CD4+ T lymphocyte infiltration correlates with regression of a UV-induced squamous cell carcinoma

It is currently unknown which arm of the immune system is responsible for regression of tumours. We have studied the T lymphocytes infiltrating a spontaneously regressing murine squamous cell carcinoma during the growth, plateau and regression phases of tumour development. In the plateau phase, where tumour growth is partially controlled by the immune system so that tumour size remains static, there was a considerable influx of CD4+ cells into the tumour. There was also an increase in the number of cells expressing the receptor for interleukin 2 (IL-2R), indicating that these cells were probably activated. The number of CD4+ cells remained high during the regression phase, where immunological destruction exceeded tumour growth. In contrast, CD8+ cells were only present in low numbers, and did not change during growth or regression of the tumours. These results indicate that CD4+ cells are probably responsible for tumour destruction. Thus CD4+ T lymphocytes are able to mediate tumour rejection and should be given more consideration for immunotherapy.