated control mice). Data are pooled from two individual experiments. *p < 0.05, **p < 0.01, ***p < 0.001 (Student's t test, unpaired). (B) Plots show the percentage and total cell number (±SEM) of LCs (Epcam⁺Langerin⁺) among CD45⁺ epidermal cells on day 21 (n = 7), day 45 (n = 7) and day 90 (n = 5) after UV exposure. Pooled data of two individual experiments. *p < 0.05, **p < 0.01, ***p < 0.001 (Student's t test, unpaired). (C) Quantitative RT-PCR analysis of Il34 and Csf1 mRNA levels in the epidermis of UV exposed WT mice at different time points post UV exposure or in naive WT mice (d 0). Data were normalized to the expression of RNA polymerase 2 (n = 3 for d 7, d 21 and d 42, n = 2 for d 0). (D and E) Il34^LacZ/LacZ and Il34^LacZ/+ mice were treated with Calcipotriol (CP) for 2 weeks and epidermal cells were analyzed 21 days post treatment. (D) Plots show the percentage of LCs among CD45⁺ epidermal cells. Graph displays the total cell number (±SEM) of LCs of CP-treated mice (n = 6). Pooled data of two individual experiments are shown. (E) Images depict anti-MHCII staining on ear-derived epidermal sheets of Il34^LacZ/LacZ and Il34^LacZ/+ mice 21 days after CP treatment (scale bar represents 100 μm). (F and G) Ears of Il34^LacZ/LacZ and Il34^LacZ/+ mice were treated with Aldara cream for 9 days (n = 4). Shown is one representative of two individual experiments. (F) Kinetics of Aldara-induced skin inflammation measured by increase in ear thickness. (G) Plots show the percentage and total cell number (±SEM) of LCs among CD45⁺ epidermal cells of Aldara treated ears on day 12 (n = 7). *p < 0.05, **p < 0.01, ***p < 0.001 (Student's t test, unpaired).

Figure 6. IL-34 Is Expressed in Human Skin and Brain

(A) Quantitative RT-PCR analysis of Il34 and Csf1 mRNA expression of human epidermis and dermis. Data were normalized to the expression of GAPDH. One of two independent experiments is shown. (B) Human skin sections were stained with mAbs against K14 and IL-34 (or isotype, mouse IgG1) and counterstained with DAPI (scale bar represents 50 μm). (C) Human brain sections of the temporal lobe were stained with mAbs against IL-34 (or isotype, mouse IgG1) and NeuN and counterstained with Dapi (scale bar left represents 20 μm, scale bar right represents 5 μm).