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Take-home messages
Mast cells in both cutaneous and systemic mastocytosis may express CD4.
They may also express certain other T-cell related antigens, such as CD2 and CD25 but they are negative for CD3, CD5 and CD7.
In mastocytosis, mast cell expression of CD4, coupled with morphological plasticity, could lead to diagnostic confusion.
In contrast to lymphocytic, histiocytic and myeloid neoplasms, mast cells express CD117 and mast cell tryptase and stain with toluidine blue.
CASE REPORT
A 59-year-old man presented with a five-year history of an asymptomatic widespread truncal rash. There was no history of urtication or itching. On examination there was a widespread monomorphic rash comprising erythematous macules and papules in a perifollicular distribution across his back, chest and flanks. Patches and plaques were absent and the lower limbs and head and neck were spared. The differential diagnoses included mycosis fungoides and mastocytosis.
Two cutaneous punch biopsies showed a superficial dermal mononuclear cell infiltrate, consisting of small, predominantly perivascular lymphocytes and larger, polygonal and slightly spindled mononuclear cells with bland vesicular nuclei in a perivascular and interstitial distribution ( fig 1 ). There were small foci of intra-epidermal lymphocytes, but no Pautrier microabscesses, no replacement of the basal epidermal layer by lymphocytes and no interface change. Scattered eosinophils were present. There was no appreciable spongiosis. The majority of the small lymphocytes showed a CD3+CD4+ or CD3+CD8+ immunophenotype, while a few expressed B-cell markers (CD20 and CD79a) ( fig 1 ). The larger cells, which were CD2-, CD3-, CD5- and CD7-negative, showed widespread CD4 expression, albeit with a lower staining intensity than the smaller CD3-positive, CD4-positive cells. They also expressed mast cell tryptase and CD117 and showed metachromatic staining with toluidine blue ( fig 1 ). Hence, a diagnosis of mastocytosis with aberrant CD4 expression was made.
Investigations showed a raised serum tryptase of 64 [micro]g/l (normal range 0-16 [micro]g/l), and osteoporosis on bone densitometry; systemic mastocytosis was confirmed on bone marrow aspirate and trephine biopsy following the 2008 World Health Organization classification criteria. 1 Further biochemical and haematological tests were normal. He remains asymptomatic and clinically well 12 months after diagnosis.
This case stimulated us to investigate the expression of CD4 on a further six cases of presumptive mastocytosis, five of which were cutaneous...