TWO CASES OF LETHAL MIDLINE GRANULOMA THOUGHT TO BE OF NATURAL KILLER CELL ORIGIN

So-called lethal midline granuloma is of great clinical and theoretical interest. Recent evidence has shown that most lethal midline granulomas are associated with a T- cell phenotype and they are therefore referred to as nasal T- cell lymphomas (NTCL). Immunohistochemical studies, however, have shown peculiar phenotypic features such as expression of natural killer (NK) -cell-related markers and extensive T- cell antigen loss including absence of expression of α β T-cell receptor (TCR). In this study, we reported genotypic and immunohistochemical features in two cases of lethal midline granuloma. The histopathological diagnosis of the biopsy specimens was polymorphic reticulosis/midline malignant reticulosis. Both cases displayed a CD2+, CD3-, CD3e+, CD4-, CD8-, CD16-, CD56+ phenotype, suggesting that these tumors may be peripheral T- cell lymphomas with extensive loss of T- cell antigens and expression of NK cell antigen (CD56), or, alternatively, NK cell neoplasias. No TCR β gene rearrangement was detected in these cases. Monoclonal Epstein- Barr virus (EBV) genome was detected in each specimen by Southern blot hybridization. The tumor cells in one of the two cases expressed latent membrane protein (LMP).These findings support the concept that lethal midline granuloma constitutes a distinct group of lymphomas that, in addition to their peculiar clinical features, exhibits the phenotype of extensive loss of T- cell antigens and expression of the NK cell antigen, as well as harbors the EBV. In view of the LMP- transforming potential, these data suggest that EBV may play a role in the pathogenesis of lethal midline granuloma.