CLL: apoptosis and proliferation

Accumulation of mature B cells that have escaped programmed cell death and undergone cell-cycle arrest in the G0/G1 phase is the hallmark of chronic lymphocytic leukaemia.[83] These cells have a low proliferative activity, and data lend support to the hypothesis that in-vivo defective apoptosis accounts for accumulation of B cells. In chronic lymphocytic leukaemia cells, translocations of the BCL2 gene are rare (fewer than 1% of cases)[84] despite high amounts of the BCL2 protein. The role of BCL2 in apoptosis inhibition is not clear, since no correlation exists between in-vitro apoptosis and the amount of BCL2 expression.[85] However, other members of the BCL2 family, such as anti-apoptotic proteins BCL-XL (alternative splice variant of BCL2L1), BAG1, and MCL1, are overexpressed whereas proapoptotic proteins, such as BAX and BCL-XS (alternative splice variant of BCL2L1), are underexpressed.[83]

Deregulation of cell-cycle regulatory genes might also contribute to accumulation of malignant cells in early phases (G0/G1) of the cell cycle. In chronic lymphocytic leukaemia cells, raised amounts of the cyclin negative regulator CDKN1B protein are recorded in most patients.[85] In view of the key role of this protein in cell-cycle progression, its overexpression in chronic lymphocytic leukaemia cells could account for the accumulation of B cells in early phases of the cell cycle. These data suggest that chronic lymphocytic leukaemia is a disease resulting from accumulation rather than proliferation.

By contrast with in-vivo results, apoptosis happens after in-vitro culture, indicating a role of the microenvironment in chronic lymphocytic leukaemia cell survival.[86] and [87] Findings showing that apoptosis in vitro is prevented by exposure to interleukin 4 and by stimulation via surface CD40 also favour this view. In vivo, such inhibition can happen in pseudo-follicles seen in the lymph nodes and in cell clusters described in bone marrow.[88] These pseudo-follicles include, in close contact with proliferating B cells, increased numbers of CD4 T cells expressing CD40 ligand. These activated CD4 T cells could be recruited by tumour B cells, since they constitutively express the T-cell-attracting chemokines CCL17 and CCL22.[88] and [89] This idea could be in agreement with a model of selective survival of certain clonal submembers, which would receive survival signals in these particular sites.

With a non-radioactive, stable isotopic labelling method to measure chronic lymphocytic leukaemia kinetics, Messmer and colleagues [90] showed that B-cell chronic lymphocytic leukaemia is not a static disease, resulting simply from accumulation of long-lived lymphocytes, but is a disease with a dynamic process in which cells proliferate and die, sometimes at appreciable levels. This finding is in conflict with the view that chronic lymphocytic leukaemia is characterised almost exclusively by cell accumulation due to a defect in apoptosis. This mechanism might compensate for the clonal decrease that could take place in the periphery by apoptosis and, depending on its importance, could have a major role in regulation of tumour burden.

A picture is emerging that emphasises the importance of proliferation centres in lymph nodes spleen and bone marrow. Here, stimulation by CD31 on endothelial cells and CD154 on T-cells activates chronic lymphocytic leukaemia cells, upregulating CD38 and perhaps ZAP-70, provoking cell division and reinforcing resistance to apoptosis. From the proliferation centre the cell emerges into the circulation, where levels of activation markers begin to decline at a rate determined by intrinsic qualities of the cell. Cells that are better at retaining activation markers are drawn back into the tissues by chemokines and once there repeat the whole cycle. CD38 can be seen as an index of how recently the CLL cell has visited a proliferation center. [91], [92], [93] and [94]

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