The enzyme elastase (EC 3.4.21.37) has proved to be a convenient and extremely sensitive marker for the quantification of neutrophils in cutaneous infiltrates. Fluorometric assay using the synthetic substrate MeOSuc-Ala-Ala-Pro-Val-N-methylcoumarin permitted the measurement of this enzyme in as few as five cells and was linear up to about 1000 cells per sample. The mean activity of lysates of human blood-derived neutrophils was 0.57 +/- 0.08 pmol of 7-amino-4-methyl-coumarin released per hour per neutrophil. Extracts of normal human skin contained no measurable elastase activity but resulted in a slight inhibition of the neutrophil enzyme (mean 12%). Application to the in vivo situation has been demonstrated by the use of leukotriene B4 as chemotactic agent. A reproducible neutrophil infiltrate was found at a dose of 2 ng, well below the threshold for the appearance of microabscesses.

The intraepidermal penetration of polymorphonuclear leukocytes is an early event in the pathogenesis of psoriasis. In the present study we quantified these cells in biopsies from lesional epidermis of chronic stable plaque psoriasis. In this phase intraepidermal penetration of polymorphonuclear leukocytes was shown in 41% of the lesions. Micropustules of Kogoj were seen in 10% of the lesions. Pronounced parakeratosis was associated with the presence of polymorphonuclear leukocytes. Microabscesses of Munro were located exclusively within areas of parakeratosis. The absence of polymorphonuclear leukocytes in the majority of skin biopsies from chronic stable plaque psoriasis is compatible with the cyclic pattern of the exocytosis of these cells.

The in situ infiltration of polymorphonuclear leukocytes following the topical application of leukotriene B4 to the clinically uninvolved skin of psoriatic patients and the skin of normal controls has been quantified using elastase as a marker enzyme. The magnitude of this response in the psoriatic patients proved to be significantly diminished compared with the response in the controls. The time course of the response was similar in both groups, maximum elastase activity being seen about 18 h after application of the LTB4.

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The leukotriene B4 (LTB4)-induced intraepidermal accumulation of polymorphonuclear leukocytes (PMN) was quantified, using an elastase assay, in psoriatic patients receiving methotrexate and in untreated psoriatic patients. Methotrexate in therapeutic dosages inhibited PMN infiltration in six patients who had improved considerably on treatment with the drug, a gradual increase being seen during the days on which no methotrexate was taken. Two patients were in relapse, despite methotrexate treatment, and both failed to show any change in LTB4-induced accumulation of PMN.