Human bone marrow contains a subset of quiescent early memory CD8(+) T cells characterized by high CD127 expression and efflux capacity

Preferential Homing of Tumor-specific and Functional CD8+ Stem Cell-like Memory T Cells to the Bone Marrow

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The bone marrow (BM) harbors not only hematopoietic stem cells but also conventional memory T and B cells. Studies of BM-resident memory T cells have revealed the complex relationship between BM and immunologic memory. In the present study, we identified CD122^high stem cells antigen-1 (Sca-1)^high, B-cell lymphoma protein-2 (Bcl-2)^high, CD8⁺ stem cell-like memory T cells (T_SCMs) as a distinct memory T-cell subset preferentially residing in the BM, where these cells respond vigorously to blood-borne antigens. We found that the most T_SCMs favorably relocate to the BM by adhesion molecules such as vascular cell adhesion protein 1, P-selectin glycoprotein 1, and P-selectin or E-selectin. Moreover, the BM-resident T_SCMs exhibited much higher levels of antitumor activity than the spleen-resident T_SCMs. These results indicate that the BM provides an appropriate microenvironment for the survival of CD8⁺ T_SCMs, thereby broadening our knowledge of the memory maintenance of antigen-specific CD8⁺ T lymphocytes. The present findings are expected to be instructive for the development of tumor immunotherapy.

New Insights into Mechanisms of Long-term Protective Anti-tumor Immunity Induced by Cancer Vaccines Modified by Virus Infection

The topic is how to achieve long-term protective anti-tumor immunity by anti-cancer vaccination and what are its mechanisms. Cancer vaccines should instruct the immune system regarding relevant cancer targets and contain signals for innate immunity activation. Of central importance is T-cell mediated immunity and thus a detailed understanding of cognate interactions between tumor antigen (TA)-specific T cells and TA-presenting dendritic cells. Microbes and their associated molecular patterns initiate early inflammatory defense reactions that can contribute to the activation of antigen-presenting cells (APCs) and to costimulation of T cells. The concommitant stimulation of naive TA-specific CD4+ and CD8+ T cells with TAs and costimulatory signals occurs in T-APC clusters that generate effectors, such as cytotoxic T lymphocytes and T cell mediated immunological memory. Information about how such memory can be maintained over long times is updated. The role that the bone marrow with its specialized niches plays for the survival of memory T cells is emphasized. Examples are presented that demonstrate long-term protective anti-tumor immunity can be achieved by post-operative vaccination with autologous cancer vaccines that are modified by virus infection.

Maintenance of memory CD8 T cells: Divided over division

Once generated during an infection, memory CD8⁺ T cells can provide long-lasting protection against reinfection with an intracellular pathogen, but the longevity of this defense depends on the ability of these pathogen-specific memory cells to be maintained. It is generally believed that the bone marrow plays an important role in this respect, where memory CD8 T cells receive reinvigorating signals from cytokines that induce homeostatic proliferation. However, in the current issue of the European Journal of Immunology, Siracusa et al. (Eur. J. Immunol. 2017. 47: 1900-1905) argue against this dogma, as they provide evidence that CD8 memory T cells in murine bone marrow are not proliferating, but largely quiescent, which protects them from elimination by the cytostatic drug Cyclophosphamide. Interestingly, this is in sharp contrast to the proliferating cell counterparts in the spleen, which are eliminated by this treatment. Here, we will discuss the impact of these results, how they relate to opposing findings by others in the field, and what the relevance of these findings is for humans and clinical applications.

Functional heterogeneity of human tissue-resident memory T cells based on dye efflux capacities

Tissue-resident memory T cells (TRMs) accelerate pathogen clearance through rapid and enhanced functional responses in situ. TRMs are prevalent in diverse anatomic sites throughout the human lifespan, yet their phenotypic and functional diversity has not been fully described. Here, we identify subpopulations of human TRMs based on the ability to efflux fluorescent dyes [efflux(+) TRMs] located within mucosal and lymphoid sites with distinct transcriptional profiles, turnover, and functional capacities. Compared with efflux(-) TRMs, efflux(+) TRMs showed transcriptional and phenotypic features of quiescence including reduced turnover, decreased expression of exhaustion markers, and increased proliferative capacity and signaling in response to homeostatic cytokines. Moreover, upon activation, efflux(+) TRMs secreted lower levels of inflammatory cytokines such as IFN-γ and IL-2 and underwent reduced degranulation. Interestingly, analysis of TRM subsets following activation revealed that both efflux(+) and efflux(-) TRMs undergo extensive transcriptional changes following TCR ligation but retain core TRM transcriptional properties including retention markers, suggesting that TRMs carry out effector function in situ. Overall, our results suggest a model for tissue-resident immunity wherein heterogeneous subsets have differential capacities for longevity and effector function.

MDR1 in immunity: friend or foe?

MDR1 is an ATP-dependent transmembrane transporter primarily studied for its role in the detoxification of tissues and for its implication in resistance of tumor cells to chemotherapy treatment. Several studies also report on its expression on immune cells where it plays a protective role from xenobiotics and toxins. This review provides an overview of what is known on MDR1 expression in immune cells in human, and its implications in different pathologies and their treatment options.

Bone Marrow T Cells and the Integrated Functions of Recirculating and Tissue-Resident Memory T Cells

Changes in T cell trafficking accompany the naive to memory T cell antigen-driven differentiation, which remains an incompletely defined developmental step. Upon priming, each naive T cell encounters essential signals – i.e., antigen, co-stimuli and cytokines – in a secondary lymphoid organ; nevertheless, its daughter effector and memory T cells recirculate and receive further signals during their migration through various lymphoid and non-lymphoid organs. These additional signals from tissue microenvironments have an impact on immune response features, including T cell effector function, expansion and contraction, memory differentiation, long-term maintenance, and recruitment upon antigenic rechallenge into local and/or systemic responses. The critical role of T cell trafficking in providing efficient T cell memory has long been a focus of interest. It is now well recognized that naive and memory T cells have different migratory pathways, and that memory T cells are heterogeneous with respect to their trafficking. We and others have observed that, long time after priming, memory T cells are preferentially found in certain niches such as the bone marrow (BM) or at the skin/mucosal site of pathogen entry, even in the absence of residual antigen. The different underlying mechanisms and peculiarities of resulting immunity are currently under study. In this review, we summarize key findings on BM and tissue-resident memory (TRM) T cells and revisit some issues in memory T cell maintenance within such niches. Moreover, we discuss BM seeding by memory T cells in the context of migration patterns and protective functions of either recirculating or TRM T cells.

Cancer-reactive memory T cells from bone marrow: Spontaneous induction and therapeutic potential (Review)

Cognate interactions between naïve tumor antigen (TA)-specific T cells and TA-presenting dendritic cells (DCs) are facilitated by secondary lymphoid organs such as lymph nodes or the spleen. These can result either in TA-specific tolerance or, depending on environmental costimulatory signals, in TA-specific immune responses. In the present review, we describe such events for the bone marrow (BM) when blood-borne TA, released from the primary tumor or expressed by blood circulating tumor cells or DCs enters the BM stroma and parenchyma. We argue that cognate T-DC interactions in the BM result in immune responses and generation of memory T cells (MTCs) rather than tolerance because T cells in the BM show an increased level of pre-activation. The review starts with the spontaneous induction of cancer-reactive MTCs in the BM and the involvement of such MTCs in the control of tumor dormancy. The main part deals with the therapeutic potency of BM MTCs. This is a new area of research in which the authors research group has performed pioneering studies which are summarized. These include studies in animal tumor models, studies with human cells in tumor xenotransplant models and clinical studies. Based on observations of an enormous expansion capacity, longevity and therapeutic capacity of BM MTCs, a hypothesis is presented which suggests the involvement of stem-like MTCs.

Memory CD8(+) T cells colocalize with IL-7(+) stromal cells in bone marrow and rest in terms of proliferation and transcription

It is believed that memory CD8⁺ T cells are maintained in secondary lymphoid tissues, peripheral tissues, and BM by homeostatic proliferation. Their survival has been shown to be dependent on IL-7, but it is unclear where they acquire it. Here we show that in murine BM, memory CD8⁺ T cells individually colocalize with IL-7⁺ reticular stromal cells. The T cells are resting in terms of global transcription and do not express markers of activation, for example, 4-1BB (CD137), IL-2, or IFN-γ, despite the expression of CD69 on about 30% of the cells. Ninety-five percent of the memory CD8⁺ T cells in BM are in G₀ phase of cell cycle and do not express Ki-67. Less than 1% is in S/M/G₂ of cell cycle, according to propidium iodide staining. While previous publications have estimated the extent of proliferation of CD8⁺ memory T cells on the basis of BrdU incorporation, we show here that BrdU itself induces proliferation of CD8⁺ memory T cells. Taken together, the present results suggest that CD8⁺ memory T cells are maintained as resting cells in the BM in dedicated niches with their survival conditional on IL-7 receptor signaling.