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TLR7-mediated skin inflammation remotely triggers chemokine expression and leukocyte accumulation in the brain.

McColl A, Thomson CA, Nerurkar L, Graham GJ, Cavanagh J - J Neuroinflammation (2016)

Bottom Line: Here we use a well-characterised animal model of psoriasis-like skin inflammation-imiquimod-to investigate the effects of tissue-specific peripheral inflammation on the brain.We found that a number of genes are upregulated in the brain following treatment, amongst which is a subset of inflammatory chemokines (CCL3, CCL5, CCL9, CXCL10, CXCL13, CXCL16 and CCR5).Strikingly, this model induced the infiltration of a number of immune cell subsets into the brain parenchyma, including T cells, NK cells and myeloid cells, along with a reduction in neurogenesis and a suppression of burrowing activity.

View Article: PubMed Central - PubMed

Affiliation: Institute of Infection, Immunity & Inflammation, College of Medical & Veterinary Life Sciences, University of Glasgow, 120 University Place, Glasgow, G12 8TA, UK.

ABSTRACT

Background: The relationship between the brain and the immune system has become increasingly topical as, although it is immune-specialised, the CNS is not free from the influences of the immune system. Recent data indicate that peripheral immune stimulation can significantly affect the CNS. But the mechanisms underpinning this relationship remain unclear. The standard approach to understanding this relationship has relied on systemic immune activation using bacterial components, finding that immune mediators, such as cytokines, can have a significant effect on brain function and behaviour. More rarely have studies used disease models that are representative of human disorders.

Methods: Here we use a well-characterised animal model of psoriasis-like skin inflammation-imiquimod-to investigate the effects of tissue-specific peripheral inflammation on the brain. We used full genome array, flow cytometry analysis of immune cell infiltration, doublecortin staining for neural precursor cells and a behavioural read-out exploiting natural burrowing behaviour.

Results: We found that a number of genes are upregulated in the brain following treatment, amongst which is a subset of inflammatory chemokines (CCL3, CCL5, CCL9, CXCL10, CXCL13, CXCL16 and CCR5). Strikingly, this model induced the infiltration of a number of immune cell subsets into the brain parenchyma, including T cells, NK cells and myeloid cells, along with a reduction in neurogenesis and a suppression of burrowing activity.

Conclusions: These findings demonstrate that cutaneous, peripheral immune stimulation is associated with significant leukocyte infiltration into the brain and suggest that chemokines may be amongst the key mediators driving this response.

No MeSH data available.


Related in: MedlinePlus

Topically applied imiquimod induces a transcriptional brain response but does not induce psoriasis-like skin inflammation. Mice were treated with 80 mg of control cream containing 5 % imiquimod or control cream alone every 24 h for five consecutive days. a Mouse weights were recorded after each treatment and are shown as percentage of initial weight, which was considered 100 %. Mice were euthanised 24 h after the final application, and b areas of treated skin were sectioned to 5 μm, stained with H&E and visualised at ×100 magnification using a light microscope. Epidermis [1], dermis [2], muscle [3] and hair follicles [4] are shown. Scale bar = 100 μm. PBL and perfused brains were collected, and c QRT-PCR analysis of the target chemokine genes was performed. n = 5 mice per group. Significance was measured using two-way ANOVA with Bonferroni multiple comparison post-tests. Individual unpaired student’s t tests were used to compare control vs treated within each tissue (c). *** (###) = p ≤ 0.001 ** (##) = p ≤ 0.01 *(#) = p ≤ 0.05
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Fig3: Topically applied imiquimod induces a transcriptional brain response but does not induce psoriasis-like skin inflammation. Mice were treated with 80 mg of control cream containing 5 % imiquimod or control cream alone every 24 h for five consecutive days. a Mouse weights were recorded after each treatment and are shown as percentage of initial weight, which was considered 100 %. Mice were euthanised 24 h after the final application, and b areas of treated skin were sectioned to 5 μm, stained with H&E and visualised at ×100 magnification using a light microscope. Epidermis [1], dermis [2], muscle [3] and hair follicles [4] are shown. Scale bar = 100 μm. PBL and perfused brains were collected, and c QRT-PCR analysis of the target chemokine genes was performed. n = 5 mice per group. Significance was measured using two-way ANOVA with Bonferroni multiple comparison post-tests. Individual unpaired student’s t tests were used to compare control vs treated within each tissue (c). *** (###) = p ≤ 0.001 ** (##) = p ≤ 0.01 *(#) = p ≤ 0.05

Mentions: There are reports that, along with the active TLR agonist, imiquimod, Aldara cream contains another active component: isostearic acid. This component of the vehicle is thought to activate the inflammasome, induce keratinocyte cell death and stimulate the production of certain pro-inflammatory cytokines [25]. To further explore the mechanisms by which the brain-specific chemokine response is induced following Aldara treatment, and to discriminate between the activities of the isostearic acid and IMQ, soluble IMQ was dissolved in isostearic acid-free aqueous control cream and applied topically to the dorsal skin. As before, mice treated with soluble IMQ lost a significant amount of weight after the first two treatments (Fig. 3a). Their weights remained significantly lower than the control group throughout the model. In contrast to Aldara treatment, topical soluble IMQ treatment was not associated with significant skin inflammation. The skin sections, examined after five applications of IMQ cream, do not show any overt signs of epidermal hyperplasia, keratosis or erythema (Fig. 3b). These data suggest that this aspect of the response may be independent of TLR stimulation and reliant instead on the actions of isostearic acid. Nonetheless, all chemokine/chemokine receptor target genes were significantly elevated in the brain following topical IMQ treatment, independently of alterations in PBL (Fig. 3c). Of note, Ccl3, Ccl5 and Ccr5 presented with a fold-change induction upwards of 1000-fold. In this model, all genes encoding chemokine ligands were downregulated in the PBL of treated mice compared with control mice. With regard to all genes, the fold-change induction in the brain following treatment was significantly higher than the fold change in PBL. These results demonstrate that, although the different active components of Aldara may be responsible for driving different aspects of the response, the actions of TLR ligand IMQ alone are sufficient to induce the remote differential chemokine response in the brain.Fig. 3


TLR7-mediated skin inflammation remotely triggers chemokine expression and leukocyte accumulation in the brain.

McColl A, Thomson CA, Nerurkar L, Graham GJ, Cavanagh J - J Neuroinflammation (2016)

Topically applied imiquimod induces a transcriptional brain response but does not induce psoriasis-like skin inflammation. Mice were treated with 80 mg of control cream containing 5 % imiquimod or control cream alone every 24 h for five consecutive days. a Mouse weights were recorded after each treatment and are shown as percentage of initial weight, which was considered 100 %. Mice were euthanised 24 h after the final application, and b areas of treated skin were sectioned to 5 μm, stained with H&E and visualised at ×100 magnification using a light microscope. Epidermis [1], dermis [2], muscle [3] and hair follicles [4] are shown. Scale bar = 100 μm. PBL and perfused brains were collected, and c QRT-PCR analysis of the target chemokine genes was performed. n = 5 mice per group. Significance was measured using two-way ANOVA with Bonferroni multiple comparison post-tests. Individual unpaired student’s t tests were used to compare control vs treated within each tissue (c). *** (###) = p ≤ 0.001 ** (##) = p ≤ 0.01 *(#) = p ≤ 0.05
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Fig3: Topically applied imiquimod induces a transcriptional brain response but does not induce psoriasis-like skin inflammation. Mice were treated with 80 mg of control cream containing 5 % imiquimod or control cream alone every 24 h for five consecutive days. a Mouse weights were recorded after each treatment and are shown as percentage of initial weight, which was considered 100 %. Mice were euthanised 24 h after the final application, and b areas of treated skin were sectioned to 5 μm, stained with H&E and visualised at ×100 magnification using a light microscope. Epidermis [1], dermis [2], muscle [3] and hair follicles [4] are shown. Scale bar = 100 μm. PBL and perfused brains were collected, and c QRT-PCR analysis of the target chemokine genes was performed. n = 5 mice per group. Significance was measured using two-way ANOVA with Bonferroni multiple comparison post-tests. Individual unpaired student’s t tests were used to compare control vs treated within each tissue (c). *** (###) = p ≤ 0.001 ** (##) = p ≤ 0.01 *(#) = p ≤ 0.05
Mentions: There are reports that, along with the active TLR agonist, imiquimod, Aldara cream contains another active component: isostearic acid. This component of the vehicle is thought to activate the inflammasome, induce keratinocyte cell death and stimulate the production of certain pro-inflammatory cytokines [25]. To further explore the mechanisms by which the brain-specific chemokine response is induced following Aldara treatment, and to discriminate between the activities of the isostearic acid and IMQ, soluble IMQ was dissolved in isostearic acid-free aqueous control cream and applied topically to the dorsal skin. As before, mice treated with soluble IMQ lost a significant amount of weight after the first two treatments (Fig. 3a). Their weights remained significantly lower than the control group throughout the model. In contrast to Aldara treatment, topical soluble IMQ treatment was not associated with significant skin inflammation. The skin sections, examined after five applications of IMQ cream, do not show any overt signs of epidermal hyperplasia, keratosis or erythema (Fig. 3b). These data suggest that this aspect of the response may be independent of TLR stimulation and reliant instead on the actions of isostearic acid. Nonetheless, all chemokine/chemokine receptor target genes were significantly elevated in the brain following topical IMQ treatment, independently of alterations in PBL (Fig. 3c). Of note, Ccl3, Ccl5 and Ccr5 presented with a fold-change induction upwards of 1000-fold. In this model, all genes encoding chemokine ligands were downregulated in the PBL of treated mice compared with control mice. With regard to all genes, the fold-change induction in the brain following treatment was significantly higher than the fold change in PBL. These results demonstrate that, although the different active components of Aldara may be responsible for driving different aspects of the response, the actions of TLR ligand IMQ alone are sufficient to induce the remote differential chemokine response in the brain.Fig. 3

Bottom Line: Here we use a well-characterised animal model of psoriasis-like skin inflammation-imiquimod-to investigate the effects of tissue-specific peripheral inflammation on the brain.We found that a number of genes are upregulated in the brain following treatment, amongst which is a subset of inflammatory chemokines (CCL3, CCL5, CCL9, CXCL10, CXCL13, CXCL16 and CCR5).Strikingly, this model induced the infiltration of a number of immune cell subsets into the brain parenchyma, including T cells, NK cells and myeloid cells, along with a reduction in neurogenesis and a suppression of burrowing activity.

View Article: PubMed Central - PubMed

Affiliation: Institute of Infection, Immunity & Inflammation, College of Medical & Veterinary Life Sciences, University of Glasgow, 120 University Place, Glasgow, G12 8TA, UK.

ABSTRACT

Background: The relationship between the brain and the immune system has become increasingly topical as, although it is immune-specialised, the CNS is not free from the influences of the immune system. Recent data indicate that peripheral immune stimulation can significantly affect the CNS. But the mechanisms underpinning this relationship remain unclear. The standard approach to understanding this relationship has relied on systemic immune activation using bacterial components, finding that immune mediators, such as cytokines, can have a significant effect on brain function and behaviour. More rarely have studies used disease models that are representative of human disorders.

Methods: Here we use a well-characterised animal model of psoriasis-like skin inflammation-imiquimod-to investigate the effects of tissue-specific peripheral inflammation on the brain. We used full genome array, flow cytometry analysis of immune cell infiltration, doublecortin staining for neural precursor cells and a behavioural read-out exploiting natural burrowing behaviour.

Results: We found that a number of genes are upregulated in the brain following treatment, amongst which is a subset of inflammatory chemokines (CCL3, CCL5, CCL9, CXCL10, CXCL13, CXCL16 and CCR5). Strikingly, this model induced the infiltration of a number of immune cell subsets into the brain parenchyma, including T cells, NK cells and myeloid cells, along with a reduction in neurogenesis and a suppression of burrowing activity.

Conclusions: These findings demonstrate that cutaneous, peripheral immune stimulation is associated with significant leukocyte infiltration into the brain and suggest that chemokines may be amongst the key mediators driving this response.

No MeSH data available.


Related in: MedlinePlus