1100 Background: Recent studies show strong correlation between tumor infiltrating lymphocytes (TILs) and triple-negative breast cancer (TNBC) patient survival. CD8+ T cells serve as a favorable prognostic marker for TNBC. In addition, other cells such as CD4+ T cells, macrophages, B cells, and Tregs also infiltrate tumors. In this study, we delineate a strong relationship between the cycling kinetics of proliferating cells in TNBCs and antitumor immune response. Methods: A multi-institutional study performed by our group has previously shown that KAMS (Ki67-Adjusted Mitotic Score) provides a measure of the cycling kinetics of proliferating tumor cells and robustly stratifies TNBC patients into slow cycling (low KAMS) cyclophosphamide-methotrexate-fluorouracil (CMF)-responsive and fast cycling (high KAMS) CMF-resistant subgroups. In this study, we reviewed clinical data from 124 CMF-treated TNBC patients from Nottingham Hospital and sought correlations between cycling kinetics (High/Low KAMS) and tumor infiltrating immune cells. Results: We found that slow cycling TNBCs had higher mean expression of tumor infiltrating immune cells than fast cycling TNBCs. Intratumoral CD68 (p = 0.003), CD3 (p = 0.006), CD20 (p = 0.01), FOXP3 (p = 0.01), and total numbers of intratumoral and stromal CD68 (p = 0.01) and CD3 (p = 0.03) expressing cells were found to be significantly higher in low KAMS tumors than in high KAMS tumors. Of these biomarkers, CD68 was significantly associated with patients’ breast cancer-specific survival (BCSS): (a) low KAMS, high CD68 TNBCs had better BCSS than low KAMS, low CD68 (p = 0.01) TNBCs, and (b) high KAMS, low CD68 cases had better BCSS than high KAMS, high CD68 cases. Conclusions: Our observation that there are more TILs in slow cycling TNBCs suggests that there may be a dynamic cross-regulation between cycling kinetics and antitumor immune response. From our surprising observation that CD68 exerts polar roles in low/high KAMS subgroups, we propose that distinctions in M1 and M2 macrophage subsets in slow and fast cycling TNBCs may correlate with distinct outcomes. In addition, metabolic competition between tumor and immune cells may determine the level and function of TILs.