Studies on the properdin system in rats bearing the transplantable human carcinoma HR132.

In an attempt to explain the natural resistance •¿ of animals to heterologously transplanted tumors, it was theorized that the protective mechanism may be due to the properdin system (1,3). To test this hypothesis it was chosen to study the effect of zymosan on the transplantable human carcino ma of the colon (HR132) carried in weanling Wistar rats (2). Zymosan was selected rather than properdin, because properdin was unobtainable. It may be recalled that zymosan is an insoluble carbohydrate •¿ complex d rived from yeast cell walls, that has the property of combining with properdin in vivo and thus influencing the properdin level of the blood (6). By actual determination, Pillemer and Ross (5) had previously shown that intravenous injec tions of small doses (5 and 25 mg/kg) of zymosan produced a drop in properdin titer followed, in 2-14 days, by a rise to as high as 300 per cent above normal. Intravenous injections of larger doses (125 mg/kg) produced even a greater initial fall in properdin titer with a return to only 75 per -cent of normal in 6-10 days. Using these findings as a basis, we succeeded in conditioning normally resistant weanling Wistar rats to HR132 by first injecting the animals with varying amounts of .zymosan (2). In the experiment, out of a total of 160 animals in each category the following tumor "takes" were obtained: nineteen in nontreated