Alterations in the normal balance of T lymphocyte subpopulations are common in advanced cancers. These changes may occur in multiple compartments such as blood, lymph node and tumors and include decreases in effector T cells (CD4+ and CD8+) and increases in regulatory T cells (Treg).1–4 Treg, a distinct subset of the CD4+ T-cell population, are of particular concern in cancer patients because they play an important role in negatively regulating the development of antitumor immune responses. Normally functioning to prevent harmful autoimmune responses, Treg have been shown to directly suppress immune responses to tumors in mouse tumor models.5–7 Treg-induced immune suppression also appears to occur in humans as evidenced by clinical studies that correlate high Treg numbers with impaired immune function.4,8–10 In people with metastatic melanoma, Treg are present in high numbers in tumor-draining lymph nodes where they directly inhibit the function of infiltrating T cells.10
In addition to an association with immune dysfunction, Treg numbers correlate with clinical outcomes for many tumors. In people with head and neck cancer, the percentage of circulating Treg increases with stage of disease and is predictive of early recurrence.11 The magnitude of Treg infiltration within tumors is of prognostic importance for human malignancies such as melanoma, lymphoma, and carcinomas of breast and ovaries.1,12,13 For some tumors, the ratio of CD8+ T cells to Treg in tumor tissues is a better predictor of outcome than measurement of Treg alone.14 Therefore, there is considerable evidence to support the prognostic value of Treg measurement as well as their critical role in regulating immune responses to tumors.
There has been little research investigating the role of Treg in diseases of companion animals. In a study of cats with chronic viral infections, Treg were identified based on surface expression of CD4 and CD25 (the high-affinity interleukin-2 receptor) and were found to be increased compared with healthy animals.15,16 Similar studies in dogs have been limited by the availability of Treg-specific cell surface markers. However, we previously reported that intracellular detection of FoxP3, a transcription factor expressed in high levels within human and murine Treg, can be used to identify canine Treg.17–20 FoxP3 protein expression in feline and porcine CD4+CD25+ lymphocytes also was recently reported, suggesting that this marker may be a useful tool for Treg identification in multiple species.21,22
The results of our previous work show that Treg are increased in dogs with cancer compared with healthy dogs and suggest that tumor-specific differences may influence Treg numbers in different tumor types.17,23 However, too few dogs with osteosarcoma (OSA) were included in these studies to determine the importance of Treg in this disease. In addition, evaluation of the association between Treg numbers and clinical outcome has not been previously reported for dogs. Therefore, we designed a prospective study to evaluate T cell subpopulations in the blood, lymph nodes, and tumors of dogs with OSA and to investigate their association with clinical outcomes.