CD3xPDL1 bi-specific T cell engager (BiTE) simultaneously activates T cells and NKT cells, kills PDL1+ tumor cells, and extends the survival of tumor-bearing humanized mice.

Bi-specific T cell engagers (BiTEs) activate T cells through CD3 and target activated T cells to tumor-expressed antigens. BiTEs have shown therapeutic efficacy in patients with liquid tumors; however, they do not benefit all patients. Anti-tumor immunity is limited by Programmed Death 1 (PD1) pathway-mediated immune suppression, and patients who do not benefit from existing BiTES may be non-responders because their T cells are anergized via the PD1 pathway. We have designed a BiTE that activates and targets both T cells and NKT cells to PDL1+ cells. In vitro studies demonstrate that the CD3xPDL1 BiTE simultaneously binds to both CD3 and PDL1, and activates healthy donor CD4+ and CD8+ T cells and NKT cells that are specifically cytotoxic for PDL1+ tumor cells. Cancer patients' PBMC are also activated and cytotoxic, despite the presence of myeloid-derived suppressor cells. The CD3xPDL1 BiTE significantly extends the survival time and maintains activated immune cell levels in humanized NSG mice reconstituted with human PBMC and carrying established human melanoma tumors. These studies suggest that the CD3xPDL1 BiTE may be efficacious for patients with PDL1+ solid tumors, in combination with other immunotherapies that do not specifically neutralize PD1 pathway-mediated immune suppression.

Frontline Science: Myeloid-derived suppressor cells (MDSCs) facilitate maternal-fetal tolerance in mice.

During successful pregnancy, a woman is immunologically tolerant of her genetically and antigenically disparate fetus, a state known as maternal-fetal tolerance. How this state is maintained has puzzled investigators for more than half a century. Diverse, immune and nonimmune mechanisms have been proposed; however, these mechanisms appear to be unrelated and to act independently. A population of immune suppressive cells called myeloid-derived suppressor cells (MDSCs) accumulates in pregnant mice and women. Given the profound immune suppressive function of MDSCs, it has been suggested that this cell population may facilitate successful pregnancy by contributing to maternal-fetal tolerance. We now report that myeloid cells with the characteristics of MDSCs not only accumulate in the circulation and uterus of female mice following mating but also suppress T cell activation and function in pregnant mice. Depletion of cells with the phenotype and function of MDSCs from gestation d 0.5 through d 7.5 resulted in implantation failure, increased T cell activation, and increased T cell infiltration into the uterus, whereas induction of MDSCs restored successful pregnancy and reduced T cell activation. MDSC-mediated suppression during pregnancy was accompanied by the down-regulation of L-selectin on naïve T cells and a reduced ability of naïve T cells to enter lymph nodes and become activated. Because MDSCs regulate many of the immune and nonimmune mechanisms previously attributed to maternal-fetal tolerance, MDSCs may be a unifying mechanism promoting maternal-fetal tolerance, and their induction may facilitate successful pregnancy in women who spontaneously abort or miscarry because of dysfunctional maternal-fetal tolerance.

Myeloid-Derived Suppressor Cell Survival and Function Are Regulated by the Transcription Factor Nrf2.

Tumor-induced myeloid-derived suppressor cells (MDSC) contribute to immune suppression in tumor-bearing individuals and are a major obstacle to effective immunotherapy. Reactive oxygen species (ROS) are one of the mechanisms used by MDSC to suppress T cell activation. Although ROS are toxic to most cells, MDSC survive despite their elevated content and release of ROS. NF erythroid 2-related factor 2 (Nrf2) is a transcription factor that regulates a battery of genes that attenuate oxidative stress. Therefore, we hypothesized that MDSC resistance to ROS may be regulated by Nrf2. To test this hypothesis, we used Nrf2(+/+)and Nrf2(-/-)BALB/c and C57BL/6 mice bearing 4T1 mammary carcinoma and MC38 colon carcinoma, respectively. Nrf2 enhanced MDSC suppressive activity by increasing MDSC production of H2O2, and it increased the quantity of tumor-infiltrating MDSC by reducing their oxidative stress and rate of apoptosis. Nrf2 did not affect circulating levels of MDSC in tumor-bearing mice because the decreased apoptotic rate of tumor-infiltrating MDSC was balanced by a decreased rate of differentiation from bone marrow progenitor cells. These results demonstrate that Nrf2 regulates the generation, survival, and suppressive potency of MDSC, and that a feedback homeostatic mechanism maintains a steady-state level of circulating MDSC in tumor-bearing individuals.

Increased expression of TLR9 associated with pro-inflammatory S100A8 and IL-8 in diabetic wounds could lead to unresolved inflammation in type 2 diabetes mellitus (T2DM) cases with impaired wound healing.

BACKGROUND: Type 2 diabetes mellitus (T2DM) is characterized by persistent hyperglycemia which causes a chain of abrupt biochemical and physiological changes. Immune dys-regulation is the hallmark of T2DM that could contribute to prolonged inflammation causing transformation of wounds into non-healing chronic ulcers. Toll like receptor -9 (TLR9) is a major receptor involved in innate immune regulation. TLR9 activation induces release of pro-inflammatory molecules like S100A8 and interleukin-8 (IL-8) by myeloid cells causing migration of myeloid cells to the site of inflammation. We hypothesized that pro-inflammatory S100A8 and IL-8 proteins could cause persistent inflammation in chronic wounds like diabetic foot ulcer (DFU) and may contribute to impaired wound healing in T2DM patients. MATERIALS AND METHODS: Expression of TLR9 and its downstream effector molecules S100A8, and IL-8 were analyzed in chronic diabetic wound and non-diabetic control wound tissue samples by semiquantitative reverse transcriptase - polymerase chain reaction (RT-PCR), quantitative RT-PCR, western blot and immunofluorescence. CD11b(+)CD33(+) myeloid cells were analyzed by flow cytometry. RESULTS: TLR9 message and protein were higher in diabetic wounds compared to control wounds (p=0.03, t=2.21 for TLR9 mRNA; p=<0.001, t=4.21 for TLR9 protein). TLR9 down-stream effector molecules S100A8 and IL-8 were also increased in diabetic wounds (p=0.003, t=3.1 for S100A8 mRNA; p=0.04, t=2.04 for IL-8). CD11b(+) CD33(+) myeloid cells were decreased in T2DM as compared to non-diabetic controls (p=0.001, t=3.6). DFU subjects had higher levels of CD11b(+) CD33(+) myeloid cells as compared to non-DFU T2DM control (p=0.003, t=2.8). Infection in the wound microenvironment could be the cause of increase in CD11b(+)CD33(+) myeloid cells in DFU (p=0.03, t=2.5). CONCLUSION: The up-regulation of myeloid cell-derived pro-inflammatory molecules S100A8 and IL-8 in combination with lower levels of CD11b(+) CD33(+) myeloid cells may cause the impairment of wound healing in T2DM subjects leading to chronic ulcers.

Estimation of outcrossing rates in interspecific backcross plants of Jatropha curcas (L.) using AFLP and SSR markers.

In this paper, we report the estimates of outcrossing rates using open-pollinated progeny arrays of 40 BC1 individuals of Jatropha developed as a result of interspecific hybridization between J. curcas and J. integerrima. For analysis PCR-based dominant AFLP and codominant SSR markers were used. The multilocus outcrossing rate (tm) estimated from AFLP markers (0.892 ± 0.112) are almost in the same range with SSR (0.884 ± 0.293) markers which indicate a high level of heterozygosity. A low value of inbreeding coefficient (F) also points out to the fact that outcrossing was the prevalent mode of reproduction in Jatropha and suggests maintenance of adequate genetic variability within families.

Reduced Myeloid-derived Suppressor Cells in the Blood and Endometrium is Associated with Early Miscarriage.

PROBLEM: The contribution of myeloid-derived suppressor cells (MDSC) in patients suffering from early or recurrent miscarriage is unknown. MDSC are implicated in modulation of T-cell response in healthy pregnancies; however, the role of MDSC in patients suffering from miscarriage has not been studied. We hypothesized that MDSC play major role in inducing maternal-fetal tolerance and this tolerance is compromised in patients suffering from miscarriage. METHOD OF STUDY: MDSC level was assessed by flow cytometry and immunostaining in blood and endometrial decidua, respectively. Activation of T cells was determined by MTT proliferation and IL-2 ELISA assays. RESULTS AND CONCLUSION: The miscarriage patients harbor reduced level of functionally suppressive MDSC in blood and endometrium as compared to healthy control women with successful pregnancies. These results suggest MDSC regulate maternal tolerance in healthy pregnancies and that drug inducing MDSC could have therapeutic implication in the miscarriage patients.

Cross-talk among myeloid-derived suppressor cells, macrophages, and tumor cells impacts the inflammatory milieu of solid tumors.

MDSC and macrophages are present in most solid tumors and are important drivers of immune suppression and inflammation. It is established that cross-talk between MDSC and macrophages impacts anti-tumor immunity; however, interactions between tumor cells and MDSC or macrophages are less well studied. To examine potential interactions between these cells, we studied the impact of MDSC, macrophages, and four murine tumor cell lines on each other, both in vitro and in vivo. We focused on IL-6, IL-10, IL-12, TNF-α, and NO, as these molecules are produced by macrophages, MDSC, and many tumor cells; are present in most solid tumors; and regulate inflammation. In vitro studies demonstrated that MDSC-produced IL-10 decreased macrophage IL-6 and TNF-α and increased NO. IL-6 indirectly regulated MDSC IL-10. Tumor cells increased MDSC IL-6 and vice versa. Tumor cells also increased macrophage IL-6 and NO and decreased macrophage TNF-α. Tumor cell-driven macrophage IL-6 was reduced by MDSC, and tumor cells and MDSC enhanced macrophage NO. In vivo analysis of solid tumors identified IL-6 and IL-10 as the dominant cytokines and demonstrated that these molecules were produced predominantly by stromal cells. These results suggest that inflammation within solid tumors is regulated by the ratio of tumor cells to MDSC and macrophages and that interactions of these cells have the potential to alter significantly the inflammatory milieu within the tumor microenvironment.

HMGB1 enhances immune suppression by facilitating the differentiation and suppressive activity of myeloid-derived suppressor cells.

Chronic inflammation often precedes malignant transformation and later drives tumor progression. Likewise, subversion of the immune system plays a role in tumor progression, with tumoral immune escape now well recognized as a crucial hallmark of cancer. Myeloid-derived suppressor cells (MDSC) are elevated in most individuals with cancer, where their accumulation and suppressive activity are driven by inflammation. Thus, MDSCs may define an element of the pathogenic inflammatory processes that drives immune escape. The secreted alarmin HMGB1 is a proinflammatory partner, inducer, and chaperone for many proinflammatory molecules that MDSCs develop. Therefore, in this study, we examined HMGB1 as a potential regulator of MDSCs. In murine tumor systems, HMGB1 was ubiquitous in the tumor microenvironment, activating the NF-κB signal transduction pathway in MDSCs and regulating their quantity and quality. We found that HMGB1 promotes the development of MDSCs from bone marrow progenitor cells, contributing to their ability to suppress antigen-driven activation of CD4(+) and CD8(+) T cells. Furthermore, HMGB1 increased MDSC-mediated production of IL-10, enhanced crosstalk between MDSCs and macrophages, and facilitated the ability of MDSCs to downregulate expression of the T-cell homing receptor L-selectin. Overall, our results revealed a pivotal role for HMGB1 in the development and cancerous contributions of MDSCs.

Budding yeast kinetochore proteins, Chl4 and Ctf19, are required to maintain SPB-centromere proximity during G1 and late anaphase.

In the budding yeast, centromeres stay clustered near the spindle pole bodies (SPBs) through most of the cell cycle. This SPB-centromere proximity requires microtubules and functional kinetochores, which are protein complexes formed on the centromeres and capable of binding microtubules. The clustering is suggested by earlier studies to depend also on protein-protein interactions between SPB and kinetochore components. Previously it has been shown that the absence of non-essential kinetochore proteins of the Ctf19 complex weakens kinetochore-microtubule interaction, but whether this compromised interaction affects centromere/kinetochore positioning inside the nucleus is unknown. We found that in G1 and in late anaphase, SPB-centromere proximity was disturbed in mutant cells lacking Ctf19 complex members,Chl4p and/or Ctf19p, whose centromeres lay further away from their SPBs than those of the wild-type cells. We unequivocally show that the SPB-centromere proximity and distances are not dependent on physical interactions between SPB and kinetochore components, but involve microtubule-dependent forces only. Further insight on the positional difference between wild-type and mutant kinetochores was gained by generating computational models governed by (1) independently regulated, but constant kinetochore microtubule (kMT) dynamics, (2) poleward tension on kinetochore and the antagonistic polar ejection force and (3) length and force dependent kMT dynamics. Numerical data obtained from the third model concurs with experimental results and suggests that the absence of Chl4p and/or Ctf19p increases the penetration depth of a growing kMT inside the kinetochore and increases the rescue frequency of a depolymerizing kMT. Both the processes result in increased distance between SPB and centromere.

Myeloid-derived suppressor cell function is reduced by Withaferin A, a potent and abundant component of Withania somnifera root extract.

Myeloid cells play a crucial role in tumor progression. The most common tumor-infiltrating myeloid cells are myeloid-derived suppressor cells (MDSC) and tumor-associated macrophages (TAMs). These cells promote tumor growth by their inherent immune suppressive activity which is enhanced by their cross-talk. The root extract of the plant Withania somnifera (Ashwagandha) (WRE) has been reported to reduce tumor growth. HPLC analysis identified Withaferin A (WA) as the most abundant constituent of WRE and led us to determine whether the anti-tumor effects of WRE and WA involve modulating MDSC and TAM activity. A prominent effect of MDSC is their production of IL-10 which increases upon cross-talk with macrophages, thus polarizing immunity to a pro-tumor type 2 phenotype. In vitro treatment with WA decreased MDSC production of IL-10 and prevented additional MDSC production of IL-10 generated by MDSC-macrophage cross-talk. Macrophage secretion of IL-6 and TNFα, cytokines that increase MDSC accumulation and function, was also reduced by in vitro treatment with WA. Much of the T-cell suppressive activity of MDSC is due to MDSC production of reactive oxygen species (ROS), and WA significantly reduced MDSC production of ROS through a STAT3-dependent mechanism. In vivo treatment of tumor-bearing mice with WA decreased tumor weight, reduced the quantity of granulocytic MDSC, and reduced the ability of MDSC to suppress antigen-driven activation of CD4+ and CD8+ T cells. Thus, adjunctive treatment with WA reduced myeloid cell-mediated immune suppression, polarized immunity toward a tumor-rejecting type 1 phenotype, and may facilitate the development of anti-tumor immunity.

Tumor-induced myeloid-derived suppressor cell function is independent of IFN-γ and IL-4Rα.

Myeloid-derived suppressor cells (MDSCs) are present in most cancer patients and experimental animals where they exert a profound immune suppression and are a significant obstacle to immunotherapy. IFN-γ and IL-4 receptor alpha (IL-4Rα) have been implicated as essential molecules for MDSC development and immunosuppressive function. If IFN-γ and IL-4Rα are critical regulators of MDSCs, then they are potential targets for preventing MDSC accumulation or inhibiting MDSC function. Because data supporting a role for IFN-γ and IL-4Rα are not definitive, we have examined MDSCs induced in IFN-γ-deficient, IFN-γR-deficient, and IL-4Rα-deficient mice carrying three C57BL/6-derived (B16 melanoma, MC38 colon carcinoma, and 3LL lung adenocarcinoma), and three BALB/c-derived (4T1 and TS/A mammary carcinomas, and CT26 colon carcinoma) tumors. We report that although MDSCs express functional IFN-γR and IL-4Rα, and have the potential to signal through the STAT1 and STAT6 pathways, respectively, neither IFN-γ nor IL-4Rα impacts the phenotype, accumulation, or T-cell suppressive potency of MDSCs, although IFN-γ and IL-4Rα modestly alter MDSC-macrophage IL-10 crosstalk. Therefore, neither IFN-γ nor IL-4Rα is a key regulator of MDSCs and targeting these molecules is unlikely to significantly alter MDSC accumulation or function.

Regulating the suppressors: apoptosis and inflammation govern the survival of tumor-induced myeloid-derived suppressor cells (MDSC).

Immune suppressive myeloid-derived suppressor cells (MDSC) are present in most cancer patients where they inhibit innate anti-tumor immunity and are a significant obstacle to cancer immunotherapy. Inflammation is a known inducer of Gr1(+)CD11b(+) MDSC; however, the factors/conditions that regulate MDSC survival and half-life have not been identified. We have used mass spectrometry (MS) and proteomic analysis to identify proteins and pathways that regulate MDSC survival. This analysis revealed high expression of caspase family proteins and the Fas-FasL, p38 MAPK, and TGFß pathways, suggesting that Fas-FasL apoptosis regulates MDSC survival. Flow cytometry, confocal microscopy, and western blot analyses confirmed the MS findings and demonstrated that Fas(+) MDSC are susceptible to Fas-mediated killing in vitro. In vivo studies with FasL-deficient and Fas-deficient mice demonstrated that Fas-FasL interactions are essential for MDSC apoptosis and for rejection of established metastatic disease and survival and that FasL(+) T cells are the effector population mediating MDSC apoptosis. MS findings validated by biological experiments demonstrated that inflammation increases MDSC levels by protecting MDSC from Fas-mediated apoptosis, possibly by activating p38 MAPK. These results demonstrate that MDSC half-life in vivo is regulated by FasL(+) T cells and that inflammation increases MDSC levels by conferring resistance to Fas-mediated apoptosis and identifies T cells as the relevant effector cells causing MDSC apoptosis in vivo. This newly recognized mechanism for regulating MDSC levels identifies potential new targets for decreasing MDSC in cancer patients.

Cross-talk between myeloid-derived suppressor cells (MDSC), macrophages, and dendritic cells enhances tumor-induced immune suppression.

The tumor microenvironment is a complex milieu of tumor and host cells. Host cells can include tumor-reactive T cells capable of killing tumor cells. However, more frequently the tumor and host components interact to generate a highly immune suppressive environment that frustrates T cell cytotoxicity and promotes tumor progression through a variety of immune and non-immune mechanisms. Myeloid-derived suppressor cells (MDSC) are a major host component contributing to the immune suppressive environment. In addition to their inherent immune suppressive function, MDSC amplify the immune suppressive activity of macrophages and dendritic cells via cross-talk. This article will review the cell-cell interactions used by MDSC to inhibit anti-tumor immunity and promote progression, and the role of inflammation in promoting cross-talk between MDSC and other cells in the tumor microenvironment.

High copies of SUM1 enhance the stability of wild-type microtubules against adverse conditions in Saccharomyces cerevisiae.

The budding yeast transcriptional repressor Sum1p binds to several promoters and recruits Hst1p, an NAD(+)-dependent histone deacetylase, at these promoters with the help of another protein Rfm1p. Hst1p causes repression of transcription by histone deacetylation of chromatin at its target promoters. In an earlier work we have shown that about 13-fold increase in Sum1p levels, brought about by expressing SUM1 from the high copy 2 micron plasmid (2 µ-SUM1), suppressed cold-sensitive growth phenotype associated with mutations in the α-tubulin gene TUB1. In this work we show that the dosage suppression is accompanied by an elevation of α-tubulin levels in mutant cells at their non-permissive growth temperature of 14°C. Further, 2 µ-SUM1 significantly rescued the benomyl-supersensitive growth phenotype of mutant cells having wild-type tubulin subunits but a deficiency in tubulin folding cofactors. Finally, wild-type 2 µ-SUM1 transformants, having no known mutation in microtubule-related genes, displayed spindle microtubules which were substantially more stable than of wild-type control cells when challenged with microtubule-depolymerizing drugs. Therefore, we conclude that high copies of Sum1p stabilize microtubules against a variety of adverse and destabilizing conditions like mutations, low temperatures and drugs.

Functional characterization of the Saccharomyces cerevisiae protein Chl1 reveals the role of sister chromatid cohesion in the maintenance of spindle length during S-phase arrest.

BACKGROUND: Metaphase cells have short spindles for efficient bi-orientation of chromosomes. The cohesin proteins hold sister chromatids together, creating Sister Chromatid Cohesion (SCC) that helps in the maintenance of short spindle lengths in metaphase. The budding yeast protein Chl1p, which has human homologs, is required for DNA damage repair, recombination, transcriptional silencing and aging. This protein is also needed to establish SCC between sister chromatids in S-phase. RESULTS: In the present study we have further characterized Chl1p for its role in the yeast Saccharomyces cerevisiae when cells are under replication stress. We show that when DNA replication is arrested by hydroxyurea (HU), the chl1 mutation causes growth deficiency and a mild loss in cell viability. Although both mutant and wild-type cells remained arrested with undivided nuclei, mutant cells had mitotic spindles, which were about 60-80% longer than wild-type spindles. Spindle extension occurred in S-phase in the presence of an active S-phase checkpoint pathway. Further, the chl1 mutant did not show any kinetochore-related defect that could have caused spindle extension. These cells were affected in the retention of SCC in that they had only about one-fourth of the normal levels of the cohesin subunit Scc1p at centromeres, which was sufficient to bi-orient the chromosomes. The mutant cells showed defects in SCC, both during its establishment in S-phase and in its maintenance in G2. Mutants with partial and pericentromeric cohesion defects also showed spindle elongation when arrested in S-phase by HU. CONCLUSIONS: Our work shows that Chl1p is required for normal growth and cell viability in the presence of the replication block caused by HU. The absence of this protein does not, however, compromize the replication checkpoint pathway. Even though the chl1 mutation gives synthetic lethal interactions with kinetochore mutations, its absence does not affect kinetochore function; kinetochore-microtubule interactions remain unperturbed. Further, chl1 cells were found to lose SCC at centromeres in both S- and G2 phases, showing the requirement of Chl1p for the maintenance of cohesion in G2 phase of these cells. This work documents for the first time that SCC is an important determinant of spindle size in the yeast Saccharomyces cerevisiae when genotoxic agents cause S-phase arrest of cells.

Myeloid-derived suppressor cells express the death receptor Fas and apoptose in response to T cell-expressed FasL.

Myeloid-derived suppressor cells (MDSCs) inhibit adaptive and innate immunity and accumulate in the blood of persons with cancer, chronic inflammation, trauma, infection, and stress. Some of the factors inducing their accumulation are known; however, mechanisms regulating their turnover have not been identified. Mass spectrometry showed prominent expression of apoptosis pathway proteins, suggesting that MDSC turnover may be regulated by Fas-FasL-mediated apoptosis. This hypothesis was confirmed by showing that blood MDSCs induced by 3 mouse tumors were Fas(+) and apoptosed in response to Fas agonist in vitro and to activated FasL(+) T cells in vivo. FasL-deficient mice contained significantly more blood MDSCs than FasL(+/+) mice, and after removal of primary tumors MDSCs regressed in STAT6(-/-) and CD1(-/-) mice but not in STAT6(-/-)FasL(-/-) or CD1(-/-)FasL(-/-) mice. Fas(+) macrophages and dendritic cells did not apoptose in response to activated T cells, indicating that Fas-FasL regulation of myeloid cells was restricted to MDSCs. These results identify a new mechanism regulating MDSC levels in vivo and show a retaliatory relationship between T cells and MDSCs in that MDSCs suppress T-cell activation; however, once activated, T cells mediate MDSC apoptosis.

Proteomic pathway analysis reveals inflammation increases myeloid-derived suppressor cell resistance to apoptosis.

Myeloid-derived suppressor cells (MDSC) accumulate in patients and animals with cancer where they mediate systemic immune suppression and obstruct immune-based cancer therapies. We have previously demonstrated that inflammation, which frequently accompanies tumor onset and progression, increases the rate of accumulation and the suppressive potency of MDSC. To determine how inflammation enhances MDSC levels and activity we used mass spectrometry to identify proteins produced by MDSC induced in highly inflammatory settings. Proteomic pathway analysis identified the Fas pathway and caspase network proteins, leading us to hypothesize that inflammation enhances MDSC accumulation by increasing MDSC resistance to Fas-mediated apoptosis. The MS findings were validated and extended by biological studies. Using activated caspase 3 and caspase 8 as indicators of apoptosis, flow cytometry, confocal microscopy, and Western blot analyses demonstrated that inflammation-induced MDSC treated with a Fas agonist contain lower levels of activated caspases, suggesting that inflammation enhances resistance to Fas-mediated apoptosis. Resistance to Fas-mediated apoptosis was confirmed by viability studies of MDSC treated with a Fas agonist. These results suggest that an inflammatory environment, which is frequently present in tumor-bearing individuals, protects MDSC against extrinsic-induced apoptosis resulting in MDSC with a longer in vivo half-life, and may explain why MDSC accumulate more rapidly and to higher levels in inflammatory settings.

The budding yeast protein Sum1 functions independently of its binding partners Hst1 and Sir2 histone deacetylases to regulate microtubule assembly.

The budding yeast protein Sum1 is a transcription factor that associates with the histone deacetylase Hst1p or, in its absence, with Sir2p to form repressed chromatin. In this study, SUM1 has been identified as an allele-specific dosage suppressor of mutations in the major alpha-tubulin-coding gene TUB1. When cloned in a 2mu vector, SUM1 suppressed the cold-sensitive and benomyl-hypersensitive phenotypes associated with the tub1-1 mutation. The suppression was Hst1p- and Sir2p-independent, suggesting that it was not mediated by deacetylation events associated with Sum1p when it functions along with its known partner histone deacetylases. This protein was confined to the nucleus, but did not colocalize with the microtubules nor did it bind to alpha- or beta-tubulin. Cells deleted of SUM1 showed hypersensitivity to benomyl and cold-sensitive growth, phenotypes exhibited by mutants defective in microtubule function and cytoskeletal defects. These observations suggest that Sum1p is a novel regulator of microtubule function. We propose that as a dosage suppressor, Sum1p promotes the formation of microtubules by increasing the availability of the alphabeta-heterodimer containing the mutant alpha-tubulin subunit.

Myeloid-derived suppressor cells inhibit T-cell activation by depleting cystine and cysteine.

Myeloid-derived suppressor cells (MDSC) are present in most cancer patients and are potent inhibitors of T-cell-mediated antitumor immunity. Their inhibitory activity is attributed to production of arginase, reactive oxygen species, inducible nitric oxide synthase, and interleukin-10. Here we show that MDSCs also block T-cell activation by sequestering cystine and limiting the availability of cysteine. Cysteine is an essential amino acid for T-cell activation because T cells lack cystathionase, which converts methionine to cysteine, and because they do not have an intact xc- transporter and therefore cannot import cystine and reduce it intracellularly to cysteine. T cells depend on antigen-presenting cells (APC), such as macrophages and dendritic cells, to export cysteine, which is imported by T cells via their ASC neutral amino acid transporter. MDSCs express the xc- transporter and import cystine; however, they do not express the ASC transporter and do not export cysteine. MDSCs compete with APC for extracellular cystine, and in the presence of MDSCs, APC release of cysteine is reduced, thereby limiting the extracellular pool of cysteine. In summary, MDSCs consume cystine and do not return cysteine to their microenvironment, thereby depriving T cells of the cysteine they require for activation and function.

Myeloid-derived suppressor cells down-regulate L-selectin expression on CD4+ and CD8+ T cells.

Effective cell-mediated antitumor immunity requires the activation of tumor-reactive T cells and the trafficking of activated T cells to tumor sites. These processes involve the extravasation of lymphocytes from the blood and lymphatics, and their homing to lymph nodes and tumors. L-selectin (CD62L) is an important molecule in these processes. It directs naive lymphocytes to peripheral lymph nodes where they become activated and it traffics naive lymphocytes to inflammatory environments, such as tumors. Individuals with advanced cancer are immune suppressed due to myeloid-derived suppressor cells (MDSC), a population of immature myeloid cells that accumulate to high levels in response to tumor-secreted and proinflammatory factors. We now demonstrate that the reduction in T cell levels of L-selectin that is commonly seen in individuals with cancer inversely correlates with MDSC levels. Three lines of evidence demonstrate that MDSC directly down-regulate L-selectin on naive T cells: 1) naive T cells cocultured with tumor-induced MDSC have reduced L-selectin; 2) T cells in tumor-free aged mice with elevated levels of MDSC have reduced L-selectin, and 3) peritoneal exudate T cells of tumor-free mice treated with plasminogen activator urokinase to elevate MDSC have reduced levels of L-selectin. MDSC are likely to down-regulate L-selectin through their plasma membrane expression of ADAM17 (a disintegrin and metalloproteinase domain 17), an enzyme that cleaves the ectodomain of L-selectin. Therefore, MDSC down-regulate L-selectin levels on naive T cells, decreasing their ability to home to sites where they would be activated. This is another mechanism by which MDSC inhibit antitumor immunity.