Angioimmunoblastic T cell lymphoma (AILT) is Key words: angioimmunoblastic T-cell lymphoma; immunohistochemistry; double labeling; clinicopathologic correlationAngioimmunoblastic lymphadenopathy with dysproteinemia was originally described by various groups 1-3 as a systemic lymphoproliferative disorder characterized by generalized lymphadenopathy, hepatosplenomegaly, fever, skin rash, anemia and immunologic abnormalities. Histologically, the lymph node structure is diffusely effaced by a hypocellular infiltrate including a mixture of small, medium-sized and large atypical lymphoid cells, plasma cells, eosinophils and histiocytes and a proliferation of high endothelial venules. 4 For some years it could not be clarified whether the disease should be considered a lymphoma 1,5,6 or an abnormal hyperimmune reaction, 2,4 because the fatal outcome was mostly attributed to severe infection rather than lymphoproliferation. 2,4 Subsequent studies, however, showed evidence of a clonal process demonstrating chromosomal abnormalities characteristic for T cell lymphoma 7-9 or a clonal rearrangement of T cell receptor genes. 10 -16 Based on these and immunophenotypic studies, the disease was incorporated into the updated Kiel classification of non-Hodgkin's lymphoma as "T cell lymphoma of angioimmunoblastic type". 17 Angioimmunoblastic lymphoma (AILT) has also been recognized by the REAL 18 and the World Health Organization (WHO) classification 19 as a separate disease entity of peripheral T cell lymphoma (PTCL). A morphological variant, AILT with hyperplastic germinal centers has been described recently. 20 It remains controversial whether the disease represents a homogeneous entity or should be divided into a preneoplastic and a neoplastic phase because some cases lack definite cytologic atypia and approximately 20 -30% of AILT showed no obvious evidence of clonality. [12][13][14][15][21][22][23][24][25] Criteria to predict an indolent clinical course have been proposed and were based on the absence of clear cells and lack of clonal T cell receptor gene rearrangement. 6,12,13,26 -29 Based on clinical data, others suggested the presence of clustered or sheet-like proliferations of immunoblasts or pale cells of T cell nature as a distinguishing criterion for AILD-like T cell lymphoma from reactive angioimmunoblastic lymphadenopathy. 6,28 The predictive value of these prognostic factors has not been settled so far.Ongoing research on AILT has also focused on the functional immunophenotype of putative tumor cells. These investigations proved to be rather difficult because of the morphologic heterogeneity of the cellular composition in AILT. Additionally, the nuclear morphology is not well preserved in frozen tissue sections. Although in most series the majority of cases seemed to be derived from CD4ϩ T cells, a minority of tumors was regarded to be CD8 ϩ . 12,13,16,20,23,24,27,30 -39 . In some studies, a considerable number of T cells was thought to be part of the reactive inflammatory background infiltrate of T cell lympho...

Nodal peripheral T-cell lymphomas are not well understood, and most of them are classified in the "not otherwise specified group" (PTCL-NOS). Data on their normal cellular derivation are ambiguous. Most peripheral T-cell lymphomas are composed of tumor cells and a (sometimes dominant) reactive background, which also includes resting and activated T-lymphocytes. We defined the phenotype of the tumor cells in 101 PTCL-NOS based on their cytological atypia and using immunohistochemical double stains on paraffin sections with CD4/Ki67 and CD8/Ki67. The results were correlated to clinical presentation and outcome. Lineage could be defined in 98 cases (97%). Tumor cells were CD4(+) in 43 cases and CD8(+) in 38. These presented at a younger age but a higher clinical stage compared with the CD4(+) lymphomas. In 15 cases, the atypical cells were CD4(-)CD8(-); two cases were CD4(+)CD8(+). Of 17 lymphoepithelioid (Lennert's) lymphomas, 15 expressed CD8, one each was CD4(+) and CD4(-)CD8(-).