DEVELOPMENT OF B-CLL FROM NORMAL B LYMPHOCYTES: SIGNALS AND MECHANISMS INITIATING THE GROWTH AND ACCUMULATION OF LEUKEMIC LYMPHOCYTES
The major events in tumorigenesis have traditionally been grouped into inducing and promoting factors. Inducing factors cause transforming mutations, whereas promoting factors sustain the proliferation and survival of cells undergoing or having undergone transforming mutations. It is useful to apply such principles to B-CLL.
Despite active research, to date no genetic aberration (inducing factor), shared by all patients, has been found in B-CLL. The monoclonality of B lymphocytes that develop in this disease implies that such a lesion existed in the cells initiating the clone. Although characteristic DNA abnor-malities can occur later in the development of B-CLL clones, these are rarely found in the initial phases of the disease.
In contrast, several stimulatory signals delivered from the microenvironment may represent important promoting factors in the development and evolution of the disease. One of these – antigen stimula-tion – appears to play a major role in the pathogenesis of B-CLL; this conclusion is based on the existence of remarkable similarities in the structures of BCRs of unrelated patients. This similarity in BCR structure is especially striking for about 25 percent of patients, with some patients’ clones using identical IgVH, D, and JH genes. Extraordinarily, in some of these cases, these rearranged Ig heavy-chain genes are paired with identical IgVL genes, yielding antigen-binding sites that are virtually identical at the amino acid level. Given the enormous number of possible com-binations of IgV gene segments encoding antibody-binding domains, one would not expect to find B-CLL patients having such structurally similar “stereotypic” BCRs by chance until well over 1 million cases have been screened. Hence, their occurrence is not likely random, making a plausible argument for the importance of antigen stimulation and drive in this disease.
Foreign antigens or autoantigens could prompt normal B lymphocytes to become B-CLL cells by selecting B-cell clones with restricted stereotypic BCRs. The nature of the antigens that select for these highly restricted BCRs (possibly unique to B-CLL) is largely unknown, although currently under investigation. It may be they result from infection due to a specific microbe common among patients, which has been found for gastric lymphomas. Alternative possibilities are that both environmental and autologous antigens may be involved. Indeed, it appears that intermittent and interchangeable encounters with microbial antigens and autoantigens, especially those generated during cell death and oxidative and other forms of stress, are key.
How would the transition from nor-mal B cells to leukemic cells via antigen stimulation occur? Normal B lymphocytes using unmutated IgV genes produce anti-bodies that are frequently polyreactive, binding carbohydrates, nucleic acids, and phospholipids. Such antibodies provide the first line of defense against microor-ganisms and promote the clearance of autoantigens and their fragments. B-CLL cells frequently display polyreactive BCRs, thereby making it possible that they derive from normal polyreactive B lymphocytes that have been repeatedly stimulated in vivo by a combination of nonprotein self-and microbial antigens.
Similarly, such a mechanism could also promote the origin of M-CLL. As already mentioned, IgVH gene mutations can occur without T-cell help in marginal zones outside of GCs. Because mutations can sometimes favor autoreactivity, such auto-reactive B cells would become expanded. However, expansion would stop if IgV gene mutations alter BCR structure in such a way that antigen binding is no longer sufficient to induce B-cell signaling.
With continued expansion leading (or not) to accumulation of IgVH gene muta-tions as explained by the T-cell-dependent versus T-cell-independent models men-tioned earlier, it becomes increasingly likely that a cell develops a genetic abnormality as in an initial inducing lesion that would lead to relatively unrestrained expansion. Such a cell is primed for leukemic transfor-mation.
A hypothesis such as this implies that it might be possible to detect clonal expan-sions in healthy subjects. In fact, small numbers of apparently clonal B cells with B-CLL cell characteristics do exist in the blood of about 3.5 percent of disease-free individuals. An even higher proportion of such clones have been found in the blood of first-degree relatives of patients with B-CLL (as often as 12 percent). Although such studies of the BCRs of B lymphocyte expansions in normal disease-free individ-uals are limited, they further support this hypothesis in that these expansions are not only monoclonal but also use some of the same genes commonly encoding the BCRs of B-CLL clones.
In addition to antigen stimulation, B-CLL cells also receive receptor-medi-ated signals as well as soluble factors, such as cytokines and chemokines, from other lymphoid and nonlymphoid cells. In particular, it is thought that in vivo B-CLL cell interactions with stromal cells and “nurselike” cells can rescue normally (ex vivo) apoptosis-prone B-CLL cells from death. The natural ligand of CD38, CD31, is displayed on stromal and nurselike cells as well as on endothelial cells and might be involved in setting up these rescue signals. Such contact-derived and soluble signals can go on to up-regulate anti-apoptotic genes, such as Bcl-2, survivin, and Mcl-1, which could rescue B-CLL cells from apop-tosis and facilitate their growth.