Evaluation of CD52 Expression in Hematopoietic Neoplasms by Standard Immunohistochemistry: Implications for the Expanded Use of Alemtuzumab (CAMPATH-1H) in the Treatment of Hematological Malignancies.

CD52 is a GPI-linked glycoprotein expressed by B cells, T cells, monocytes and macrophages. The humanized monoclonal antibody alemtuzumab (CAMPATH-1H) is specific for CD52 and is FDA-approved for the treatment of relapsed or refractory chronic lymphocytic leukemia (CLL). The utility of alemtuzumab in the treatment of other lymphoid and non-lymphoid malignancies has been recently explored; however, a comprehensive survey of CD52 expression among the various classes of hematopoietic neoplasms has not been completed. In addition, most methods of detecting CD52 rely on flow cytometric techniques that cannot be performed in a retrospective manner. Here we present methods for the reliable detection of CD52 on fixed, paraffin-embedded human bone marrow and lymphoid tissue samples using standard immunohistochemical staining methods and a non-humanized form of CAMPATH (CAMPATH-1G; Serotec, UK). Over 200 cases of B and T cell lymphomas and leukemias were evaluated. As summarized below (see table), the vast majority of low-grade B cell lymphoproliferative disorders (CLL/SLL, follicular lymphoma, lymphoplasmacytic lymphoma, mantle cell lymphoma) express CD52. Interestingly, some heterogeneity was noted within certain of these tumor types. For example, plasma cells were largely negative in lymphoplamsmacytic lymphomas, and proliferation centers were more intensely positive in CLL/SLL. In addition, we found that the majority of precursor B-cell acute lymphoblastic leukemia/lymphomas express this antigen. In contrast, there is some heterogeneity in CD52 expression among diffuse large B cell lymphomas, with approximately 22% of these tumors expressing undetectable or extremely low levels of CD52. Neoplasms of the T cell lineage also show marked heterogeneity in CD52 expression. The majority of precursor T cell acute leukemia/lymphomas and anaplastic large cell lymphomas showed no detectable CD52 expression; however, among the remaining T cell tumors, there was more pronounced tumor to tumor variation in CD52 expression. Among acute myeloid leukemias, we found CD52 expression only in AML-M4Eo. Finally, all cases of Hodgkin lymphoma and multiple myeloma tested were negative for CD52 expression. Together, these findings expand the subclasses of hematopoietic neoplasms that are rational candidates for alemtuzumab (CAMPATH-1H) therapy, and, in addition, reveal that CD52 is not universally expressed by lymphoid malignancies. Thus, target validation using this standard immunohistochemical procedure can now be readily performed on a case-by-case basis and is likely to be essential to guide alemtuzumab (CAMPATH-1H) therapy for hematological tumors.