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Abstract Background Thrombotic microangiopathy (TMA) is a clinicopathologic diagnosis that can be characterized by microangiopathic hemolytic anemia (MAHA), thrombocytopenia and end-organ damage. Though most patients will present with the hematologic abnormalities of MAHA and thrombocytopenia, there are some that never demonstrate those features and only present with acute renal injury. Here we describe the clinical presentation, laboratory assessment, management, and outcomes of patients with renal-limited TMA (rTMA). Methods After IRB approval, the electronic medical record was searched from 2008-March 2017 for patients managed at our institution with a clinical diagnosis of rTMA supported by renal biopsy. Inclusion criteria were as follows: 1) Presence of TMA on renal biopsy, 2) Absence of MAHA 3) No history of renal transplant prior to diagnosis of TMA. Clinical notes were reviewed to document the physician-determined cause of TMA. If no cause was listed, cause of TMA was based off of labs and clinical observation. Results Thirty-five patients (18 female) met clinical criteria for a diagnosis of rTMA. Median age at time of biopsy was 59 years (range 9 to 87). Creatinine at time of biopsy was 2.4 mg/dL (range 0.55 to 6.4). Evidence of chronic TMA was noted on 17 specimens and an additional 3 had findings of chronic and acute TMA. Ten patients had ADAMTS13 activity performed and all were >10% with negative inhibitor screen (median 66%(26-100)). 24 patients (68.6%) had complement serology assessed. Three patients (12.5%) had low C3 while C4 was normal in 19 patients (79%). One patient in our cohort had complement mutational testing performed with no evidence of a pathogenic variant. Most common etiology of rTMA was medications (28.5%) and underlying rheumatologic disorder (20%)(Table 1). In 6 patients (17%), an identifiable cause was not elucidated. Management of TMA was dependent on underlying etiology of rTMA. Discontinuation or dose adjustment of TMA-causative medications was recommended in 4 patients and 25% of patients had normalization of Cr. Four patients received therapeutic plasma exchange (TPE) for median 5 (2-5) days. Causes of rTMA for those receiving TPE were gemcitabine, bevacizumab, unspecified connective tissue disease, and rTMA due to an unknown cause. 50% of patients were noted to have improvement in Cr with TPE. Antihypertensive medications were used for rTMA secondary to uncontrolled hypertension (n=4) and scleroderma (n=4). 11 patients (31%) were observed or treatment for causative underlying etiology was pursued, while 7 patients were treated with glucocorticoids. Of 6 patients, with no identifiable underlying cause, 4 were treated with observation, 1 was treated with GC, and one was treated with TPE Fourteen patients (38.9%) required dialysis; 6 of these patients were able to discontinue dialysis less than two months after initiation. Three patients (8.3%) with rTMA, respectively due to tacrolimus toxicity, longstanding hypertension, and systemic lupus erythematosus, received a renal transplant, and one additional pt (2.8%) with rTMA due to scleroderma remained on the renal transplant waiting list at time of death. In a median follow-up time of 8.73 months (range 0 to 47.9), 16 patients (44.4%) died. Median time from diagnosis of TMA to death was 12.1 months (range 1.47 to 58.2). No deaths were directly attributable to underlying TMA. Conclusion Renal-limited TMA is a pathologic diagnosis that in our cohort was most likely to medications or underlying rheumatologic conditions. rTMA was managed by treating the underlying disorder or removing the causative medication. Though cause of rTMA was evident in the majority of patients, 17% had no identifiable precipitating factor predisposing to rTMA. Larger studies are required to delineate optimal management and surveillance strategies for these patients. Disclosures No relevant conflicts of interest to declare.

Abstract To further understand the molecular biology of mucosa-associated lymphoid tissue (MALT) lymphoma, we conducted a comprehensive gene expression study on the Affymetrix platform using total RNA extracted from biopsy specimen of 35 well-characterized pulmonary MALT lymphoma samples and compared them to gene expression profiles of a range of B and T cell lymphoma, such as Burkitt’s lymphoma, diffuse large B-cell lymphoma, Waldenstrom macroglobulinemia, chronic lymphocytic leukemia, multiple myeloma, and peripheral T-cell lymphoma, and normal cellular counterparts, such as different B-cell and T-cell populations, normal plasma cells and lung tussue. Analytical strategies were adopted to negate tissue specific effect such that our results can be generalized to MALT lymphoma from other anatomical sites. We further analyzed the data using functional tools such as gene-set enrichment analysis and gene ontology. Network/pathway analysis was performed using Metacore. Unsupervised clustering of whole dataset (including other lymphoid malignancies and normal cellular counterpart) using genes with variable expression, showed that compared to other B-cell lymphoid malignancies, MALT lymphoma have a prominent T-cell signature. Using established gene expression of different B-cell compartment, we confirmed that MALT has a marginal zone B-cell and memory B-cell cell-of-origin signature. Using ANOVA with post-hoc pair-wise comparison, we identified 4 novel transcripts over-expressed in MALT but not other B-cell tumors, and validated one of these using immunohistochemistry. MALT lymphoma with t(11;18) or t(14;18) over-expressed genes enriched for pathways other than the NFKB pathway such as JAK-STAT and SRC signaling pathways, which may represent novel therapeutic targets. We identified a number of genes with ‘outlier’ expression including RARA and RGS13. However, translocation involving RARA and the immunoglobulin heavy chain locus was not detected by FISH, suggesting other mechanisms of deregulating gene expression were involved. Using unsupervised clustering of only the MALT samples, we found that the molecular heterogeneity within MALT is composed of a group characterized by MALT1 translocations and another group with plasmacytic differentiation. Interestingly, further refinement of this grouping can be achieved by clustering the samples based on ‘outlier’ gene expression. Samples with MALT1 translocations have high expression of RARA, samples with plasmacytic differentiation have high FKBP11 expression and samples with high RGS13 expression tend to have trisomy 3 and reactive follicles. Our study therefore identified novel molecular markers, deregulated pathways and genetic defects in MALT. Furthermore, subgroups with distinct pathological features and anchored by unique pattern of ‘outlier’ gene expression were identified.

Abstract Abstract 1318 Previous studies have demonstrated that heme-oxygenase-1 (HO-1), the rate limiting enzyme in the catabolism of heme, is a regulator of the balance between hematopoiesis and stem cell preservation under stress. HO-1−/− mice display oxidative stress, anemia, and leukocytosis with chronic inflammation. We hypothesize that therapy with inhaled carbon monoxide (CO), a by-product of HO-1 activity with antioxidant and anti-apoptotic properties; will reduce stress hematopoiesis in HO-1−/− mice, reducing inflammation and redistributing hematopoietic potential. In order to test this hypothesis we treated 25 week old HO-1−/− mice and HO-1+/+ mice (n=7/strain) with 250 ppm inhaled CO for 1 h/day, 3 days/week for eight weeks and compared them to an equal number of untreated HO-1−/− and HO-1+/+ mice. After 8-weeks of treatment the mice were sacrificed and flow cytometry was performed on bone marrow to assess hematopoietic potential. Cell cycle analysis of the bone marrow demonstrates that untreated HO-1−/− mice have a significantly decreased percent of cells in S-phase compared to untreated HO-1+/+ mice. Treatment with CO significantly (p<0.05) increases the percent of cells in S-phase in HO-1−/− mice but not HO-1+/+ mice. Reactive oxygen species (ROS) production in lineage−, c-kit+, Sca-1+ (KLS) cell population was assessed using 5-(and 6-)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate dye. HO-1−/− mice have a significantly (p<0.001) increased proportion of ROS positive KLS cells compared with HO-1+/+ mice. Analysis of long-term (LT), short-term, and multipotent (MPP) progenitor cell populations was conducted. Treatment with CO significantly (p<0.05) increases the percent of LT-HSC and MPP progenitor cells in HO-1−/− mice but not HO-1+/+ mice. Concordantly, the total white blood cell count of the CO-treated mice increased significantly. The differential of the mature blood cells demonstrates significant shift in cell maturation, with significant increase in red blood cells, platelets, and lymphocytes and a significant decrease in monocytes. Combined this data indicates that CO therapy is able to modify the hematopoietic potential of HO-1−/− mice leading to a change in mature cell populations. We propose a model in which CO-mediated signaling initiates a homeostatic conditioning program in stem cells to balance hematopoiesis and stem cell preservation, ultimately leading to a change in the inflammatory milieu of the mice. Disclosures: Belcher: Sangart, Inc: Research Funding. Vercellotti:Sangart, Inc: Research Funding.

Abstract Background: Erythropoiesis-stimulating agents (ESAs) reduce anemia in cancer patients; however, there are concerns that ESAs increase mortality in some patients. Within the framework of an international collaboration we thus conducted a patient-level meta-analysis to assess the effects of ESAs on mortality in cancer patients. Methods: We performed a meta-analysis, based on the intention-to-treat principle, of cancer patients enrolled in randomized controlled trials comparing epoetin alfa, epoetin beta or darbepoetin alfa plus red blood cell transfusions as needed versus transfusion alone, for prophylaxis or treatment of anemia while or after receiving anticancer therapy. Patient-level data were obtained and analyzed by independent statisticians at two academic departments. Primary endpoints were on-study mortality and overall survival in patients receiving chemotherapy, defined as all patients from studies in which =/> 70% of study population received chemotherapy, and in all cancer patients regardless of anticancer therapy. On-study mortality was defined as death from any cause between date of randomization and 28 days after end of active study phase. Overall survival was defined as death from any cause between date of randomization and longest follow up available. Hazard ratios (HR) and 95% confidence intervals (CIs) were calculated per study and meta-analyzed with fixed-effects and random-effects models. Stratified multivariable Cox-regression analyses were conducted to assess the impact of baseline imbalances. Tests for interactions were used to identify differences in effect across pre-specified subgroups. All analyses were pre-specified in a peer-reviewed protocol (Bohlius et al. 2008). An independent steering committee consisting of clinicians and methodologists agreed on all analyses and interpretations. Independent investigators and representatives from manufacturers contributing data offered advice, but had no decision-making authority. Results: Data from 13,933 cancer patients enrolled in 53 studies were included in the analysis; 38 trials including 10,441 patients used mainly chemotherapy. Including all cancer patients ESAs increased on-study mortality by 17% (HR 1.17; 95% CI 1.06–1.30), with little evidence for a difference between chemotherapy and other trials (p for interaction=0.42), and worsened overall survival by 6% (HR 1.06; 95% CI 1.00–1.12). In the chemotherapy population on-study mortality was increased by 10% (HR 1.10, 95% CI 0.98–1.24) and overall survival was worsened by 4% (HR 1.04; 95% CI 0.97–1.11). Adjusting for known prognostic factors had little effect on the overall estimates. There was no conclusive evidence for effect modification by patient level characteristics such as age, sex, Hb and Hct at baseline, Hb ceiling, type and stage of tumor or study level characteristics (anticancer treatment, ESA treatment schedules, study design and quality) for the outcomes tested. Conclusion: ESA treatment in cancer patients increased on-study mortality by 17% and worsened overall survival by 6%. For patients undergoing chemotherapy the increase was less pronounced, but could not be excluded. In clinical practice, risks of ESAs must be balanced against benefits of ESAs depending on the clinical circumstances of the individual patient.

Abstract Abstract 4767 Previous studies have demonstrated that heme-oxygenase-1 (HO-1), the rate limiting enzyme in the catabolism of heme, is a regulator of the balance between hematopoiesis and stem cell preservation under stress. HO-1 produces three by-products, carbon monoxide, biliverdin/bilirubin, and iron/ferritin, all of which have demonstrated antioxidant and anti-apoptotic properties. HO-1-/- mice display oxidative stress, anemia, and leukocytosis with chronic inflammation. We hypothesize that therapy with inhaled carbon monoxide (CO) may reduce stress hematopoiesis in HO-1-/- mice, restoring them to normal hematopoiesis. We have previously reported that treatment with 250 ppm CO for 1 h raises mouse carboxyhemoglobin to 14–20% which returns to baseline within 24 hours. Therefore, in order to test this hypothesis we treated 25 week old HO-1-/- mice and HO-1+/+ mice (n=7/strain) with 250 ppm inhaled CO for 1 h/day, 3 days/week for eight weeks and compared them to an equal number of untreated HO-1-/- and HO-1+/+ mice. We demonstrate that after four weeks, CO-treated HO-1-/- mice have a total white blood cell count of 11.9 ± 3.2 K/μ L compared to 22.6 ± 3.2 K/μ L in untreated HO-1-/- mice (p<0.05), with HO-1+/+ mice having no treatment effect. After 8-weeks of treatment the mice were sacrificed and methylcellulose colony-forming unit assays and flow cytometry were performed on bone marrow to assess their hematopoietic potential. Total methylcellulose colony-forming units were similar in both HO-1-/- and HO-1+/+ mice with CO-treatments significantly (p<0.05) increasing the total number of colonies per animal. Of note, in both the CO-treated HO-1-/- and HO-1+/+ mice the CFU-GM were not significantly affected. Cell cycle analysis of the bone marrow using propidium iodide demonstrates that untreated HO-1-/- mice have a significantly decreased percent of cells in S-phase compared to untreated HO-1+/+ mice. Treatment with CO significantly (p<0.05) increases the percent of cells in S-phase in HO-1-/- mice but not HO-1+/+ mice. Similarly, untreated HO-1-/- mice have a lower frequency of cells in sub-G1 phase compared to untreated HO-1+/+ mice, however CO-treatments significantly (p<0.01) increase this frequency in HO-1-/- mice to levels comparable with untreated and CO-treated HO-1+/+ mice. Reactive oxygen species (ROS) production in lineage-, c-kit+, Sca-1+ (KLS) cell population was assessed using 5-(and 6-)-chloromethyl-2′,7′-dichlorodihydrofluorescein diacetate dye. HO-1-/- mice have a significantly (p<0.001) increased proportion of ROS positive KLS cells compared with HO-1+/+ mice. Interestingly, the CO-treated HO-1-/- mice had a significantly higher proportion of KLS cells staining positive for ROS, but the mean fluorescent intensity of this population was significantly (p<0.05) decreased compared to untreated HO-1-/- mice. This indicates that there is an overall decrease in ROS in the KLS cells of CO-treated HO-1-/- mice compared to untreated HO-1-/- mice. Analysis of long-term, short-term, and multipotent progenitor cell populations was conducted and reveals significant changes at the level of the short-term hematopoietic progenitor population (ST-HSC). Specifically, untreated HO-1-/- mice have a significantly increased percent of ST-HSC cells compared to untreated HO-1+/+ mice. Treatment with CO significantly (p<0.05) decreases the percent of ST-HSC cells in HO-1-/- mice but not HO-1+/+ mice. Combined this data indicates that CO therapy is able to modify the hematopoietic potential of HO-1-/- mice. We propose a model in which CO-mediated signaling initiates a homeostatic conditioning program in stem cells to balance hematopoiesis and stem cell preservation. Disclosures: Belcher: Sangart: Research Funding. Vercellotti:Sangart: Consultancy, Research Funding.

Abstract Introduction: Novel substances such as the next generation IMiD pomalidomide or the recently approved next generation proteasome inhibitor carfilzomib (Cfz) have considerably expanded our treatment options in MM, both of them influencing multiple myeloma (MM) interaction with bone marrow stroma cells (BMSCs), that provides an interesting target for anti MM therapy. More compounds directed at this disease critical crosstalk are currently under investigation, however the development of novel drugs remains inefficient, displayed by a substantial drop out rate of the 376 preclinical single agents tested since 1961 (Rongvaux, Annu Rev Immunol. 2013; Schüler, Expert Opin Biol Ther. 2013; Kortüm, CLML 2014). Our focus in the projected presented here was to develop a novel bone-derived in vitro 3D co-culture model specifically adapted to mimic the BM niche to more closely study the role of bone and BM bystander cells and to perform more reliable ex vivo compound screening in MM. Methods: Previous 2D models were compared to a novel 3D co-culture model (agarose matrix interlayer, 100 microwells/cm², 1.5mm in depth, permeable for oxygen+cytokines, but not for BMSCs utilizing U266, RPMI-8226, OPM-2 and primary BM patient (pt) cells, with and without HS-5 vs. M210B4 stroma support (Fig. A + Fig. B.a. for pt characteristics). Analyses covered Trypan Blue, Annexin/PI, MTT, FACS, cell cycle analyses and H2B-mCherry/cytochrome c-GFP assays (Udi, Br J Hematol. 2013). In a next step, primary bone-derived stroma cells were acquired from bones of C57BL/6 J mice. Bones were flushed, digested and FACS sorted in order to acquire single BM and bone bystander cell subtypes (MSPCs [mesenchymal stem and progenitor cells], endothelial cells, osteoblasts, PAS [PDFGRalphaSca1] and CaRs [CXCL12-abundant reticular cells]) which were then compared to HS-5 and M210B4 with regard to growth support, cytokine secretion and protection from anti-MM substances. Results: MMCLs and pt specimens were cultured at different concentrations (10 vs. 100 cells per microwell) with and without M210B4 demonstrating a growth advantage with vs. without M210B4 (Fig. B.b). Liquid overlay technique allowed cluster formation of pt specimens leading to more reliable propagation of pt material for up to 20d of culture. Apoptotic changes were assessed by confocal microscopy of RPMI8226 co-expressing fluorescently labelled histone 2B-mCherry (red) and cytochrome c-GFP (green) as indicators of late and early apoptosis. Comparing BMSCLs with regard to their MM growth support capacities, human HS-5 proved even more beneficent than M210B4 stroma (Fig. B.b). Phenotypic analyses of pt specimens showed decreased CXCR4 expression with vs. without BMSCs suggesting a dynamic regulation of homing molecules. The model was then used as an ex-vivo platform allowing both cytotoxicity and cell cycle analyses for the combination of bortezomib (Btz) vs. Cfz with ARRY-520 (kinesin spindle protein inhibitor). Btz (10nM) and Cfz (20nM) proved significantly cytotoxic compared to the control (U266 and pt specimens, respectively) after 48h of single agent treatment Fig.B.c). Compared to Btz (10nM, B10) and Cfz (20nM, C20) as single agents, the additional combination with ARRY-520 showed stronger additive cytotoxicity for Btz (A5+B10, median: 37.5% vs. 13.1%) than for Cfz (A5+C20, 38.6% vs. 34.1%). To note, the model could also be utilized for more profound analyses as depicted for G2/M cell cycle studies in Fig.B.d. 5nM ARRY-520 (A5) led to significant accumulation of OPM-2 cells in G2/M arrest after 48h of treatment confirming prior analyses (Hernández-Garcia Blood Suppl 4710,2014; Fig. B.d). Co-culture studies with different subsets of BM and bone-derived bystander cells are currently ongoing and will be presented at the meeting (Fig. B.e). Conclusions: More complex, 3D bone-derived high-throughput in vitro models are urgently needed to better predict the potency of preclinically tested agents and to better estimate the likelihood of their later clinical adoption into phase I-II trials. With this work, we provide an innovative model which reflects the BM microenvironment as a crucial predictor for in-vivo sensitivity as shown for ARRY-5200. This ex-vivo approach helps to better incorporate MM growth support by bone and BM-derived bystander cells and thus depicts a valid tool to better characterize the role of the BM niche in myeloma. Figure 1. Figure 1. Disclosures Engelhardt: Deutsch Krebshilfe: Other: grant. Wäsch:German Cancer Aid: Research Funding; Comprehensiv Cancer Center Freiburg: Research Funding; Janssen-Cilag: Research Funding; MSD: Research Funding.

Abstract Introduction: Slow rate of myeloid recovery at the end of induction chemotherapy reflected by a low absolute neutrophil count (ANC) is highly predictive of relapse in children with ALL (Laughton et al, 2005). Whether rate of myeloid recovery is predictive of outcome post-hematopoietic stem cell transplantation (HSCT) in children and young adults has not been studied. Objective: To explore the association between early ANC recovery and survival in children and young adults undergoing allogeneic and autologous HSCT for leukemia or lymphoma. Methods: Retrospective chart review was performed for all consecutive patients (age 6 months to 21 years) undergoing first HSCT for hematologic malignancies and lymphomas at Mayo Clinic, Division of Pediatric Hematology/Oncology from 1995 to 2004. Day 15 and day 28 ANC were calculated for all patients. The primary end point was 1-year relapse free (RFS) and overall survival (OS). To explore the association between ANC and primary outcome, patients were stratified in 2 groups: ≥ or < than median ANC and Kaplan Meier survival curves were generated for both the groups and compared with the log-rank test. Matched related donor (MRD), matched unrelated donor (MUD) and haploidentical (as one group) and autologous transplantations were analyzed separately. Results: There were a total of 50 patients. 28 received MRD, 2 MUD, 10 haploidentical and 10 autologous. The underlying malignancies were (AML n =25, Biphenotypic Leukemia n=1, ALL n =7, NHL n=3, HD n=7, JMML n=5, CML n=2, CLL n=1). No patient was lost to follow up during that time and data was available for all except (day 28 ANC) for one patient. The 1-year RFS and OS rate was the same; 81% for the MRD, 50% for MUD and haploidentical and 70% for autologous transplant populations, respectively. In patients receiving MRD transplants, higher ANC at either day 15 or day 28 post transplantation was associated with better, albeit statistically non-significant, RFS/OS rates at 1 year (86% for ≥ median (0.68) vs. 76% for < median p=0.44 and 92% for ≥ median (2.2) vs. 72% for < median p= 0.18 respectively). Opposite results were observed for MUD and Haploidentical transplantation group (33% for ANC ≥ median (0.47) at day 15 vs. 66% for < median p=0.28, 43% for ANC ≥ median (1.9) at day 28 vs. 60% for < median p=0.48). For autologous transplant group, higher ANC at day 15 was associated with better, albeit statistically non-significant, RFS/OS rates at 1 year (80% for ANC ≥ median (0.42) vs. 60% for < median p=0.60) while opposite results were observed on day 28 (60% for ANC ≥ median (0.46) vs. 75% for < median p=0.54). Conclusions: Our data generated conflicting findings of the association between ANC and RFS/OS in different patient populations undergoing HSCT. However, our results are limited by the small sample size. Further collaborative studies are needed to explore this association.

Abstract Abstract 2883 CLL-like monoclonal B-cell lymphocytosis (MBL) shares common immunophenotypic features and cytogenetic abnormalities with CLL and is generally perceived as its premalignant state. The World Health Organization has set a consensus cut-off of 5×109/L circulating B cells to discriminate between what constitutes ‘disease’ and what not. However, the clonal size within MBL is extremely variable. High-count (HC), clinical MBL is associated with absolute lymphocytosis and progresses to CLL requiring treatment at a rate of 1–2% per year, whereas low-count (LC) MBL is found in the general population through high-sensitivity techniques and carries a risk of progression that is limited if any. Given the high frequency of CLL-like MBL in the general population, it is important to understand the underlying mechanisms and also identify biological markers endowing malignant potential that may distinguish between the different forms. To this end, we performed a detailed immunogenetic profiling of 334 CLL-like MBL cases (78 LC and 256 HC) for a total of 355 productive VDJ rearrangements (including double rearrangements), 91 from LC MBL and 264 from HC MBL. We also compared the immunoglobulin (IG) gene repertoires of MBL to 544 CLL Rai Stage 0 (CLL-0) that were part of an IG sequence dataset of 7424 CLL cases previously analyzed by our group. LC and HC MBL had distinct IG gene repertoires, with over-representation of the IGHV1–69 and IGHV4–34 genes in HC and the IGHV4–59/61 genes in LC MBL, respectively (p<0.001). The HC MBL repertoire exhibited clear similarities to CLL-0 in terms of IGHV gene usage (similar frequencies of IGHV1–69 and IGHV4–34). Regarding somatic hypermutation, no differences were identified between LC MBL versus HC MBL versus CLL-0, in that the frequency of mutated rearrangements (<98% identity to the germline) was overall similar (LC MBL: 72.5%, HC MBL: 76.1%, CLL-0: 75%). In this respect, all the aforementioned subgroups differed significantly (p<0.001) from the frequency previously reported by us in CLL where only 55% of rearrangements carried mutated IGHV genes. We finally analyzed the expression of stereotyped B cell receptor (BcR) IGs, identified through a cluster analysis of the MBL sequences together with all CLL sequences from our cohort and 5494 non-CLL IG sequences retrieved from public databases. Overall, only 6/91 (5.5%) LC MBL rearrangements could be clustered with other sequences in subsets with stereotyped BcRs. Two of these six LC MBL cases were clustered together with IG sequences from various entities, including CLL, other lymphomas and autoimmune diseases; thus, they were considered to carry ‘public’ BcR stereotypes. In contrast, HC MBL showed a significantly (p=0.0002) higher frequency of ‘CLL-specific’ BcR stereotypes versus LC MBL, with 60/264 (22.7%) HC MBL cases carrying stereotyped VH CDR3s. This frequency was comparable to the one observed in CLL-0 (20.2%). Notably, a gradation was observed in the frequency of BcR IG stereotypy depending on the absolute count of CLL-like cells, starting with 5.5% in LC MBL, raising to 22.7–20.2% in in HC MBL/CLL-0 and, finally, peaking at 30.4% in the entire CLL cohort as previously reported. Altogether, these findings suggest that rather than a true premalignant condition, LC MBL may merely reflect immune senescence or result from persistent antigen stimulation. On the other hand, HC MBL appears to be a continuum with Rai stage 0 in the evolution to clinically overt CLL, being maybe one step behind where it requires either additional genetic hits or simply extra time to cross the numerical border that discriminates it from CLL. Hence, the identification of molecular genetic markers that may predict progression of HC MBL/CLL-0 into full-fledged CLL is strongly warranted. Disclosures: Shanafelt: Genentech: Research Funding; GlaxoSmith Klein: Research Funding; Teva/Cephalon: Research Funding; Celgene: Research Funding.

Abstract Introduction: Early lymphocyte recovery predicts outcome in adults undergoing high dose chemotherapy followed by autologous stem cell transplantation for a variety of malignancies (Porrata et al; 2001, 2002, 2005). Whether rate of lymphoid recovery is predictive of outcome post-hematopoietic stem cell transplantation (HSCT) in children and young adults has not been studied. Objective: To explore the association between early ALC recovery and survival in children and young adults undergoing allogeneic and autologous (HSCT) for leukemia or lymphoma. Methods: Retrospective chart review was performed for all consecutive patients (age 6 months to 21 years) undergoing first HSCT for hematologic malignancies and lymphomas at Mayo Clinic, Division of Pediatric Hematology/Oncology from 1995 to 2004. Day 15 and day 28 ALC were calculated for all patients. The primary end point was 1-year relapse free (RFS) and overall survival (OS). To explore the association between ALC and primary outcome, patients were stratified in 2 groups: ≥ or < than median ALC and Kaplan Meier survival curves were generated for both the groups and compared with the log-rank test. Matched related donor (MRD), matched unrelated donor (MUD) and haploidentical (as one group) and autologous transplantations were analyzed separately. Results: There were a total of 50 patients. 28 received MRD, 2 MUD, 10 haploidentical and 10 autologous transplant. The underlying malignancies were AML n =25, Biphenotypic Leukemia n=1, ALL n =7, NHL n=3, HD n=7, JMML n=5, CML n=2, CLL n=1). No patient was lost to follow up during that time and data was available for all except (day 28 ALC) for one patient. The 1-year RFS and OS rate was the same; 81% for the MRD, 50% for MUD and haploidentical and 70% for autologous transplant populations, respectively. In patients receiving MRD transplants, higher ALC at either day 15 or day 28 post transplantation was associated with better, albeit statistically non-significant, RFS/OS rates at 1 year (85% for ≥ median (0.23) vs. 78% for < median p=0.69 and 86% for ≥ median (0.47) vs. 77% for < median p= 0.59 respectively). Opposite results were observed for MUD and Haploidentical transplantation group (33% for ALC ≥ median (0.05) at day 15 vs. 67% for < median p=0.2, 33% for ALC ≥ median (0.35) at day 28 vs. 66% for < median p=0.28) and for autologous transplantations (67% for ALC ≥ median (0.23) at day 15 vs. 75% for < median p=0.68, 40% for ALC ≥ median (0.48) at day 28 vs. 100% for < median p=0.07). Conclusions: Our data generated conflicting findings of the association between ALC and RFS/OS in different patient populations undergoing HSCT. However, our results are limited by the small sample size. Further collaborative studies are needed to explore this association.