Kinetics of serum factors mediating blocking, unblocking and antibody‐dependent cellular cytotoxicity in hamsters given isografts of para‐7 tumor cells

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Abstract
Sera collected at intervals after PARA-7 tumor-cell inoculation were monitored for blocking, unblocking, and antibody-dependent cellular cytotoxicity (ADCC) using the microcytotoxicity test. Within 7 days after isografting, significant levels of blocking were demonstrated. This activity increased in parallel with tumor growth and reached a maximum at day 21. In contrast, the capacity of the sera to mediate ADCC was maximal at day 7, and no longer detectable at 3 weeks. Unblocking was not demonstrable in tumor-bearer sera. Further studies showed that, when the tumor was excised, blocking activity fell rapidly while ADCC was elevated. Sera collected 7 days after surgery could neutralize the blocking activity of tumorbearer sera. Significant levels of unblocking were still evident 10 weeks after surgery while ADCC had generally disappeared after 5 weeks. These findings illustrate the complexity of the hamster's humoral immune response to tumor cells, and suggest that (1) the sequestering of antibody by circulating tumor antigen accounts for the demise of ADCC in tumorbearer sera and the concomitant appearance of blocking activity; and (2) the serum factors mediating ADCC and unblocking differ.

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