Allogeneic stem cell transplantation together with donor lymphocyte infusion represents the best curative option for many haematological malignancies. Donor T lymphocytes play a central role in the elimination of leukemic cells (Graft versus Leukemia, GvL) and acceptance of the graft. Unfortunately, these donor T cells also respond to the patients’ own tissue, resulting in Graft versus Host Disease (GvHD).

In a promising strategy to control GvHD, while at the same time maximally utilizing the GvL effect, genetically-modified donor T cells, expressing a suicide gene, are selectively eliminated in vivo in case of severe GvHD. Genetic transfer of the human CD20 molecule to donor T cells prior to transplantation permits selective elimination via CD20-reactive reagents, such as the widely used chimeric Ab rituximab (RTX). RTX activates complement (CDC), and recruits effector cells (ADCC) for elimination of CD20-positive cells.

Using a novel experimental system we previously showed that the efficiency of RTX-mediated killing of CD20-positive T cells clearly depends on the level of CD20 expression. As a consequence, RTX is not able to kill all genetically modified CD20+ T cells.

Here we investigated whether a novel fully human CD20 Ab, HuMab 7D8, would be better in activating complement and/or effector cells. Our results show HuMab 7D8 to be more powerful in activating complement in CDC-assays. To obtain a similar percentage cell kill compared with RTX, a 100-fold lower dose of HuMab 7D8 was needed and a 4 times lower human serum concentration. To study the effect of the number of CD20 molecules per cell we used CEM-CD20 clones (N=26), all with different CD20-expression levels, in an assay in which all other parameters were kept constant. The results show that HuMab 7D8 is significantly stronger in inducing CD20-mediated cell kill compared to RTX. Especially cells expressing very low levels of CD20, which are completely resistant to RTX-induced CDC, are lysed with high efficiency by HuMab 7D8 and complement. In contrast to RTX, HuMab 7D8 was able to eliminate 100% of transduced T cells in the presence of human serum, and effector cells. At present, studies with CD20-luciferase positive human T cells are conducted in mice to study the in vivo effect of HuMab 7D8 and RTX. These results demonstrate that the CD20 molecule represents an attractive suicide molecule and CD20-transgenic T cells can with high efficiency be eliminated by HuMab 7D8.

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