A systemic antitumor immune response prevents outgrowth of lung tumors after i.v. rechallenge but is not able to prevent growth of experimental liver tumors.

Although a systemic antitumor immune response by antibodies or T cells is often detected in cancer patients, this response mostly does not result in tumor rejection. The beneficial effect of tumor vaccination on survival rates is limited as tumor response is low. In contrast to solid tumors, circulating tumor cells may be more easily accessible and therefore destroyed by the immune system and thus prevent metastases. This discrepancy is further clarified in our study, by assessing the effect of a systemic immune response on established liver tumors and circulating tumor cells. Male Wag/Rij rats were inoculated with CC531 colorectal tumor cells subcapsulary in the liver, with or without immune suppression (60 mg/kg cyclophosphamide). To study the effect of a systemic immune response, rats received CC531 tumor cells intravenously and three weeks later the number of lung tumors was assessed. Presence of specific anti-CC531 antibodies in serum was determined by flow cytometric analysis at times of inoculation, i.v. tumor cell administration and sacrifice. Rats with liver tumors and subsequent rechallenge produced anti-CC531 IgG antibodies and did not develop lung tumors, whereas without existing liver tumors, rats developed lung tumors upon i.v. administration of CC531 tumor cells. Liver tumors in rats with and without i.v. CC531 tumor cell administration were equal in size. These results showed that a systemic immune response, induced upon liver tumor induction and rechallenge, prevented formation of lung tumors but did not affect tumor growth in the liver. Possibly the immune response lacked the ability to penetrate the protective extracellular matrix surrounding the established liver tumors, which prevented the tumor cells from recognition by and contact with cells of the immune system.