Obstacles to cancer immunotherapy: expression of membrane complement regulatory proteins (mCRPs) in tumors.

Monoclonal antibodies (mAbs) are being increasingly used in cancer therapy owing to their ability to recognize specifically cancer cells and to activate complement- and cell-mediated cytotoxicity and/or to induce growth arrest or apoptosis. The therapeutic potential of anticancer antibodies is significantly limited due to the ability of cancer cells to block killing by complement. Of the multiple resistance strategies exploited by cancer cells, the expression of membrane complement regulatory proteins (mCRPs), such as CD46 (membrane cofactor protein (MCP)), CD55 (decay-accelerating factor (DAF)), CD35 (complement receptor type-1 (CR1)) and CD59, has received most attention. CD46, CD55 and CD35 block the complement cascade at the C3 activation stage and CD59 prevents assembly of the membrane attack complex of complement (MAC). These proteins protect normal tissues from accidental injury by activated complement, but also confer resistance on cancer cells, thereby limiting the effect of complement-fixing monoclonal antibodies. Expression of mCRPs on malignant cells is highly variable, yet there is clear indication that certain tumors express higher mCRP levels than the normal tissue from which they have evolved. mCRP level of expression and cellular location may also vary during malignant transformation and between differentiated and undifferentiated tumors. Neutralizing anti-mCRP mAbs have been used in vitro to elucidate the significance of mCRP expression to the tumor complement resistance phenotype. In general, CD59 appears to be the most effective mCRP protecting tumor cells from complement-mediated lysis. Nevertheless, it acts additively, and in certain tumors even synergistically, with CD55 and CD46. It is envisaged that treatment of cancer patients with mCRP blocking antibodies targeted specifically to cancer cells in combination with anticancer complement-fixing antibodies will improve the therapeutic efficacy.