Tumor-blocking and -inhibitory serum factors in the clinical course of canine venereal tumor.

An in vitro method for growing colonies of canine transplantable venereal tumor cells in a semisolid agar medium is described. Using autologous or pooled homologous normal dog sera as a feeder layer, 49.3 +/- 3.5 and 47.5 +/- 4.5 tumor colonies were obtained, respectively, when 2 X 10(4) tumor cells were plated. With this assay system, assessment of colony counts provided an accurate and rapid technique for monitoring serum factors in tumor-bearing dogs that inhibited colony formation, or blocked the inhibition of colony formation. In 52 dogs given tumor transplants, a direct correlation was demonstrated between serum activity tested in vitro and the in vivo growth characteristics of the tumor. Tumor cells preincubated with serum from dogs with active tumor growth consistently showed normal colony growth when cultured in agar containing colony-inhibitory sera (blocking effect). In vivo regression was characterized by loss of serum blocking and the development of serum colony-inhibitory activity in culture. In metastatic disease, only blocking activity could be identified, while persistent local invasive disease was characterized by low levels of both blocking and inhibitory serum activity. The sensitivity of this technique coupled with its in vivo predictive capabilities provides a model for monitoring serological responses to a naturally occurring neoplasm in a large, randomly bred animal.