Histophysiology of mast cells in skin and other organs

Functional Recognition Theory and Type 2 Immunity: Insights and Uncertainties

Type 2 immunity plays an important role in host defense against helminths and toxins while driving allergic diseases. Despite progress in understanding the biology of type 2 immunity, the fundamental mechanisms regulating the type 2 immune module remain unclear. In contrast with structural recognition used by pattern recognition receptors, type 2 immunogens are sensed through their functional properties. Functional recognition theory has arisen as the paradigm for the initiation of type 2 immunity. However, the vast array of structurally unrelated type 2 immunogens makes it challenging to advance our understanding of type 2 immunity. In this article, we review functional recognition theory and organize type 2 immunogens into distinct classes based on how they fit into the concept of functional recognition. Lastly, we discuss areas of uncertainty in functional recognition theory with the goal of providing a framework to further define the logic of type 2 immunity in host protection and immunopathology.

A decision tree model for neuroimmune guidance of allergic immunity

The immune system defends the body from infectious and non-infectious threats. Distinct recognition strategies have evolved to generate antigen-specific immunity against pathogens or toxins versus antigen-independent tissue repair. Structural recognition, or the sensing of conserved motifs, guides the immune response to viruses, bacteria, fungi, and unicellular parasites. Functional recognition, which is sensing that is based on the activities of an input, guides antigen-independent tissue healing and antigen-specific Type 2 immunity to toxins, allergens, and helminth parasites. Damage-associated molecular patterns (DAMPs), released from damaged and dying cells, permit functional recognition by immune cells. However, the DAMP paradigm alone does not explain how functional recognition can lead to such disparate immune responses, namely wound healing and Type 2 immunity. Recent work established that sensory neurons release neuropeptides in response to a variety of toxins and allergens. These neuropeptides act on local innate immune cells, stimulating or inhibiting their activities. By integrating our knowledge on DAMP function with new information on the role of neuropeptides in innate immune activation in Type 2 immunity, we describe a decision tree model of functional recognition. In this model, neuropeptides complement or antagonize DAMPs to guide the development of antigen-specific Type 2 immunity through the activation of innate immune cells. We discuss why this decision tree system evolved and its implications to allergic diseases.

Immunosensation: Neuroimmune Cross Talk in the Skin

Classically, skin was considered a mere structural barrier protecting organisms from a diversity of environmental insults. In recent decades, the cutaneous immune system has become recognized as a complex immunologic barrier involved in both antimicrobial immunity and homeostatic processes like wound healing. To sense a variety of chemical, mechanical, and thermal stimuli, the skin harbors one of the most sophisticated sensory networks in the body. However, recent studies suggest that the cutaneous nervous system is highly integrated with the immune system to encode specific sensations into evolutionarily conserved protective behaviors. In addition to directly sensing pathogens, neurons employ novel neuroimmune mechanisms to provide host immunity. Therefore, given that sensation underlies various physiologies through increasingly complex reflex arcs, a much more dynamic picture is emerging of the skin as a truly systemic organ with highly coordinated physical, immunologic, and neural functions in barrier immunology.

Anatomic relationship between substance P- and CGRP-immunoreactive nerve fibers and mast cells in the palatal mucosa of the rat

The study describes the distribution of mast cells and of substance P (SP) and calcitonin gene-related peptide (CGRP)-immunoreactive nerve fibers in the rat palatal mucosa, focusing on the anatomic relationship between these tissue elements. The maxilla of 10-14-wk-old rats was dissected free, fixed, demineralized and frozen. Consecutive sections were stained with avidin peroxidase or processed for immunohistochemistry. In order to define the correlation between nerve fibers and mast cells, double staining techniques were used. The distance between each avidin-positive mast cell and the nearest detectable nerve fiber was determined. 5-Hydroxytryptamine- (5-HT) and avidin peroxidase-positive mast cells were frequently seen in the palatal mucosa but were rarely found in the gingival area. A large number of nerve fibers showing SP- and CGRP-like immunoreactivity were seen, particularly in association with blood vessels. Some nerve fibers were located in contact with or very close to the mast cells but the vast majority of mast cells showed no close anatomic association to nerve fibers. The nerve fibers and mast cells were mainly concentrated to the same regions in the palatal mucosa where blood vessels occurred. The observations suggest that in the rat palatal mucosa the main functional relationship relates to SP/CGRP and the blood vessels, and only to a minor degree to SP/CGRP and mast cells.

Immunohistochemical localization of tyrosine hydroxylase in corneal nerves

The sympathetic innervation of the mammalian cornea is thought to play an important role in the regulation of epithelial ion transport, mitogenesis, and wound healing following corneal injuries. Anatomically, the three-dimensional organization and relative density of corneal sympathetic innervation in many species remains inadequately described. In the present study, the sympathetic innervation of five different mammals (guinea pig, rat, mouse, hamster, and human) was studied in corneas sectioned parallel to the main axis of fiber orientation by labeling the fibers immunohistochemically with antiserum against tyrosine hydroxylase and an avidin-biotin-diaminobenzidine technique. The results showed that each species displayed a distinctive pattern and density of tyrosine hydroxylase immunoreactive (TH-IR) corneal innervation that was unique to that species. The overall level of TH-IR innervation was highest in the guinea pig, moderate in the human, hamster, and rat, and lowest in the mouse. In all species examined, TH-IR nerves were most numerous in the corneoscleral limbus where they either formed intimate associations with blood vessels or coursed through the connective tissue matrix apparently unrelated to vascular elements. Other TH-IR nerves entered the cornea proper in radially directed stromal nerve bundles to give rise to subepithelial plexuses of varying complexity. Occasional intraepithelial penetrations were observed in the guinea pig, human, and rat. Removal of the superior cervical ganglion resulted in the total loss of TH-IR staining from guinea pig and hamster corneas and in the substantial but incomplete loss of TH-IR staining from rat and mouse corneas, thus demonstrating their predominantly sympathetic origin. Combined sympathetic and sensory ocular denervation in the rat eliminated almost all corneal and limbal TH-IR immunostaining, thus suggesting a minor TH-IR sensory component in this species. In agreement with this conclusion, small numbers of TH-IR sensory neurons and an abundance of TH-IR fibers were observed in the trigeminal ganglia of the rat and guinea pig. Removal of the rat main ciliary ganglion resulted in the loss of additional TH-IR fibers from the chamber angle and iris, thereby confirming a partial parasympathetic contribution to the rat iridial TH-IR innervation. Following unilateral removal of the superior cervical ganglion in rats and guinea pigs, the contralateral cornea contained increased numbers of TH-IR nerves, suggesting an upregulation of tyrosine hydroxylase (TH) expression in some contralateral axons.(ABSTRACT TRUNCATED AT 400 WORDS)

Interactions of mast cells with the nervous system ?Recent advances

This article reviews recent advances in the understanding of mast cell-nervous system interactions. It is drawn largely from work published within the last ten years, and discusses the anatomical and biochemical evidence of a functional connection between mast cells and the nervous system, and the implications that such a relationship may have for normal and abnormal physiological functioning. Mast cells are found at varying levels of association with the nervous system; in CNS parenchyma (mainly thalamus), in connective tissue coverings (e.g. meninges, endoneurium), and in close apposition to peripheral nerve endings in a variety of tissues. There is, as yet, no clearly defined role for mast cells in nervous system function, or vice-versa, and it seems most likely that their interactions fulfil mutually modulatory roles. By extension, pathological situations where one of the partners in this relationship is overly stimulated may lead to a dysregulation of the other, and contribute to disease symptomatology.

Formation of contacts between mast cells and sympathetic neurons in vitro

Functional interactions between mast cells and peripheral nerves may occur at sites of association seen in vivo. To study the interactions, we developed a tissue culture model of murine sympathetic neurons co-cultured with rat basophilic leukaemia (RBL-2H3) cells (homologues of mucosal mast cells) or rat peritoneal mast cells. In co-cultures of up to 3 days, light microscopy identified neurite contacts with peritoneal mast cells or RBL-2H3 cells, but not with glial cells or fibroblasts. Electron microscopy confirmed membrane-membrane contact between neurites and RBL-2H3 cells. Time-lapse analysis of interactions between neurons and RBL-2H3 cells showed that 60-100% of the cells in a given field acquired neurite contact within 17 h. In matching control studies, there was no increase in the frequency of neurite contact with cells of the rat plasmacytoma line (YB2/0): these were not selected as targets, and contacts were broken if formed. Time-lapse records of the derivation of neurites from their path suggested a neurotropic effect of mast cells, with neurite contact ensuing when the intervening distance was less than 36 +/- 4 microns. Once formed, contacts were invariably maintained throughout the period of examination (up to 72 h), in contrast to YB2/O or fibroblast contacts. We conclude that neurons selectively form and maintain connections with cells representative of rat connective tissue-type and mucosal mast cells in vitro. Similar interactions in vivo could promote nerve/mast cell contacts, which may allow bidirectional communication between the nervous and immune systems.

Skin reactivity to neuropeptides in atopic dermatitis

Adults with atopic dermatitis (AD), with respiratory atopy only and healthy non-atopic controls were given intradermal injections of substance P (SP), neurokinin A (NKA), neurotensin (NT), vasoactive intestinal polypeptide (VIP), calcitonin gene-related peptide (CGRP) and histamine into the normal-appearing skin on the back. The weal and flare responses were evaluated after 3, 5 and 15 min and the areas calculated using an automatic image analyser. With the three different concentrations used (1, 3 and 30 pmols) a statistically significant (P less than 0.05) reduction in both the weal and flare response to SP, NKA, NT and histamine and a reduced flare to CGRP was observed only in AD patients. Among those with AD there was no uniformity of response to the individual neuropeptide and in general the more severely affected showed a lower reactivity. Dose-response relationships were evaluated for SP and NT (10-320 pmols) in AD and healthy controls. In AD dose-response curves and time-course relationships were similar to controls, but at significantly reduced levels. The itch response to the neuropeptides and histamine was not different in atopics and controls. We suggest that this hyporesponsiveness in AD is the result of natural tachyphylaxis of the target structures (mast cells and blood vessels) and possibly due to a higher availability of neuropeptides in the skin or to a primary abnormal sensitivity of the blood vessels and mast cells to these peptides.

Mast cells are closely apposed to nerves in the human gastrointestinal mucosa

Mast cell/nerve associations have been recorded in several publications; however, the human gastrointestinal tract has received little attention. Accordingly, mucosal samples from small bowel, appendix, and large bowel were studied. Combined histochemical/immunocytochemical techniques revealed that the proportion of mast cells apposed to nerves ranged from 47.08% +/- 6.10% to 77.66% +/- 4.26%. The highest incidence of contact was observed in the appendix; where the apparent nerve density was also greater than in the large or small bowel. Electron-microscopic studies revealed many mast cells adjacent to nerve fibers and membrane-to-membrane contact between axonlike processes and mast cells. Often, these processes were dilated, as were axons in adjacent nerve fibers. These data provide a microanatomic basis for potential communication between nerves and mast cells in the human gastrointestinal mucosa. This may be of physiologic significance in the normal individual and important in disease processes, such as inflammation and fibrosis.

Nerve growth factor enhances antigen and other secretagogue-induced histamine release from rat peritoneal mast cells in the absence of phosphatidylserine

The effects of 2.5S nerve growth factor (NGF) and epidermal growth factor (EGF), isolated from mouse submaxillary glands, on histamine release from rat peritoneal mast cells (PMCs) were studied. In the absence of phosphatidylserine, NGF (1 ng/ml to 1 microgram/ml) did not cause histamine release from PMCs isolated from normal rats and those infected with the nematode Nippostrongylus brasiliensis. However, when PMCs (greater than 97% pure) were preincubated with NGF and then challenged with worm antigen (Ag), there was a marked enhancement of histamine release (approximately twofold with a maximum effect at 10 ng/ml of NGF [3.8 X 10(-10) mol/L]) compared with the release induced by Ag alone. EGF (1 ng/ml to 1 microgram/ml) neither produced histamine release from PMCs in the presence of phosphatidylserine nor enhanced Ag-induced histamine release. This suggests that NGF acts directly on PMCs by activation of cell-surface receptors. The early kinetics of Ag-induced histamine release were altered by NGF that increased the initial rate at 15 seconds but did not prolong the overall duration of histamine release. Simultaneous addition of Ag and NGF did not cause enhanced histamine release; thus, some preincubation time with NGF (5 minutes or less) was required for the activation of PMCs. Moreover, after PMCs were activated by NGF, that state persisted for 1 hour, even when unbound NGF was removed by washing, and thereafter subsided gradually. Further studies revealed that NGF enhanced histamine release induced by concanavalin A, compound 48/80, and ionophore A23187. These results suggest that NGF might be an important molecule in inflammatory responses through the regulation of mediator release from mast cells.

Intestinal mucosal mast cells in normal and nematode-infected rat intestines are in intimate contact with peptidergic nerves

Inflammatory or allergic conditions, as well as situations where healing and repair processes occur, are characterized by the presence of increased numbers of mast cells. Previous work on the effect of neuropeptides on mast cell mediator release showed that only substance P caused such release from intestinal mucosal mast cells [Shanahan, F., Denburg, J. A., Fox, J., Bienenstock, J. & Befus, A. D. (1985) J. Immunol. 135, 1331-1337]. Accordingly, we investigated the microanatomical relationship between mast cells and enteric nerves in normal rat intestine and parasite-infected rat intestine, in which mucosal mast cell hyperplasia occurs. Combined immunohistochemistry for neuron-specific enolase and staining with alcian blue at pH 0.5 was employed on paraffin-embedded sections of normal and Nippostrongylus brasiliensis-infected rat jejunum. Sixty-seven percent of intestinal mucosal mast cells were touching subepithelial nerves, and an additional 20% were within 2 micron of nerves. Assessment of the proportion of the lamina propria occupied by mast cells (12.5%), the average mast cell area (121 +/- 28 microns 2), and the density of enteric nerves (one per 788 +/- 151 microns 2) suggested that the association was 5 times greater than would be expected by chance alone (P less than 0.0001). In consecutive sections, the nerves in contact with mast cells were also shown to contain substance P and/or calcitonin-gene-related peptide. Electron microscopy confirmed this association: 8% of the mast cells in infected rats exhibited membrane-membrane contact with unmyelinated axons containing 70- to 170-nm dense-core vesicles, and an additional 31% were situated less than 250 nm from nerves. Other mast cells appeared to embrace nerve bundles through the projection of lamellopodia. These data provide systematic quantitative evidence that a structural foundation for communication between the immune and nervous systems exists in the rat gastrointestinal tract.

[The histodynamic of alopecia areata in the dependence on the griseofulvin-induced epithelial proliferation (author's transl)]

The histodynamic of the alopecia areata--without or with the medicamentous induction of the hair growth respectively--was investigated comparatively. 1. Griseofulvin induced an epithelial proliferation in alopecia areata, which is noticeable in the surface epithelium as well as in the follicular epithelium. 2. Under the influence of griseofulvin a deeper penetration of the follicles into the fatty tissue--in connexion with a cutan-subcutaneous volume increase--results; at the same time the supra- and infraseboglandular follicular areas are lengthened. 3. The matter for discussion is that these epithelial changes originate from an increase of metabolic activity or blood supply respectively in the adjacent cutan- and subcutaneous tissue.

[Light- and electron microscopical investigation on the pathogenesis of Kaposi's sarcoma (author's transl)]

In Kaposi's sarcoma (K.S.) the regular metabolism between endothelial cell, extracellular matrix and fibrocyte is disturbed. Thereby the composition of the extracellular matrix change and via a feed-back to the CNS and hormonal system in return changes of endothelial cells and fibrocytes appear. Processing of misinformation sum up in transformation of fibrocytes to tumor cells and destruction of endothelial cells. Initial a widening of the interendothelial borders of the capillary bed seem to be of great importance for the pathogenesis of K.S., because for long time spaces lymphedemas preceed the tumor. With this immunoreactions occur which probably are accompanied by autoimmuno-processes

[Pathogenesis of neurofibromatosis. Light- and electron microscopical investigations (author's transl)]

Light- and electron microscopical investigations of the pathogenesis of Recklinghausen's neurofibromatosis can support those views which consider the Schwanncells in these tumors to develop from mesenchymal cells, that means from reticular fibroblasts. It seems to be of special interest that these cells while transforming to Schwann cells become successive coated with a glycocalyx (surface coat); The whirle-or onion skin like formation of the tumor cells lead to a splitting of the ground substance into numerous compartments accompanied by a change in ground substance composition. This obviously cohere with an increasing number of mast cells within the tumor. It may be that these arrangements of the tumor tissue plays an important part in tumor growth. With this, the great importance of a regular composition of the extracellular matrix and cell surface coats for an undisturbed flow of information from cell to cell is discussed.

Histophysiology of the vegetative peripheral nervous system of skin

Preterminal nerve fibers of the peripheral vegetative nervous system make inmediate contact (neuro-effector-areas) to interstitial cells (I.C.). This connection is characterized through a common glycocalyx with the nerve fiber. The I.C. are specific innervated cells and differ morphologically from Schwann-cells, fibrocytes, and histiocytes. The I.C. are able to come into morphologically different contacts with neighbouring cells by microvilli-like cell protrusions. These neighbouring cells then are able to contact other cells by themselves. The results are interpreted in the sense of electro-mechanical feed-back system of information processing in the vegetative periphery.