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Type 1 diabetes (T1D) is an autoimmune disease characterized by T and B cell responses to proteins expressed by insulin-producing pancreatic β cells, inflammatory lesions within islets (insulitis), and β cell loss. We previously showed that Ag-specific tolerance targeting single β cell protein epitopes is effective in preventing T1D induced by transfer of monospecific diabetogenic CD4 and CD8 transgenic T cells to NOD.scid mice. However, tolerance induction to individual diabetogenic proteins, for example, GAD65 (glutamic acid decarboxylase 65) or insulin, has failed to ameliorate T1D both in wild-type NOD mice and in the clinic. Initiation and progression of T1D is likely due to activation of T cells specific for multiple diabetogenic epitopes. To test this hypothesis, recombinant insulin, GAD65, and chromogranin A proteins were encapsulated within poly(d,l-lactic-co-glycolic acid) (PLGA) nanoparticles (COUR CNPs) to assess regulatory T cell induction, inhibition of Ag-specific T cell responses, and blockade of T1D induction/progression in NOD mice. Whereas treatment of NOD mice with CNPs containing a single protein inhibited the corresponding Ag-specific T cell response, inhibition of overt T1D development only occurred when all three diabetogenic proteins were included within the CNPs (CNP-T1D). Blockade of T1D following CNP-T1D tolerization was characterized by regulatory T cell induction and a significant decrease in both peri-insulitis and immune cell infiltration into pancreatic islets. As we have recently published that CNP treatment is both safe and induced Ag-specific tolerance in a phase 1/2a celiac disease clinical trial, Ag-specific tolerance induced by nanoparticles encapsulating multiple diabetogenic proteins is a promising approach to T1D treatment.

In this study, we evaluated the ability of negatively charged bio-degradable nanoparticles, ONP- 302, to inhibit tumor growth. Therapeutic treatment with ONP-302 in vivo resulted in a marked delay in tumor growth in three different syngeneic tumor models in immunocompetent mice. ONP- 302 efficacy persisted with depletion of CD8+ T cells in immunocompetent mice and also was effective in immune deficient mice. Examination of ONP-302 effects on components of the tumor microenvironment (TME) were explored. ONP-302 treatment caused a gene expression shift in TAMs toward the pro-inflammatory M1 type and substantially inhibited the expression of genes associated with the pro-tumorigenic function of CAFs. ONP-302 also induced apoptosis in CAFs in the TME. Together, these data support further development of ONP-302 as a novel first-in- class anti-cancer therapeutic that can be used as a single-agent as well as in combination therapies for the treatment of solid tumors due to its ability to modulate the TME.

Older people exhibit dysregulated innate immunity to respiratory viral infections, including influenza and SARS-CoV-2, and show an increase in morbidity and mortality. Nanoparticles are a potential practical therapeutic that could reduce exaggerated innate immune responses within the lungs during viral infection. However, such therapeutics have not been examined for effectiveness during respiratory viral infection, particular in aged hosts. Here, we employed a lethal model of influenza viral infection in vulnerable aged mice to examine the ability of biodegradable, cargo-free nanoparticles, designated ONP-302, to resolve innate immune dysfunction and improve outcomes during infection. We administered ONP-302 via i.v. injection to aged mice at day 3 after infection, when the hyperinflammatory innate immune response was already established. During infection, we found that ONP-302 treatment reduced the numbers of inflammatory monocytes within the lungs and increased their number in both the liver and spleen, without impacting viral clearance. Importantly, cargo-free nanoparticles reduced lung damage, reduced histological lung inflammation, and improved gas exchange and, ultimately, the clinical outcomes in influenza-infected aged mice. In conclusion, ONP-302 improves outcomes in influenza-infected aged mice. Thus, our study provides information concerning a practical therapeutic, which, if translated clinically, could improve disease outcomes for vulnerable older patients suffering from respiratory viral infections.

e14552 Background: Recent advances in the field of cancer immunology have highlighted the importance of the immune system for eliminating tumors. Numerous studies have shown that tumor-infiltrating immune cells such as antigen-presenting cells (APCs), T cells, and natural killer (NK) cells play critical roles in tumor control. However, the inflammatory anti-tumor immune response is counteracted by the induction of immune regulatory mechanisms within the tumor microenvironment (TME). These findings have led to the development of immune-targeted therapies, which are aimed at activating anti-tumor immune signaling pathways and enhancing anti-tumor immune function. While immunotherapies, have revolutionized the treatment of several solid tumors and leukemias, at best response rates remain low at 25%-30%, and a portion of patients eventually develop resistance to therapy leading to disease progression and mortality. Methods: We have previously reported the potent effects of 3-4 daily intravenous infusions of immune modifying poly(lactic- co-glycolic acid) (PLGA) nanoparticles, ONP-302, free from drugs or other bioactive agents, for the amelioration of acute inflammatory diseases by targeting myeloid cells. The present studies describe a novel use for ONP-302 nanoparticles, employing an altered dosing scheme to reprogram myeloid cells resulting in significant enhancement of tumor immunity. The efficacy of ONP-302 nanoparticles at inducing an anti-tumor immune response was evaluated using syngeneic mouse tumor models. Results: ONP-302 infusion decreased tumor growth via the activation of the cGAS/STING pathway within myeloid cells, and subsequently increased NK cell activation via an IL-15-dependent mechanism. Additionally, ONP-302 treatment increased PD-1/PD-L1 expression in the tumor microenvironment, thereby allowing for functionality of anti-PD-1 for treatment in the B16.F10 melanoma tumor model which is normally unresponsive to monotherapy with anti-PD-1. Conclusions: These findings indicate that ONP-302 allows for tumor control via reprogramming myeloid cells via activation of the STING/IL-15/NK cell mechanism, as well as increasing anti-PD-1 response rates.

e14553 Background: The tumor microenvironment (TME) plays a crucial role in tumor growth and progression and has a significant influence on response to therapy. The TME consists of myeloid-derived cells, stroma (e.g. fibroblasts and extracellular matrix (ECM)), and the vasculature that together support tumor growth and progression. Studies in animal models and in humans show that myeloid- derived cells such as myeloid derived suppressor cells (MDSCs) and tumor associated macrophages (TAMs) engage in activities that support tumor growth and progression. These cells also promote immune suppression in the TME that blunts the efficacy of the anti-cancer drugs and immune-targeted therapies such as immune checkpoint inhibitors. In addition to MDSCs, cancer-associated fibroblasts (CAFs) in the TME support tumor progression via production of pro-tumor and pro-angiogenic growth-factors, remodeling of the ECM via production of proteases, and suppression of anti-tumor immune function. CAF abundance in the TME is a negative prognostic factor for several solid tumors and is associated with negative outcomes and poor response to immune-targeted therapies like immune checkpoint inhibitors. ONP-302 nanoparticles fabricated from biodegradable poly (lactic-co-glycolic acid)(PLGA) polymer have been previously described in the literature for the treatment of acute inflammatory conditions via immuno-modulatory effects on myeloid derived cells. Here, we evaluated the efficacy of ONP-302 nanoparticles at inhibiting tumor growth via targeted inhibition of myeloid-derived cells and reshaping of the TME. Methods: ONP-302 anti-tumor efficacy was evaluated in syngeneic mouse tumor models using both immunocompetent and immunodeficient mice. We examined the effect of ONP-302 treatment tumor growth kinetics and effects on the major cellular constituents of the TME such as myeloid-derived cells and CAFs. Results: Therapeutic treatment with ONP-302 in vivo resulted in a marked delay in tumor growth in three different syngeneic tumor models in immunocompetent mice. ONP- 302 efficacy persisted with depletion of CD8+ T cells in immunocompetent mice and also was effective in immune deficient mice. We found ONP-302 treatment caused a gene expression shift in TAMs toward the pro-inflammatory M1 type and substantially inhibited the expression of genes associated with the pro-tumorigenic function of CAFs. ONP-302 also induced apoptosis in CAFs in the TME. Conclusions: Our data indicate that the slowing of tumor growth after ONP-302 treatment is due to disruptions in known signaling pathways involving TAMs and CAFs, pathways typically supporting tumor growth. These data taken in concert indicate the activity of ONP-302 is pleotropic and affects multiple pathways.

e14555 Background: Metastasis is responsible for most cancer-related deaths and depends on the formation of a metastatic niche at distal sites. The metastatic niche consists primarily of immunosuppressive myeloid cells, such as neutrophils and monocytes. These cells are recruited to distal sites in response to cancer-induced immune dysregulation and promote the recruitment and survival of disseminated tumor cells. We have employed drug-free poly(lactide-co-glycolide) (PLGA) nanoparticles, ONP-302, to modulate myeloid cell phenotypes and trafficking to disrupt the formation of the metastatic niche. The anti-metastatic efficacy of ONP-302 nanoparticles was evaluated in a translationally relevant mouse model of orthotopic triple-negative breast cancer. Methods: ONP-302 nanoparticles were evaluated in the syngeneic 4T1 model of triple-negative breast cancer in female BALB/c or RAG1KO BALB/c mice. ONP-302 nanoparticles intravenously injected into tumor-bearing mice every 3 days starting 1 day after tumor inoculation. Lungs were extracted 14 and 21 days after inoculation for analysis by single cell RNA sequencing and quantification of metastases by luminescent imaging, respectively. In resection studies, primary tumors were resected 11 days after inoculation and tissues were extracted 42 days after resection for quantification of metastases. Results: ONP-302 nanoparticles significantly reduced primary tumor growth and completely inhibited the formation of lung metastases. ONP-302 was also effective at suppressing metastasis when applied following surgical resection. In mice which received ONP-302 after surgical resection, lung metastases were undetectable up to 42 days after resection, while 44% of control mice developed metastases. These findings suggest that ONP-302 targets both primary tumor growth and metastatic colonization. Single cell RNA sequencing was employed to analyze ONP-302 modulation of the lung metastatic niche. The lungs of ONP-302 treated mice had lower neutrophil infiltration and a higher proportion of monocytes and dendritic cells compared to control mice. Gene expression pathways associated with inflammatory interferon and tumor necrosis factor responses were upregulated in neutrophils, monocytes, and dendritic cells from ONP-302 treated mice. The upregulation of these pathways suggests that ONP-302 treatment prevents the formation of an immunosuppressive niche and promotes anti-tumor cytotoxicity. Adaptive immunity was essential for ONP-302 efficacy, as T and B cell deficient RAG1KO mice demonstrated complete loss of ONP-302 efficacy both in slowing primary tumor growth and preventing metastasis. Conclusions: Our findings demonstrate that our drug-free PLG nanoparticles, ONP-302, impede cancer metastasis as a monotherapy by inhibiting the accumulation of immunosuppressive myeloid cells.

Older people exhibit dysregulated innate immunity to respiratory viral infections, including influenza and SARS-CoV-2, to increase morbidity and mortality. Nanoparticles are a potential practical therapeutic that could divert exaggerated innate immune responses away from the lungs during viral infection. However, such therapeutics have not been examined for effectiveness during respiratory viral infection, particular in aged hosts. Here, we employed a highly lethal model of influenza viral infection in vulnerable aged mice to examine the ability of biodegradable, cargo-free nanoparticles, designated ONP-302, to resolve innate immune dysfunction and improve outcomes during infection. We administered ONP-302 via intravenous injection to aged mice at day 3 post-infection when the hyperinflammatory innate immune response is already established. We found that ONP-302 treatment diverted tissue-damaging inflammatory monocytes from the lungs to the spleen without impacting viral clearance or reducing chemo-attractive signals in the infected lung. Importantly, cargo-free nanoparticles reduced lung damage and histological lung inflammation and improved gas exchange and, ultimately, the clinical outcomes in influenza-infected aged mice. ONP-302 improves outcomes in influenza-infected, vulnerable, aged mice. Thus, our study provides vital fundamental information concerning a practical therapeutic which, if translated clinically, could improve disease outcomes for vulnerable older patients suffering from respiratory viral infections. Supported by COUR Pharmaceuticals

There are many autoimmune diseases where pathologic self T cells are the underlying cause of disease symptoms and progression. T cells are found at the site of tissue destruction, resulting not only in the exacerbation of disease, but also the release of self-epitopes in the context of inflammation resulting in the activation of additional T cell populations perpetuating disease. Published data have shown treatment with antigen-containing biodegradablepoly(lactide-co-glycolide) (PLGA) nanoparticles, i.e. tolerogenic immune-modifying particles (TIMP), is both safe and induces antigen-specific tolerance in mouse models of autoimmunity and allergy, as well as in a celiac disease Phase I/IIa clinical trial. This study further investigated the role and mechanism of TIMP-induced tolerance. The present data show that TIMP treatment modulated T cells specific for spread epitopes associated with disease progression, but not encapsulated within the TIMP, i.e. tissue-specific bystander suppression. The PLP139–151 TCR transgenic model systems was utilized to determine the cellular and molecular mechanisms driving antigen specific tolerance mediated by tolerogenic nanoparticle treatment. These data show antigen-specific TIMP treatment induced both FoxP3+ iTregs and IL-10+ Tr1 regulatory phenotypes within the antigen-specific T cell populations in both naïve mice and mice pre-primed with antigen/CFA. Additionally, both functional Tregs and IL-10 are required for TIMP-induced tolerance. Treatment thus activates various antigen-specific Treg subsets capable of regulating responses to disease-relevant autoepitopes not encapsulated within the TIMP via release of immunoregulatory cytokines within the inflammatory site. Supported by funding provided by Cour Pharmaceuticals Development, Inc.

Cancer-associated mutations of the core splicing factor 3 B1 (SF3B1) result in selection of novel 3′ splice sites (3′SS), but precise molecular mechanisms of oncogenesis remain unclear. SF3B1 stabilizes the interaction between U2 snRNP and branch point (BP) on the pre-mRNA. It has hence been speculated that a change in BP selection is the basis for novel 3′SS selection. Direct quantitative determination of BP utilization is however technically challenging. To define BP utilization by SF3B1-mutant spliceosomes, we used an overexpression approach in human cells as well as a complementary strategy using isogenic murine embryonic stem cells with monoallelic K700E mutations constructed via CRISPR/Cas9-based genome editing and a dual vector homology-directed repair methodology. A synthetic minigene library with degenerate regions in 3′ intronic regions (3.4 million individual minigenes) was used to compare BP usage of SF3B1K700E and SF3B1WT. Using this model, we show that SF3B1K700E spliceosomes utilize non-canonical sequence variants (at position −1 relative to BP adenosine) more frequently than wild-type spliceosomes. These predictions were confirmed using minigene splicing assays. Our results suggest a model of BP utilization by mutant SF3B1 wherein it is able to utilize non-consensus alternative BP sequences by stabilizing weaker U2-BP interactions.

Splicing factor 3B1 (SF3B1) is a core splicing protein that stabilizes the interaction between the U2 snRNA and the branch point in the mRNA target during splicing. SF3B1 is heavily phosphorylated at its N terminus and a substrate of cyclin-dependent kinases (CDKs). Although SF3B1 phosphorylation coincides with splicing catalysis, the functional significance of SF3B1 phosphorylation is largely undefined. Here, we show that SF3B1 phosphorylation follows a dynamic pattern during cell cycle progression that depends on CDK activity. SF3B1 is known to interact with chromatin, and we found that SF3B1 maximally interacts with nucleosomes during G(1)/S and that this interaction requires CDK2 activity. In contrast, SF3B1 disassociates from nucleosomes at G(2)/M, coinciding with a peak in CDK1-mediated SF3B1 phosphorylation. Thus, CDK1 and CDK2 appear to have opposing roles in regulating SF3B1–nucleosome interactions. Importantly, these interactions were modified by the presence and phosphorylation status of linker histone H1, particularly the H1.4 isoform. Performing genome-wide analysis of SF3B1–chromatin binding in synchronized cells, we observed that SF3B1 preferentially bound exons. Differences in SF3B1 chromatin binding to specific sites, however, did not correlate with changes in RNA splicing, suggesting that the SF3B1–nucleosome interaction does not determine cell cycle–dependent changes to mRNA splicing. Our results define a cell cycle stage–specific interaction between SF3B1 and nucleosomes that is mediated by histone H1 and depends on SF3B1 phosphorylation. Importantly, this interaction does not seem to be related to SF3B1's splicing function and, rather, points toward its potential role as a chromatin modifier.

Recurrent mutations in core splicing factors have been reported in several clonal disorders, including cancers. Mutations in SF3B1, a component of the U2 splicing complex, are the most common. SF3B1 mutations are associated with aberrant pre-mRNA splicing using cryptic 3' splice sites (3'SSs), but the mechanism of their selection is not clear. To understand how cryptic 3'SSs are selected, we performed comprehensive analysis of transcriptome-wide changes to splicing and gene expression associated with SF3B1 mutations in patient samples as well as an experimental model of inducible expression. Hundreds of cryptic 3'SS were detectable across the genome in cells expressing mutant SF3B1. These 3'SS are typically sequestered within RNA secondary structures and poorly accessible compared with their corresponding canonical 3'SS. We hypothesized that these cryptic 3'SS are inaccessible during normal splicing catalysis and that this constraint is overcome in spliceosomes containing mutant SF3B1. This model of secondary structure-dependent selection of cryptic 3'SS was found across multiple clonal processes associated with SF3B1 mutations (myelodysplastic syndrome and chronic lymphocytic leukemia). We validated our model predictions in mini-gene splicing assays. Additionally, we found deregulated expression of proteins with relevant functions in splicing factor-related diseases both in association with aberrant splicing and without corresponding splicing changes. Our results show that SF3B1 mutations are associated with a distinct splicing program shared across multiple clonal processes and define a biochemical mechanism for altered 3'SS choice.

Mutational landscape of CLL is now known to include recurrent non-synonymous mutations in SF3B1, a core splicing factor. About 5-10% of newly diagnosed CLL harbor these mutations which are typically limited to HEAT domains in the carboxyl-terminus of the protein. Importantly, the mutations are not specific to CLL but also present in several unrelated clonal disorders. Analysis of patient samples and cell lines has shown the primary splicing aberration in SF3B1-mutant cells to the use of novel or "cryptic" 3' splice sites (3SS). Advances in genome-editing and next-generation sequencing (NGS) have allowed development of isogenic models and detailed analysis of changes to the transcriptome with relative ease. In this manuscript, we focus on two relevant methods to study splicing factor mutations in CLL: development of isogenic scalable cell lines and informatics analysis of RNA-Seq datasets.

Mutational landscape of CLL is now known to include recurrent non-synonymous mutations in SF3B1, a core splicing factor. About 5-10% of newly diagnosed CLL harbor these mutations which are typically limited to HEAT domains in the carboxyl-terminus of the protein. Importantly, the mutations are not specific to CLL but also present in several unrelated clonal disorders. Analysis of patient samples and cell lines has shown the primary splicing aberration in SF3B1-mutant cells to the use of novel or "cryptic" 3' splice sites (3SS). Advances in genome-editing and next-generation sequencing (NGS) have allowed development of isogenic models and detailed analysis of changes to the transcriptome with relative ease. In this manuscript, we focus on two relevant methods to study splicing factor mutations in CLL: development of isogenic scalable cell lines and informatics analysis of RNA-Seq datasets.

E2F-2 is a retinoblastoma (Rb)-regulated transcription factor induced during terminal erythroid maturation. Cyclin E-mediated Rb hyperphosphorylation induces E2F transcriptional activator functions. We previously reported that deregulated cyclin E activity causes defective terminal maturation of nucleated erythroblasts in vivo. Here, we found that these defects are normalized by E2F-2 deletion; however, anemia in mice with deregulated cyclin E is not improved by E2F-2-loss, which itself causes reduced peripheral red blood cell (RBC) counts without altering relative abundances of erythroblast subpopulations. To determine how E2F-2 regulates RBC production, we comprehensively studied erythropoiesis using knockout mice and hematopoietic progenitors. We found that efficient stress erythropoiesis in vivo requires E2F-2, and we also identified an unappreciated role for E2F-2 in erythroblast enucleation. In particular, E2F-2 deletion impairs nuclear condensation, a morphological feature of maturing erythroblasts. Transcriptome profiling of E2F-2-null, mature erythroblasts demonstrated widespread changes in gene expression. Notably, we identified citron Rho-interacting kinase (CRIK), which has known functions in mitosis and cytokinesis, as induced in erythroblasts in an E2F-2-dependent manner, and we found that CRIK activity promotes efficient erythroblast enucleation and nuclear condensation. Together, our data reveal novel, lineage-specific functions for E2F-2 and suggest that some mitotic kinases have specialized roles supporting enucleation of maturing erythroblasts.

The malaria SYBR green assay, which is used to profile in vitro drug susceptibility of Plasmodium falciparum , is a reliable drug screening and surveillance tool. Malaria field surveillance efforts provide isolates with various low levels of parasitemia. To be advantageous, malaria drug sensitivity assays should perform reproducibly among various starting parasitemia levels rather than at one fixed initial value. We examined the SYBR green assay standardized procedure developed by the Worldwide Antimalarial Resistance Network (WWARN) for its sensitivity and ability to accurately determine the drug concentration that inhibits parasite growth by 50% (IC 50 ) in samples with a range of initial parasitemia levels. The initial sensitivity determination of the WWARN procedure yielded a detection limit of 0.019% parasitemia. P. falciparum laboratory strains and field isolates with various levels of initial parasitemia were then subjected to a range of doses of common antimalarials. The IC 50 s were comparable for laboratory strains with between 0.0375% and 0.6% parasitemia and for field isolates with between 0.075% and 0.6% parasitemia for all drugs tested. Furthermore, assay quality (Z′) analysis indicated that the WWARN procedure displays high robustness, allowing for drug testing of malaria field samples within the derived range of initial parasitemia. The use of the WWARN procedure should allow for the inclusion of more malaria field samples in malaria drug sensitivity screens that would have otherwise been excluded due to low initial parasitemia levels.

Splicing factor 3B1 (SF3B1) is a member of the U2 snRNP complex that is a key regulator of RNA splicing. RNA splicing begins with the recognition of splice sites (ss) at the 5' and 3' ends of introns and ends with the removal of introns and joining of exons flanking them. SF3B1 plays an important role in this process by facilitating the recognition of the 3'ss. SF3B1 is frequently mutated in numerous cancers as well as the myelodysplastic syndromes (MDS). Mutations within the HEAT domain of the protein potentially contribute to disease pathogenesis. In addition to influencing splicing by binding to pre-mRNA, SF3B1 has been shown to affect splicing of exons by associating with them directly on chromatin via histone/nucleosome interactions. However, it is not understood if or how SF3B1 association with chromatin is regulated. Given that N-terminal serine and threonine residues on SF3B1 are known substrates of cyclin E-Cdk2, which phosphorylates histone subunits and other chromatin associated proteins, we hypothesized that CDK2 activity regulates SF3B1-nucleosome interactions. Although SF3B1 is phosphorylated during splicing catalysis, the function of this phosphorylation has remained unknown. We have now discovered, using nucleosome preparations and histone subunit co-immunoprecipitation assays in synchronized cells, that endogenous SF3B1 interacts with nucleosomes in a highly cell-cycle dependent manner, while total cellular abundance of SF3B1 remains invariant during cell cycle progression. In human and mouse cells, including hematopoietic cell lines, SF3B1 is excluded from chromatin during both G0 (quiescence) and G2/M phases of cell cycle. Notably, SF3B1 is enriched within chromatin maximally during S-phase. Unexpectedly, we found that the inhibition of Cdc2 (Cdk1) during G2/M enforces the SF3B1-chromatin interaction, pointing to a direct role for Cdc2 in restraining this interaction during mitosis. Further, SF3B1 loading onto chromatin during early cell cycle progression from G0 to S-phase is inhibited by Cdk2 inhibition. Thus, Cdk2 and Cdc2 appear to have antagonistic roles in controlling SF3B1-chromatin interactions during the cell cycle. Our findings suggest that Cdk activity may regulate the recruitment of the spliceosome machinery in order to coordinate splicing of particular transcripts with cell cycle progression. Current studies are focusing on how disease-associated mutations in the HEAT domain of SF3B1 affect the dynamics of its cell cycle-dependent interaction with nucleosomes and corresponding alterations to splicing outcomes. Disclosures No relevant conflicts of interest to declare.

Whole genome sequencing has recently revealed the prevalence of mutations in proteins directing splicing of RNA in up to half of the patients with Myelodysplastic Syndrome (MDS). Mutations in the protein SF3B1 are particularly common in MDS patients with the phenotypic abnormality termed ring sideroblasts (dysplastic erythroid precursors with perinculear rings formed by iron-laden mitochondria). The most common SF3B1 mutation in MDS patients results in a change from lysine to glutamic acid at amino acid position 700 (K700E). Given that splicing of RNA is a ubiquitous phenomenon, it is unclear how these mutations result in clonal proliferation and dysplastic hematopoiesis; two hallmark features of MDS. Furthermore, direct experimental evidence demonstrating a causative role for SF3B1 mutations in MDS-related phenotypes is lacking. To better understand how mutations of spliceosomal proteins contribute to MDS pathogenesis, we sought to define how expression of mutant SF3B1 changes erythroid maturation in vitro and in vivo. Native SF3B1 cDNA constructs are not amenable to bacterial propagation due to toxicity of its HEAT-domain repeats. We overcame this problem by codon optimization (changing the DNA sequence while preserving the native peptide sequence). Human cord blood derived CD34+ cells were transduced with retroviral vectors to express either the wild-type or K700E mutant of SF3B1. After a week of expansion in cytokines (IL-3, SCF and IL6), cells were induced to erythroid differentiation by addition of erythropoietin (EPO) and analyzed for surface markers of erythroid differentiation (CD 71, CD117, CD105, CD45 and CD235A) at regular intervals. K700E mutant expressing cells were found to have significantly reduced expression of CD105 when compared to wild-type SF3B1-expressing cells (average 50% recuction, n =8). CD105 or endoglin is a TGF-beta receptor accessory receptor expressed at high levels during intermediate stages of erythroid maturation. A more modest reduction of CD71 expression was also noted in K700E-SF3B1 cells. MDS bone marrow is known to express low levels of both CD105 and CD71 making our results clinically relevant. To further characterize how mutant SF3B1 may cause dysplastic hematopoiesis, we studied transduced and transplanted murine progenitor cells in vivo and in colony forming assays. Murine data demonstrate significantly reduced K700E-transduced hematopoietic progenitors (as defined by flow-cytometry) in vivo and impaired erythroid colony formation in vitro. Together, our results suggest that enforced expression of K700E-SF3B1 induces aberrant erythroid maturation and impairs homeostasis of hematopoietic precursor cells. Thus, we provide direct evidence that MDS-associated SF3B1 mutations perturb normal hematopoiesis and offer rationale for using our complementary experimental approach as a platform for elucidating the molecular mechanisms through which mutations in RNA splicing factors promote hematologic disease. Disclosures: No relevant conflicts of interest to declare.