Identification of tumours with the CD43 only phenotype during the investigation of suspected lymphoma: A heterogeneous group not necessarily of T cell origin

Aims: CD43 is usually employed as a T cell marker in the immunophenotypic work-up of suspected cases of non-Hodgkin's lymphoma (NHL). In this setting, tumours expressing CD43 in the absence of other T or B cell markers (CD43 only phenotype) are rare. We present four cases with this aberrant phenotype seen at our institution. Methods: The CD43 only phenotype was defined as expression of CD43 in the absence of expression of B cell markers CD20 and CD79a, and T cell markers CD3 and CD5, on initial immunohistochemistry performed on biopsies of suspected NHL. Combinations of further immunohistochemistry, flow cytometry, cytogenetic analysis and molecular studies were used to enable further diagnosis and lineage assignment. Results: The four cases were subsequently diagnosed as: one case of extramedullary acute myeloid leukaemia, one case of null cell anaplastic large-cell lymphoma, and two cases of extranodal diffuse large B cell lymphoma. None were demonstrated to be of T cell origin. Conclusions: Our series further confirms the lack of specificity of CD43 expression for T cell lineage. Documentation of the CD43 only phenotype in suspected cases of NHL therefore requires further investigation to both correctly diagnose and clarify lineage of these tumours.