Abstracts (complete list) - Wissenschaft Online

Alexandra Doerr, Carsten Watzl, Michael Kirschfink
iC3b binding to Raji cells modulates Rituximab- induced
antibody-dependent cellular cytotoxicity (ADCC)
Malignant cells are protected against autologous complement by various mechanisms
including the overexpression of membrane-associated complement regulators, and
soluble complement inhibitors, secreted into the tumour microenvironment. This
complement resistance represents a major barrier for successful anti-tumour
immunotherapy. Conflicting results exist on the possible impact of complement on NK
cell-mediated cytotoxicity.
In the present study we investigated the ability of complement to increase antibodymediated
NK cell cytotoxicity, induced by Rituximab, a chimeric anti-tumour antibody,
which is directed against CD20 expressed on B-cells and successfully applied in
immunotherapy of Non-Hodgkin-B-cell lymphomas.
Raji cells were preincubated with C5 depleted human serum as source of complement
before exposure to freshly isolated NK cells. Complement- and NK cell- mediated
cytotoxicity was measured by 51 Cr release assay and binding of complement proteins to
tumour cell surface by cytofluorometry.
As Raji cells activate the alternative pathway of the complement cascade, a significant
amount of iC3b was deposited on the tumour cell surface and complement-mediated
tumour cell lysis (CDC) occured, even in the absence of Rituximab. Rituximab dosedependently
induced ADCC, as well as CDC and the deposition of iC3b in the presence
of normal human serum. Tumour-directed NK cell cytotoxicity increased after
preexposure to serum complement even in the absence of antibody. If Raji cells were
incubated with low concentrations of Rituximab in the presence of C5-depleted serum
leading to binding of iC3b on targeted Raji cells, ADCC of the tumour cells was
augmented. However, if Raji cells were exposed to higher concentrations of Rituximab
in the presence of C5 depleted serum, higher amounts of iC3b exerted an inhibitory
effect on NK cell lysis.
This demonstrates that the amount of deposited iC3b on the target cell determines if
complement has a positive or negative effect on NK cell-mediated ADCC.
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