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BK virus (BKV), a human polyomavirus that remains latent in renal epithelial cells, can be reactivated after hematopoietic stem cell transplantation (HSCT) leading to hemorrhagic cystitis. The incidence of BK viremia is higher after Umbilical cord blood transplantation (UCBT) than HSCT from adult donors. Data regarding the role of immune recovery after UCBT in BKV reactivation is lacking. We examined the correlation between the development of BK viremia and immune reconstitution in 27 adult recipients of UCBT. The incidence of BK viremia was 52% and developed most frequently within the first 8 weeks after the transplantation, but persisted in seven patients at 6 months, and three patients at 1-year post UCBT. Detection of BK viremia 1 year after transplant was negatively associated with the number of CD8+ cells (p = 0.03) and CD8+CD45RO+ cells (p = 0.05) at 6 months, and the number of CD4+ (p = 0.03) and CD4+CD45RO+ cells (p = 0.03) at 12 months after UCBT. Conversely, BK viremia at 6 and 12 months was positively correlated with the number of T regulatory (Treg) cells at 1 month (p = 0.005 and p = 0.016, respectively). Because UCB Treg have highly potent immunosuppressive function, our findings indicate that sustained BK viremia in UCBT recipients might be associated with the increase of Treg cells early after transplantation, which mediate impaired and delayed reconstitution of CD4+ and CD8+ T effector cells.

Metabolism is a common cellular feature. Cancer creates a suppressive microenvironment resulting in inactivation of antigen-specific T cells by metabolic reprogramming. Development of approaches that enhance and sustain physiologic properties of T cell metabolism to prevent T cell inactivation and promote effector function in the tumor microenvironment is an urgent need for improvement of cell-based cancer immunotherapies.

Peroxisome proliferator activated receptors (PPARs) are transcription factors that belong to nuclear hormone superfamily, with three distinct types identified: PPARapha (PPARα), PPARgamma (PPARγ), and PPARbeta/delta (PPARβ/δ). PPARs possess a critical role in the regulation of lipid metabolism, and thus play critical roles in the differentiation and fate of immune cells. PPARα is involved in lipid and carbohydrate metabolism and PPARα agonists, such as fibrates, have been used for the treatment of hypertriglyceridemia and cardiovascular diseases. PPARα has an anti-inflammatory role during infection, and similar to PPARγ, affects the polarization of macrophages. In acute myelogenous leukemia (AML), PPARα mutations correlate with chemoresistance, poor treatment outcomes and unfavorable prognosis. In experimental tumor models, it has been proposed that PPARα agonists might enhance anti-tumor T cell responses during PD-1 blocking immunotherapy. To dissect the mechanistic role of PPARα in tumor immunity, we used mice with global deletion of PPARα and examined tumor growth and profile of the immunological landscape, using various syngeneic tumor models. Significantly larger B16-F10 melanoma and MC-17 fibrosarcoma tumors were observed in PPARα KO mice compared with wild-type control, suggesting that PPARα deletion attenuated the immunological response against cancer. To dissect the role of PPARα in key populations of the innate and adaptive immune system involved in anti-tumor responses, we analyzed the immunological landscape of tumor, tumor draining lymph nodes (TDLN) and spleen, 14-16 days after tumor implantation. Assessment of CD4 + and CD8 + T cells, CD11b +F4/80 + tumor-associated macrophages (TAMs), CD11b +Ly6C hiLy6G - monocytic myeloid derived suppressor cells (M-MDSC), and CD11b +Ly6C loLy6G + polymorphonuclear myeloid derived suppressor cells (PMN-MDSC), by using flow cytometry, showed no quantitative differences between the two experimental groups. Functionally, MDSC from PPARα KO and WT mice showed comparable immunosuppressive properties as determined by suppression assay using splenocytes from OTI transgenic mice. However, PPARα KO TAMs demonstrated a less activated state, as determined by the lower expression levels of MHC-II that is critical for antigen presentation, and CD86 that is critical for T cell costimulation and prevention of T cell anergy and exhaustion. In agreement with these properties of TAMs, CD4 + T cells from TDLN of PPARα KO mice had diminished expression of activation markers, including PD-1, PD-L1 and ICOS, and numerically decreased central memory-like CD4 + T cells (T CM), compared to control tumor bearing mice. Furthermore, CD69, an emerging marker of T cell exhaustion, was significantly upregulated in CD4 + and CD8 + T cells from the TDLN of PPARα KO mice. To determine whether PPARα ablation altered the cell intrinsic properties of myeloid cells and/or T cells resulting in impaired anti-tumor function, we examined in vitro responses of isolated populations. In response to activation via TCR/CD3 and CD28, PPARα deficient T cells had no significant differences in expansion and cytokine production compared to control. In contrast, PPARα deficient Ly6C + monocytes isolated from the bone marrow displayed diminished responses to TLR-mediated signaling as determined by production of IL-6 and TNFα. Our in vitro and in vivo findings reveal a dominant role of PPARα in regulating the fate of innate immune cells thereby altering T cell responses and anti-tumor function. Our findings have implications for the development of new therapeutic approaches to enhance innate immune cell function for the improvement of cancer immunotherapy. Disclosures No relevant conflicts of interest to declare.

PD-1 is a T cell inhibitor for which blocking agents have achieved success as anti-cancer therapeutics. The current view is that cancer limits host immune responses by upregulating PD-L1 in the tumor microenvironment thereby causing PD-1 ligation and inactivation of CD8+ Teff cells. Recently, we determined that PD-1 alters the differentiation of myeloid progenitors during cancer-mediated emergency myelopoiesis. We found that PD-1 is expressed in granulocyte/macrophage progenitors (GMP), which accumulate during cancer-driven emergency myelopoiesis and give rise to myeloid-derived suppressor cells (MDSC) that promote tumor growth. In tumor-bearing mice with myeloid-specific PD-1 ablation, accumulation of GMP and MDSC was prevented, while output of effector myeloid cells was increased. PD-1-mediated T cell inactivation is attributed to the function of SHP-2 phosphatase, which is activated by recruitment to PD-1 cytoplasmic tail. Temporal activation of SHP-2 is critical for myeloid cell fate. Activating SHP-2 mutations prevent myeloid cell differentiation and lead to the accumulation of immature myelocytes and development of leukemia. To determine whether PD-1-mediated inhibition of anti-tumor immunity relies on SHP-2-mediated effects in T cells or myeloid cells, we generated mice with conditional targeting of the Ptpn11 gene (encoding for Shp-2) and selectively eliminated Shp-2 in T cells (Shp-2fl/flLckCre) or myeloid cells (Shp-2fl/flLysMCre). No significant difference in tumor growth was observed between control Shp2fl/fl and Shp-2fl/flLckCre mice bearing B16-F10 melanoma. Strikingly, Shp-2fl/flLysMCre mice had significantly diminished tumor growth that was not further decreased by anti-PD-1 antibody, in contrast to control Shp-2fl/fl mice in which anti-PD-1 treatment significantly reduced tumor size. To determine how Shp-2 ablation affected the properties of myeloid cells, we examined CD11b+Ly6ChiLy6G- monocytic (M-MDSC), CD11b+Ly6CloLy6G+ polymorphonuclear (PMN-MDSC), CD11b+F4/80+ tumor-associated macrophages (TAM) and CD11c+MHCII+ dendritic cells (DC). No quantitative differences were observed in these myeloid subsets in tumor bearing mice among the different groups. However, M-MDSC from Shp-2fl/flLysMCre mice had elevated expression of CD86 and IFNγ, consistent with effector differentiation. Suppression assays, by measuring antigen-specific responses of OTI transgenic T cells, showed significantly attenuated suppressor function of MDSC isolated from tumor-bearing Shp-2f/fLysMCre mice compared to control or Shp-2f/fLckCre mice. CD38 is a key mediator of MDSC-mediated immunosuppression. It is an ADP-ribosyl cyclase that has ectoenzyme and receptor functions, is induced early during differentiation of myeloid progenitors by retinoic acid receptor alpha (RARα) signaling, and mediates T cell immunosuppression. Because Shp-2 is involved in the differentiation of myeloid progenitors, we examined CD38 expression. We found that expression of CD38 was significantly reduced in MDSC from Shp-2fl/flLysMCre mice compared to control and Shp-2fl/flLckCre-tumor bearing mice. Since the suppressive potency of MDSC is decreased by autophagy, and SHP-2 has been implicated in regulating autophagy in cancer cells, we examined autophagy of MDSC in our system. Assessment of autophagy in ex vivo isolated MDSC, using Cyto-ID that stains the autophagosome membrane and indicates autophagic activity, showed enhanced autophagy in MDSC isolated from tumor bearing Shp-2fl/flLysMCre mice compared to control or Shp-2fl/flLckCre mice. Enhanced autophagy was also detected in bone marrow-derived MDSC from Shp-2fl/flLysMCre mice as determined by accumulation of LC3B-II and p62 during culture under conditions of starvation-induced stress. Consistent with the diminished MDSC suppressor function, myeloid cell-specific Shp-2 ablation in tumor-bearing mice induced an increase of CD8+ T cells showing an effector phenotype with improved functionality, despite preserved expression of PD-1 and Shp-2. Together these results indicate that inhibition of PD-1-mediated SHP-2 activation in myeloid progenitors, thereby preventing the accumulation of immature immunosuppressive MDSC and promoting the differentiation of effector myeloid cells, might be a previously unidentified mechanism by which PD-1 blockade mediates anti-tumor function. Disclosures No relevant conflicts of interest to declare.

Utilization of the adaptive immune system against malignancies, both by immune-based therapies to activate T cells in vivo to attack cancer and by T-cell therapies to transfer effector cytolytic T lymphocytes (CTL) to the cancer patient, represent major novel therapeutic advancements in oncologic therapy. Allogeneic hematopoietic stem cell (HSC) transplantation (HSCT) is a form of cell-based therapy, which replaces the HSC in the patient's bone marrow but also serves as a T-cell therapy due to the Graft-vs.-leukemia (GVL) effect mediated by donor T cells transferred with the graft. Allogeneic HSCT provides one potentially curative option to patients with relapsed or refractory leukemia but Graft-vs.-Host-Disease (GVHD) is the main cause of non-relapse mortality and limits the therapeutic benefit of allogeneic HSCT. Metabolism is a common cellular feature and has a key role in the differentiation and function of T cells during the immune response. Naïve T cells and memory T cells that mediate GVHD and GVL, respectively, utilize distinct metabolic programs to obtain their immunological and functional specification. Thus, metabolic targets that mediate immunosuppression might differentially affect the functional program of GVHD-mediating or GVL-mediating T cells. Components of the innate immune system that are indispensable for the activation of alloreactive T cells are also subjected to metabolism-dependent regulation. Metabolic alterations have also been implicated in the resistance to chemotherapy and survival of malignant cells such as leukemia and lymphoma, which are targeted by GVL-mediating T cells. Development of novel approaches to inhibit the activation of GVHD-specific naïve T cell but maintain the function of GVL-specific memory T cells will have a major impact on the therapeutic benefit of HSCT. Here, we will highlight the importance of metabolism on the function of GVHD-inducing and GVL-inducing alloreactive T cells as well as on antigen presenting cells (APC), which are required for presentation of host antigens. We will also analyze the metabolic alterations involved in the leukemogenesis which could differentiate leukemia initiating cells from normal HSC, providing potential therapeutic opportunities. Finally, we will discuss the immuno-metabolic effects of key drugs that might be repurposed for metabolic management of GVHD without compromising GVL.