Metastases from
renal cell carcinomas (RCC) are resistant to radiation and
chemotherapy but are relatively immunogenic. We have investigated the possibility to eliminate human RCC
micrometastases using MAb G250. G250 penetrates human
micrometastases completely in a spheroid model and induces
complement deposition rapidly on the outmost cell layers. However,
complement dependent cytotoxicity (CDC) was barely detected using either (51)
chromium release assays or confocal microscopy, due to relatively low expression of the G250
antigen and the effect of membrane bound
complement regulatory
proteins. Addition of blocking anti-CD59 MAbs enhanced formation of
C5b-9 and consequently
complement mediated lysis (13%).
Complement assisted cellular cytotoxicity (
CACC) was not detectable, although the
iC3b ligand and
CR3 receptor were present on respectively target and effector cells. Addition of soluble
beta-glucan induced the killing of MAb and
iC3b opsonized spheroids by effector cells (6-21%). Despite a lower affinity for G250
antigen, a bispecific anti-G250*anti-CD55 MAb enhanced cell killing in spheroids comparable to the parental
G250 MAb. Our results suggest that
complement-activating G250 in combination with anti-mCRP MAbs is able to kill human RCC cells in
micrometastasis in vitro. For
CACC the presence of CR3-priming
beta-glucan seems to be obligatory. In vivo, bi-MAb may be more effective as therapeutic agent due to its increased C5a generating properties.