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Allergic to infections: How bacteria, viruses and fungi activate mast cells

I am often asked about whether an infection, even a mild cold, can cause worsening mast cell symptoms.  The answer is yes.  Viral, fungal and bacterial infections can all cause mast cell activation, and patients with prior activated mast cells are especially susceptible.  This is why it is so important for mast cell patients to avoid contagious illness as much as possible.

Several cell types in the human body have Toll-like receptors (TLRs) on their cell surfaces. These receptors bind many types of molecules that indicate presence of infection. These molecules are called pathogen-associated molecular patterns (PAMPs) and they share similar shapes that identify them as being released by infecting organisms. When these PAMPs are bound by TLRs on cell surfaces, it sends signals for the cells to mount an immune response.

The expression of TLRs on mast cells has been well studied using both mouse (murine) and human mast cells. TLR1, 2, 3, 4, 5, 6, 7, 9 and 10 have been identified on mast cells by at least one study. Some of these TLRs were only detected by finding related mRNA. (When cells express a gene to make a protein like a TLR, the DNA gene is copied into mRNA, which tells the cell how to make the TLR.) Since only the mRNA and not the TLR was directly identified, these TLRs require more research to be fully characterized.

TLR2 is one of the most well studied and understood of toll-like receptors found on mast cell surfaces. TLR2 is also known as CD282. Substances that bind to TLR2 include many molecules released by bacteria and fungi. Several types of peptidoglycans and found in bacterial cell membranes bind TLR2. In particular, lipoteichoic acid is a potent activator of TLR2. This molecule is found on the surfaces of gram-positive bacteria, like Staphylococcus spp. (Staph, MRSA) and Streptococcus spp. (Strep). Other bacteria that are known to activate TLR2 include Neisseria meningitides, Haemophilus influenzae, and Borrelia burgdorferi, among others. Mycobacteria are also activating to TLR2. Zymosan is found in cell membranes of yeast and binds TLR2. Aspergillus fumigatus (fungi) and several viruses, including Herpes simplex, Varicella zoster, Cytomegalovirus and measles, activate TLR2 responses. Heat shock protein 70 (HSP 70) is released by cells in the body when they are under certain types of stress, and this can activate TLR2.

When TLR2 is bound, mast cells produce and release several types of molecules that are not prestored in granules. The molecules released depend on which protein has bound TLR2. These molecules include IL-1b, which causes inflammatory pain hypersensitivity; IL-5, which activates eosinophils; leukotriene B4, which forms reactive oxygen species and participates in inflammation; leukotriene C4, which causes slow contraction of smooth muscle, including in the airway; GM-CSF (Granulocyte macrophage colony-stimulating factor), a growth factor for white blood cells; TNF, which has many inflammatory effects; RANTES, which brings other white cells to the site of inflammation; and others. TLR2 activation is responsible for the worsening of asthma symptoms in the presence of bacterial infection.

Multiple studies reported that stimulation of TLR2 with peptidoglycan (a constituent of gram positive bacterial cell membranes) induced degranulation. Stimulation with peptidoglycan induced histamine release as well as cytokine release in a 2003 study (Varadaradjalou 2003). Another study found that peptidoglycan did not cause statistically significant degranulation, but zymosan (a fungal product) and Pam3Cys (a synthetic molecule that acts like LPS, another component of bacterial membranes) did induce significant degranulation (McCurdy 2003). Other studies have not been able to replicate these results.

There is also evidence that stimulation of TLR2 can change the behavior of mast cells. When mast cells are grown in the presence of bacterial cell membrane products, they make different amounts of different proteins. Another study demonstrated that two bacterial cell membrane products downregulated the amount of FceRI (the IgE receptor) on the surface of mast cells, so after two days, mast cells were less responsive to stimulation by IgE molecules. This was partially due to the effects of TLR2 (Yoshioka 2007).

However, mast cells that are sensitized react more strongly to TLR2 activation with LPS (Medina-Tamayo 2011). This effect seems to be reliant on prior binding of IgE. Other very technical studies have investigated the effect of antigen (such as bacterial, viral or fungal products) on the interplay between the IgE receptor and TLR receptors.   While most of this work has been done in mouse cells, several investigators have shown that activation of TLR receptors and the IgE receptor causes enhanced release of cytokines but not degranulation. It is thought that the exaggerated response to IgE receptor and TLR2 stimulation can cause the exacerbation of allergic type conditions during active infection. (Qiao 2006)



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