Cutaneous allergic vasculitis is a pathologically necrotizing vasculitis localized in the skin and characterized by the involvement of cutaneous small blood vessels.1 Clinically, the skin lesions are polymorphic and palpable papules on the lower legs are generally recognized. The histological features are vascular and perivascular infiltration of neutrophils with leukocytoclasis, endothelial cell swelling, fibrin deposition and erythrocyte extravasation.2-7 Although infections, drugs, and associated diseases are suspected to be the etiological agents,8 its mechanism still remains unclear. To the best of our knowledge, there has been no case report of cutaneous allergic vasculitis in the literature in which the number of mast cells was investigated. Here we report a case of the disease in which an accumulation of mast cells in the skin lesion was demonstrated.

A 33-year-old woman, who had a past history of allergic rhinitis due to Japanese cedar and Phleum pratense (timothy), presented at our hospital in December 2002 because of a symmetric appearance of solid papules with itching on both lower legs. The size of the papules ranged from a few millimeters up to ten millimeters. She had no symptoms of allergic rhinitis and was not taking any medication at that point. The peripheral blood leukocyte was 6600 /μl (basophils 0.3%, eosinophils 10.3%, neutrophils 60.6%, lymphocytes 23.4%, monocytes 5.4%). The serum levels were as follows: IgG = 1251 mg/dl, IgA = 196 mg/dl, IgM = 115 mg/dl, IgE = 130 IU/ml. Serum complement levels were also normal (CH₅₀ = 37.9 U/ml; C3 = 79 mg/dl; C4 = 20 mg/dl; C1q = 2.4 μg/ml). Antinuclear antibody was negative. The urinalysis and renal values were normal. On the next day, the peripheral blood leukocyte count increased to 9100 /μl and the number of eosinophils also increased (basophils 1.1%, eosinophils 14.4%, neutrophils 69.6%, lymphocytes 10.5%, monocytes 4.4%). The CRP level was 0.09 mg/dl. LE factor, RA test, and anti-DNA antibody were all negative. Biopsy from the skin lesions of the thigh was performed in December 2002. A diagnosis of cutaneous allergic vasculitis was made based on the clinical symptoms and pathological findings of biopsy specimens described below. The skin lesions improved after treatment with oral corticosteroid and the number of eosinophils decreased (WBC 5900 /μl, eosinophil 6.2%) in January 2003. The steroid was then tapered and discontinued 3 weeks after its commencement. The patient was well without any therapy until June 2005, when a second skin biopsy was performed after informed consent was obtained. The patient has not suffered from any palpable papules since.

Hematoxylin-eosin staining of the biopsy specimens, which was performed in December 2002, showed an increased number of neutrophils and decreased number of eosinophils in the blood vessel circumference (Fig. 1). These findings were compatible with the diagnosis of cutaneous allergic vasculitis. Toluidine blue staining showed the presence of metachromatic cells in the interstitium (Fig. 2).

Immunohistochemical staining of the biopsy specimens for human mast cell tryptase using monoclonal antibody against human mast cell tryptase (Chemicon, Temecula, CA, USA) showed the presence of mast cells (Fig. 3). On the other hand, the number of mast cells in the biopsy specimens obtained in June 2005 was markedly decreased both on toluidine blue staining (Fig. 4) and on immunohistochemical staining (Fig. 5). The mast cells were counted using an Olympus BX-50 microscope equipped with a 0.1 × 0.1 mm ocular grid (Olympus Eyepiece Micrometer U-OCMSQ 10/10, Tokyo, Japan) in 8 grid fields, and the mean of the number of mast cells was analyzed. The number of mast cells in the skin biopsy specimens taken in December 2002 and June 2005 was 82.8 /mm² and 20.2 /mm², respectively. The number of metachromatic cells was comparable with that in the corresponding biopsy specimens on immunohistochemical staining.

Cutaneous leukocytoclastic vasculitis was formerly called allergic cutaneous vasculitis or hypersensitivity vasculitis. The report of the Chapel Hill Consensus Conference in 19949 defined cutaneous allergic vasculitis as leukocytoclastic vasculitis localized in the skin without systemic vasculitis or glomerulonephritis, and stated that this type of vasculitis is dominant for adult women especially in the lower legs and dorsum pedis areas.

Although the mechanism of this disease still remains obscure, possible involvement of allergic reaction has been suspected. Soter et al.10 showed the findings of mast cell degranulation in cutaneous necrotizing venulitis. Tosca and Stratigos11 advocated that mast cells might play a central role in allergic cutaneous vasculitis. Grunwald et al.12 suggested possible involvement of mast cells in hypersensitive angitis by the degranulation test of mast cells. However, to the best of our knowledge, there has been no case report of allergic cutaneous vasculitis in which the number of mast cells was investigated.

In this report, we examined the number of mast cells in the skin of allergic cutaneous vasculitis. The mean number of mast cells in the skin lesions and the skin after healing was 82.8 /mm² and 20.2 /mm², respectively, indicating an increase in the number of mast cells in the skin lesion in our present case. However, the second biopsy was performed 2.5 years after the first biopsy. It is therefore uncertain whether the biopsy specimens represented the effect of improvement or not. Further studies are required. In conclusion, this is the first report of cutaneous allergic vasculitis showing an accumulation of mast cells in the skin lesion.

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