CD200 Expression May Help in Differential Diagnosis between Mantle Cell Lymphoma (MCL) and B-Cell Chronic Lymphocytic Leukemia (B-CLL).

Mantle cell lymphoma (MCL) is a B-cell malignancy that shares many morphologic and immunophenotyping features with B-cell chronic lymphocytic leukemia (B-CLL). However, these two entities are characterized by a very different prognosis and response to therapies. Differential diagnosis is assessed by a panel of monoclonal antibodies (moAbs) to screen the expression of CD5, CD23 and Cyclin D1. Nevertheless, some of these antibodies work better on fresh or frozen tissues; for example, on fixed tissues (expecially by B5) CD5 and Cyclin D1 can give equivocal or even negative results. In addition, it is cumbersome to evaluate Cyclin D1 by flow cytometry. CD200 (previously referred to as OX2) is a membrane glycoprotein, belonging to the immunoglobulin superfamily, expressed on T and B lymphocytes (but not on NK cells) and it might play an immunosuppressive role. Its expression has also been reported on human myeloma plasmacells and B-CLL cells. We investigated the expression of CD200 on fresh neoplastic cells of 90 patients (82 B-CLL and 8 MCL in leukemic phase) by flow cytometry, using a mouse IgG1 anti-human moAb (MRC OX-104, BD Pharmingen, U.S.A.). In our series, CD200 was present on neoplastic cells of all 82 B-CLL (expression on 62–100% of CD5+ cells, mean 96%, standard deviation 7%). On the contrary, in MCL patients (Cyclin D1+ and/or t(11;14) FISH+) CD200 was positive in less than 20% of CD5+ cells in 3 subjects (4–18%) and totally absent in the remaining 5 (two-sided Fisher’s exact test p<0.0001). To further extend the study, we examined CD200 by immunohistochemistry on paraffin-embedded formalin-fixed lympoid tissues and Lowy-fixed bone marrow (BM) trephine biopsies from 18 B-CLL (12 lymphnodes and 7 BM samples) and 19 MCL (11 lymphoid tissues and 9 BM) patients, using a goat anti-human IgG affinity-purified polyclonal antibody (R&D Systems, U.S.A.). Again, all B-CLL neoplastic cells were positive for CD200 both in lymphnodes and in BM while all MCL cells were negative, both in lympoid tissues and in BM (two-sided Fisher’s exact test p<0.0001). Notably, in all MCL CD200-negative lympoid tissue biopsies, it was possible to observe CD200+ residual dendritic cells (useful as immunohistochemistry reaction positive control). Moreover, results were consistent in old archival samples and even in non-perfectly prepared tissues referred to our Center. We would propose to add CD200 in flow cytometry and immunohistochemistry routine panels, as it can be of diagnostic utility in distinguishing between MCL and B-CLL, in particular in mantle cell leukemia patients. Finally, these data have to be considered in view of the proposed targeted therapy with anti-CD200, to eventually exclude MCL patients.