The identification of memory T cells with stem cell-like properties has been reported recently in both humans and mice. Such stem cell-like memory T cells (Tscm) upon adoptive transfer into a humanized mouse model had superior self-renewal, proliferation and mediated potent anti tumor immunity. However, very little is known about their specificities, physiological function and contribution to anti-tumor immune responses in cancer patients.

We investigated the presence of Tscm cells in the peripheral blood (PB) and bone marrow (BM) of breast cancer patients by multicolor flow cytometry analyses with a panel of human T cell markers to distinguish naïve (Tn), central memory (Tcm), effector memory (Tem) and Tscm (CCR7+, CD45RO-, CD127+, CD45RA+, CD95+, CXCR3+, IL2Rβ+) population. Tscm cells were detected in both CD4+ and CD8+ T cells in the PB and BM of breast cancer patients. Interestingly, we observed a significant increase (3-fold) in the number of Tscm cells among the CD4+ and CD8+ T cells in breast cancer patients as compared to healthy donors. This was significantly higher among the CD8+ T cells as compared to CD4+ T cells. We further evaluated the expression of exhaustion markers (PD-1 & LAG3) and found that majority of Tscm cells (>95%) did not express PD-1 & LAG3 and comprised the least exhausted T cell compartment as compared to Tn, Tem and Tcm. To investigate the contribution of different subsets of memory T cells to tumor-specific T cells in breast cancer patients, we used a panel of breast cancer associated MHC class-I pentamers against human epidermal growth factor receptor 2 (HER2348-356 , HER2435-443), Mucin-1 (MUC112-20) and Carcinoembryonic antigen (CEA571-579). Cumulative analyses of tumor-specific pentamer+ CD8+ T cells in PB and BM showed that 20-25% of total tumor-specific T cells were of Tscm phenotype. We observed higher percentages of tumor-specific pentamer+CD8+ T cells among the total Tscm cells in BM as compared to PB, however this difference was not significant. We performed ex vivo re-stimulation assays using autologous dendritic cells loaded with several breast cancer-specific peptides with PB and BM of breast cancer patients to further assess the frequency and function of tumor-specific Tscm cells. Cumulative analyses showed that 20-23% of tumor-specific IFN-γ+ CD8+ and 26-32% IL-2+ CD4+ had a Tscm phenotype.

In summary, we report the presence of Tscm cells in the PB and BM of breast cancer patients and a significant increase in their number as compared to healthy donors. Importantly, we could show that Tscm contribute to the population of functional and tumor reactive CD8+ and CD4+ T cells in PB and BM of breast cancer patients. Our further aim is to investigate the therapeutic potential of Tscm cells in adoptive T cell experiments with humanized mouse model of breast cancer.

Citation Format: Mudita Pincha, Paranchai Boonsawat, Christoph Domschke, Philipp Beckhove. Characterizing tumor-specific memory stem like T cells in blood and bone marrow of breast cancer patients. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4076. doi:10.1158/1538-7445.AM2014-4076