Alexandra C. Kendall

Background: Dandruff features epidermal scaling but lacks overt signs of inflammation, such as erythema. Characterisation of its inflammatory profile may improve understanding of the underlying inflammatory mechanisms and therefore indicate alternative approaches for treatment. Methods: Full‐thickness scalp biopsies were sampled from healthy volunteers and those with dandruff. Immunohistochemistry for Ki67 and keratin 16 was used to examine keratinocyte proliferation and differentiation. Mass spectrometry lipidomics was applied to profile scalp skin barrier lipids and lipid mediators. The presence of immune cells and their cytokine production was examined by flow cytometry. Cytokine and chemokine levels in scalp skin were analysed by cytometric bead array. Results: Increased keratinocyte proliferation and aberrant keratinocyte differentiation were observed in dandruff relative to healthy scalp; however, this epidermal dysregulation was not mirrored by alterations to barrier lipid levels. Decreased numbers of innate lymphoid cells and reduced expression of CD86 on antigen‐presenting cells, were the few cellular alterations observed in dandruff scalp, suggesting dandruff only subtly changes major skin immune cell populations. Despite this, increased proportions of interleukin‐17‐producing T‐cells and increased C‐C Motif Chemokine Ligand 17 (CCL17) levels were observed in dandruff scalp, indicative of a low‐grade, mixed Type‐2/Type17 inflammatory response. Increased levels of N‐acyl ethanolamines, including the endocannabinoid anandamide, as well as linoleoyl‐ and oleoyl‐ethanolamine, were also observed in dandruff scalp which could be responsible for suppressing inflammation to sub‐clinical levels. Conclusion: These findings indicate that, contrary to being a mere flaking disorder, dandruff involves an altered inflammatory environment consistent with low‐grade inflammation.

Eicosanoids are a family of bioactive lipids, including derivatives of the ubiquitous fatty acid arachidonic acid (AA). The intimate involvement of eicosanoids in inflammation motivates the development of predictive in silico models for a systems-level exploration of disease mechanisms, drug development and replacement of animal models. Using an ensemble modelling strategy, we developed a computational model of the AA cascade. This approach allows the visualisation of plausible and thermodynamically feasible predictions, overcoming the limitations of fixed-parameter modelling. A quality scoring method was developed to quantify the accuracy of ensemble predictions relative to experimental data, measuring the overall uncertainty of the process. Monte Carlo ensemble modelling was used to quantify the prediction confidence levels. Model applicability was demonstrated using mass spectrometry mediator lipidomics to measure eicosanoids produced by HaCaT epidermal keratinocytes and 46BR.1N dermal fibroblasts, treated with stimuli (calcium ionophore A23187), (ultraviolet radiation, adenosine triphosphate) and a cyclooxygenase inhibitor (indomethacin). Experimentation and predictions were in good qualitative agreement, demonstrating the ability of the model to be adapted to cell types exhibiting differences in AA release and enzyme concentration profiles. The quantitative agreement between experimental and predicted outputs could be improved by expanding network topology to include additional reactions. Overall, our approach generated an adaptable, tuneable ensemble model of the AA cascade that can be tailored to represent different cell types and demonstrated that the integration of in silico and in vitro methods can facilitate a greater understanding of complex biological networks such as the AA cascade.

Resolving inflammation is thought to return the affected tissue back to homoeostasis but recent evidence supports a non-linear model of resolution involving a phase of prolonged immune activity. Here we show that within days following resolution of Streptococcus pneumoniae-triggered lung inflammation, there is an influx of antigen specific lymphocytes with a memory and tissue-resident phenotype as well as macrophages bearing alveolar or interstitial phenotype. The transcriptome of these macrophages shows enrichment of genes associated with prostaglandin biosynthesis and genes that drive T cell chemotaxis and differentiation. Therapeutic depletion of post-resolution macrophages, inhibition of prostaglandin E2 (PGE₂) synthesis or treatment with an EP4 antagonist, MF498, reduce numbers of lung CD4⁺/CD44⁺/CD62L⁺ and CD4⁺/CD44⁺/CD62L^-/CD27⁺ T cells as well as their expression of the α-integrin, CD103. The T cells fail to reappear and reactivate upon secondary challenge for up to six weeks following primary infection. Concomitantly, EP4 antagonism through MF498 causes accumulation of lung macrophages and marked tissue fibrosis. Our study thus shows that PGE₂ signalling, predominantly via EP4, plays an important role during the second wave of immune activity following resolution of inflammation. This secondary immune activation drives local tissue-resident T cell development while limiting tissue injury

Our work highlights the importance of multi-omics to comprehend the multi-molecular composition of the nanoparticle biomolecule corona and paves the way for the corona-enabled enrichment of blood-circulating disease-specific lipids.

Resolving inflammation is thought to return the affected tissue back to homeostasis. However, we show that within days following resolution of Streptococcus pneumonia-triggered lung inflammation there is an influx of antigen specific lymphocytes with a memory and tissue-resident phenotype as well as macrophages bearing an alveolar or interstitial phenotype. Transcriptomic analysis revealed that macrophages are enriched with genes that drive T cell chemotaxis and differentiation as well as prostaglandin biosynthesis. Therapeutic depletion of post-resolution macrophages or inhibition of PGE2/EP4 signalling reduced numbers of lung CD4+/CD44+/CD62L+ and CD4+/CD44+/CD62L-/CD27+ T cells as well as their expression of the alpha-integrin, CD103. This intervention resulted in a failure of these cells to reappear and reactivate upon secondary challenge up to six weeks following primary infection. Concomitantly, EP4 inhibition caused accumulation of lung macrophages and marked tissue fibrosis. Hence, resolving inflammation triggers a second wave of immune activity controlled, in part, by PGE2, which through EP4, drives local tissue-resident T cell development on the one hand, whilst limiting tissue injury on the other.

Linked Article: Danby et al. Br J Dermatol 2022; 186:875–886.

The cause of chronic inflammation in ulcerative colitis (UC) is incompletely understood. Here we tested the hypothesis that an excessive acute inflammatory response to bacteria contributes to the pathogenesis. Acute inflammatory responses were provoked in vivo in UC patients and healthy controls by intradermal inoculation with bacteria. Vascular responses were quantified by laser Doppler. Inflammatory exudates were recovered in superimposed suction blisters and cells measured by polychromatic flow cytometry, cytokines by multiplex array, and inflammatory lipids by mass spectrometry. Vascular responses in UC patients were heightened at 24h after bacterial injection (p=0·03), and remained abnormally high at 48h (p=0·0005) and this amplified response was seen in UC with Gram-positive as well as Gram-negative organisms (p=0·01). The cellular infiltrate over the injection site, composed largely of neutrophils at 4 hours a was greater in UC (p=0·002). At 48h, the increased numbers of cells in UC were composed of neutrophils (p=0·001) and CD4 lymphocytes (p=0·001). The exaggerated inflammation in UC was not a cytokine-driven phenomenon. Exaggerated onset was normalised in patients taking 5-aminosalicylates, accompanied by increased concentrations of hydroxy fatty acids 9-oxo-octadecadienoic acid (OxoODE; p=0·05) and 13-OxoODE (p=0·01) in resolving exudates. In vitro, these compounds suppressed macrophage inflammatory cytokine secretion through PPARγ (p<0·0001). Conversely, 5-aminosalicylates did not inhibit early inflammatory reactions in control participants. Acute inflammatory responses to bacteria in UC are both overly exuberant and slow to resolve. Neutrophils accumulate in excess and persist, in keeping with the pathological appearances of disease flares. These studies also provide new insight into the mechanism of 5-aminosalicylate (5ASA) drugs, which act as pro-resolution rather than indiscriminate anti-inflammatory agents by promoting formation of immunomodulatory hydroxy lipids. While production of these lipids is not defective as part of the underlying disease process, this identifies a novel mechanism of drug action harnessing pro-resolution pathways.Summary: Wysoczanski and colleagues demonstrate that the inflammatory response to injected bacteria is exaggerated and prolonged in ulcerative colitis. This disordered inflammation appears to be associated with increased secretion of PGE2. 5-aminosalicylate drugs, which are used to treat this condition, normalize inflammation and PGE2 secretion, and appear to work through PPARγ

Nutritional supplementation with fish oil or ω‐3 (n‐3) polyunsaturated fatty acids (PUFAs) has potential benefits for skin inflammation. Although the differential metabolism of the main n‐3PUFA eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) could lead to distinct activities, there are no clinical studies comparing their relative efficacy in human skin. Following a 10‐wk oral supplementation of healthy volunteers and using mass spectrometry‐based lipidomics, we found that n‐3PUFA mainly affected the epidermal mediator lipidome. EPA was more efficient than DHA in reducing production of arachidonic acid–derived lipids, and both n‐3PUFA lowered N ‐acyl ethanolamines. In UV radiation–challenged skin (3 times the minimum erythemal dose), EPA attenuated the production of proinflammatory lipids, whereas DHA abrogated the migration of Langerhans cells, as assessed by immunohistochemistry. Interestingly, n‐3PUFA increased the infiltration of CD4 ⁺ and CD8 ⁺ T cells but did not alter the erythemal response, either the sunburn threshold or the resolution of erythema, as assessed by spectrophotometric hemoglobin index readings. As EPA and DHA differentially impact cutaneous inflammation through changes in the network of epidermal lipids and dendritic and infiltrating immune cells, they should be considered separately when designing interventions for cutaneous disease.—Kendall, A. C., Pilkington, S. M., Murphy, S. A., Del Carratore, F., Sunarwidhi, A. L., Kiezel‐Tsugunova, M., Urquhart, P., Watson, R. E. B., Breitling, R., Rhodes, L. E., Nicolaou, A. Dynamics of the human skin mediator lipidome in response to dietary ω‐3 fatty acid supplementation. FASEB J. 33, 13014–13027 (2019). www.fasebj.org