Delayed hypersensitivity in tumor-bearing mice. In vitro activation of "eclipsed" spleen cells.

At various stages during the progressive growth of a transplanted sarcoma in BALB/c mice, the delayed hypersensitivity response to tumor antigen was determined using the food-pad swelling test (FPS) and the leukocyte migration inhibition assay (LMI). A close correlation was observed between the in vivo and in vitro assays. "Early" recognition of tumor antigen was detected 24 h after tumor inoculation by both techniques and this positive response was maintained until day 15. As the tumor grew larger, the delayed hypersensitivity response in vivo vanished, while the delayed hypersensitivity response in vitro disappeared about 3 days later. This suppression or "eclipse" of the anti-tumor cellular immune response was specific for the type of tumor used, and could be reversed in vitro by means of a low pH treatment of lymphoid cells.