CD200/OX2, a cell surface molecule with immuno-regulatory function, is consistently expressed on hairy cell leukaemia neoplastic cells

CD200 (formerly called OX2) is a transmembrane glycoprotein with immunosuppressive functions. It is expressed on normal B-lymphocytes, T-lymphocytes, dendritic cells and several solid tissues. CD200 receptor expression is limited to myeloid leucocytes and a subset of T-lymphocytes. In mouse systems, the binding of CD200 to its receptor decreases the production of T-helper cell type 1 (Th1)-like cytokines, such as interleukin (IL)-2 and interferon γ, increases the release of Th2-like cytokines, such as IL-10 and IL-4 and promotes the in vitro differentiation of T cells toward CD4+CD25+Foxp3+ Treg lymphocytes. CD200 is constantly overexpressed on chronic lymphocytic leukaemia (CLL) cells. The addition of CLL cells to mixed lymphocyte reactions causes an immunological shift from a Th1-like response to a Th2-like response, confirming that CD200 plays an important role in controlling T-cytotoxic immune response. Starting from these data, we extended the investigation of CD200 expression to another B-chronic lymphoproliferative disorder i.e. hairy cell leukaemia (HCL). As hairy cells display a specific immunophenotype, multicolour flow cytometry is currently the best tool for HCL diagnosis.A total of 10 specimens (six peripheral blood samples and four bone marrow aspirates), collected from 10 patients with newly diagnosed HCL, were studied. As normal controls we analysed 10 peripheral blood specimens and two bone marrow aspirates from 12 healthy donors. The following antigens were analysed: CD200, SmIg-kappa, SmIg-lambda, CD45, CD19, CD5, CD23, CD20, CD22, CD103, CD11c, CD25, CD43, CD10, CD3, CD56 and CD81. In this study, we found CD200 positivity on all hairy cells from all HCL patients studied, with a large amount of this antigen on neoplastic cells. CD200 expression pattern, combined with its molecular connexions toward suppression of T-cytotoxic response and hairy cell growth makes Anti-CD200 MAb an intriguing option for new therapeutic strategies in HCL.