Cross-linking of surface IgG induces apoptosis in a bcl-2 expressing human follicular lymphoma line of mature B cell phenotype.

We have established three lymphoma cell lines, HF-1 from one follicular lymphoma (FL) patient, and HF-4 and HF-9 from another. All cell lines carry the characteristic t(14;18) chromosomal translocation and express constitutively the bcl-2 gene product (Bcl-2 protein). Cross-linking of their surface membrane Igs (sIgs) with relevant antibodies triggers a vigorous calcium signal in all three lines but only HF-1 is induced to apoptosis. Treatment with anti-Ig arrests the proliferation of HF-1 within 6-12 h, nucleosomal DNA fragmentation is evident in 18 h and a morphologically complete apoptosis is seen in 24-48 h. While bcl-2 was expressed at equal levels in all lines, the apoptosis-sensitive HF-1 line displayed a much lower expression of c-myc than seen in the apoptosis-resistant line. This finding challenges the concept that expression of bcl-2 per se renders resistance to apoptosis but that the balance between the expression of bcl-2 and c-myc may dictate the outcome of sIg cross-linking. HF-1 is a unique, phenotypically mature human B cell line expressing surface IgG. This cell line offers a new tool for investigations on apoptosis and induction of tolerance in mature B lymphocytes. Our results suggest that some FLs may be amenable to anti-cancer treatment based on anti-sIg antibody induced apoptosis.

Somatic hypermutations in the immunoglobulin genes of two new human lymphoma lines of lymphatic follicle origin.

Variable immunoglobulin heavy-chain regions (VDJ) of two newly established human lymphoma cell lines (HF-1 and HF-4) were sequenced. The most homologous germline VH gene found for both the HF-1 and HF-4 sequences was VH26 of the VH3a (V gene) family (82% and 91% homologies, respectively). The JH region of the HF-4 heavy-chain sequence contained two nucleotide differences compared to the published germline JH3 gene. The DHJH region of the HF-1 gene had a record high number (20%) of somatic mutations. The numerous hypermutations found in the HF-1 cell line support the hypothesis that in some human follicular lymphomas, mutations continue to accumulate in immunoglobulin genes during the malignant growth. Follicular lymphoma cell lines, which have an active mutational machinery, in future may help to solve the molecular events behind the somatic hypermutations modifying immunoglobulin genes of B lymphocytes.

Inhibition of endocytic vesicle fusion in vitro by the cell-cycle control protein kinase cdc2.

Membrane transport between the endoplasmic reticulum and the plasma membrane, which involves the budding and fusion of carrier vesicles, is inhibited during mitosis in animal cells. At the same time, the Golgi complex and the nuclear envelope, as well as the endoplasmic reticulum in some cell types, become fragmented. Fragmentation of the Golgi is believed to facilitate its equal partitioning between daughter cells. In fact, it has been postulated that both the inhibition of membrane traffic and Golgi fragmentation during mitosis are due to an inhibition of vesicle fusion, while vesicle budding continues. Although less is known about the endocytic pathway, internalization and receptor recycling are also arrested during mitosis. We have now used a cell-free assay to show that the fusion of endocytic vesicles from baby hamster kidney cells is reduced in Xenopus mitotic cytosol when compared with interphase cytosol. We reconstituted this inhibition in interphase cytosol by adding a preparation enriched in the starfish homologue of the cdc2 protein kinase. Inhibition was greater than or equal to 90% when the added cdc2 activity was in the range estimated for that in mitotic Xenopus eggs, which indicates that during mitosis the cdc2 kinase mediates an inhibition of endocytic vesicle fusion, and possibly other fusion events in membrane traffic.