A number of methods have been established for identifying sentinel nodes (SNs). In the present study, we attempted to clarify the immunological status of SNs with or without micrometastasis in breast cancer patients. SNs were identified by the dye- and γ probe-guided method. Total RNA was extracted from the SNs, and the expression of T-BET, GATA-3, and FOXP3 were evaluated using quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR). Micrometastasis was identified as microscopically negative but positive by RT-PCR specific for mammaglobin. Of 88 patients, 17 (19.3%) showed positive metastasis in SNs (pN1, 14; pN2, 3). Of the 71 metastasis-negative SNs, 11 showed positive bands on RT-PCR specific for mamma-globin [pN0(mol+)]. There was no significant correlation among clinicopathological features with or without micrometastasis. Immunological parameters were compared among the 60 pN0, 11 pN0(mol+), and 17 pN1-2. Although T-BET expression was higher in pN0(mol+) than pN0, FOXP3 expression was also higher in pN0(mol+) than pN0. In pN1-2, T-BET expression decreased compared with pN0(mol+), but FOXP3 expression did not. On the other hand, GATA-3 expression inversely increased in pN1-2 compared with pN0(mol+). In patients with breast cancer, micrometastasis can stimulate Th1 response in SNs. However, the Treg cell response is also induced at the micrometastasis level and persists during the progression of metastasis in SNs. Then, the shift in the Th1/Th2 balance may preferentially lean toward Th2 responses in pN1-2 SNs and suppress antitumor immune responses. Micrometastasis [pN0(mol+)] is a status immunologically distinguishable from pN0 and pN1-2.

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