Regulatory T cells (Tregs) play an important role in the suppression of anti-tumour T cell responses, yet their exact mechanism of action is not fully understood. We developed a novel mouse model of CD4 T cell-mediated tumour rejection in order to determine how Tregs affect CD4 T cell responses to tumour-derived neoantigens.

Adoptive transfer of naïve tumour-specific CD4 T cells rapidly eradicated large subcutaneous B16 tumours in lymphopaenic mice despite lack of direct tumour recognition due to MHC class II mismatch. Tumour clearance was dependent on efficient T cell expansion and IFNg production, and the recruitment of neutrophils and macrophages into the tumour. Tumour recurrence observed in some animals was invariably associated with poor T cell expansion and early conversion into FoxP3-expressing Tregs that accumulated in the draining lymph nodes and within the tumour tissue. Depletion of induced Tregs did not rescue the T cell response and failed to improve survival.

Adoptive transfer of CD4 T cells into immunosufficient tumour-bearing hosts did not result in tumour rejection, in agreement with lack of significant T cell expansion or Th1 differentiation. In contrast, depletion of Treg prior to T cell transfer triggered acute tumour rejection and dramatically improved long-term survival. These findings were verified in a series of experiments where antigen-specific T cells and antigen-specific Tregs were adoptively co-transferred into lymphopaenic hosts. Tregs markedly suppressed CD4 T cell expansion and Th1 differentiation, and prevented tumour rejection.  Early (day 3) but not late (day 10) Treg depletion rescued the T cell response and improved survival.

Our results indicate that the effect of Tregs on anti-tumour responses is complex and that therapeutic benefit of Treg depletion is limited to the early stages of T cell responses to tumour neoantigens.