Ultrastructural analysis of the development of human basophils and mast cells in vitro

Calcium signaling and regulation of neutrophil functions: Still a long way to go

Neutrophils are the most abundant leukocytes in blood and disruption in their functions often results in an increased risk of serious infections and inflammatory autoimmune diseases. Following recent discoveries in their influence over disease progression, a resurgence of interest for neutrophil biology has taken place. The multitude of signaling pathways activated by the engagement of numerous types of receptors, with which neutrophils are endowed, reflects the functional complexity of these cells. It is therefore not surprising that there remains a huge lack in the understanding of molecular mechanisms underlining neutrophil functions. Moreover, studies on neutrophils are undoubtedly limited by the difficulty to efficiently edit the cell's genome. Over the past 30 years, compelling evidence has clearly highlighted that Ca²⁺ -signaling is governing the key processes associated with neutrophil functions. The confirmation of the role of an elevation of intracellular Ca²⁺ concentration has come from studies on NADPH oxidase activation and phagocytosis. In this review, we give an overview and update of our current knowledge on the role of Ca²⁺ mobilization in the regulation of pro-inflammatory functions of neutrophils. In particular, we stress the importance of Ca²⁺ in the formation of NETs and cytokine secretion in the light of newest findings. This will allow us to embrace how much further we have to go to understand the complex dynamics of Ca²⁺ -dependent mechanisms in order to gain more insights into the role of neutrophils in the pathogenesis of inflammatory diseases. The potential for therapeutics to regulate the neutrophil functions, such as Ca²⁺ influx inhibitors to prevent autoimmune and chronic inflammatory diseases, has been discussed in the last part of the review.

Calcium-dependent, non-apoptotic, large plasma membrane bleb formation in physiologically stimulated mast cells and basophils

Large membrane derangements in the form of non-detaching blebs or membrane protrusions occur in a variety of cell stress and physiological situations and do not always reflect apoptotic processes. They have been studied in model mast cells under conditions of cell stress, but their potential physiological relevance to mast cell function and formation in primary mast cells or basophils have not been addressed. In the current study, we examine the large, non-detaching, non-apoptotic, membrane structures that form in model and primary mast cells under conditions of stimulation that are relevant to allergy, atopy and Type IV delayed hypersensitivity reactions. We characterized the inflation kinetics, dependency of formation upon external free calcium and striking geometric consistency of formation for large plasma membrane blebs (LPMBs). We describe that immunologically stimulated LPMBs in mast cells are constrained to form in locations where dissociation of the membrane-associated cytoskeleton occurs. Mast cell LPMBs decorate with wheat germ agglutinin, suggesting that they contain plasma membrane (PM) lectins. Electrophysiological capacitance measurements support a model where LPMBs are not being formed from internal membranes newly fused into the PM, but rather arise from stretching of the existing membrane, or inflation and smoothing of a micro-ruffled PM. This study provides new insights into the physiological manifestations of LPMB in response to immunologically relevant stimuli and in the absence of cell stress, death or apoptotic pathways.

Ketotifen suppression of NF1 neurofibroma growth over 30 years

A patient with NF1 was treated with oral ketotifen for 30 years since infancy. Review of the patient's course and treatment details establishes a basis for reconsideration of several fundamental precepts about NF1 pathogenesis. The data suggest a distinctive benefit to treating an NF1 patient with an inhibitor of mast cell degranulation before cutaneous neurofibromas are clinically apparent: the neurofibromas appear to be arrested at a very early stage of development. The patient's skin was especially remarkable for both the paucity of cutaneous neurofibromas and the distinctive monotonous uniformity of those present, which were small and flat or barely sessile. While the data do not, of themselves, prove that ketotifen treatment commencing in childhood preempts neurofibroma maturation, in the context of earlier publications, they certainly warrant further investigation.

Respirable antisense oligonucleotides: a new, third drug class targeting respiratory disease

PURPOSE OF REVIEW

To describe the potential of a new class of respiratory drugs, respirable antisense oligonucleotides.

RECENT FINDINGS

The first respirable antisense oligonucleotide, EPI-2010, has now reached clinical trials. It has shown intriguing initial indications of efficacy and the potential to be the first once-per-week asthma preventative. Respirable antisense oligonucleotides are capable of addressing targets that have proven to be intractable to traditional 'small molecule' approaches, and against which newer monoclonal antibody strategies may also not be optimal. Respirable antisense oligonucleotides functionally, but not genetically, ablate gene expression by blocking the template function of target respiratory messenger RNAs by as yet incompletely defined mechanisms. They do so with an avidity and specificity which can be several orders of magnitude greater than those shown by small molecule antagonists for their protein targets. The target properties of respiratory messenger RNAs are strikingly different from those of respiratory proteins, enabling respirable antisense oligonucleotides to offer the potential of longer duration of effect, increased specificity of effect, and lack of systemic side effects compared with either traditional small molecule protein antagonists or monoclonal antibodies.

SUMMARY

Respirable antisense oligonucleotides represent a new, third class of respiratory drugs with the potential to extend the range of therapeutic responses to otherwise intractable respiratory targets, and to address precedented targets with the possibility of improving on such features as safety and durability of response.

Human Intestinal Fibroblasts Prevent Apoptosis in Human Intestinal Mast Cells by a Mechanism Independent of Stem Cell Factor, IL-3, IL-4, and Nerve Growth Factor

In rodents, fibroblasts (FBs) mediate stem cell factor (SCF)-dependent growth of mast cells (MCs). In humans, SCF is mandatory for MC differentiation and survival. Other factors such as IL-3, IL-4, and nerve growth factor (NGF) act in synergism with SCF, thus enhancing proliferation and/or preventing apoptosis in MCs. In this study, we studied in vitro interactions between human MCs and human FBs, both isolated from the intestine and purified to homogeneity. In coculture with FBs, MCs survived for up to 3 wk, whereas purified MCs cultured alone died within a few days. TNF-alpha and IL-1beta, which both did not affect MC survival directly, enhanced FB-dependent MC growth. We provide evidence that FB-derived MC growth factors are soluble, heat-sensitive molecules which down-regulate MC apoptosis without enhancing MC proliferation. However, only low amounts of SCF were measured in FB-conditioned medium (<0.2 ng/ml). Moreover, blocking of SCF/c-kit interaction by anti-SCF or anti-c-kit Abs and neutralization of IL-3, IL-4, and NGF did not affect MC survival in the coculture system. In conclusion, our data indicate that human FBs promote survival of human MCs by mechanisms independent of SCF, IL-3, IL-4, and NGF. Such interactions between MCs and FBs may explain why MCs accumulate at sites of inflammatory bowel disease and intestinal fibrosis.

Induced maturation of frog mast cells by nerve growth factor during ontogenesis

The effect of nerve growth factor (NGF) on ontogenesis of frog mast cells was investigated in vivo by histochemical, morphometric, and ultrastructural analysis. Three groups of tadpoles at various stages of development were used. In the first group, the larvae received i.p. injections of 1 ng NGF/g; the second group received 10 ng NGF/g, while the control group received only the vehicle. The first recognizable mast cells arose symmetrically in the tongue at stage 26 of Witschi's standard table. At stages 26 and 29, the mast cell number in the NGF-injected tadpoles was significantly higher than the control group. From stage 29 onward, the mast cell number rapidly increased in all groups. No significant differences in mast cell number were observed between the control group and the NGF-injected groups at stages 31 and 33. Electron microscopy revealed that at metamorphic climax (stage 33), the mast cells in the NGF-treated groups were more mature than those in the control group. Therefore, nerve growth factor at early stages of tadpole development is likely to induce differentiation of mast cell precursors, while at later stages it is likely to induce maturation of immature mast cells. The close anatomical association between mast cells and perineurium, observed during nerve development, is intriguing. Already in the early stages of nerve development, the mast cells form a network around Schwann cell-axon complexes, together with the perineurial cells. At climax, the mast cells are located between the perineurial layers, suggesting that they may play a role in the tissue-nerve barrier of the perineurium. Nerve growth factor also seems to induce perineurial cell maturation.

The case for extending storage and secretion functions of human mast cell granules to include synthesis

Ultrastructural studies using standard procedures have for years indicated close associations of ribosomes and secretory granules in human mast cells. These descriptive studies have informed new studies, using established and new ultrastructural methods based on different principles, designed to investigate the possible role of RNA metabolism in secretory granules of human mast cells. In aggregate, these studies indicate human mast cell secretory granule associations with ribosomes, the protein synthetic machine of cells, with ribosomal proteins, with RNA, with poly(A)-positive mRNA and with various long-lived, or short-lived, uridine-rich, and poly(A)-poor RNA species with key roles in RNA processing and splicing. These studies indicate that secretory-storage granules in human mast cells may also be synthetic granules.

Stem cell factor and IL-6 do not promote complete maturation of human cultured mast cells from umbilical cord blood cells: an ultrastructural study

Human mast cells are classified into two phenotypes based on their neutral protease compositions. One type is a tryptase-positive and chymase-positive MCTC cell that is predominant in the skin, another is a tryptase-positive and chymase-negative MCT cell that is predominant in the lung. Cord blood-derived human mast cells cultured in the presence of stem cell factor and interleukin-6 are a mixture of MCTC and MCT at various ratios, as revealed by immunocytochemical staining. We performed an electron microscopic analysis of cord blood-derived human cultured mast cells and found that they were so immature that we could not distinguish MCT and MCTC from their ultrastructural morphology. The response to secretagogues was not the response of MCTC but rather of MCT. Although human cultured mast cells are the most useful cells for use in in vitro experiments, the present culture condition supplemented with stem cell factor and interleukin-6 does not develop fully mature mast cells in vitro.

Role of mast cells in the onset of IgE-mediated late-phase cutaneous response in mice

BACKGROUND

In mice that are passively sensitized to IgE, cutaneous antigen challenge produces a biphasic response with peaks at 1 and 24 hours after challenge.

OBJECTIVE

We investigated the role of mast cells in the IgE-mediated late-phase reaction in mice.

METHODS

We histologically and ultrastructurally investigated the morphologic changes of mast cells during the biphasic responses.

RESULTS

Degranulation of mast cells, which was observed between 4 and 24 hours after challenge, reached a peak at 8 hours. Piecemeal degranulation was seen during the immediate phase reaction. The number of IL-6-positive mast cells was increased after 4 hours in both IgE-sensitized and unsensitized mice, but positive cells showed a greater increase in sensitized mice and reached a peak after 8 hours. With in situ hybridization experiments, mast cells were positive for IL-6 messenger RNA at 6 hours after challenge.

CONCLUSION

These findings indicate that anaphylactic degranulation of mast cells and the expression of IL-6 mRNA within 4 hours after antigen challenge are important for the onset of the late-phase allergic cutaneous reaction in mice.

Distribution and local differentiation of mast cells in the parenchyma of the forebrain

Mast cells are found in the brain of many species. Although a considerable body of information is available concerning the development and differentiation of peripheral mast cells, little is known about brain mast cells. In the present study, the ontogeny of mast cells in the dove brain was followed by using three markers: acidic toluidine blue, alcian blue/safranin, and an antiserum to gonadotropin-releasing hormone (GnRH). Mast cells first appear in the pia on embryonic day (E)13-14 in ovo, then along blood vessels extending from the pia into the telencephalon on posthatch day 4-5, and in the medial habenula at week 3. Medial habenular mast cell numbers increase during development, peaking in peripubertal birds, and declining thereafter. Several measures indicate that mast cells mature within the medial habenula: there is an increase in the intensity of metachromasia, a switch from alcian blue granules in young animals to mixed alcian blue and safranin granules in older animals, and an increase in GnRH-like immunoreactivity. These results were extended by using electron microscopy. The architecture of mast cell granules evolved from electron lucent with small electron dense deposits at E15 to more electron dense granules with complex patterns of internal structure by 2 months. Ultrastructural immunocytochemistry for the GnRH-like peptide at 1 month revealed both immunopositive and negative cells, suggesting that the acquisition of this phenotype is not simultaneous across the population. Thus, immature mast cells infiltrate the central nervous system and undergo in situ differentiation within the neuropil.

Histamine content and secretion in basophils and mast cells

Biochemical determinations of the histamine content and secretion from basophils and mast cells have been available for some time, and much of the complex anatomy of these cellular populations and their release reactions has been documented using the electron microscope. The ultrastructural analyses led to the description of vesicular transport between secretory granules and the plasma membrane as a mechanism for secretion from basophils and mast cells--a process termed piecemeal degranulation. Proof of concepts incorporated in a general degranulation model put forth in 1975 (DVORAK, H.F. and DVORAK, A.M.) requires high magnification imaging of a granule constituent in trafficking vesicles in the process of a stimulated release reaction in which the constituent release is monitored biochemically. Development and application of a new enzyme-affinity method to detect histamine at high magnifications in well-preserved ultrastructural samples have provided the necessary means to establish proof that appropriate secretagogues can stimulate the vesicular transport of histamine in basophils and mast cells during release reactions monitored biochemically. The background information necessary to the understanding of this result is presented here, as well as the development and verification of the diamine oxidase-gold method to image histamine in human mast cell granules as the test system. Also presented are applications using this technology to examine histamine stores and secretion in vitro, in vivo, and ex vivo in human basophils and mast cells and in mouse mast cells. Specifically examined are histamine stores developing in maturing mast cells induced to develop de novo from cultured human cord blood cells, secretagogue-stimulated release and recovery of histamine stores from isolated, purified human lung mast cells ex vivo, cytokine-stimulated degranulation of human skin mast cells and their histamine stores in vivo, piecemeal degranulation of human gut mast cells and their histamine stores in inflammatory bowel disease in vivo, piecemeal degranulation of mouse skin mast cells and their histamine stores in inflammatory eye disease in an interleukin-4 transgenic mouse model in vivo, and the stimulated secretion and recovery of histamine from human basophils ex vivo.

Cell biology of the basophil

The cell biology of basophils, based on published studies spanning 1990-1997, is reviewed. These rarest cells of granulocyte lineages are now available in sufficient numbers for such studies to be done, based on new methods for isolating and purifying the cells from peripheral blood and organ sources and for their derivation in growth factor-containing cultures from their precursors de novo. These studies are dependent on electron microscopy for the accurate identification of basophils, studies which have recently established the presence of basophils in two new species--mice and monkeys. Secretory, endocytotic and storage properties of basophils constitute their mechanistic role(s) in human disease; their role(s) in health is, however, obscure. Development of immunoaffinity and enzyme-affinity ultrastructural labeling techniques to image the Charcot-Leyden crystal protein and histamine in human basophils, coupled with ultrastructural analysis of kinetic samples of cells obtained after stimulation with diverse secretogogues, has provided insight into the role of vesicles in secretory transport mechanisms in human basophils as well as the definition of key ultrastructural phenotypes of secreting basophils.

Diamine Oxidase-Gold Ultrastructural Localization of Histamine in Human Skin Biopsies Containing Mast Cells Stimulated to Degranulate In Vivo by Exposure to Recombinant Human Stem Cell Factor

Stem cell factor (SCF ) has a major role in hematopoiesis and in the regulation of mast cell development and function. For example, recombinant human SCF (rhSCF ) can induce the development of human mast cells from precursor cells in vitro, stimulate mediator release from human skin mast cells in vitro, and promote both the development and functional activation of human skin mast cells in vivo. In the present study, we used a new ultrastructural enzyme-affinity method, employing diamine oxidase (DAO)-conjugated gold particles (DAO-gold), to detect histamine in skin biopsies obtained from patients with breast carcinomas who were receiving daily subcutaneous (SC) injections of rhSCF in a phase I study of this cytokine. We examined control biopsies obtained at sites remote from rhSCF injection as well as biopsies of rhSCF-injected skin that were obtained within 2 hours and 30 minutes of the SC injection of rhSCF at that site. The rhSCF-injected sites (which clinically exhibited a wheal-and-flare response), but not the control sites, contained mast cells undergoing regulated secretion by granule extrusion. The DAO-gold-affinity method detected histamine in electron-dense granules of mast cells in control and injected skin biopsies; however, the altered matrix of membrane-free, extruded mast cell granules was largely unreactive with DAO-gold. Notably, DAO-gold bound strongly to fibrin deposits and collagen fibers that were adjacent to degranulated mast cells. These findings represent the first morphologic evidence of histamine secretion by classical granule exocytosis in human mast cells in vivo.