Efficacy and safety of avatrombopag in combination with immunosuppressive therapy in treatment-naïve and relapsed/refractory severe aplastic anaemia: protocol for the DIAAMOND-Ava-FIRST and DIAAMOND-Ava-NEXT Bayesian Optimal Phase II trials

INTRODUCTION

Immunosuppressive therapy (IST) with antithymocyte globulin (ATG) and ciclosporin is standard of care for patients with severe aplastic anaemia (sAA) not eligible or suitable for allogeneic stem cell transplant. While patients respond to IST, few achieve complete responses and a significant proportion are refractory or relapse. The addition of eltrombopag, a thrombopoietin-receptor agonist (TPO-A), to IST has been shown to improve haematological responses in sAA. Avatrombopag is a second-generation TPO-A with potential advantages over eltrombopag. However, to date avatrombopag has not been studied in sAA.

METHODS AND ANALYSIS

Investigator-initiated, single-arm registry-based Bayesian Optimal Phase II trial of avatrombopag conducted in two cohorts, patients with untreated sAA (FIRST cohort) and in patients with sAA that has relapsed or is refractory to IST (NEXT cohort). In the FIRST cohort, participants receive IST (equine ATG and ciclosporin) plus avatrombopag from day 1 until day 180 at 60 mg oral daily, with dose adjusted according to platelet count. Participants in the NEXT cohort receive avatrombopag at 60 mg oral daily from day 1 until day 180, with or without additional IST at the discretion of the treating clinician.For each cohort, two primary endpoints (haematological response and acquired clonal evolution) are jointly monitored and the trial reviewed at each interim analysis where a 'go/no-go' decision is made by evaluating the posterior probability of the events of interests.

ETHICS AND DISSEMINATION

The trial has received ethics approval (Monash Health RES-18-0000707A). The trial conduct will comply with ICH-GCP and all applicable regulatory requirements. The results of the trial will be submitted to a peer-review journal for publication.

TRIAL REGISTRATION NUMBER

ACTRN12619001042134, ACTRN12619001043123.

The Australian Aplastic Anaemia and other Bone Marrow Failure Syndromes Registry

The bone marrow failure syndromes (BMFS) are a diverse group of acquired and inherited diseases which may manifest in cytopenias, haematological malignancy and/or syndromic multisystem disease. Patients with BMFS frequently experience poor outcomes, and improved treatment strategies are needed. Collation of clinical characteristics and patient outcomes in a national disease-specific registry represents a powerful tool to identify areas of need and support clinical and research collaboration. Novel treatment strategies such as gene therapy, particularly in rare diseases, will depend on the ability to identify eligible patients alongside the molecular genetic features of their disease that may be amenable to novel therapy. The Australian Aplastic Anaemia and other Bone Marrow Failure Syndromes Registry (AAR) aims to improve outcomes for all paediatric and adult patients with BMFS in Australia by describing the demographics, treatments (including supportive care) and outcomes, and serving as a resource for research and practice improvement.

Disease Progression of WHIM Syndrome in an International Cohort of 66 Pediatric and Adult Patients

Warts, hypogammaglobulinemia, infections, and myelokathexis (WHIM) syndrome (WS) is a combined immunodeficiency caused by gain-of-function mutations in the C-X-C chemokine receptor type 4 (CXCR4) gene. We characterize a unique international cohort of 66 patients, including 57 (86%) cases previously unreported, with variable clinical phenotypes. Of 17 distinct CXCR4 genetic variants within our cohort, 11 were novel pathogenic variants affecting 15 individuals (23%). All variants affect the same CXCR4 region and impair CXCR4 internalization resulting in hyperactive signaling. The median age of diagnosis in our cohort (5.5 years) indicates WHIM syndrome can commonly present in childhood, although some patients are not diagnosed until adulthood. The prevalence and mean age of recognition and/or onset of clinical manifestations within our cohort were infections 88%/1.6 years, neutropenia 98%/3.8 years, lymphopenia 88%/5.0 years, and warts 40%/12.1 years. However, we report greater prevalence and variety of autoimmune complications of WHIM syndrome (21.2%) than reported previously. Patients with versus without family history of WHIM syndrome were diagnosed earlier (22%, average age 1.3 years versus 78%, average age 5 years, respectively). Patients with a family history of WHIM syndrome also received earlier treatment, experienced less hospitalization, and had less end-organ damage. This observation reinforces previous reports that early treatment for WHIM syndrome improves outcomes. Only one patient died; death was attributed to complications of hematopoietic stem cell transplantation. The variable expressivity of WHIM syndrome in pediatric patients delays their diagnosis and therapy. Early-onset bacterial infections with severe neutropenia and/or lymphopenia should prompt genetic testing for WHIM syndrome, even in the absence of warts.

Results of a phase 2 trial of an oral CXCR4 antagonist, mavorixafor, for treatment of WHIM syndrome

Warts, hypogammaglobulinemia, infections, and myelokathexis (WHIM) syndrome is a rare primary immunodeficiency caused by gain-of-function mutations in the CXCR4 gene. We report the safety, tolerability, pharmacokinetics, pharmacodynamics, and preliminary efficacy of mavorixafor from a phase 2 open-label dose-escalation and extension study in 8 adult patients with genetically confirmed WHIM syndrome. Mavorixafor is an oral small molecule selective antagonist of the CXCR4 receptor that increases mobilization and trafficking of white blood cells from the bone marrow. Patients received escalating doses of mavorixafor, up to 400 mg once daily. Five patients continued on the extension study for up to 28.6 months. Mavorixafor was well tolerated with no treatment-related serious adverse events. At a median follow-up of 16.5 months, we observed dose-dependent increases in absolute neutrophil count (ANC) and absolute lymphocyte count (ALC). At doses ≥300 mg/d, ANC was maintained at >500 cells per microliter for a median of 12.6 hours, and ALC was maintained at >1000 cells per microliter for up to 16.9 hours. Continued follow-up on the extension study resulted in a yearly infection rate that decreased from 4.63 events (95% confidence interval, 3.3-6.3) in the 12 months prior to the trial to 2.27 events (95% confidence interval, 1.4-3.5) for patients on effective doses. We observed an average 75% reduction in the number of cutaneous warts. This study demonstrates that mavorixafor, 400 mg once daily, mobilizes neutrophil and lymphocytes in adult patients with WHIM syndrome and provides preliminary evidence of clinical benefit for patients on long-term therapy. The trial was registered at www.clinicaltrials.gov as #NCT03005327.

Revisiting acquired aplastic anaemia: current concepts in diagnosis and management

Acquired aplastic anaemia is a rare, serious, immunologically mediated bone marrow failure syndrome, characterised by marrow hypoplasia of varying severity and significant pancytopenia. Careful attention and investigation, including molecular testing, is required to confirm the diagnosis and exclude other mimicking conditions, such as inherited bone marrow failure syndromes. In a proportion of patients, the disease evolves to myelodysplasia or acute myeloid leukaemia and in some there is an association with paroxysmal nocturnal haemoglobinuria. The disease has a major impact on patient quality of life. Haemopoietic stem/progenitor cell transplantation for eligible patients with an available donor is the only current curative therapy. Other patients may receive immunosuppression, most commonly with anti-thymocyte globulin and cyclosporin. An initial response to immunosuppression is often encouraging, but relapse is common. Supportive care, including management of transfusion requirements and infections, is central to management. Promising new diagnostic tools and emerging therapies will likely transform approaches to this important, chronic and life-threatening condition.

Use of arsenic trioxide in remission induction and consolidation therapy for acute promyelocytic leukaemia in the Australasian Leukaemia and Lymphoma Group (ALLG) APML4 study: a non-randomised phase 2 trial

BACKGROUND

Initial treatment of acute promyelocytic leukaemia traditionally involves tretinoin (all-trans retinoic acid) combined with anthracycline-based risk-adapted chemotherapy, with arsenic trioxide being the treatment of choice at relapse. To try to reduce the relapse rate, we combined arsenic trioxide with tretinoin and idarubicin in induction therapy, and used arsenic trioxide with tretinoin as consolidation therapy.

METHODS

Patients with previously untreated genetically confirmed acute promyelocytic leukaemia were eligible for this study. Eligibilty also required Eastern Cooperative Oncology Group performance status 0-3, age older than 1 year, normal left ventricular ejection fraction, Q-Tc interval less than 500 ms, absence of serious comorbidity, and written informed consent. Patients with genetic variants of acute promyelocytic leukaemia (fusion of genes other than PML with RARA) were ineligible. Induction comprised 45 mg/m(2) oral tretinoin in four divided doses daily on days 1-36, 6-12 mg/m(2) intravenous idarubicin on days 2, 4, 6, and 8, adjusted for age, and 0·15 mg/kg intravenous arsenic trioxide once daily on days 9-36. Supportive therapy included blood products for protocol-specified haemostatic targets, and 1 mg/kg prednisone daily as prophylaxis against differentiation syndrome. Two consolidation cycles with tretinoin and arsenic trioxide were followed by maintenance therapy with oral tretinoin, 6-mercaptopurine, and methotrexate for 2 years. The primary endpoints of the study were freedom from relapse and early death (within 36 days of treatment start) and we assessed improvement compared with the 2 year interim results. To assess durability of remission we compared the primary endpoints and disease-free and overall survival at 5 years in APML4 with the 2 year interim APML4 data and the APML3 treatment protocol that excluded arsenic trioxide. This study is registered with the Australian New Zealand Clinical Trials Registry, number ACTRN12605000070639.

FINDINGS

124 patients were enrolled between Nov 10, 2004, and Sept 23, 2009, with data cutoff of March 15, 2012. Four (3%) patients died early. After a median follow-up of 4·2 years (IQR, 3·2-5·2), the 5 year freedom from relapse was 95% (95% CI 89-98), disease-free survival was 95% (89-98), event-free survival was 90% (83-94), and overall survival was 94% (89-97). The comparison with APML3 data showed that hazard ratios were 0·23 (95% CI 0·08-0·64, p=0·002) for freedom from relapse, 0·21 (0·07-0·59, p=0·001) for disease-free survival, 0·34 (0·16-0·69, p=0·002) for event-free survival, and 0·35 (0·14-0·91, p=0·02) for overall survival.

INTERPRETATION

Incorporation of arsenic trioxide in initial therapy induction and consolidation for acute promyelocytic leukaemia reduced the risk of relapse when compared with historical controls. This improvement, together with a non-significant reduction in early deaths and absence of deaths in remission, translated into better event-free and overall survival.

FUNDING

Phebra.

Dose-adjusted arsenic trioxide for acute promyelocytic leukaemia in chronic renal failure

OBJECTIVE

To determine the potential for arsenic trioxide (ATO) to be safely and effectively incorporated into induction therapy of newly diagnosed acute promyelocytic leukaemia (APL) in patients with severe chronic renal failure (CRF) by reduction of the ATO dosage to compensate for reduced renal elimination of arsenic in CRF.

PATIENTS AND METHODS

Two of the four CRF patients with APL in the study were dialysis-dependent, and two had eGFRs of 18 and 19 mL/min/1.73 m(2) . ATO dosage schedules were adjusted to obtain comparable whole-blood arsenic levels to those in APL patients with normal renal function who achieved molecular remission (MR) while receiving 10 mg ATO daily for 28 d.

RESULTS

Average ATO administered per day in CRF patients ranged from 36 to 50% of the ATO administered to APL patients with normal renal function. No clinically significant cardiac, hepatic or other toxicities were detected. RT-PCR-negative MR was achieved after one treatment course in two patients and after two courses in the others. Relapse-free survival is 155, 60, 43 and 5 months.

CONCLUSION

The observations in this pilot study have demonstrated whole-blood arsenic levels can provide a guide to adjustments of ATO dosage schedules that permit safe and effective therapeutic outcomes in APL patients with severely compromised renal function.

Oral administration of arsenic trioxide in the treatment of acute promyelocytic leukaemia and accelerated phase chronic myeloid leukaemia: an Australian single-centre study

Experience in the treatment of patients with acute promyelocytic leukaemia (APL) and accelerated phase chronic myeloid leukaemia with orally administered arsenic trioxide (ATO) in our institution since 1999 has demonstrated that bioavailability of oral ATO is comparable with intravenous administration, and similar outcomes are produced in treatment of APL. Oral administration was well tolerated, with good compliance, in patients not requiring hospitalisation for postinduction treatment and was particularly convenient for patients living considerable distances from our institution. Orally administered ATO can be considered a practicable option in management of APL.

Protein kinase C-beta II expression in diffuse large B-cell lymphoma predicts for inferior outcome of anthracycline-based chemotherapy with and without rituximab

Protein kinase C-beta II (PKC-beta II) expression has been reported to indicate inferior prognosis in diffuse large B-cell lymphoma (DLBCL) treated with anthracycline-based chemotherapy. This study compared prognostic significance of immunohistochemically determined PKC-beta II expression in de novo DLBCL treated with cyclophosphamide, doxorubicin, vincristine and prednisolone (CHOP) chemotherapy with and without rituximab. Outcomes were assessed in 80 consecutive patients, 48 treated with CHOP, and 32 with rituximab plus CHOP (R-CHOP). PKC-beta II expression correlated with inferior overall survival (OS) and progression-free survival (PFS) in CHOP-treated patients with low-risk International Prognostic Index (IPI) disease (0-2 adverse factors), but not in the overall patient group unstratified by IPI. PKC-beta II expression correlated with inferior OS and PFS in R-CHOP-treated patients unstratified by IPI status. Immunohistochemically demonstrated PKC-beta II expression thus identified patient subgroups where alternative treatment strategies may confer superior outcome. We now report that PKC-beta II expression has prognostic significance not only for CHOP therapy in low-risk IPI disease, but also for all patients receiving CHOP plus rituximab.

Reversal of dialysis-dependent renal failure in light-chain deposition disease by autologous peripheral blood stem cell transplantation

Specific treatment of light-chain deposition disease has been reported as ineffective in altering the course of the severe or end-stage renal failure it causes. The authors describe a case of biopsy-proven primary light-chain deposition disease of the kidney, severe renal failure, and incipient dialysis dependency, treated by autologous peripheral blood stem cell transplantation, that led to reversal of dialysis dependency and sustained improvement in renal function.

Delayed-onset neutropenia associated with rituximab therapy

The characteristics of severe neutropenia with a delayed onset following administration of rituximab have been evaluated in 53 consecutively treated patients. All but one patient received rituximab for the treatment of non-Hodgkin's lymphoma. Eight episodes of grade 4 neutropenia were detected between 1 and 5 months after rituximab, when administered alone on five occasions, and on three occasions in combination with chemotherapy, where neutrophil counts had recovered prior to the development of neutropenia. In three episodes, the patients presented with sepsis. Development of neutropenia did not correlate with either the presence of detectable disease or the administration of further treatment. Neutropenia was associated with selective depletion of neutrophil precursors in all but one episode, where it was associated with generalized bone marrow hypoplasia. All episodes developed after a period of either normal or mildly depressed neutrophil counts following treatment with rituximab, and persisted for between several days and several months, before undergoing spontaneous recovery in four instances, and after administration of filgrastim in the remainder. Episodes of neutropenia were associated with disordered immune status manifested by lymphopenia and hypogammaglobulinaemia, raising the possibility that either disturbance of the balance of lymphocyte subsets or an immune dyscrasia induced by rituximab resulted in the development of this type of neutropenia.

Membranoproliferative glomerulonephritis in association with chronic lymphocytic leukaemia: a report of three cases

AIMS

The reported association of glomerular disease with chronic lymphocytic leukaemia (CLL) is rare despite the relative frequency of this type of leukaemia. Hence, we have examined the renal biopsies in three patients with CLL and glomerulonephritis.

METHODS

Renal biopsies were examined by light microscopy, immunofluorescence microscopy and electron microscopy.

RESULTS

One of two patients with mild impairment of renal function and an active urinary sediment had ultrastructural features of idiopathic type I membranoproliferative glomerulonephritis (MPGN), and the other had features of fibrillary/ immunotactoid glomerulonephritis with deposits of IgG and C3. One patient with nephrotic syndrome had characteristic electron microscopic appearances of type III MPGN. In all three there was an association with monoclonal gammopathy. The parameters of glomerular damage improved in association with response to drug treatment of the CLL.

CONCLUSION

There is a spectrum of types of MPGN seen in patients with CLL and there appears to be an association with the presence of monoclonal gammopathy. This is the first reported case of type III MPGN in CLL.

Assessment of cellular expression of parathyroid hormone-related protein mRNA and protein in multiple myeloma

The capacity of multiple myeloma cells to generate parathyroid hormone-related protein (PTHrP) has been examined by in situ assessment of PTHrP mRNA and PTHrP protein in myeloma cells of patients in whom the disease was associated with the development of hypercalcaemia. The presence of PTHrP mRNA was evaluated by in situ hybridization using an antisense riboprobe, and PTHrP by immunohistochemistry using a monoclonal antibody, in archival bone marrow trephine specimens from 17 patients. PTHrP mRNA was detected in myeloma cells in 16 of the 17 patients, indicating a high frequency of PTHrP gene expression in myeloma cells in these subjects. PTHrP protein was, on the other hand, detected in the myeloma cells of only five of these patients. The impact of the mercury-based fixation and decalcification procedure used for processing the bone marrow trephine specimens was assessed to determine the influence of this process on the outcome of the immunohistochemical assay for PTHrP. It was shown that this preparative procedure resulted in a marked reduction of immunohistochemically detectable PTHrP, which provides a possible explanation for the lower frequency of positivity for PTHrP in myeloma cells in the bone marrow specimens. The present findings are consistent with the view that PTHrP can be generated in myeloma cells in vivo, and could contribute to osteolysis and hypercalcaemia, as in patients with cancer.

Aetiology of severe iron overload in a family with hereditary haemolytic anaemia

Severe iron overload is a reported complication of certain erythroid disorders which are characterized by increased erythropoietic activity. Proposed mechanisms include enhancement of iron absorption secondary to increased erythroid activity and coexistent heterozygosity or homozygosity for haemochromatosis. We performed PCR-based analysis for the haemochromatosis-related HFE C282Y mutation in an extended family with inherited haemolytic anaemia in which several members exhibited iron overload. The results demonstrated iron overload was associated with homozygosity but not heterozygosity for this mutation. Such an association may also exist in other erythroid disorders in which iron overload has been reported.

Parathyroid hormone-related protein in hypercalcemia associated with hematological malignancy

Hypercalcemia is an important complication in multiple myeloma as well as T-cell leukemia/lymphoma, and is moderately common in high and intermediate grade non-Hodgkin's lymphoma. The underlying mechanism has been unclear because the neoplastic cells are usually present in the bone marrow, where they are in a position to produce short range effects on bone resorption which are difficult to identify. This contrasts with the situation in hypercalcemia associated with non-metastatic carcinoma, where it has been clearly demonstrated that the most common cause is release from the tumor of a humoral mediator, Parathyroid Hormone-related Protein (PTHrP). Roles have been advocated in multiple myeloma for release of a number of other cytokines with osteolytic capacity on the basis of their enhancement of osteolytic activity in cultured fetal rat bone, although a causal relationship in patients has not been established. PTHrP has more recently been implicated in the genesis of hypercalcemia in patients with hematological malignancies by the demonstration in a proportion of cases of increased circulating levels of PTHrP, comparable to those in hypercalcemia due to cancer. Immunohistochemical studies indicate neoplastic hemopoietic cells can contain PTHrP, and thus have the capacity to act in a paracrine manner to enhance local bone resorption and contribute to the development of hypercalcemia.

Disseminated echovirus infection after allogeneic bone marrow transplantation

Disseminated enteric human cytopathogenic orphan (echo) virus infection after allogeneic bone marrow transplantation has been reported once previously: a patient developed a fatal infection with the virus being isolated from brain, lung and heart. We report a second case of disseminated echovirus infection in which virus was isolated from the stomach and liver. On this occasion the infection was associated with the development of biopsy-proven acute graft-versus-host disease of the skin, stomach, colon and liver. The infection resolved without sequelae.

High response rates with short infusional 2-chlorodeoxyadenosine in de novo and relapsed low-grade lymphoma. Australian and New Zealand Lymphoma Study Group

Thirty-five patients (eight de novo, 27 relapsed disease) with low-grade non-Hodgkin's lymphoma (diffuse small lymphocytic, follicular small cleaved cell, follicular mixed cell, and lymphoplasmacytoid) were treated with 2-chlorodeoxyadenosine (2CdA) at a daily dose of 0.14 mg/kg for 5d (2 h infusion) for an average of three cycles. Minor treatment delays, generally due to haematological toxicities, occurred in nine of 105 cycles. Major toxicities were lymphopenia, neutropenia and thrombocytopenia. Opportunistic infections occurred in seven patients. Overall response rate was 69% (five complete, 19 partial) reaching 88% for de novo patients (two complete, five partial). Elevated beta 2-microglobulin level was negatively predictive of response (P = 0.0014). Eight of 24 responders relapsed, with a median follow-up of 13 months. 2CdA administered as an intermittent infusion shows considerable single-agent activity in low-grade lymphomas achieving high response rates of prolonged duration. Consideration of schedules where 2CdA is alternatively administered with combination chemotherapy appears warranted.

Parathyroid hormone-related protein in hypercalcaemia associated with haematological malignancy

The incidence of hypercalcaemia and its association with humoral mechanisms involving parathyroid hormone-related protein (PTHrP), parathyroid hormone (PTH), or 1.25(OH)2 vitamin D were assessed in a prospective study of patients admitted to a clinical haematology unit. Hypercalcaemia was detected in 18/165 patients, and was due to primary hyperparathyroidism in 3/17 patients in whom results of humoral mediator assessments were obtained. In the other patients, hypercalcaemia was associated in nine instances with myeloma, in five with B-cell non-Hodgkin's lymphoma (NHL), and in one with myeloid neoplasia. No evidence was obtained of a humoral mechanism involving 1.25(OH)2 vitamin D, but elevated circulating levels of PTHrP, comparable with those in humoral hypercalcaemia of malignancy, were present in 2/4 patients with NHL, and in 3/9 with myeloma. The relationship between presence or absence of elevated circulating PTHrP, and presence or absence of hypercalcaemia during the course of treatment, indicated PTHrP was involved in the production of hypercalcaemia. Such an association raises the possibility that PTHrP released by neoplastic cells in these disorders acts in a paracrine manner to produce local bone resorption, and when produced in greater amounts causes elevated circulating levels which make an additional humorally-mediated contribution to the development of hypercalcaemia.

An effective age-unrestricted m-AMSA-based second-line regimen for poor prognosis acute myeloid leukaemia

The efficacy and toxicity of a regimen consisting of amsacrine (m-AMSA), cytarabine, and thioguanine for remission-induction therapy in poor prognosis categories of acute myeloid leukaemia (AML) were determined in a single arm study of 46 patients. The study group consisted of 17 patients with disease refractory to daunorubicin plus cytarabine-based induction regimens, 22 patients with disease which had relapsed during daunorubicin plus cytarabine maintenance therapy, or following completion of this maintenance programme after receiving > or = 500 mg daunorubicin/m2, and 7 previously untreated patients where cardiac disease contraindicated anthracycline therapy. Complete remission (CR) was attained in 46%, and probability of survival was comparable to published results for first-line treatment with daunorubicin plus cytarabine regimens. There was no statistically significant difference in CR rate or probability of survival between these three categories of poor prognosis AML, and cardiotoxic complications were uncommon despite extensive anthracycline exposure in the majority. In the 43% of patients who were 60-76 years of age, there was no statistically significant difference in CR rate or probability of survival relative to patients < 60 years. This observation fails to support the view that less myelotoxic regimens with lesser efficacy should be the basic approach to treatment of AML in patients > or = 60 years of age.

Hypercalcemia in the blastic phase of chronic myeloid leukemia associated with elevated parathyroid hormone-related protein

We describe a case of hypercalcemia without lytic bone lesions complicating myeloid blast crisis of chronic myeloid leukemia (CML). Serum levels of parathyroid hormone-related protein (PTHrP) were elevated during the initial hypercalcemic period and became undetectable during chemotherapy-induced chronic phase, only to become elevated again during subsequent recurrent blastic periods repeatedly associated with hypercalcemia. Previously reported cases of hypercalcemia complicating CML are reviewed. It is suggested that PTHrP was responsible for the hypercalcemia in this case and may be an important mediator of hypercalcemia in CML.

Accelerated establishment of murine long-term bone marrow cultures by addition of stem cell factor

The effect of stem cell factor (SCF) on the establishment of hematopoietic activity in murine long-term bone marrow cultures (LTBMC) was investigated by addition of SCF to (a) normal LTBMC from the onset of culture and (b) pre-established irradiated bone marrow stroma inoculated with lineage negative (Lin-) primitive hematopoietic progenitor cells enriched on the basis of low rhodamine-123 uptake (Rh-dull). Hematopoietic activity was established more rapidly in LTBMC grown in the presence of SCF (70 ng/mL), and the typical decline in cellularity and progenitor cell content during the first weeks of culture was not observed. SCF also promoted the rapid expansion of progenitor cells derived from Lin-, Rh-dull primitive hematopoietic cells inoculated onto irradiated preestablished bone marrow stroma. The data demonstrate that exogenous SCF augments hematopoietic activity in LTBMC, and that the levels of endogenous SCF elaborated in LTBMC may be suboptimal for expansion of hematopoietic cells.

Expansion of hemopoietic activity in long-term culture of human bone marrow by c-kit ligand (stem cell factor)

The response of hemopoietic activity in long term culture of human bone marrow (LTMC) to exogenous c-kit ligand has been investigated. Addition of c-kit ligand as recombinant Stem Cell Factor after the adherent stromal cell layer had become confluent failed to increase the content of hemopoietic cells in the culture system. Production of hemopoietic progenitors and differentiated progeny was on the other hand amplified by addition of c-kit ligand at regular intervals from the initiation of the culture, and resulted in net formation of these cells in the culture system. The characteristics of the response were consistent with consequences of increased survival of hemopoietic progenitors during a vulnerable initial phase in which the availability of endogenous c-kit ligand was limited by the initially low number of stromal cells. The increase in differentiated progeny was essentially due to increased formation of neutrophil series, and the lack of increased basophil production under these circumstances presumably reflected the nature of the contributory effects of endogenously generated growth regulatory molecules which acted in a synergistic manner with added c-kit ligand.

Characterization of leucocyte phagocytic stimulatory material released by activated human platelets

The action of material released by activated human platelets on the phagocytic activity of human neutrophils and monocytes has been examined under conditions where there was a low background level of exposure of the leucocytes to platelet release products ( PRPr ). Incubation with PRPr produced an approximate three-fold stimulation of particle ingestion by both cell types. Material with stimulatory activity passed through ultrafiltration membranes capable of retaining molecules greater than 500 Daltons. This low molecular weight stimulator was inactivated by ATP diphosphopyrolase to suggest it could be one or more of the nucleotide di- or triphosphates in PRPr . Comparison with the action of specific nucleotides revealed only ADP and ATP exerted equivalent stimulatory effects at concentrations consistent with those of nucleotides normally present in PRPr . This effect was mediated by induction of a change in leucocyte behaviour which was sustained in the absence of continued exposure to the stimulatory agent. Stimulation of phagocytosis was also produced by high molecular weight material in PRPr , but this appeared to be mediated by dissociation of low molecular weight stimulator from a high molecular weight complex. These observations extend the range of biological functions known to respond to nucleotides in PRPr , and have implications for the interpretation of leucocyte phagocytic activity assessed in vitro in view of the potential modification of this process by exposure of leucocytes to PRPr during the isolation of leucocytes from peripheral blood.