N-terminal PAX8 polyclonal antibody shows cross-reactivity with N-terminal region of PAX5 and is responsible for reports of PAX8 positivity in malignant lymphomas

Recently, reports using immunohistochemistry and a polyclonal antibody directed against the N-terminal region of PAX8 describe PAX8 expression in malignant lymphomas. As the N-terminal regions of PAX family members, including the B-cell transcription factor PAX5, have high sequence homology, we investigated PAX8 positivity in malignant lymphomas. Comparative sequence analysis between the N- and C-terminal regions of PAX8 and PAX5 proteins confirmed homologies of 70% and 39%, respectively. We then compared the results using N-terminal (high homology) and C-terminal (lower homology) anti-PAX8 antibodies to assess PAX8 expression in reactive tissues, diffuse large B-cell lymphoma and classical Hodgkin lymphoma, using routine immunohistochemical methods. Expression of PAX8 was also assessed in diffuse large B-cell lymphoma and classical Hodgkin lymphoma cell lines using real-time qRT-PCR methods. Our results show that reactive and neoplastic B-cells are positive for PAX8 using the N-terminal antibody, but negative for PAX8 when the C-terminal antibody was used. PAX8 mRNA levels were not detected in any of the B-cell lymphoma cell lines studied. These results indicate that benign and malignant B-cells do not express PAX8. We conclude that positivity for PAX8 reported by others in B-cell lymphomas is likely due to cross-reactivity between the N-terminal regions of PAX8 and PAX5, due to the high sequence homology of these two regions.