Mast cell heterogeneity

Increasing evidence for the existence of inter- and intra-species mast cell heterogeneity has expanded the potential biological role of this cell. Early studies suggesting that mast cells at mucosal sites differ morphologically and histochemically from connective tissue mast cells have been confirmed using isolated intestinal mucosal mast cells in the rat and more recently in man. These studies also established that mucosal mast cells are functionally distinct from connective tissue mast cells. Thus, mucosal and connective tissue mast cells differ in their responsiveness to a variety of mast cell secretagogues and antiallergic agents. Speculation about the therapeutic use of antiallergic drugs in disorders involving intestinal mast cells cannot, therefore, be based on extrapolation from studies of their effects on mast cells from other sites. Regulatory mechanisms for mast cell secretion may also be heterogeneous since mucosal mast cells differ from connective tissue mast cells in their response to a variety of physiologically occurring regulatory peptides. The development of techniques to purify isolated mast cell subpopulations will facilitate future analysis of the biochemical basis of the functional heterogeneity of mast cells.     

Mast cell heterogeneity in the human uvea

To identify chymase- and tryptase-positive mast cells in the human uvea, and to study their associations with different types of resident uveal cells, uveal specimens from 24 human donor eyes were cryosectioned in sagittal and tangential planes. Enzyme histochemical staining of chymase was combined with immunohistochemical staining for tryptase, detected with the APAAP method. Fluorescence immunohistochemistry was performed with antibodies against c-kit, alpha smooth muscle actin, protein gene product (PGP) 9.5, CD45, and HLA-DR. In different uveal compartments, the total amounts of mast cells were calculated and the distributions of chymase and tryptase were quantified. All uveal mast cells were c-kit and CD45 positive and HLA-DR negative. No association existed between mast cells and actin-containing cells. Only a few mast cells were in close association with PGP 9.5-labeled nerve fibers. In the choroid, most mast cells were located in the inner central part (mean density = 48.9/mm²), and contained both chymase and tryptase (96%). The ciliary muscle contained numerous mast cells (mean density = 33.7/mm²), many of them tryptase positive but chymase negative (63%). In the pars plana, a high number of chymase-positive, tryptase-negative mast cells were found (20%). In the iris only a few mast cells were present. Although the choroid contains the most common subtype of mast cells, a unique situation concerning the distribution of chymase and tryptase is present in the anterior uveal tissues. A possible role for these cells in the special immunological situation of the anterior eye chamber merits further investigation. Accepted: 16 September 1999     

Mast cell heterogeneity: effects of neuroenteric peptides on histamine release

Recent reports suggesting that the actions of certain neuroenteric peptides may be mediated in part by the secretion of histamine and other mast cell contents could have important implications for gastrointestinal motility and secretion. However, evidence for a mast cell-hormonal interaction is based on studies using peritoneal or cutaneous mast cells. Because intestinal mucosal mast cells (MMC) differ functionally from peritoneal mast cells (PMC), we compared the effects of several neurotransmitters and intestinal hormones on histamine secretion from two mast cell types in the rat. MMC hyperplasia was induced in rats by infection with the nematode Nippostrongylus brasiliensis, and MMC were isolated from the small intestine by collagenase digestion. Substance P, somatostatin, vasoactive intestinal polypeptide (VIP), neurotensin, and bradykinin had a potent secretagogue effect on (10(-7) to 10(-4)M) PMC which was temperature-, energy-, and calcium-dependent. In contrast to PMC, MMC released significant amounts of histamine only when challenged with substance P. Acetylcholine, bombesin, motilin, and pentagastrin had no secretory effect on either PMC or MMC. The differences between PMC and MMC in responsiveness to peptides could not be attributed to the MMC isolation procedure because PMC treated similarly or mixed with MMC suspensions retained their responsiveness to these stimuli. Our results extend the concept of neurocrine control of mast cell function, but indicate that mast cells from different sites have distinct profiles of responsiveness to regulatory peptides.     

Electron-microscopical evidence for heterogeneity of micronodular argyrophil cell growth in man

Hypergastrinaemia-associated changes of non-antral argyrophil cells in man are of increasing interest, because of the development of potent inhibitors of gastric acid secretion. Using an antibody against chromogranin A, we identified micronodular endocrine cell hyperplasia of the oxyntic mucosa in gastric biopsy specimens of patients with hypergastrinaemia of different backgrounds. Consecutive ultrathin sections were examined at the electron-microscopical level. Endocrine cell types within the (extraepithelial) micronodules closely resembled those in the adjacent mucosa. Micronodules were classified into two groups. The first group was composed of endocrine cells only and predominated in patients with drug-induced hypergastrinaemia and/or chronic gastritis, and in a gastrinoma/MEN I patient. The second group represented “neuroendocrine complexes”, showing a close intermingling of non-myelinated nerve fibres with endocrine cells, and was found predominantly in pernicious anaemia. Micronodular argyrophil cell growth in man is therefore heterogeneous and depends on the background of the hypergastrinaemia.     

Mast cell glycosaminoglycans

Mast cells contain granules packed with a mixture of proteins that are released on degranulation. The proteoglycan serglycin carries an array of glycosaminoglycan (GAG) side chains, sometimes heparin, sometimes chondroitin or dermatan sulphate. Tight packing of granule proteins is dependent on the presence of serglycin carrying these GAGs. The GAGs of mast cells were most intensively studied in the 1970s and 1980s, and though something is known about the fine structure of chondroitin sulphate and dermatan sulphate in mast cells, little is understood about the composition of the heparin/heparan sulphate chains. Recent emphasis on the analysis of mast cell heparin from different species and tissues, arising from the use of this GAG in medicine, lead to the question of whether variations within heparin structures between mast cell populations are as significant as variations in the mix of chondroitins and heparins.     

Mast cells in normal and sectioned peripheral nerve

Summary An investigation is reported on the properties and quantitative distribution of mast cells in normal and sectioned peripheral nerve. A considerable number of mast cells has been found in the epineurial connective tissue in normal rats, as well as scattered mast cells in the endoneurium. After nerve section there was an about five-fold increase in the number of endoneurial mast cells throughout the distal part of the sciatic nerve.The mast cell granules in normal and sectioned nerve showed the same histochemical properties as mast cell granules in other tissues, i.e. strong toluidine blue metachromasia resistant to alcohol dehydration, and persistence of dye binding and metachromasia at pH below 1. Furthermore, the metachromasia is unaffected by extraction with chloroform and methanol prior to staining. The metachromatic component of the mast cell granules can be differentiated by these properties from other metachromatic structures in normal and sectioned nerve. The significance of the findings is discussed, in particular the possible relation of endoneurial mast cells to the degradation of myelin. Acknowledgements. The authors are indebted to Miss Kristina Müntzing for skilful technical assistance.     

Mast cell activation and autism

Autism spectrum disorders (ASD) are neurodevelopmental disorders characterized by varying degrees of dysfunctional communication and social interactions, repetitive and stereotypic behaviors, as well as learning and sensory deficits. Despite the impressive rise in the prevalence of autism during the last two decades, there are few if any clues for its pathogenesis, early detection or treatment. Increasing evidence indicates high brain expression of pro-inflammatory cytokines and the presence of circulating antibodies against brain proteins. A number of papers, mostly based on parental reporting on their children's health problems, suggest that ASD children may present with “allergic-like” problems in the absence of elevated serum IgE and chronic urticaria. These findings suggest non-allergic mast cell activation, probably in response to environmental and stress triggers that could contribute to inflammation. In utero inflammation can lead to preterm labor and has itself been strongly associated with adverse neurodevelopmental outcomes. Premature babies have about four times higher risk of developing ASD and are also more vulnerable to infections, while delayed development of their gut-blood-brain barriers makes exposure to potential neurotoxins likely. Perinatal mast cell activation by infectious, stress-related, environmental or allergic triggers can lead to release of pro-inflammatory and neurotoxic molecules, thus contributing to brain inflammation and ASD pathogenesis, at least in a subgroup of ASD patients. This article is part of a Special Issue entitled: Mast cells in inflammation.     

Mast cell tryptase and asthma

Recent physiological and pharmacological studies have indicated the potential importance of tryptase, the major protein component in mast cells, in inflammatory diseases (especially asthma). Being released at inflammatory sites after the activation of mast cells, tryptase is capable of causing bronchohyperresponsiveness and infiltration of eosinophils, neutrophils, etc. in animal airways. The mechanisms by which tryptase causes bronchoconstriction involve probably the potentiation of other chemical mediators such as histamine, production of bradykinin via the hydrolysis of kininogen, and cleavage of the bronchodilating peptides VIP (vasoactive intestinal peptide) and PHM (peptide histidine-methionine). Tryptase has also been found to be a potent mitogen in vitro for airway smooth muscle cells and epithelial cells, implying its role in the hyperplasia of the asthmatic airways. The experimental data providing evidence for the above roles of tryptase are summarized in the present review, as well as the effects of tryptase inhibition in animal asthma models. The potential strategies for the development of anti-asthmatic agents based on the inhibition of tryptase are discussed.     

Single cell heterogeneity

Multi-level heterogeneity is a fundamental but underappreciated feature of cancer. Most technical and analytical methods either completely ignore heterogeneity or do not fully account for it, as heterogeneity has been considered noise that needs to be eliminated. We have used single-cell and population-based assays to describe an instability-mediated mechanism where genome heterogeneity drastically affects cell growth and cannot be accurately measured using conventional averages. First, we show that most unstable cancer cell populations exhibit high levels of karyotype heterogeneity, where it is difficult, if not impossible, to karyotypically clone cells. Second, by comparing stable and unstable cell populations, we show that instability-mediated karyotype heterogeneity leads to growth heterogeneity, where outliers dominantly contribute to population growth and exhibit shorter cell cycles. Predictability of population growth is more difficult for heterogeneous cell populations than for homogenous cell populations. Since “outliers” play an important role in cancer evolution, where genome instability is the key feature, averaging methods used to characterize cell populations are misleading. Variances quantify heterogeneity; means (averages) smooth heterogeneity, invariably hiding it. Cell populations of pathological conditions with high genome instability, like cancer, behave differently than karyotypically homogeneous cell populations. Single-cell analysis is thus needed when cells are not genomically identical. Despite increased attention given to single-cell variation mediated heterogeneity of cancer cells, continued use of average-based methods is not only inaccurate but deceptive, as the “average” cancer cell clearly does not exist. Genome-level heterogeneity also may explain population heterogeneity, drug resistance, and cancer evolution.     

Liver cell heterogeneity. The distribution of fructose-bisphosphatase in fed and fasted rats and in man.

Periportal and perivenous hepatocytes were isolated by microdissection from lyophilized liver slices (16 micrometer) from fed and fasted rats and from a human patient. NADP/NADPH cycling was used to determine fructose-1,6-bisphosphatase activity in the isolated hepatocytes (10 ng dry weight). The periportal hepatocytes contain 3 times as much fructose-1,6-bisphosphatase activity as the perivenous hepatocytes. A 24 h fast led to two-fold increase in the activity in the periportal hepatocytes and a four-fold increase in the perivenous hepatocytes. Fructose-1,6-bisphosphatase parallels closely with the key enzyme phosphoenolpyruvate carboxykinase, and therefore can be considered a suitable marker for gluconeogenic capacity.     

Mast cell chymase regulates dermal mast cell number in mice

Chymase inhibitor reduced the increase in the number of dermal mast cells in 2,4-dinitrofluorobenzene-induced dermatitis in a dose-dependent manner. Intradermal injection of human chymase to mouse ear significantly increased histamine content, the marker for mast cell number in the skin. These results suggest that chymase released by mast cells may participate in local mast cell accumulation in a positive feedback fashion. Immunohistochemical analysis revealed that the intradermal injection of chymase reduces expression of stem cell factor (SCF) on surface of the skin keratinocytes. In addition, incubation of human keratinocytes with chymase in vitro resulted in release of SCF into the culture medium. Since soluble SCF is thought to regulate mast cell number, the chymase-induced mast cell accumulation may occur via the ability of chymase to process membrane-bound SCF on the epidermal keratinocytes.     

Mast cell responses to helminth infection

Mast cells have been suggested to be major effector cells in the immune response to infection with helminths. It is now clear, however, that mast cells are heterogeneous and have a diversity of important functions. In this review, Timothy Lee, Mark Swieter and Dean Befus point out that much of the confusion about the role of mast cells in immunity stems from methods and interpretations which are inadequate for the diversity of roles played by these cells in host responses to parasites. Classical histochemistry may fail to reveal active mast cells, and studies using chemical antagonists are difficult to interpret until we know more about the action of the drugs. The authors show that current research is extending our knowledge of mast cell heterogeneity, and helping to define the powerful array of mediators that they can use to orchestrate the immune response to helminth infections.