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Parasite like functions of the house dust mite

The parasitic mange-like attributes of the scavenging house dust mite (HDM) were ‘unknowingly’ defined by scientists in 1998, but it wasn’t until 2013 that the link was made by doctors looking at the evolutionary profile of the HDM.[1,2]  They discovered that the mite made a shift from a mange or ectoparasite mite (living under the fur of animals) to a free-living, independent scavenger about 80 million years ago.

Before this life-style shift the mite survived by placing digestive enzymes in its droppings to irritate the skin of the host causing itching and weeping sores which then fed the mite. A whole HDM colony could live under crusted wounds made by the enzymes until the animal died, signalling death to the mite and its colony. Not a very good prospect, so a survival shift was made by this clever mite from mange-like parasite to scavenger. An example of this type of mange mite living today is the HDM’s close relative, the sheep scab mite. This well-known pest causes acute allergic dermatitis in goats and sheep and is a serious veterinary problem for farmers worldwide.[3,4] 

The house dust mite was now living in the nesting sites of birds or rats scavenging on whatever food was available, such as discarded skin scales, fungi, bacteria, bits of food, dead insects or pollen.[5]  In order to digest such a wide-ranging diet the mite relied upon several specialized gut-derived enzymes to do the job, many are placed in its droppings to break down left-over food for later nourishment. Like rabbits, HDM consider their droppings as food. Travelling from nest to nest was easy for the mite because when the animals moved, they took their mites with them to new warm damp, nesting conditions. For the host animal the disadvantage was that the unwelcomed mite, its colony and droppings, caused irritation and/or sores.[6] 

HDM Dermatophagoides pteronyssinus - Courtesy of the LSHTM

Recently one of the mite’s digestive enzymes (which has biochemical properties) was investigated in detail and found to reflect the mange-like past of the house dust mite.[7,8] The enzyme named Der p1, is made within the mite’s gut and is similar to one made by the sheep mange mite. Both enzymes (sheep and mite) are compared to papain, a meat tenderiser used in the food industry.[4,9] 

Although Der p1 is a single enzyme it has many tasks. It is first out of the mite’s dropping as it dissolves in moisture. Its first task is to release the activity of companion enzymes or products held in the dropping. The second task of Der p1 is to go to work breaking down the protective surface of defence cells or the ‘glue’ that binds delicate cells together. This activity offers bystanders (other mite products, dirt, or bacteria) an entry into the body.[10]  This activity is selective and can raise immune signals that are associated with allergy, or an allergic reaction to the mite.[1] 

Activated enzymes/allergens can deepen a reaction by attracting non-allergic defence mechanisms that can harm nearby healthy cells through a cascade of unwanted reactions.[11]  These non-allergic responses are associated with lung diseases such as non-allergic asthma and COPD.[12]  Fortunately, nature has balances that can dampen down this harm. Der p1, however, can interfere with this dampening down process, thus allowing its full damaging force.[13,14,15]  This was Der p1’s third task, to keep a harmful immune reaction rolling on.

Reviewing three tasks of Der p1

  1. Activating companion enzymes/allergens in the mite’s dropping
  2. Damaging and breaching delicate defence cells
  3. Dampening down mechanisms designed to reduce non-allergic harm

Quote: “From the viewpoint of the organisms that actually produce allergens, these proteins exist only to function as enzymes and structural or regulatory proteins. The exception is the allergen of parasites that may modulate, and thus subvert, host protective immune effector mechanisms.

Allergy,1998; 53 (Suppl 48): p60-63

Reviewing the activity of enzymes produced by HDM and the statement above, a question should be asked. Is the house dust mite a scavenger with harmful parasite-like functions, and is this why it is a cause of disease worldwide?

  1. ‘Mite allergens: significance of enzymatic activity, Hewitt CRA et al, Allergy,1998; 53 (Suppl 48): p60-63
  2. ‘Is Permanent Parasitism Reversible? Critical Evidence from Early Evolution of House Dust Mites’ Pavel B Klimov, Barry O’Connor, Oxford University Press, on behalf of the Society of Systemic Biologists (2013): Feb. 15, 2013
  3. ’Sheep Scab, Psoroptes ovis infestation’, ‘The Center for Food Security and Public Health’ Iowa State University, 2009
  4. ‘The effect of Psoroptes ovis infestation on ovine epidermal barrier function, Stoeckli et al. ‘Veterinary Research’, 2013: 44 Article 11 “the surface-living mite deposits excretory and secretory products containing homologues of known house dust mite (HDM) allergens and enzymes on to the stratum corneum.” [Der p1 & Der p3]
  5. ‘Dust Mites’, Dr Matthew J Colloff, 2009, Springer, ISBN-13: 978-9048122233
  6. ‘Dust Mite Allergies and Asthma – A Worldwide Problem’, Platts-Mills TAE, de WeckA, UCB Institute of Allergy, Bad Kreuznach September 1987. Reported in The Journal of Allergy and Clinical Immunology, 1989, 83, pages 416 to 427
  7. Contribution of cysteine and serine proteases to proteolytic digestion in an allergy-eliciting house dust mite, JC Vidal-Quist, F Ortego, P Hernandez-Crespo, J. Insect Physiology, 133 (2021) 104285, p2-11
  8. ‘Innate Immune Responses in House Dust Mite Allergy’, Alain Jacquet, Hindawi, ISRN
    2013, Article ID 735031, Chapter 9, “Group 1 HDM allergens indeed degraded airway α1-antitrypsin inhibitor, elastase-specific inhibitor [elafin], and secretory leukocyte protease inhibitor resulting in enhanced tissue damage and immune activation.”
  9. Sequence analysis of cDNA coding for a major house dust mite allergen Der p1’, Chua K.Y. et al, J. Exp. Med., Vol 167, Jan 1988 p175-182
  10. Class specific inhibition of house dust mite proteinases which cleave cell adhesion, induce cell death and which increase the permeability of lung epithelium. Winston HL et al,‘British Journal of Pharmacology’, 1998,124,1048-1059
  11. Cellular and Molecular Events in the Airway Epithelium Defining the Interaction Between House Dust Mite Group 1 Allergens and Innate Defences, J Zhang, ‘Int.J.Mol.Sci’ 2018, Nov 19 (11): 3549
  12. Proteolytic, lipidergic and polysaccharide molecular recognition shape innate responses to house dust mite allergens, Jacquet A, Robinson C, Allergy, 2020, 75 (1) pp 33-53. “The protein targets of the serine peptidase HDM allergens of groups 3, 6 and 9 have some overlap with the group 1 cysteine peptidase allergens…”
  13. ‘Neutrophil Elastase (NE) and NE Inhibitors: Canonical and Noncanonical Functions in Lung Chronic Inflammatory Diseases (Cystic Fibrosis and Chronic Obstructive Pulmonary Disease)’, Ali Roghanian, Jean-Michel Sallenave, J. Aerosol Medicine & Pulmonary Drug Delivery, 2008, Vol. 21; No. 1: p 125-144. “Enzymatic activity from mites can interfere with the natural control of NE.”
  14. ‘Elafin (elastase-specific inhibitor) has anti-microbial activity against Gram-positive and Gram-negative respiratory pathogens’, Simpson AJ, Maxwell AI, Govan JRW, Haslett C, Sallenave JM,; FEBS Letters 452 (1999) 309-313
  15. ‘House Dust Mite Der p1 Downregulates Defence of the Lung by Inactivating Elastase Inhibitors’ Brown A, et al, ‘Am J Respir.Cell Mol.Biol.,2003, Vol.29 (3 Pt:1) 381-389