A novel CD11c monoclonal antibody effective in formalin-fixed tissue for the diagnosis of hairy cell leukemia.

The diagnosis of hairy cell leukemia (HCL) and its distinction from other B-cell lymphomas can be difficult in formalin-fixed tissue. This is because the histology is not always classical, and despite having a characteristic phenotype, there are few relevant monoclonal antibodies with sufficient sensitivity and specificity that can be applied to fixed material. In this study, we assessed the utility of a newly developed antibody against a formalin-resistant CD11c epitope (5D11) for the diagnosis and monitoring of HCL in formalin-fixed tissue. We performed immunohistochemical staining for CD11c expression in formalin-fixed and also decalcified tissue of 196 small B-cell lymphomas, including 104 cases of HCL showing extensive to minimal infiltrates. The CD11c antibody was both sensitive and specific for HCL, even in cases with minimal infiltration. We recommend that this CD11c antibody be added to a panel of antibodies for immunostaining of formalin-fixed material, for differentiation of HCL from other small B-cell lymphomas, and for detection of residual disease following therapy.