Demodex Mites Modulate Sebocyte Immune Reaction: Possible Role in the Pathogenesis of Rosacea

N. Lacey; A. Russell-Hallinan; C.C. Zouboulis; F.C. Powell


The British Journal of Dermatology. 2018;179(2):420-430. 

In This Article

Abstract and Introduction


Background: Rosacea is a common facial skin disorder mainly affecting middle-aged adults. Its aetiology is unknown and pathogenesis uncertain. Activation of the host innate immune response has been identified as an important factor. The Demodex mite population in the skin of rosacea patients is significantly higher than in patients with normal skin, suggesting that they may be of aetiological importance in this disorder.

Objectives: To determine the potential of Demodex mites to interact with the host immune system.

Methods: Live Demodex mites were extracted from normal facial skin of control subjects and used in cell stimulation experiments with the immortalized SZ95 sebocyte line. Time- and mite-dose-dependent experiments were performed. Direct effects of Demodex and effects of the medium in which Demodex had been cultured were evaluated on the Toll-like receptor (TLR) signalling pathway on both a gene and protein expression level.

Results: Mites modulated TLR signalling events on both mRNA and protein levels in SZ95 sebocytes. An initial trend towards downmodulation of genes in this pathway was observed. A subsequent switch to positive gene upregulation was recorded after 48 h of coculture. Demodex secreted bioactive molecules that affected TLR2 receptor expression by sebocytes. High numbers of Demodex induced proinflammatory cytokine secretion, whereas lower numbers did not.

Conclusions: Demodex mites have the capacity to modulate the TLR signalling pathway of an immortalized human sebocyte line. Mites have the capacity to secrete bioactive molecules that affect the immune reactivity of sebocytes. Increasing mite numbers influenced interleukin-8 secretion by these cells.


Rosacea is a chronic inflammatory cutaneous disorder that predominantly affects the centrofacial skin, in up to 15% of the adult population.[1–3] Recent publications have suggested that this skin disorder, in addition to having a negative psychological impact,[4] may have implications of systemic disease for the patient with rosacea.[5,6]

The cause of rosacea is unknown. Multiple immune-mediated components – including an increase in Toll-like receptor (TLR)2 expression in the epidermis, increased protease activity and abnormal proinflammatory cathelicidin peptides – augment the innate immune response in the skin of patients with rosacea.[7–9] What activates this inflammatory cascade in the skin of patients with rosacea is unclear.

Rosacea has been classified into four major subtypes.[10] One of these is papulopustular rosacea, which responds well to systemic or topical antibiotic treatment.[11] Improved clinical outcome following antibiotic treatment correlates with reduced cathelicidin and protease activity,[12] supporting the importance of these immune-mediated inflammatory components for the pathogenesis of this disease.

Ivermectin, a medication used to treat animal Demodex infestation in veterinary medicine, is effective in clearing the inflamed facial lesions of patients with papulopustular rosacea[13] and has been shown to inhibit the production of inflammatory cytokines.[14]

Demodex mites are part of the normal adult human facial skin flora residing within the follicular canals of the sebaceous follicles. Under normal circumstances these complex and potentially antigenic organisms (Figure 1a, b) do not evoke a host immune response. Their biological function as a component of the skin microflora is unknown and their pathogenic potential is unclear.[15] The Demodex mite population is markedly increased in patients with rosacea compared with healthy controls.[16–19] The reason for this is unknown, and the significance of increased mite numbers for the activation of the innate immune response and subsequent inflammatory reaction in the skin of patients with rosacea has not been investigated.[20]

Figure 1.

Identifying and extracting live Demodex mites from standardized skin-surface biopsies. (a) A representative image of an adult Demodex folliculorum mite in a skin-surface biopsy taken on a stereomicroscope. (b) A group of four adult D. folliculorum mites in a follicular cast taken on a compound microscope. (c) An example of our standardized skin-surface biopsy slides, showing the letter and number markers to aid sample orientation in the 1-cm2 nine-piece grid. The image was taken using a stereomicroscope (Olympus SZX16). (d) Image of a skin-surface biopsy using a stereomicroscope (original magnification × 50). Samples were not clarified with oil, and Demodex mites present in the samples are indicated with red arrows. (e) Image of two Demodex mites on the tip of Moria fine forceps after manipulation from a skin-surface biopsy, indicated with a red arrow. (f) Groups of Demodex mites indicated by red arrows in a cell-culture system.

We extracted living Demodex mites from the skin of human patients with normal skin and maintained these mites alive ex vivo.[21] We then investigated whether the mites have the potential to induce immune responses when cocultured with living human cells in vitro.

The aims of this study were to answer the following questions: (i) Would live human Demodex mites cocultured with the immortalized human sebocyte cell line SZ95 have the capacity to influence the immune response of these cells? (ii) If mites affected the immune reactivity of these sebocytes was this by secretion of bioactive molecules? (iii) If mites had the capacity to influence the immune reaction of sebocytes, would the number of mites cocultured with these cells influence that response?