There is evidence that the immune systems of patients with breast cancer are dysfunctional. Regulatory T cells (Tregs), and IDO, an immunosuppressive enzyme, are associated with more advanced disease in some cancers and may promote immunologic tolerance to tumors. Our aim was to assess whether expression of Foxp3, a marker of Tregs, and IDO were linked with nodal metastasis in breast cancer patients. Inhibitors of IDO are available and could potentially demonstrate utility in breast cancer if IDO drives progression of disease.

Sentinel lymph nodes (SLN) of 47 breast cancer patients with varying degrees of nodal disease and 10 controls were evaluated for expression of Foxp3 and IDO using immunohistochemistry. Positively stained cells were quantified and their distribution within the SLN noted.

The proportion of Foxp3⁺ cells was higher in SLN of cancer patients than controls (19% v. 10%, p < 0.001). Specifically, there were more Foxp3⁺ cells in SLN with metastasis than tumor-free SLN (20% v. 14%, p = 0.02). The proportion IDO⁺ cell in SLN of cancer patients was not statistically different than controls (4.0% v. 1.6%, p = 0.08). In order to demonstrate the combined immunosuppressive effect of Foxp3 and IDO, we categorized each SLN as positive or negative for Foxp3 and IDO. The Foxp3⁺/IDO⁺ group almost exclusively consisted of cancer patients with node positive disease.

In conclusion, our study shows that Foxp3⁺ cells are associated with more advanced disease in breast cancer, a finding that is proving to be true in many other cancers. As IDO has been found to promote differentiation of Tregs, IDO may become a suitable target to abrogate the development of T-cell tolerance and to promote an effective immune response to breast cancer. Our results about the combined expression of IDO and Foxp3 in metastastic SLN support this assumption.