CD21 antigen in T‐lineage neoplastic lymphoid cells: Characteristic expression at thymic stage

The expression of CD21 antigen, a receptor for the C3d fragment of complement and Epstein‐Barr vlrus (EBV), was Investigated in a total of 85 cases of neoplastic lymphoid cells Including 39 cases of T‐lineage acute lymphoblastic leukemia (ALL)/lymphoblastic lymphoma (LBL), although CD21 antigen is usually regarded as a pan‐B antigen. The CD21 antigen was expressed by one of the eight cases of neoplastic lymphoid cells expressing the CD7 antigen as a sole pan‐T antigen, by three of the 20 cases of pro‐ or early thymic stage (CD7+ CD5+ CD2‐, CD7+ CD5‐ CD2+, or CD7+ CD5+ CD2+), and ten of 11 cases of thymic stage (CD3r CD4+ CD8+), but not by one case of late thymic stage (CD3 ± CD4+ CD8‐) T‐ALULBL cells. The CD21 antigen was not expressed by any of the 11 cases of adult T‐cell leukemia (ATL) or two cases of chronic T‐lineage leukemia. At most 4% of the normal thymocytes obtained from seven infants or children expressed the CD21 antlgen. While only a very limited population of normal thymocytes expresses CD21 antigen, T‐ALL/LBL cells at the thymic stage characteristically express CD21 antigen in contrast to pro‐ or early thymic ALL/LBL or peripheral‐stage neoplastic T cells. The estimation of the expression of CD21 antigen is useful for delineating stages of differentiation in T‐ALL/LBL. Furthermore, these observation are notable, considering the possibiilty that the reported EBV‐carrying T‐cell lymphomas result from the penetration of EBV into EBV‐negative neoplastic T cells. © 1994 Wiley‐Liss, Inc.