Although the immune system is capable of mounting a response against many cancers, that response is insufficient for tumor eradication in most patients due to factors in the tumor microenvironment that defeat tumor immunity. We previously identified the immune-suppressive molecule CD200 as up-regulated on primary B cell chronic lymphocytic leukemia (B-CLL) cells and demonstrated negative immune regulation by B-CLL and other tumor cells overexpressing CD200 in vitro. In this study we developed a novel animal model that incorporates human immune cells and human tumor cells to address the effects of CD200 overexpression on tumor cells in vivo and to assess the effect of targeting Abs in the presence of human immune cells. Although human mononuclear cells prevented tumor growth when tumor cells did not express CD200, tumor-expressed CD200 inhibited the ability of lymphocytes to eradicate tumor cells. Anti-CD200 Ab administration to mice bearing CD200-expressing tumors resulted in nearly complete tumor growth inhibition even in the context of established receptor-ligand interactions. Evaluation of an anti-CD200 Ab with abrogated effector function provided evidence that blocking of the receptor-ligand interaction was sufficient for control of CD200-mediated immune modulation and tumor growth inhibition in this model. Our data indicate that CD200 expression by tumor cells suppresses antitumor responses and suggest that anti-CD200 treatment might be therapeutically beneficial for treating CD200-expressing cancers.

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These data suggest that melanoma, ccRCC and ovarian tumor cells can express CD200, thereby potentially suppressing anti-tumor immune responses. CD200 blockade with an antagonistic antibody may permit an effective anti-tumor immune response in these solid tumor types.

CD200 is an immunosuppressive molecule overexpressed in multiple hematologic malignancies such as B cell chronic lymphocytic leukemia, multiple myeloma, and acute myeloid leukemia. We previously demonstrated that up-regulation of CD200 on tumor cells suppresses antitumor immune responses and that antagonistic anti-human CD200 mAbs enabled human PBMC-mediated tumor growth inhibition in xenograft NOD/SCID human (hu)-mouse models. Ab variants with effector function (IgG1 constant region (G1)) or without effector function (IgG2/G4 fusion constant region (G2G4)) exhibited high antitumor activity in a human tumor xenograft model in which CD200 was expressed. In this report, we seek to select the best candidate to move forward into the clinic and begin to decipher the mechanisms of tumor cell killing by comparing anti-CD200-G1 vs anti-CD200-G2G4 in two related animal models. In a CD200-expressing xenograft NOD/SCID hu-mouse model where CD200 ligand/receptor interactions are already established before initiating treatment, we find that anti-CD200-G1 is a less effective Ab compared with anti-CD200-G2G4. Separately, in a model that evaluates the effect of the Abs on the immune cell component of the xenograft NOD/SCID hu-mouse model distinctly from the effects of binding to CD200 on tumor cells, we find that the administration of anti-CD200-G1 Abs completely abolished human PBMC-mediated tumor growth inhibition. Along with supporting in vitro studies, our data indicate that anti-CD200-G1 Abs efficiently mediate Ab-dependent cellular cytotoxicity of activated T cells, critical cells involved in immune-mediated killing. These studies suggest important implications regarding the selection of the constant region in anti-CD200 immunotherapy of cancer patients.

Multiple cancer vaccine trials have been carried out using ex vivo generated autologous dendritic cells (DCs) loaded with tumor antigen before readministration into patients. Though promising, overall immunologic potency and clinical efficacy might be improved with more efficient DC-based therapies that avoid ex vivo manipulations, but are instead based on in vivo targeting of DCs. For initial in vivo proof of concept studies, we evaluated targeting of proteins or peptides to DCs through DC-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN). Because the biology of DC-SIGN is different between mice and humans, we assess human DC-SIGN targeting in the setting of elements of a human immune system in a mouse model. Administration of anti-DC-SIGN antibodies carrying either tetanus toxoid peptides or keyhole limpet hemocyanin (KLH) to Rag2gammaC mice reconstituted with human immune cells raised stimulatory human T-cell responses to the respective antigen without additional adjuvant requirements. Furthermore, administration of anti-DC-SIGN antibody-KLH conjugate enhanced the adjuvant properties of KLH resulting in inhibition of RAJI (Human Burkitt's Lymphoma Cell Line) cell tumor growth in Nonobese Diabetic/Severe Combined Immunodeficient mice transplanted with human immune cells. Thus, mouse models reconstituted with human immune cells seem to be suitable for evaluating DC-targeted vaccines, and furthermore, targeting to DCs in situ via DC-SIGN may provide a promising vaccine platform for inducing strong immune responses against cancer and infectious disease agents.