One of the great mysteries about CLL is what the cell of origin is. Based on its CD5 positivity it was originally thought to be derived from a follicular mantle cell, which was why everybody was so surprised to find that so many cases had mutated IGHV genes. Then there were those who thought it derived from the equivalent of the mouse B1 cell. B1 cells are self-renewing CD5+ B cells that mainly live in the peritoneal cavities of mice and produce 'polyreactive' or 'natural' antibodies. They respond to T-independent antigens but rarely accumulate somatic mutations or switch Ig classes to IgA or IgG.
However, no human equivalent of the B1 cell has so far been recognized; although CD5+ human B cells do exist in the circulation, follicular mantles and possibly the omentum, they do not produce 'polyreactive' Ig, they do not mature into plasma cells with appropriate stimulation and they do not show stereotypy.
When I first discovered that patients with mutated and unmutated IGHV genes had clinically different CLL, I postulated that there were two types of CLL derived from different cell types with a different history of exposure to antigen and a different pathway of differentiation, but this was clearly shown not to be the case by gene expression profiling. The difference in gene expression between U-CLL and M-CLL amounted to only a few hundred genes, whereas the difference between CLL as a whole and either memory of naive amounts to thousands.
To reconcile the difference between the BCR implications and the gene profiling data, some experts proposed an origin in the follicular marginal zone (MZ). The figure illustrates a splenic follicle. The germinal center is surrounded by the darker follicular mantle which itself is surrounded by the marginal zone. MZ cells exist in the outermost part of the white pulp of the spleen, in tonsilar subepithelial areas, in the dome regions of Peyer's patches in the gut, and the subcapsular regions of lymph nodes.
MZ B cells have bright surface IgM and dim surface IgD and respond to T-independent stimuli. Their IGHV genes can be mutated or unmutated varying according to anatomical site, though they can reach 70% mutated in the spleen. However, MZ cells are CD5-, CD23-, CD22+; at odds with CLL cells. It would have to be postulated that CLL cells are frozen in a semi-activated state and that the CD5+, CD23+ CD22- phenotype is a sign of activation, not lineage.
In the issue of Blood of 10 February 2011 p 1781, Chiorazzi and Ferrarini publish an article taking a look at this hypothesis in the light of recent research. Among new facts to emerge is that the surface Ig of, particularly U-CLL, cells is a polyspecific antibody often directed against antigens revealed during apoptosis (e.g. non-muscle myosin heavy chain IIA).
Some cases of CLL share BCRs of remarkably similar DNA sequence. These 'stereotyped' CLL comprise about 30% of all CLLs and the HCDR3s are often coded by identical gene sequences, though identical amino acid sequences may be found even though the genes are different. Stereotypy is mostly among U-CLL giving rise to the possibility that there are two types of CLL: stereotyped and non-stereotyped. Possibly the stereotyped CLLs derive from a population of B cells whose function is the clearing-up of the mess after programmed cell death.
When we originally had this debate about cells with unmutated IGHV genes resembling memory cells but not fitting the definition, at a meeting I coined the phrase 'antigen experienced cells', which has passed into the vernacular without acknowledgement. Next time I will write down my 'bon mots' rather than spilling them into the air. Memory cells are defined by their ability to recognize an invader more successfully the second time around, having undergone molecular alterations to facilitate this. For cells programmed to recognize inflammatory debris - with polyspecific antibodies and anti-apoptotic products - alteration of the molecular state of the BCR is perhaps unnecessary, memory being achieved by clonal expansion and perhaps epigenetic changes. U-CLL, particularly stereotyped U-CLL might derive from a cell with an 'antigen experience' like this.
We are beginning to recognize how the leukemic process is imposed on whatever cell is the CLL progenitor. We have seen that many people have a small CLL-like clone of cells that gets more easily detectable as the person ages and that such a clone is more commonly found in people with a relative who has CLL. We know that not all such cases of monoclonal B-cell lymphocytosis (MBL) are CLL-like; some resemble marginal cells. We also know that it is relatively rare for such proliferations to turn into frank CLL. We have seen that there is a distinction between an MBL population of >1000/ cu mm and those with a smaller population than this in that those with a smaller population do not have the same biased use of VH genes and lack the stereotypy seen in CLL. They are also much less likely to transform to CLL. Those with the large population transform at a rate of about 1% a year.
We have recognized certain genetic lesions to be common in both sporadic and familial cases of CLL. Most common is a disorder of 13q14 which takes out the function of the LEU2/miR15-a/miR16-1 complex; effectively a tumor-suppressor gene. This lesion does not seem to be a feature of low count MBL but is present in high count MBL. One case of familial CLL had a mutation of miR16-1 in the germline and the same lesion is found in the NZB mouse germline which develop a CLL-like illness in old age. In a mouse model in which the LEU2/miR15-a/miR16-1 complex has been deleted a CLL-like disease develops. Other miR genes may be involved in other features of the disease. For example, miR29a/29b and miR181a/181b are involved in modulating expression of TCL1 which activates pathways involved in apoptosis and proliferation. These deletions are not infrequent in CLL and transgenic mice that over-express TCL1 develop a disease that resembles U-CLL.
Although we do not know what gene may be involved in trisomy 12 CLL we do know that the CLL associated with trisomy 12 is rather different from other forms in that the morphology is often atypical with more prolymphocytes than normal and the markers tend to be rather different with slightly more dense surface Ig, CD79b and CD20, and greater tendency to express FMC7 and CD22. They are likely to develop other trisomies (especially of chromosomes 16, 18 and 19). However, the patients behave quite normally clinically.
There is evidence that epigenetic mechanisms may also be involved. A single kindred was found to carry a mutation that silenced the DAPK1 gene. Although no other familial cases had this mutation a single sporadic case was found in a different far away country. However this gene is frequently methylated in both sporadic and familial cases of CLL.
It has become clear that the development of CLL develops stepwise. Although virtually all cases of CLL go through an MBL step, very few cases of MBL actually progress. Familial cases may be mutated and unmutated in the same kindred and have a higher incidence of other types of lymphoma.
In the search for a cell of origin for CLL we are no nearer an answer than we were 20 years ago. We cannot find a human B1 equivalent. We recognize the expression of cell markers as fluid, more likely to represent activation that lineage. We are not surprised by clinical differences, which may represent the summation of acquired genetic lesions. The U-CLL/M-CLL split may have several explanations. Quite apart from it representing an origin at a different stage of differentiation, it may simply indicate an acquired defect of spontaneous mutation. The high levels of AID in U-CLL might indicate this.