Immune evasion by CD200: New approaches to targeted therapies for chronic lymphocytic leukemia (CLL)

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Background: Although the human immune system is capable of raising an immune response against many cancer types, that response is insufficient to eradicate the cancer in most patients, possibly due to immune evasion through negative regulation of the immune system by the tumor. We identified the immune-suppressive molecule CD200 to be upregulated 1.5–5.4-fold on CLL cells in all 80 patients examined. Interaction of CD200 with its receptor alters cytokine profiles from Th1 to Th2 in mixed lymphocyte reactions, and results in the induction of regulatory T cells, which are thought to hamper tumor-specific effector T cell immunity. We addressed whether CD200 expression on tumor cells plays a role in immune evasion, thereby preventing elimination of tumor cells by the immune system in a xenograft hu/SCID mouse model, and whether treatment with an antagonistic anti-CD200 antibody affects tumor growth. Methods: The human non-Hodgkins lymphoma cell lines RAJI and Namalwa were transduced with human CD200 and injected subcutaneously together with human peripheral blood lymphocytes (PBL) into NOD/SCID mice. Tumor growth over time was compared among mice that either received CD200-expressing tumor cells or received tumor cells lacking CD200 expression. In subsequent experiments, mice were treated with chimeric or humanized anti-CD200 antibodies (doses ranged from 1 to 20 mg/kg) by intravenous injection. Treatment was either started immediately or 7 days after tumor cell injection. Results: As expected, PBLs reduced CD200-negative RAJI or Namalwa tumor growth by up to 75%. In contrast, growth of RAJI or Namalwa tumors expressing CD200 at levels comparable to that of CLL was not reduced by PBLs. Administration of anti-CD200 antibodies at 5 mg/kg resulted in nearly complete tumor growth inhibition (1/10 mice developed a small tumor) over the course of the study even when treatment was started 7 days after tumor cell injection. Conclusions: CD200 expression on tumor cells inhibits the ability of human lymphocytes to eradicate tumor cells. Treatment of CD200-expressing tumors with antagonistic anti-CD200 antibodies inhibits tumor growth, indicating the potential for anti-CD200 therapy as a promising approach for CLL.

No significant financial relationships to disclose.

The ENCEL system: a somatic cell protein delivery system

A wide variety of somatic cells are being explored for the introduction of foreign genes with a view toward gene therapy. A prime requirement for successful gene therapy is the sustained expression, effective dosing, and systemic delivery of the therapeutic gene product. Microvascular endothelial cells offer several advantages over other cell types as a somatic cell gene delivery vehicle in that they provide direct secretion of protein into the blood stream and they are amendable to highly stable retroviral-based protein expression. Importantly, they also offer a large surface volume to size ratio in that they can be induced with angiogenic factors to form organized capillary-like structures in vitro when grown in a three dimensional culture system using collagen gels. These genetically-modified capillary endothelial cells (the ENCEL system) maintained in collagen gels can be stably transplanted and removed. The unique biological properties of microvascular capillary endothelial cells allows the ENCEL system to provide large numbers of cells in a small volume which offers the highly desired opportunity for providing a sustained and effective dose of a therapeutic protein. Alexion is currently applying its Unigraft immunotherapeutic and engineering technologies to commercialize a non-human ENCEL system acceptable for implantation into any patient.

Perforating and non-perforating indications for repeated operations in Crohn's disease: evidence for two clinical forms

The surgical indications in 770 patients with Crohn's disease undergoing intestinal resection at The Mount Sinai Hospital from 1960-83 have been reviewed. Surgical indications were divided into two principal categories: 375 cases with perforating indications and 395 cases non-perforating. Among 292 patients who underwent second operations for recurrent Crohn's disease, the indications for second operation were closely dependent on the indication for primary resection. Second operations were undertaken for perforating indications much more often among cases where the initial indication had been perforating, than among those whose initial indications had been non-perforating (73% v 29%, p less than 0.00001). This trend to similarities in the indications which bring patients to surgery was maintained within each anatomical category of Crohn's disease and even between second and third operations (p less than 0.001). Operations for perforating indications were followed by reoperation approximately twice as fast as operations for non-perforating indications, whether going from first to second operation (perforating 4.7 v non-perforating 8.8 years, p less than 0.001), or from second to third (perforating 2.3 v non-perforating 5.2 years, p less than 0.005). Crohn's disease thus seems to occur in two different clinical patterns, independent of anatomic distribution. These are a relatively aggressive perforating type and a more indolent non-perforating type, which tend to retain their identities between repeated operations and to influence the speed with which reoperation occurs.