Acute lymphoblastic leukaemia (ALL) is the commonest form of childhood malignancy, and most cases arise from B-cell clones arrested at the pre-B-cell stage of differentiation. The molecular events that arrest pre-B-cell differentiation in the leukaemic pre-B cells have not been well characterized. Here we show that the differentiation regulator SLP-65 (an adaptor protein also called BLNK or BASH) inhibits pre-B-cell leukaemia in mice. Reconstitution of SLP-65 expression in a SLP-65-/- pre-B-cell line led to enhanced differentiation in vitro and prevented the development of pre-B-cell leukaemia in immune-deficient mice. Tyrosine 96 of SLP-65 was required for this activity. The murine SLP-65-/- pre-B-cell leukaemia resembles human childhood pre-B ALL. Indeed, 16 of the 34 childhood pre-B ALL samples that were tested showed a complete loss or drastic reduction of SLP-65 expression. This loss is probably due to the incorporation of alternative exons into SLP-65 transcripts, leading to premature stop codons. Thus, the somatic loss of SLP-65 and the accompanying block in pre-B-cell differentiation might be one of the primary causes of childhood pre-B ALL.

In the periphery, B cells differentiate in germinal centres (GCs) of secondary lymphoid organs. Isolated GC cells die quickly in vitro by apoptosis. Therefore, cell lines originating from follicular lymphomas, which are the malignant counterparts of GC B cells, would provide a stable in vitro model to study the immunobiology of GC B cells. We have established three novel human follicular lymphoma cell lines that were characterized with special reference to immunophenotypic features, response to B-cell receptor (BCR) triggering, response to cytokines and cytokine mRNA expression. One of the cell lines, HF-1A3, has a phenotype of a centrocyte. It expresses surface immunoglobulin G (sIgG) and dies by apoptosis following BCR cross-linking. Co-stimulation with interleukin-6 (IL-6), IL-15 or interferon-g (IFN-g) rescues HF-1A3 cells from BCRinduced apoptosis. The second cell line, HF-28, also represents phenotypically an IgG þ centrocyte. Ligation of its BCR leads to the cell-cycle arrest at G1 instead of apoptosis. HF-28 cells express both CD45RA and RO isoforms, which is unusual in B lymphocytes apart from plasma cells, thus suggesting a transition to plasma cell phenotype. The third cell line, HF-4.9, which phenotypically represents an sIgM þ centroblast, responds by proliferation to BCR cross-linking. These cell lines offer a unique in vitro model to study antigenic selection and cytokine-mediated growth regulation of human GC B cells.

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In the present study we used a human follicular lymphoma cell line, HF1A3, as an in vitro model for the antigen-driven selection process in germinal centers. Apoptosis can be induced in HF1A3 cells by B cell receptor (BCR) stimulation, but the molecular mechanisms and kinetics of this process are largely unknown. We demonstrate here that there is over 12 h delay between receptor activation and the execution phase of apoptosis, i.e. disruption of mitochondrial membrane potential, release of cytochrome c from mitochondria, caspase-3 activation and DNA fragmentation. New protein synthesis is required for mitochondrial alterations and subsequent apoptosis to occur, as these processes are completely blocked by the protein synthesis inhibitor cycloheximide. All the apoptotic events induced by BCR triggering are completely reversed by CD40 ligation with anti-CD40 antibody. CD40 ligation can reverse the apoptotic process in HF1A3 cells almost until the first mitochondrial events take place demonstrating that CD40-mediated protection operates very fast and at or before mitochondrial phase of apoptosis. Using specific inhibitors of cell signaling we could demonstrate that Raf-extracellular signal-regulated kinase, phosphatidylinositol 3-kinase, p38 or protein kinase C activation pathways are not involved in CD40-mediated protection from BCR-induced apoptosis in HF1A3 cells.