Summary:Bone marrow aspirates are composed of two cellular compartments, an abundant buffy coat suspension and a minor particulate fraction. The particulate fraction is routinely removed by filtration prior to transplantation in order to reduce the risk of embolism. This study shows that the filter-retained fraction includes many multicellular complexes, previously defined as haematons. A haematon is a finely arborized stromal-web which is tightly packed with haemopoietic progenitor cells and differentiated postmitotic cells. Comparison of the pooled buffy coat and the filter-retained materials from healthy donors showed that the haematon fraction contained 8-40 × 10 6 CD34 + cells, 20-115 × 10 3 high proliferative potential colony-forming cells (HPP-CFC) and 0.49-2.67 × 10 6 granulocyte-macrophage colony-forming unit (GM-CFU) which constituted 24 ± 8% (10-36; n = 8) of the total GM-CFU population harvested. Similar, but more variable recoveries of GM-CFU were obtained from the haematon fractions from patients with breast cancer (21 ± 13%; n = 10), Hodgkin's disease (33 ± 19%; n = 4), non-Hodgkin's lymphoma (21 ± 18; n = 7), but the recovery was lower from patients with acute myelogenous leukaemia (AML) (13 ± 13%; n = 6). The haematon fraction was enriched in CD34 + cells (2.5-fold), long-term culture initiating cells (LTC-IC/CAFC, week 5) (3.5-fold), HPP-CFC (2.8-fold) and GM-CFU (2.3-fold) over the buffy coat. Purified CD34 + cells expanded exponentially and produced 800 to 4000-fold more nucleated cells, 300 to 3500-fold more GM-CFU and 10 to 80-fold more HPP-CFC in stroma-free suspension culture with interleukin-1 (IL-1␤), IL-3, IL-6, GM-CSF and stem cell factor (SCF), than did the starting cell input. The haematon fraction produced significantly more progenitor cells than the buffy coat in long-term liquid culture (LTC). This was due to the higher frequency of LTC-IC/CAFC and to the presence of the whole spectrum of native, stroma cell-associated CAFC in haematons. Thus, the haematon includes the most productive haematogenous compartment in human BM. This simple enrichment strategy,

e19513 Background: PTLD is a rare complication of organ transplantation, but it can jeopardize the transplantation outcome. Methods: Paul Brousse University Hospital is the largest liver transplantation center in France. We retrospectively analyzed data from all of the patients who experienced PTLD and treated in our hematology/oncology department in the last 10 years. Results: 16 cases of PTLD occurred after various intervals following the liver transplant without correlation with the type of underlying liver disease or immunosuppressive treatment. Overall, the patients had poor general health (ECOG 3-4: 69%), renal and/or hepatic failure (75%), and cytopenia (63%). PTLD were often of high grade (81%), B phenotype (94%), stage IV (75%), with high LDH (88%). The most frequent disease sites were liver (63%), subdiaphragmatic lymphadenopathy (69%), and extranodal (69%). After a reduction in the immunosuppressive treatment, the patients received their initial chemotherapy mostly with a combination of prednisolone, cyclophosphamide, vincristine, and rituximab. As soon as their general condition improved, R-CHOP was given as the standard regimen. All patients experienced severe and frequent complications during chemotherapy. Two patients had liver transplant rejection, lethal in one case. Six patients died: 4 of progression or relapse of PTLD, one after 4th liver transplantation, one of leukoencephalitis. Complete remission was obtained in 81% of patients. Ten patients (63%) are alive in complete remission after a median follow up of 6.5 years. Conclusions: Despite the clinical complexity of these patients, PTLD can be treated utilizing common lymphoma chemotherapy with treatment accommodations to poor ECOG, liver and renal dysfunctions.