CD200: a New Target for Immunotherapy in Hematologic Malignancies.

CD200 is a transmembrane glycoprotein expressed on several tissues in rats and humans. It plays an immunoregulatory function by switching cytokine production from a T_H1 to T_H2 pattern, thus reducing cytotoxic response while indirectly enabling tumor escape and growth. Interestingly, CD200 is also a target for a novel humanized monoclonal antibody (Anti-CD200 MoAb, Alexion Pharmaceuticals, Cheshire, CT, USA). Anti-CD200 inhibits CD200 binding to its receptor, so modifying cytokine production and improving T-cell mediated cytotoxic response. Expression of CD200 has been already described in chronic lymphocytic leukemia/small lymphocyte lymphoma (CLL/SLL), multiple myeloma (MM) and acute myeloid leukemia (AML). Moreover, CD200 expression is considered an unfavorable prognostic factor in MM and AML. The goal of this work was to study, using flow cytometry, CD200 expression on a large number of onco-hematological samples, with the aim to exactly describe conditions in which anti-CD200 MoAb could be a therapeutic option. Analysis was conduced using a six-color FACSCanto II cytometer (Becton Dickinson, BD, San Jose, CA, USA) equipped with the FACSDiva software (BD). CD200 expression was evaluated by using CD200-PE-conjugated antibody (BD/Pharmingen). During the last year we analyzed CD200 expression in 184 samples: 131 bone marrow aspirates (BM), 33 peripheral blood specimens (PB) and 20 fine needle aspiration cytology samples (FNAC). One hundred and four were lymphoproliferative disorders, 12 MM, 16 myelodysplastic syndromes (MDS) and 52 acute leukemias. CD200 positivity was assigned to every single case when CD200 mean fluorescence intensity (MFI) was higher then 256 arbitrary units, a channel close to the cut-off point between positive and negative cells, in our experience. All results concerning our analysis are showed in the table. Three classes of hematologic neoplasms displayed a constant positivity for CD200 with a high level of MFI: CLL/SLL, hairy cell leukemia (HCL) and B-cell acute lymphoblastic leukemia (B-ALL). Lymphoplasmacytic lymphoma (LPL) was positive in 100% of cases but with lower MFI as compared to CLL/SLL, HCL and B-ALL (p<0.001). MM/MGUS plasma cells showed CD200 positivity in 80% of cases. CD200 was also expressed in cases of marginal zone lymphoma (MZL), mantle cell lymphoma (MCL), diffuse large B-cell lymphoma (DLBCL), T-non Hodgkin lymphoma (T-NHL), AML, acute hybrid leukemia (AHL) and T-cell acute lymphoblastic leukemia (T-ALL). Follicular Lymphoma (FL) samples were always negative but one, which expressed the antigen with a very low MFI. In AML we also compared CD200 expression with that of a large number of antigens (n=40), finding a statistically significant inverse correlation with myeloperoxidase (MPO-7) (Spearman’s r= −0.54; p<0.001). Difference in MFI between AML and B-ALL was statistically significant (p=0.003). No case of acute promyelocytic leukemia (APL) was positive for CD200. In conclusion, CD200 is candidate as a new specific target for immunotherapy with anti-CD200 in all cases of CLL/SLL, B-ALL and HCL as well as in selected cases of MZL, DLBCL, MCL, LPL, MM, T-NHL, AML, AHL and T-ALL.