CD99 immunoreactivity in gastrointestinal and pulmonary neuroendocrine tumours

Although considered a specific marker for Ewing's sarcoma/peripheral neuroe ctodermal tumour, the MIC2 gene product (CD99) has been immunolocalised in a variety of human tumours. The present study evaluated immunohistochemical ly the prevalence of CD99 expression in a series of 68 neuroendocrine tumou rs of different gastrointestinal and pulmonary sites. We now report on memb rane and/or granular cytoplasmic immunoreactivity in 25% of these tumours, independent of their anatomical sites. In lung neuroendocrine tumours, CD99 was preferentially confined to typical carcinoids (P=0.009). A statistical ly significant relationship was observed between the number of CD99 positiv e cells but not the immunostaining patterns and the presence of local invas ion and/or distant metastases (P<0.001). Moreover, there was a tendency for CD99-reactive tumours to show a reduced proliferative activity expressed b y a Ki67 index of 2% (P=0.119). The number of CD99 immunoreactive cells or patterns of immunoreactivity did not correlate with the presence of associa ted clinical syndrome or particular hormonal immunostaining. Although the m olecular basis underlying CD99 expression in neuroendocrine tumours is stil l poorly understood, our data suggest that CD99 may be involved in cell-to- cell adhesion of neuroendocrine tumour cells and in downregulation of their proliferative activity.