Ibrutinib inhibits collagen-mediated but not ADP-mediated platelet aggregation

The BTK (Bruton's tyrosine kinase) inhibitor ibrutinib is associated with an increased risk of bleeding. A previous study reported defects in collagen- and adenosine diphosphate (ADP)-dependent platelet responses when ibrutinib was added ex vivo to patient samples. Whereas the collagen defect is expected given the central role of BTK in glycoprotein VI signaling, the ADP defect lacks a mechanistic explanation. In order to determine the real-life consequences of BTK platelet blockade, we performed light transmission aggregometry in 23 patients receiving ibrutinib treatment. All patients had reductions in collagen-mediated platelet aggregation, with a significant association between the degree of inhibition and the occurrence of clinical bleeding or bruising (P=0.044). This collagen defect was reversible on drug cessation. In contrast to the previous ex vivo report, we found no in vivo ADP defects in subjects receiving standard doses of ibrutinib. These results establish platelet light transmission aggregometry as a method for gauging, at least qualitatively, the severity of platelet impairment in patients receiving ibrutinib treatment.

Comparison of three methods of CD34+ cell enumeration in peripheral blood: dual-platform ISHAGE protocol versus single-platform, versus microvolume fluorimetry

BACKGROUND

Quantitation of peripheral blood (PB) CD34(+) cells is now an established method for timing PBPC harvesting. Recent refinements to the dual-platform ISHAGE gating strategy for CD34(+) cells has seen the introduction of microbeads to enable absolute counting of cells on a single instrument platform. This eliminates the need for total WBCC performed on an automated hematology analyzer and potentially increases the analytical precision of the methodology. At the same time, alternative methods for CD34(+) cell enumeration have started to emerge, notably microvolume fluorimetry, which forms the basis of the fully-automated STELLer CD34 method using the Imagn 2000.

METHODS

We performed a three-way evaluation of these methods. Sixty-eight samples of PB from 42 patients undergoing PBPC mobilization were analyzed by all three methods and correlations between all three calculated. The two-platform ISHAGE method was used as the reference method.

RESULTS

Precision and linearity of the single-platform and STELLer CD34 assays were excellent. Correlation with the dual-platform reference method was also excellent (single-platform method slope = 1.03, intercept = -0.03 and R(2) = 0.9325, STELLer CD34 assay slope = 0.827, intercept = 4.27, R(2) =0.8215). Bias, determined by Bland-Altman analysis, was 1.16 and -1.62 for single platform and STELLer CD34 assay respectively.

CONCLUSION

The three methods of CD34(+) cell enumeration gave equivalent results. The single-platform methodology negated the need for a separate white cell analyzer, while the STELLer CD34 methodology was technically the simplest.

VLA-4 is involved in the engraftment of the human pre-B acute lymphoblastic leukaemia cell line NALM-6 in SCID mice

Attachment of human pre-B leukaemic cells to human or murine bone marrow stromal cells in vitro is largely mediated by the beta1 integrin VLA-4 binding to VCAM-1. Cells subsequently migrate within the stroma, a process also involving VLA-4. A variant of the pre-B acute lymphoblastic leukaemia cell line NALM-6, designated 4A1, lacking expression of VLA-4, was generated by radiation-induced mutagenesis followed by several rounds of negative selection with immunomagnetic beads, fluorescence activated cell sorting and clonal expansion. In vitro assays using 4A1 cells showed reduced binding to, and migration under, the murine stromal line M2-10B4. Sublethally irradiated mice (n=19) with severe combined immunodeficiency were injected intravenously with NALM-6 cells. Animals developed signs of leukaemia with hind-limb paralysis at a median of 30 d (95% confidence interval 28-30). Although there were no gross abnormal findings at autopsy, histological analysis revealed extensive marrow replacement and focal liver infiltration with leukaemic blasts, which were confirmed to be of human origin by flow cytometry. 12 mice were injected with a similar number of cells from the VLA-4-negative variant cell line 4A1. Six mice developed signs of leukaemia after 43-74d, with the remaining six being free of signs of disease after > 100d (P<0.001). Mice in this group with leukaemia had a lower incidence of hind-limb paralysis and less leukaemic infiltration in the marrow, but in some cases had large tumour nodules elsewhere. After a single 500 microg intraperitoneal injection of anti-murine VCAM-1 monoclonal antibody (MK2.7), five additional mice were injected with an identical number of wild-type (VLA-4+) NALM-6. All animals developed signs of leukaemia after a similar period to those injected with wild-type NALM-6 only. These results demonstrate that the beta1 integrin VLA-4 is involved in the engraftment of the pre-B-cell leukaemic cell line NALM-6 in SCID mice, although the interaction with VCAM-1 is unlikely to be the sole explanation.

Long-term survival and proliferation of precursor-B acute lymphoblastic leukemia cells on human bone marrow stroma

Leukemic cells frequently persist in the bone marrow of patients treated for acute lymphoblastic leukemia (ALL), and may regrow to produce relapse. We have used a long-term co-culture system to analyze the interaction of ALL blasts with components of the marrow microenvironment. Blast cells from 10 cases of precursor-B ALL were cultured on allogeneic human bone marrow stromal layers at 37 degrees C in microtiter wells, and replated when stroma showed evidence of deterioration. Leukemic cells from seven of 10 cases showed evidence of survival and proliferation beyond 30 days in culture, while in three cases there was a progressive decline in the number of viable leukemic cells by 7-21 days. Two cases continued to proliferate for 149 to 332+ days, while five underwent senescence after 34-52 days. In the two cases with long-term proliferation, the leukemic identity of the cells was confirmed by immunophenotyping, cytogenetics, and demonstration of clonal immunoglobulin gene rearrangements. Evidence of selection of leukemic subclones was seen in two cases, with immumophenotypic evidence of loss of CD10 and CD34 antigens, and acquisition of CD20 and surface mu chain. The leukemic cells in these cases grew either in clumps attached to the surface of the stroma, or as'cobblestone areas' beneath the stromal cells. Survival and growth of two evaluable cases was dependent on the continuing presence of stromal cells in the culture system. In one case, direct contact with stroma was shown to be necessary to main- tain viability, while blast cells from the other case survived equally well when separated from the stroma by a 0.4-micron pore size microporous membrane. These results indicate that leu- kemic cells from the majority of cases of precursor-B ALL are able to persist and undergo proliferation in vitro in the presence of normal marrow stroma. This process appears dependent on either direct cell-cell contact, or on diffusible factors derived from the stroma. The availability of ALL cells capable of indefinite proliferation under these conditions will allow further analysis of the mechanisms mediating leukemic cell proliferation.