Histaminase activity in patients with vernal keratoconjunctivitis

To investigate the activity of histamine-degradating enzymes in tears and plasma of patients with vernal keratoconjunctivitis (VKC).

Tear and plasma samples were collected from patients with VKC and from age-matched control subjects. Histamine was measured by enzyme-linked immunosorbent assay in acid samples treated with perchloric to deactivate histaminase and in untreated samples. Tear cytology, skin test reactivity to histamine, and the sum clinical score of allergic signs and symptoms in patients with VKC also were evaluated. Nineteen patients with active VKC and six age-matched control subjects participated in this study.

In untreated samples, tear histamine (mean +/- standard error of the mean) was 11.15 +/- 2.16 ng/ml in patients with VKC and 0.855 +/- 0.225 ng/ml in control tears (P < 0.001). In treated samples, mean tear histamine was 22.25 +/- 4.17 ng/ml in patients with VKC versus 10.64 +/- 2.85 ng/ml in control subjects (not statistically different). The ratio of histamine in treated to untreated samples (indicating histaminase activity) was significantly lower in patients with VKC (2.30 +/- 0.263) than in control subjects (17.57 +/- 5.97; P = 0.0001). Plasma histamine levels in untreated and treated samples were significantly higher in patients with VKC (untreated, 2.23 +/- 0.334 ng/ml; treated, 4.37 +/- 0.357 ng/ml) than in control subjects (untreated, 0.254 +/- 0.068, P = 0.0002; treated, 2.96 +/- 0.171 ng/ml, P = 0.0082). The enzymatic breakdown of histamine (treated/ untreated) in plasma was significantly decreased in patients with VKC (2.54 +/- 0.447) compared with control subjects (14.78 +/- 4.86; P = 0.0012). Skin reactivity to histamine was not increased in VKC. Tear histamine levels were significantly correlated to tear lymphocyte content in the general population and to tear basophils in the patients with tarsal-vernal VKC only. An increased number of tear eosinophils were correlated with elevated enzyme activity only in patients with tarsal-vernal VKC and to the clinical score only in limbal-vernal patients.

The enzymatic degradation of histamine was significantly decreased in patients with VKC compared with control subjects in both tears and plasma, suggesting that this dysfunction may be a primary factor in the pathophysiology of VKC.