Objectives: To investigate the expression of the open reading frame (ORF) 50 protein, a human herpesvirus 8 (HHV-8)-encoded immediate early protein, in HHV-8-associated diseases.
Study design/methods: We developed a rabbit anti-ORF50 protein polyclonal antibody, and investigated the ORF50 protein expression by immunohistochemistry using primary effusion lymphoma (PEL) cell lines and tissue sections of Kaposi's sarcoma (KS), HHV-8-associated solid lymphoma, and multicentric Castleman's disease (MCD).
Results: Western blot analysis revealed that this antibody reacted with a 110-kd protein in the lysate of phorbolester-stimulated HHV-8-infected PEL cell lines. Immunohistochemistry revealed that a very small population of cells expressed the ORF50 protein in KS and HHV-8-associated solid lymphoma cells, and almost all these cells expressed HHV-8-encoded latency-associated nuclear antigen. The ORF50 protein expression was also rare in the cells of PEL cell lines, and the staining pattern was diffuse or dot-like in the nuclei, overlapping partially with that of promyelocytic leukemia protein. In MCD, however, the ORF50 protein was expressed in some cells of the mantle zone of the lymphoid follicle.
Conclusions: The ORF50 protein expression in vivo was considerably rare in KS, HHV-8-associated solid lymphoma, and PEL, but was more frequent in MCD. Rare expression of this transactivator protein in HHV-8-associated malignancies causes low expression levels of other lytic proteins and may play a role in the maintenance of the latent infection. This is the first report describing the expression of an immediate early protein of HHV-8 in cases of HHV-8-associated diseases.