ARTICLE
During our on-going investigations of mast cell morphology, we have noticed
with great interest that a subgroup of mast cells become dendritic in
shape in prurigo nodularis lesional dermis.
The mast cell was first recognised by the legendary Paul Ehrlich as
far back as 1877 [1, 2], and since then, it has been well established
that mast cells are round or elongated in shape with characteristic, cytoplasmic,
basophilic granules. Johansson et al. [3] was the first to describe
the dendritic nature of normal, human, cutaneous mast cells using histamine-based
immunohistochemistry.
Mast cells are found in very large numbers throughout
the body, for example in the skin, and the respiratory and digestive tracts,
where they act as the "first line of defence". Mast cells synthesize,
store and release a wide range of bioactive mediators, including biogenic
amines, such as histamine; proteinases, such as tryptase and chymase;
lipid mediators, e.g. prostaglandin D2 and leukotriene
C4; and cytokines and other growth factors, e.g. TNF-alpha,
IL4-6, IL-8, and IL-13. Mast cells are known to be involved in a variety
of physiological and pathological states, such as immediate and delayed
hypersensitivity, cell growth regulation, neoplasia defence, and the sensations
of pain and itch. Increased numbers of mast cells in the skin are found
in different pathological conditions, such as mastocytosis, urticaria,
keloids, and scleroderma, the latter elegantly demonstrated by Akimoto
et al. [4] in their recent paper published in the British Journal
of Dermatology. It is evident that mast cells have a wide range of
functions, which raises the issue of human mast cell heterogeneity.
Using immunofluorescence double-labelling, 6
biopsies from 6 patients (one biopsy/patient), and corresponding normal
human skin biopsies were fixed in a mixture of 4% paraformaldehyde and
14% of a saturated solution of picric acid in phosphate buffer. Cryostat
sections (14 µm), vertical to the skin surface, were processed for
immunofluorescence double-labelling, employing chicken polyclonal anti-tryptase
(1:200; Promega Corporation, Madison, WI, USA) and mouse monoclonal anti-chymase
(1:1,000; Chemicon International Inc., Temecula, CA, USA), followed by
incubation with lissamine rhodamine (LRSC)-conjugated donkey anti-chicken
IgG and fluorescein-isothiocyanate (FITC)-conjugated donkey anti-mouse
IgG antiserums (both 1:160; Jackson Co., West Grove, PA, USA). All antibodies
were diluted in 0.01 M phosphate buffer containing 0.3% Triton X-100.
The sections were then observed and photographed with a Nikon Microphot-FXA
fluorescence microscope equipped with dark-field optics using different
excitation light. Additionally, 2 samples from prurigo nodularis patients,
and corresponding material from normal human skin, were processed for
conventional electron microscopy.
Employing this tryptase and chymase immunofluorescence double-labelling
and conventional electron microscopic techniques, we have identified a
group (5-10%) of mast cells with evident dendritic processes in the tissue
samples of prurigo nodularis lesional skin (Figs.
1a-f and 2a-c).
These mast cells, found both in the reticular as well as the papillary
dermis, usually had a larger cell body, and second or third level branches.
They were evenly distributed in the tissue, and with no particular preference
for e.g. blood vessels, fibroblasts or nerve fibers. Some dendritic
mast cells even revealed quite long dendrites, up to 50-100 µm. Under
the electron microscope, certain mast cells with particularly long dendrites
were identified, and, furthermore, most of these dendritic mast cells
had a larger cell body, but contained fewer granules in the cytoplasm,
especially within the dendrites (Fig.
2a-c). In the control group of skin samples from healthy volunteers,
the mast cells were mostly round or elongated in shape, but very few of
them had a dendritic shape.
Currently, mast cell classification seems not
to be entirely satisfactory. The modern human mast cell classification
was done by Irani et al. [5], who found that some mast cells contain
tryptase and chymase (MCTC), whereas other mast cells contain
only tryptase (MCT). These mast cells have a diverse tissue
distribution, for instance, MCTC are mainly distributed in
the skin, axillary lymph nodes, breast parenchyma and colon submucosa,
whereas MCT are mainly distributed in lung and bronchial tissue.
However, bearing in mind the complicated function of these cells, mast
cell classification is probably still in its infancy.
The dendritic mast cells identified here contain
both tryptase and chymase, and are clearly different from other cutaneous
dendritic cells, such as melanocytes and Langer-hans cells. Why some mast
cells become more dendritic is currently not clear, however, it may indicate
that certain subtypes of mast cells are more actively involved in a particular
disease (such as prurigo nodularis), than other subtypes. The dendritic
mast cells in prurigo nodularis may still have ordinary mast cell functions,
but the ones with many dendrites and few granules (Fig.
2a), could have a mediator-releasing function which is not only
equal to mass degranulation, but may also involve elaborate dendritic
process-dependent delivery mechanisms.
We believe that morphological identification of subgroups of mast cells
could contribute to the understanding of mast cell heterogeneity, and,
furthermore, hopefully lead to future studies regarding the molecular,
immunological, chemical and functional identification of such subgroups
of mast cells. The search for other possible dendritic mast cell subtypes
in other dermatoses, such as mastocytosis or atopic eczema, could support
the significance of our present findings.
CONCLUSION
Acknowledgements
This work was supported by grants from the Cancer- och Allergifonden,
and funds from the Medical Faculty of the Karolinska Institute. Ms Eva-Karin
Johansson is gratefully acknowledged for her excellent secretarial assistance.
REFERENCES
1. Ehrlich P. Beiträge zur Kenntniss der Anilinfärbungen und
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2. Ehrlich P. Beiträge zur Kenntniss der granulierten Bindegewebszellen
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