ALBERT

Description: BACKGROUND: Symptom burden in essential thrombocythemia (ET) and polycythemia vera (PV) is severe even among individuals with low risk disease (Blood 2012. 12;123(24):3803-10). New therapies exist which alleviate the severe symptom burden and reduce splenomegaly in ET and PV patients (N Engl J Med 2015; 372:426-435). This analysis is the first to date to evaluate thresholds at which symptom-based treatment can be considered for ET and PV patients who are intolerant or resistant to hydroxyurea (HU). METHODS: Patient demographics, symptom burden, and disease traits were collected from ET and PV patients at a single time point during therapy. The MPN-10 total symptom score (TSS, JCO 2012;30(33)4098-103) was utilized to assess symptom burden. Symptom criteria models were determined as previously described among a population of MF patients (Scherber et. al. EHA 2016: a2250). Cutoffs were then evaluated in a cohort of ET and PV patients to assess for utility as a symptom model among this population. RESULTS: Demographics and symptom burden: 838 PV and 867 ET patients with previous hydroxyurea therapy were included in this analysis. Patients were of mean age (54.9 years ET, 64.0 years PV) and gender (69.2% female ET, 55.7% female). Mean disease duration was 6.0 years for ET and 7.3 years for PV.Among ET and PV patients, 15.0% and 24.2% had prior thrombosis respectively. In evaluating prognostic risk, ET patients tended to be low (45.5%) or intermediate risk (42.9%) with only a minority of patient meeting criteria forhigh risk disease (11.6%). Laboratory findings: ET patients had a mean platelet value of 598.7x 109/L(SD=283.4). Among PV patients, mean hematocrit was 45.8% (SD=8.1) and 42.6% of patients had a hematocrit of greater than 45%. White blood cell count was normal between the two groups (ET mean 8.3 x 109/L, PV mean 9.0 x 109/L). Symptoms: Mean worst symptom severity was 6.4 out of 10 (SD=2.7). Among ET patients, worst symptom was most frequently fatigue (32.7%, mean 5.0/10, SD=3.1, overall prevalence 88%) followed by night sweats (13.6%, mean 2.0/10, SD=3.0, overall prevalence 53%) and concentration difficulties (8.6%, mean 3.1/10, SD=3.0, overall prevalence 68%). For PV, worst individual symptom items were most frequently fatigue (29.2%, mean 5.2/10, SD=3.0, overall prevalence 91%), pruritus (14.1%, mean 3.2/10, SD=3.2, overall prevalence 69%), and night sweats (12.8%, mean 2.5/10, SD=3.0, overall prevalence 57%). Cutoff Scoring: 47.0% of ET patients fit criteria for TSSgreater than to equal to 20; 59.0%% had a single itemgreater than 5; and 45.7% had both a TSS greater than or equal to 20 and a single item greater than 5. Among PV participants, 54.5% had aTSS greater than to equal to 20; 66.1% had a single itemgreater than 5; and 51.5% had both a TSS greater than or equal to 20 and a single item greater than 5. Each scoring method was significantly associated with individual item scores (Table 1). Prognostic scoring was not significantly associated with any of the symptom cutoffs evaluated. Correlations: Among ET patients, a prior history of thrombosis was significantly associated with having a worst symptom item greater than 5 (p=0.043). ET patients with lower hemoglobin were significantly more likely to meet criteria for a MPN-10 score greater than or equal to 20 or to meet combined criteria for a MPN-10 greater than or equal to 20 and single worst item greater than 5 (for both p=0.01 or less). For PV, lower hematocrit levels were significantly associated with having an individual worst symptom score of greater than 5 (44.9% versus 46.7%, p=0.0376). CONCLUSION: Assessment of ET and PV symptoms, now measurable through standardized and practical instruments such as the MPN-10, is an integral part of determining therapeutic impact of newer therapies in both clinical practice and trial settings. In our modeling, patients with severe symptom burden profiles are well represented by utilizing cutoff criteria including aworst individual symptom item of greater than 5 out of 10, an MPN-10 score of greater than or equal to 20, or combined criteria of both cutoffs. These cutoffs can be considered when determiningwhich HU intolerant or resistant patients would most benefit fromsymptom orientedtreatment. Disclosures Kiladjian: AOP Orphan: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Honoraria, Research Funding. Schouten:Novartis: Consultancy; Sanofi: Consultancy. Etienne:BMS: Speakers Bureau; ARIAD: Speakers Bureau; Pfizer: Speakers Bureau; novartis: Consultancy, Speakers Bureau. Harrison:Incyte Corporation: Honoraria, Speakers Bureau; Shire: Honoraria, Speakers Bureau; Gilead: Honoraria, Speakers Bureau; Baxaltra: Consultancy, Honoraria, Speakers Bureau; Novartis: Consultancy, Honoraria, Other: travel, accommodations, expenses, Research Funding, Speakers Bureau. Radia:Pfizer: Honoraria; Novartis: Honoraria. Cervantes:AOP Orphan: Membership on an entity's Board of Directors or advisory committees; Baxalta: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Vannucchi:Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau. Mesa:Promedior: Research Funding; Celgene: Research Funding; CTI: Research Funding; Gilead: Research Funding; Incyte: Research Funding; Galena: Consultancy; Ariad: Consultancy; Novartis: Consultancy.

Description: Background: The Philadelphia chromosome negative chronic myeloproliferativeneoplasms (MPNs) including essential thrombocythemia (ET), polycythemia vera(PV), and myelofibrosis (MF) are a group of diseases of myeloid clonal lineage with significant symptom burden. MPN patients have also been found to have a high likelihood of mood disturbance related to their MPN, including symptoms of depression and anxiety (BMC Cancer. 201616:167). To date, the relationship between disease factors and depressive symptoms among MPN patients has not been well characterized. Methods: We have previously published the results of a 70-item internet-based survey which evaluated fatigue-relieving strategies among MPN patients (Cancer. 2016 Feb 1;122(3):477-85). Surveyed data included mental health questionnaires including a measure of symptoms of depression, the PHQ-2, (Ann Fam Med. 2010. 8(4): 348-353), a measure of positive and negative moods the POMS-short form (J Nerv Ment Dis. 1979;167(10):612-4) and a measure of symptoms of depression and anxiety, the MHI-5 (J Consult Clin Psychol. 1983. 51;730-742). This abstract is a secondary analysis of data collected among this cohort. Data: Demographics A total of 1788 individuals diagnosed with an MPN participated in the survey. 1389 patients answered questions regarding mood and psychological comorbidities. Many MPN patients reportedthat they had been seen by a health provider for care or diagnosis of depression (32%), anxiety (29.5%), stress (26.2%), or grief(15.0%). Overall, 24.0% of ET, 22.2% of PV, and 22.9% of MF patients endorsed symptoms of depression based on their score of greater than or equal to 3 on the PHQ-2. Demographic and Psychosocial Correlates Younger patients were significantly more likely to have PHQ-2 score of equal to or greater than three (57.0 years vs 59.5 years, p=0.0006). Gender, race, and country of origin did not significantly correlate with screening positive for symptoms of depression. Higher BMI was associated with a higher likelihood of screening positive for symptoms of depression (BMI 26.2 vs 24.6, p=0.005). Not surprisingly, having received treatment for mood problems in the last 6 months (p

Description: Background: Red blood cell transfusion dependence (RBC-TD) is a reality for most MDS patients at some point during their disease trajectory. RBC-TD has been associated with inferior quality of life, iron overload and inferior overall survival. Alongside a focus on new therapies to abrogate or diminish the dependence on RBC units, optimizing quality of life while on transfusions may also have value. Our objective was to better understand the transfusion experience and what patients value, in order to inform future research initiatives. Methods: We designed a web-based survey (using SurveyMonkey) in the USA, Canada and UK that was disseminated by the international MDS Foundation, the Leukemia Lymphoma Society of Canada, the Aplastic Anemia and MDS Association of Canada (AAMAC), the University of York, the MDS registry of Canada and the MDS UK Patient Support Group. The survey consisted of 57 questions and was completed by willing patients who received at least 1 unit of blood in the previous 8 weeks. The anonymized survey was completed from 8/27/17 to 2/18/2018. Results: Of 712 responses, 475 (67%) were RBC-TD and eligible for the survey. 55% were male with a median age of 72 years, 77% lived in urban or suburban communities. 75%, 12% and 12% of respondents were from USA, Canada and the UK; 93% were Caucasian. 45% and 27% reported lower and higher risk MDS respectively while 28% did not know. Patients reported having lived with MDS for a median of 3 years, 52% became transfusion dependent within 6 months of diagnosis and 37% 〉 12 months after diagnosis. 80% knew the haemoglobin (Hb) threshold at which they were transfused (Figure 1). 14%, 30% and 54% received 1, 1-2 or 2 units at a time and the median number of units/month was 2 (Figure 2). The most common symptoms pre-transfusion were fatigue (91%), weakness (69%), shortness of breath (SOB, 72%), dizziness (43%) and headache (22%) with fatigue and SOB having the most negative impact on their lives. 25%, 53% and 12% felt better after a transfusion within 1 day, 1-2 days and 3-4 days respectively and 20% felt worse for 1-2 days post transfusion. 64% reported feeling symptomatic for 5+ days before their next transfusion was organized, but 65% were able to organize a blood transfusion within 1-2 days of reaching their Hb threshold. 61% had a crossmatch on a different day from their transfusion and 30% of these patients indicated that they would have preferred to have a same day crossmatch even if it demanded more time at the transfusion centre. 42% were driven to their transfusions by friends or family. Travel time to the transfusion centre was 〈 1 hour for 93% of patients and did not have a negative impact on quality of life for 81% of respondents. 47% spent 4-6 hours at the clinic on the day of their transfusions and 19% spent 6-7+ hours. 31% faced economic hardship ('significantly' 12%, 'somewhat' 20%) due to their transfusion dependence. 24% experienced negative side effects consisting of, allergic reactions (32%), fever (16%), circulatory overload (9%), and other (39%). 57% patients reported iron overload. When presented with scenarios, 45% indicated that receiving less frequent transfusions (for example, by receiving 3 units at a time instead of 2) and 74% indicated that having home Hb point of care testing (to facilitate organizing transfusions before symptoms) would improve their QOL. Only 20% felt that receiving more frequent transfusions would improve their QOL, but 40% preferred to be transfused at a higher threshold than currently ordered by their physician. The ideal 'aspired-to' Hb thresholds for transfusion by these patients are indicated in figure 1, and 66% indicated a Hb of 〉 85g/L. Conclusions: It is well known that transfusion dependence has a negative impact on the QOL of MDS patients but this survey revealed that it also presents an economic hardship and significant time commitment. Patient feedback from this survey suggests some approaches that might improve the experience of TD MDS patients. These might include strategies to reduce the time for the transfusion pathway (eg. same day crossmatches) and point of care testing for Hb at home. Studies are needed to address a lack of evidence on optimal or higher Hb thresholds for red cell transfusion in outpatients as no trials have been conducted. 2 clinical trials (RBC-Enhance, NCT 02099669 and REDDS, ISRCTN26088319) are ongoing. More transfusion clinical trials with primary endpoints of patient reported outcomes are needed in MDS. Disclosures Buckstein: Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding.

Description: Background: The paradigm for treatment selection in newly diagnosed (ND) multiple myeloma (MM) is largely predicated on establishment of suitability for front-line autologous stem cell transplantation. In this context, real-world evidence from multiple jurisdictions demonstrates that the most widely employed front-line (1L) therapies are bortezomib (B)-based triplets in transplant-eligible patients, and in transplant-ineligible patients, B-based triplets (or doublets) or the combination of lenalidomide and dexamethasone. While a sound hypothetical rationale for treatment stratification at diagnosis is recognised, viz alternative treatments for high-risk (HR) versus standard-risk disease, this is rarely employed in either everyday practice or clinical trials. A significant impediment to such an approach is the lack of a uniformly recognised risk stratification model that not only recognises truly HR disease (e.g. median survival from diagnosis 〈 24 months [m]) but is also accessible and affordable. Against this background we evaluated data from the Australian and New Zealand Myeloma and Related Diseases Registry (MRDR) to define characteristics of 'functional' HR disease that could inform response-adaptive strategies, thus enabling the appropriate therapeutic targeting of HR patients. Methods: NDMM patients enrolled on to the MRDR since February 2013 with available data were evaluated. Patients were defined as either sub-optimal responders (SOR) to 1L if their best response was minimal response or stable disease (but not progressive disease [PD]), or early progressors (EP) if they demonstrated PD within 12m of commencing 1L therapy, irrespective of whether this occurred on or after the completion of 1L therapy. For categorical variables p-values were determined using a Pearson's chi-squared test, for continuous variables p-values were calculated using a Wilcoxon rank-sum test. Survival analysis was performed using the Kaplan-Meier method and p-values determined with the log-rank test. Results: 1320 NDMM patients were included with a median follow-up of 23m (12-37 IQR, 0-83 range). Of these, 152 were SOR (11.5%) and 118 were EP (8.9%). At diagnosis only 2 factors associated with SOR were identified: age 〉70y (p

Description: Introduction The majority of patients with Essential Thrombocythaemia (ET) have mutations in JAK2, MPL, and CALR, causing activation of the JAK/STAT pathway; but 10-15% of ET patients lack detectable mutations in these genes, so-called 'Triple Negative' (TN). We applied a systematic approach to investigate mutational status and epigenetic signatures in a cohort of TN ET patients. Methods and Results We investigated 46 patients (72% female), median age at diagnosis 35 years (range 8-77 years) including a father and son. All patients were TNusing standard diagnostic assays. We applied deep, error corrected, next generationsequencing (NGS) of 24 genes using the HaloPlexHS platform to peripheral blood samples. Whole exome sequencing was also performed in 23 patients using skin as constitutional control. Overall we identified somatic mutations in 10/46 patients including MPL(3 patients, W515R, W515G, W515S, R537W, VAF 0.02-0.1) JAK2V617F (4 patients, VAF 0.02-0.08); and germline MPLmutations in a further 3 patients (P453R, S505N); including the father and son pair. We selected patients lacking somatic or germline mutations ("true TN") to analyse gene expression using RNA-seq and DNA methlyation status using 850K Epic Arrays. Patients with JAK2V617Fand CALRmutations and healthy donors (HC) were included as controls. Concerning RNA-seq data, we performed multiple differential analysis of HC vs TN, CALRand JAK2V671F; as well as HC vs all ET samples (adjusted for subtype). Each HC comparison highlighted clear differences between gene expression profiles of HC and disease (Figure 1A). The differentially expressed genes (DEGs) in each comparison overlapped significantly, suggesting that all ET samples have consistent gene expression differences to HC samples regardless of their driver mutation status. In total 1444 differentially expressed genes (ET vs HC) were highlighted (figure 1B). Functional analysis identified significant enrichment for genes involved in the MAPK pathway. Addtionally, we noted upregulation of GATA1,ITGA2B and GP6 genes, not previously reported to be dysregulated in ET. Correlation of gene expression data with DNA methylation status identified a consistent signature of 306 hypomethylated genes, showing significant enrichment for genes involved in transcriptional misregulation and upregulation of inflammatory regulators such as TNF and NFκB signaling pathways. Next, we identified which transcription factor motifs preferentially bind within these methylation blocks. The blocks showed an enrichment for 6 key transcriptional regulators: ATF3, ATF4, CEBPA, CEBPB, MAX, and RARA. All 6 were significantly upregulated in all ET samples. To validate the motifs, we processed ChIP-seq data from the K562 cell line and identified a significant proportion of the hypo methylated regions are bound by these, transcription factors: 374/410 (91%) regions; 43/410 (10%) are bound by all 6 transcription factors. Conclusions A significant proportion (22%) of patients assigned as 'TN' ET via traditional diagnostic techniques in fact harbor known driver mutations at a low allele frequency, suggesting that error corrected NGS approaches may be diagnostically useful in this setting. Additionally, for a group of "true" TN ET patients we demonstrate that patterns of gene expression and DNA methylation are more similar to patients with ET with known driver mutations than healthy controls. Among the upregulated genes are key platelet regulatory genes: GP6, GP1BB, ACTN1and ITGA2B. Furthermore, we identify consistently hypomethylated genes with increased expression across all molecular subtypes of ET which are highly enriched for genes involved in proinflammatory pathways and show that binding of 6 key transcription factorsmay underlie these changes regardless of driver mutation status. Our observations suggest that the ET disease phenotype may, at least in part, be driven by transcriptional misregulation and may be propagated downstream via the MAPK, TNF and NFKappa pathways in addition to activation of JAK/STAT pathways. These findings identify novel mechanisms of disease initiation which require further evaluation. Disclosures Dillon: Novartis: Consultancy, Honoraria; Abbvie: Consultancy, Honoraria; Pfizer: Consultancy, Honoraria; TEVA: Consultancy, Honoraria. Mufti:Cellectis: Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene Corporation: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau. McLornan:Jazz Pharmaceuticals: Honoraria, Speakers Bureau; Novartis: Honoraria. Harrison:Shire: Speakers Bureau; CTI: Speakers Bureau; Celgene: Honoraria, Speakers Bureau; AOP: Honoraria; Janssen: Speakers Bureau; Novartis: Honoraria, Research Funding, Speakers Bureau; Roche: Honoraria; Promedior: Honoraria; Gilead: Speakers Bureau; Sierra Oncology: Honoraria; Incyte: Speakers Bureau.

Description: Introduction: Cytopenias are a leading cause of ruxolitinib (RUX) discontinuation for patients (pts) with myelofibrosis (MF). Though RUX 5 mg BID is recommended for pts with platelet (PLT) counts of 50 to

Description: Essential thrombocythemia (ET) is associated with arterial and venous thrombosis, hemorrhage and transformation to acute leukemia and myelofibrosis. Cytoreduction with hydroxyurea (Hu) reduces thrombotic risk, but concerns about potential toxicity have led to the widespread use of a newer agent, anagrelide (Ag) as first-line therapy. However, no randomized trials comparing the efficacy and safety of Ag with Hu have been performed. The MRC PT1 trial is an international, multi-centre, randomized controlled trial comparing Ag with Hu in patients with ET at high-risk of vascular events (based on age, platelet count or cardiovascular risk factors). The trial was independently conceived, conducted and analysed by the investigators on behalf of the UK Myeloproliferative Disorders Study Group and the MRC Adult Leukaemia Working Party. Patients (newly diagnosed or previously treated) were randomized to receive either Ag plus aspirin (Ag+asp) or Hu+asp. Clinical end-points together with blood and bone marrow morphology were assessed centrally by experts blinded to treatment allocation. With 809 patients randomized and median follow-up of 39 months, it is the largest and most comprehensive randomized study of ET performed to date. Patients in the Ag+asp arm were significantly more likely to reach the primary end-point and several secondary end-points. Intention-to-treat log-rank analyses of time to event indicate that Ag+asp patients were significantly more likely than Hu+asp patients to reach the composite primary end-point of arterial thrombosis, venous thrombosis or major hemorrhage (55 v 36 events; odds ratio 1.57; 95% CI 1.04–2.37; p=0.03). Ag+asp was also associated with increased rates of arterial thrombosis (37 v 17 events; OR 2.16; 95% CI 1.27–3.69; p=0.004) and major hemorrhage (22 v 8 events; OR 2.61; CI 1.27–5.33; p=0.008) but, interestingly, decreased venous thrombosis compared to Hu+asp (3 v 14 events; OR 0.27; CI 0.11–0.71; p=0.006). Myelofibrotic transformation, the diagnosis of which required new clinical and/or laboratory features in addition to grade III/IV reticulin fibrosis, was significantly increased in the Ag+asp arm (16 v 5 events; OR 2.92, CI 1.24–6.86, p=0.01). Transformation to MDS/AML was comparable between the two arms (4 Ag+asp v 6 Hu+asp), although the small number of transformations and short follow-up prevent firm conclusions about leukemogenicity. Overall survival was not statistically different. Ag+asp was more poorly tolerated than Hu+asp. In the Ag+asp arm significantly fewer patients remained on their allocated treatment (p

Description: Background: Since hydroxyurea emerged as an effective therapy for sickle cell disease (SCD), there have been numerous studies that have demonstrated its safety and efficacy in children and adults with SCD. In their 2014 guidelines, the NHLBI recommended that hydroxyurea treatment should be offered to all infants and children with sickle cell anemia (HbSS and HbS/beta0 thalassemia) starting at 9 months of age. However, hydroxyurea is underused among children and adolescents with SCD and to date, there have been no studies that have identified the specific determinants that may predict hydroxyurea adherence in these patients. Objectives: 1. To identify predictors of hydroxyurea adherence in children with SCD. 2. To measure the rate of hydroxyurea use among CHEO patients with SCD who were born between January 1, 2003 and December 31, 2015; and 3. To compare the rates of SCD-related complications between patients who were not prescribed hydroxyurea, patients who were adherent to hydroxyurea and patients who were not adherent to hydroxyurea Methods: We extracted medical chart data to identify patients with SCD who were born between January 1, 2003 and December 31, 2015. Patients were classified as either "Not prescribed hydroxyurea" or "Prescribed hydroxyurea" based on clinical documentation and the presence of at least one hydroxyurea outpatient prescription. For those patients who were prescribed hydroxyurea, hematological indices were collected and analyzed over time to estimate adherence to hydroxyurea. To measure the adherence of children prescribed hydroxyurea, we examined the trends in the patient's hematological indices after their first prescription of hydroxyurea. Adherence was defined as increased hematological indices (from baseline) by greater than or equal to any 2 of the following: Mean corpuscular volume (MCV) by 10 fL; Hemoglobin levels (g/L) by 10 g/L and/or %HbF (fetal hemoglobin) by 10%. We measured the frequency of disease-related complications among CHEO patients with SCD according to their use of hydroxyurea and used multivariate analyses to evaluate immigration status, newborn screening status, SCD subtype, SCD complications, income, age and sex as predictors for hydroxyurea adherence. Results: Children with HbSS were more likely to have been prescribed hydroxyurea compared to children with HbSC (87.8% vs. 9.5%). Canadian citizenship, newborn hemoglobinopathy screening and lower familial income were associated with better hydroxyurea adherence (Table 1). Although the association was not statistically significant, patients were more likely to be prescribed hydroxyurea if they were from a lower income background (61.9% for lowest and second lowest quartiles vs. 38.1% for third and highest quintiles). Patients were also more likely to adhere to hydroxyurea if they did not have private medical insurance for hydroxyurea coverage (Table 1). Finally, hydroxyurea adherence was associated with reduced rates of health care utilization and SCD-related complications (Table 2). Conclusions: In line with previous studies of hydroxyurea for the treatment of SCD, patients who were adherent to hydroxyurea had fewer complications compared to those patients who were either non-adherent to or not prescribed hydroxyurea. Similarly, patients had fewer complications after being prescribed hydroxyurea compared to before they started hydroxyurea with a reduction in the rate of ED visits, acute chest syndromes, complications, transfusions and hospitalizations. Patients from non-immigrant families, patients who were identified through newborn hemoglobinopathy screening and patients from lower income families were more likely to be adherent to hydroxyurea. Although the results of this study were limited by its small sample size, further studies will clarify these determinants of hydroxyurea adherence among SCD patients and enable clinicians to improve hydroxyurea adherence for SCD patients. Disclosures Klaassen: Shire: Consultancy; Novartis: Research Funding; Hoffman-La Roche: Consultancy; Amgen Inc.: Membership on an entity's Board of Directors or advisory committees; Octapharma AG: Consultancy, Honoraria; Agios Pharmaceuticals Inc.: Consultancy; Cangene: Research Funding.

Description: Bone marrow cells (BMCs) from CXB-12/HiaJ (CXB-12) mice had 14 times the total long-term repopulating ability found in the best of 11 other CXB recombinant inbred (RI) lines. BMCs from each RI line donor were mixed with genetically marked standard competitor BMCs from the BALB/cBy×C57BL/6 F1 (CByB6F1) hybrid, the mice used to produce the RI lines, and the mixtures repopulated lethally irradiated CByB6F1 recipients. Percentages of donor-type erythrocytes and lymphocytes measured the actual long-term repopulating functions of the donor RI lines relative to the standard competitor. CXB-12 BMCs repopulated better after 3 or 6 months than after 1 month, suggesting that the most primitive precursors were involved. Compared to CByB6F1 standard competitor cells, CXB-12 cells repopulated 3 to 12 times as well, with their advantage increasing when higher doses of cells were transplanted, probably because of hybrid resistance of the recipient against low doses. This was far better than expected, because F1 cells normally function 2 to 3 times as well as cells from an inbred strain. In competitive dilution, the advantage resulted from 2 factors: more precursor cells and more function per precursor. In the model that best fit the data, CXB-12 donors had 2.4 times the concentration of hematopoietic stem cells (HSCs) as the CByB6F1 standard, and each HSC repopulated 1.4 times as well. CXB-12 mice did not have elevated erythrocyte and lymphocyte numbers in blood and marrow and did not have unusually elevated concentrations of colony-forming unit spleen, cobblestone colonies, and long-term colony-initiating cells in marrow.

Description: Mesenchymal stem cells (MSC) mostly surround the vasculature system of bone marrow (BM). MSC have been shown to exhibit immune suppressive properties. Since MSC express MHC Class II antigen, the question is whether these cells can act as APC. To this end, we hypothesize that MSC have the ability to present non-self antigens while acting as immune modulators. These dual roles of MSC prevent exacerbated inflammatory responses in the BM, thereby preventing hematopoietic dysfunction. A ‘dampened’ immune response in BM during insults by foreign agents could cause protection of the barrier that separates BM cavity with the periphery. The phagocytic role of MSC was shown by confocal microscopy and fluoresbrite plain YG 1.0-micron microspheres. APC property was demonstrated by challenging MSC with C. albicans (pulsed MSC), followed by exposure to CD4+ cells. The latter was obtained by immunoselection from peripheral blood mononuclear cells (PBMC) cultured for 5 days with C. albicans (10 mg/ml). Proliferation of the CD4+ cells (3H-thymidine incorporation and cell counts) proved APC properties of MSC, at efficiency comparable to macrophages. Overall, the studies show that the window between APC function and the period at which MSC could become immune suppressive is critical, since activated T-cells could destroy the endothelial barrier between BM and lymphatics/peripheral circulation. These studies show that MSC could be key cells in regulating immune responses in BM, and thereby protect BM from failure.