Prevalence and natural history of monoclonal and polyclonal B-cell lymphocytosis in a residential adult population

Pathogenesis of chronic lymphocytic leukemia and the development of novel therapeutic strategies

Chronic lymphocytic leukemia (CLL) is the most common adult leukemia in Western countries and is characterized by the clonal expansion of mature CD5⁺ B cells. There have been substantial advances in the field of CLL research in the last decade, including the identification of recurrent mutations, and clarification of clonal architectures, signaling molecules, and the multistep leukemogenic process, providing a comprehensive understanding of CLL pathogenesis. Furthermore, the development of therapeutic approaches, especially that of molecular target therapies against CLL, has markedly improved the standard of care for CLL. This review focuses on the recent insights made in CLL leukemogenesis and the development of novel therapeutic strategies.

Pathophysiology of chronic lymphocytic leukemia and human B1 cell development

Chronic lymphocytic leukemia (CLL), the most frequent type of leukemia in adults, is a lymphoproliferative disease characterized by the clonal expansion of mature CD5⁺ B cells in peripheral blood, bone marrow, and secondary lymphoid tissues. Over the past decade, substantial advances have been made in understanding the pathogenesis of CLL, including the identification of recurrent mutations, and clarification of clonal architectures, transcriptome analyses, and the multistep leukemogenic process. The biology of CLL is now better understood. The present review focuses on recent insights into CLL leukemogenesis, emphasizing the role of genetic lesions, and the multistep process initiating from very immature hematopoietic stem cells. Finally, we also review progress in the study of human B1 B cells, the putative normal counterparts of CLL cells.

Monoclonal B-cell lymphocytosis

Flow cytometry diagnostic practices can detect very low levels of clonal B cells in the peripheral blood. In the absence of clinical symptoms, cytopenia or organomegaly, the small clones may correspond to monoclonal B-cell leukemia (MBL) diagnosis. Most MBLs harbor a chronic lymphocytic leukemia (CLL) phenotype (e.g., CD5⁺, CD23⁺) and are referred to as CLL-type MBL. The two other types are atypical CLL-type MBL and non-CLL-type MBL. In addition to the phenotypical classification, the clonal B count is a major issue because of the impact on the prognosis and the risk of progression in CLL. It allows for the discrimination of two distinct types: high-count (HC) MBL and low-count (LC)-MBL based on a cutoff value of 0.5 × 10⁹/L clonal B cells. LC MBL appears to be very stable over time and is probably related to immunosenescence. Conversely, HC MBL could be a premalignant state before the occurrence of CLL.

Low-count monoclonal B-cell lymphocytosis persists after seven years of follow up and is associated with a poorer outcome

Low-count monoclonal B-cell lymphocytosis is defined by the presence of very low numbers of circulating clonal B cells, usually phenotypically similar to chronic lymphocytic leukemia cells, whose biological and clinical significance remains elusive. Herein, we re-evaluated 65/91 low-count monoclonal B-cell lymphocytosis cases (54 chronic lymphocytic leukemia-like and 11 non-chronic lymphocytic leukemia-like) followed-up for a median of seven years, using high-sensitivity flow cytometry and interphase fluorescence in situ hybridization. Overall, the clone size significantly increased in 69% of low-count monoclonal B-cell lymphocytosis cases, but only one subject progressed to high-count monoclonal B-cell lymphocytosis. In parallel, the frequency of cytogenetic alterations increased over time (32% vs. 61% of cases, respectively). The absolute number of the major T-cell and natural killer cell populations also increased, but only among chronic lymphocytic leukemia-like cases with increased clone size vs. age- and sex-matched controls. Although progression to chronic lymphocytic leukemia was not observed, the overall survival of low-count monoclonal B-cell lymphocytosis individuals was significantly reduced vs. non-monoclonal B-cell lymphocytosis controls (P=0.03) plus the general population from the same region (P≤0.001), particularly among females (P=0.01); infection and cancer were the main causes of death in low-count monoclonal B-cell lymphocytosis. In summary, despite the fact that mid-term progression from low-count monoclonal B-cell lymphocytosis to high-count monoclonal B-cell lymphocytosis and chronic lymphocytic leukemia appears to be unlikely, these clones persist at increased numbers, usually carrying more genetic alterations, and might thus be a marker of an impaired immune system indirectly associated with a poorer outcome, particularly among females.

Detection of monoclonal B cells in general population from two different regions of Mexico

To estimate the frequency of monoclonal B cells in Mexican general population from two different regions of Mexico. Monoclonal B cells were detected by rearrangements of the immunoglobulin heavy chains (IGH) in 288 individuals: 188 from a metropolitan area and 100 from a rural area. After DNA extraction from peripheral blood by the CTAB/DTAB method, multiplex PCR was used to amplify the IGH rearrangements, followed by capillary electrophoresis. In together, 9.4% of the studied individuals showed monoclonal B cells. This prevalence is significantly higher to those previously described for other populations, but similar to a report in the Spanish population. Among people from the metropolitan area, 12.8% exhibited monoclonal B cells in comparison with 3% of people from the rural area. All individuals showing monoclonal B cells were elder than 40 years. Higher frequency of incomplete monoclonal rearrangements was observed. Individuals from urban areas show significantly increased frequencies of monoclonal B cells regarding the people from the rural area. It is reasonable to believe that the environmental factor could have a greater impact on the development of monoclonality than the genetic component.

Monoclonal B-cell lymphocytosis in blood donors in Turkey

OBJECTIVES

Monoclonal B-cell lymphocytosis (MBL) is a precursor state of chronic lymphocytic leukemia (CLL) with peripheral lymphocytosis below 5 × 10⁹/l. The diagnostic criteria exclude the presence of lymphadenopathy, organomegaly, infections, autoimmune diseases or any sign of a lymphoproliferative disorder. This prospective study was designed in order to evaluate the frequency of MBL in blood donors in Turkey.

METHODS

The diagnosis of MBL was identified by flow cytometry method based on the International Familial CLL Consortium Report. A total of 999 volunteers [median age 34 (18-78) years; male/female: 705/294] were included in the study.

RESULTS

Monoclonal B-cell lymphocytosis was demonstrated in 18 cases (1.8%). A total of 16 cases (1.6%) was evaluated as CLL-like MBL, while 2 (0.2%) had a non-CLL-like phenotype. The subjects were divided into three groups according to age, as <40 years, 40-60 years and >60 years. The prevalence of MBL was 1.1% below 40 years, 0.6% between 40 and 60 years and 0.1% in cases over 60 years, without statistical significance (p > 0.05).

DISCUSSION

The sensitivity of the flow cytometry method is essential and may be responsible for the variations in the prevalence of MBL in different populations which can also be attributed to study design, higher detection rates in the elderly and families with genetic predisposition to CLL.

CONCLUSION

Large population-based studies and standardized laboratory methods are needed to determine the potential risk factors of progression to CLL, including molecular markers and genetic profile.

What does it mean I have a monoclonal B-cell lymphocytosis?: Recent insights and new challenges

Monoclonal B-cell lymphocytosis (MBL) is defined as a laboratory abnormality where small (<5 x 10(9)/L) clonal B-cell populations are detected in the peripheral blood of otherwise healthy subjects. According to the immunophenotype, MBL is labeled as chronic lymphocytic leukemia (CLL)-like (75% of cases), atypical CLL, and CD5-negative. Concentration of clonal B cells differentiates low- (LC) and high-count (HC)-MBL (< or ≥ 0.5 x 10(9)/L, respectively). Thanks to technical improvements, we are able to identify CLL-like clonal B-cell populations at increased frequency with age, but we are still far from understanding its relationship with clinically overt CLL. LC-MBL, requiring high-throughput screening technique to be identified in population studies, seems to be a bird of a different feather and several hints suggest that LC-MBL is related to aging and/or chronic antigenic stimulation. Immunogenetic, cytogenetic and genetic data support the notion that HC-MBL, usually identified in the clinical setting, is a premalignant condition and, based on biological parameters, it is frequently difficult to differentiate it from early stage CLL. The rapid improvement and widespread availability of cutting-edge technology, in particular next-generation sequencing (NGS), raises hope that we are getting closer to unveiling the fundamental nature of MBL and CLL and how they are related to each other.

Monoclonal B-cell lymphocytosis and early-stage chronic lymphocytic leukemia: diagnosis, natural history, and risk stratification

Monoclonal B lymphocytosis (MBL) is defined as the presence of a clonal B-cell population in the peripheral blood with fewer than 5 × 10(9)/L B-cells and no other signs of a lymphoproliferative disorder. The majority of cases of MBL have the immunophenotype of chronic lymphocytic leukemia (CLL). MBL can be categorized as either low count or high count based on whether the B-cell count is above or below 0.5 × 10(9)/L. Low-count MBL can be detected in ∼5% of adults over the age of 40 years when assessed using standard-sensitivity flow cytometry assays. A number of biological and genetic characteristics distinguish low-count from high-count MBL. Whereas low-count MBL rarely progresses to CLL, high-count MBL progresses to CLL requiring therapy at a rate of 1% to 2% per year. High-count MBL is distinguished from Rai 0 CLL based on whether the B-cell count is above or below 5 × 10(9)/L. Although individuals with both high-count MBL and CLL Rai stage 0 are at increased risk of infections and second cancers, the risk of progression requiring treatment and the potential to shorten life expectancy are greater for CLL. This review highlights challenging questions regarding the classification, risk stratification, management, and supportive care of patients with MBL and CLL.

Immunophenotypic analysis reveals heterogeneity and common biologic aspects in monoclonal B-cell lymphocytosis

Monoclonal B-cell lymphocytosis (MBL) is the presence of small B-cell clones in the peripheral blood of healthy subjects. Most MBL have the characteristic phenotype of chronic lymphocyte leukemia (chronic lymphocytic leukemia (CLL)-like MBL), and depending on the number of monoclonal B-cells, may characterize a preclinical stage of the CLL. However, there are also MBL with an atypical (CD5(+) CD20(+/bright) CD23(dim/-) ) or a CD5(neg) phenotype, which remain largely unexplored. We performed an extended immunophenotypic, cytogenetic, and hematologic analysis in 75 CLL-like, 39 atypical, 50 CD5(neg) , and 7 biphenotypic MBL cases to detect differences or similarities among the MBL subsets. The phenotypic analysis showed expression variations in many surface markers and a wide spectrum of disease-specific phenotypes within each MBL subtype. Interphase fluorescent in situ hybridization analysis showed a different panel of aberrations according to the phenotype. Overall, del(13q14) and +12 were the most common abnormalities (39%), whereas del(11q13), del(17p13), and del(6q23) were detected only in 3, 1, and 0 cases, respectively. A comparison of MBL with overt chronic lymphoproliferations revealed common aspects in the preclinical state, regarding both the kind of cytogenetic aberrations detected and the lymphocyte composition. Our findings highlight not only the heterogeneity among MBL subsets but also indicate common biologic features which differentiate MBL from clinical disease.

New insights into monoclonal B-cell lymphocytosis

Monoclonal B-cell lymphocytosis (MBL) is a premalignant condition characterized by the presence of less than 5000/μL circulating clonal B cells in otherwise healthy individuals. Three subcategories have been identified according to the immunophenotypic features: CLL-like, CD5(+) atypical, and CD5(−) MBL. CLL-like MBL is by far the most frequent and best studied category and further divided in low-count [LC] and high-count [HC] MBL, based on a cutoff value of 500/μL clonal B cells. LC-MBL typically remains stable and probably does not represent a truly premalignant condition, but rather an age-related immune senescence. On the other hand, HC-MBL is closely related to CLL-Rai0, bearing similar immunogenetic profile, and is associated with an annual risk of progression to CLL requiring therapy at a rate of 1.1%. Currently there are no reproducible factors for evaluating the risk of progression to CLL. CD5(−) MBL is characterized by an immunophenotype consistent with marginal zone origin and displays many similarities with marginal zone lymphomas (MZL), mainly the splenic MZL. The cutoff value of 5000/μL clonal B cells cannot probably be applied in CD5(−) MBL, requiring a new definition to describe those cases.

Hematopoietic stem cell aging and chronic lymphocytic leukemia pathogenesis

Human malignancies develop through the multistep acquisition of critical somatic mutations during the clinical course. Regarding hematological malignancies, recent novel findings have indicated that hematopoietic stem cells (HSCs), which have the potential to self-renew and differentiate into multilineage hematopoietic cells, are an important cellular target for the accumulation of critical somatic mutations and play a central role in myeloid malignancy development. In contrast to myeloid malignancies, mature lymphoid malignancies, such as chronic lymphocytic leukemia (CLL), are considered to directly originate from differentiated mature lymphocytes; however, we previously reported that the propensity to generate clonal B cells had already been acquired at the HSC stage in CLL patients. Similarly, HSC involvement has been reported in the pathogenesis of mature T cell lymphomas. These studies indicate that, in mature lymphoid, if not all, malignancies, HSCs should be considered as the critical cellular target in the oncogenic process. The prevalence of these hematological malignancies dramatically increases with age, and the effect of aging HSCs should thus be taken into account when investigating the stepwise malignant transformation process of these age-associated malignancies.

Monoclonal B-cell lymphocytosis in healthy blood donors: an unexpectedly common finding

Circulating monoclonal B cells may be detected in healthy adults, a condition called monoclonal B-cell lymphocytosis (MBL). MBL has also been identified in donated blood, but no systematic study of blood donors has been reported. Using sensitive and specific laboratory methods, we detected MBL in 149 (7.1%; 95% confidence interval, 6.0% to 8.3%) of 2098 unique donors ages 45 years or older in a Midwestern US regional blood center between 2010 and 2011. Most of the 149 donors had low-count MBL, including 99 chronic lymphocytic leukemia-like (66.4%), 22 atypical (14.8%), and 19 CD5(-) (12.8%) immunophenotypes. However, 5 donors (3.4%) had B-cell clonal counts above 500 cells per µL, including 3 with 1693 to 2887 cells per µL; the clone accounted for nearly all their circulating B cells. Four donors (2.7%) had 2 distinct MBL clones. Of 51 MBL samples in which immunoglobulin heavy chain (IGH)V-D-J genotypes could be determined, 71% and 29% used IGHV3- and IGHV4-family genes, respectively. Sequencing revealed 82% with somatic hypermutation, whereas 18% had >98% germ-line identity, including 5 with entirely germ-line sequences. In conclusion, MBL prevalence is much higher in blood donors than previously reported, and although uncommon, the presence of high-count MBL warrants further investigations to define the biological fate of the transfused cells in recipients.

MBL versus CLL: how important is the distinction?

Monoclonal B-cell lymphocytosis (MBL) is defined as a clonal B-cell expansion whereby the B-cell count is less than 5 × 10(9)/L and no symptoms or signs of lymphoproliferative disorders are detected. Based on B-cell count, MBL is further divided into low-count and clinical MBL. While low-count MBL seems to carry relevance mostly from an immunological perspective, clinical MBL and chronic lymphocytic leukemia appear to be overlapping entities. Only a deeper knowledge of molecular pathways and microenvironmental influences involved in disease evolution will help to solve the main clinical issue, i.e. how to differentiate nonprogressive and progressive cases requiring intensive follow-up.

Precursors to lymphoproliferative malignancies

We review monoclonal B-cell lymphocytosis (MBL) as a precursor to chronic lymphocytic leukemia and monoclonal gammopathy of undetermined significance (MGUS) as a precursor to plasma cell disorders. These conditions are present in the general population and increase with age. These precursors aggregate with lymphoproliferative malignancies in families suggesting shared inheritance. MBL and MGUS may share some of the same risk factors as their related malignancies but data are limited. Although these conditions are characterized by enhanced risk for the associated malignancy, the majority of individuals with these conditions do not progress to malignancy. A key focus for current work is to identify markers that predict progression to malignancy.

Common infectious agents and monoclonal B-cell lymphocytosis: a cross-sectional epidemiological study among healthy adults

Background

Risk factors associated with monoclonal B-cell lymphocytosis (MBL), a potential precursor of chronic lymphocytic leukaemia (CLL), remain unknown.

Methods

Using a cross-sectional study design, we investigated demographic, medical and behavioural risk factors associated with MBL. “Low-count” MBL (cases) were defined as individuals with very low median absolute count of clonal B-cells, identified from screening of healthy individuals and the remainder classified as controls. 452 individuals completed a questionnaire with their general practitioner, both blind to the MBL status of the subject. Odds ratios (OR) and 95% confidence interval (CI) for MBL were estimated by means of unconditional logistic regression adjusted for confounding factors.

Results

MBL were detected in 72/452 subjects (16%). Increasing age was strongly associated with MBL (P-trend<0.001). MBL was significantly less common among individuals vaccinated against pneumococcal or influenza (OR 0.49, 95% confidence interval (CI): 0.25 to 0.95; P-value = 0.03 and OR: 0.52, 95% CI: 0.29 to 0.93, P-value = 0.03, respectively). Albeit based on small numbers, cases were more likely to report infectious diseases among their children, respiratory disease among their siblings and personal history of pneumonia and meningitis. No other distinguishing epidemiological features were identified except for family history of cancer and an inverse relationship with diabetes treatment. All associations described above were retained after restricting the analysis to CLL-like MBL.

Conclusion

Overall, these findings suggest that exposure to infectious agents leading to serious clinical manifestations in the patient or its surroundings may trigger immune events leading to MBL. This exploratory study provides initial insights and directions for future research related to MBL, a potential precursor of chronic lymphocytic leukaemia. Further work is warranted to confirm these findings.

Clinical aspects of monoclonal B-cell lymphocytosis

BACKGROUND

Monoclonal B-cell lymphocytosis (MBL) is an asymptomatic precursor condition for chronic lymphocytic leukemia (CLL). It is defined by the presence of small clones of aberrant B cells in the peripheral blood, with a total B-cell count below the threshold for diagnosis of CLL (<5.0x10(9) cells/L).

METHODS

The authors review current literature on the prevalence of MBL, and the clinical course of this CLL precursor condition, and recommended management for individuals with MBL.

RESULTS

MBL occurs in approximately 4% to 5% of healthy adults. While most cases of CLL are preceded by MBL, progression to leukemia requiring CLL treatment occurs in only 1% to 2% of individuals with MBL per year. The absolute B-cell count is most strongly associated with progression, and patients with low-count MBL identified in population screening studies rarely develop CLL. Studies are ongoing to better define the relationship between MBL and CLL and to identify prognostic indicators that predict which patients will progress to CLL. Given their elevated risk of developing malignancy, individuals with clinical MBL should be monitored at least annually for progressive lymphocytosis and signs or symptoms of CLL.

CONCLUSIONS

Many of the epidemiologic and genetic factors associated with MBL development and its progression to CLL have not yet been identified. However, ongoing studies by many research groups are aimed at answering these questions to facilitate management of individuals with this premalignant condition. In addition, active investigation of MBL will likely yield new insights into the biology of CLL, potentially identifying new therapeutic targets for this incurable disease.

Monoclonal B-cell lymphocytosis is closely related to chronic lymphocytic leukaemia and may be better classified as early-stage CLL

The World Health Organization classification uses a cut-off point of 5·0 × 10(9)/l cells with a chronic lymphocytic leukaemia (CLL)-phenotype in peripheral blood to discriminate between monoclonal B-lymphocytosis (MBL) and B-CLL. This study analysed 298 MBL patients by multi-parameter flow cytometry, chromosome banding analysis (CBA)/fluorescence in situ hybridization (FISH), and IGHV mutation status and compared them with 356 CLL patients. In MBL, CBA more frequently revealed a normal karyotype and FISH identified less frequently del(6q), del(13q) (as sole alterations), and del(17)(p13). Within the MBL cohort, a shorter time to treatment (TTT) was found for ZAP-70-positivity, 14q32/IGH-translocations (CBA), del(11)(q22·3) (FISH) and unmutated IGHV status. Higher CD38 and ZAP-70 expression, del(11)(q22·3) (FISH), trisomy 12 (FISH), and 14q32/IGH-translocations (CBA) were correlated with a shorter TTT in the combined cohort (MBL + CLL); a sole del(13)(q14) (FISH) correlated with longer TTT. Regarding overall survival, unmutated IGHV status and 'other' alterations (CBA) had an adverse impact. There was no correlation between the concentration of CLL-cells and TTT or overall survival. Multivariate analysis confirmed a negative impact on TTT for del(11)(q22·3)/ATM, trisomy 12 (both by FISH), and 14q32/IGH-translocations by CBA. These data emphasize a close relationship between MBL and CLL regarding clinically relevant parameters and provide no evidence to strictly separate these entities by a distinct threshold of clonal B-cells.

Immunologic aspects of monoclonal B-cell lymphocytosis

Monoclonal B-cell lymphocytosis (MBL) is a preclinical hematologic condition wherein small numbers of clonal B cells can be detected in the blood of otherwise healthy individuals. Most MBL have a surface immunophenotype nearly identical to that of chronic lymphocytic leukemia (CLL), though other phenotypes can also be identified. MBL has been shown to be a precursor state for CLL, but most MBL clones are quite small and apparently have minimal potential to progress of CLL or other B-cell lymphoproliferative disorder (B-LPD). The investigation of MBL as a precursor state for CLL will likely lead to important insights into mechanisms of disease pathogenesis. The review will cover clinical and translational aspects of MBL, with a particular emphasis on the prevalence of MBL; the relationship between MBL, CLL, and other B-LPDs; and the capacity of MBL to modulate the normal B- and T-cell compartments.

Inherited predisposition to chronic lymphocytic leukemia

Inherited susceptibility to chronic lymphocytic leukemia (CLL) has been recognized for decades. Approximately 10% of individuals with CLL report a family history of CLL or a related lymphoproliferative disorder, and genetic predisposition is the best understood risk factor for CLL. Studies of familial CLL have suggested that the disease features are largely similar to sporadic CLL, although recent data suggest that familial CLL may more commonly show somatic hypermutation of the immunoglobulin heavy-chain variable region, suggesting a more indolent disease course. Monoclonal B-cell lymphocytosis (MBL) has been identified recently as a likely precursor to CLL; it is found in the general population with increasing age and enriched in unaffected relatives of individuals with familial CLL. Studies of MBL as well as mouse models of CLL may lead to better understanding of early CLL pathogenesis that is relevant to familial predisposition. To date, the identification of genes that predispose to familial CLL has been slow, primarily due to the relatively few families available for study, the small size of those families and disease causation most likely by multiple genes that each confer smaller risks. In the coming years, the application of systematic genomics approaches to familial CLL should, hopefully, lead to the identification of novel loci involved in the disease.

Non-CLL-like monoclonal B-cell lymphocytosis in the general population: prevalence and phenotypic/genetic characteristics

BACKGROUND

Monoclonal B-cell lymphocytosis (MBL) indicates <5 × 10(9) peripheral blood (PB) clonal B-cells/L in healthy individuals. In most cases, MBL cells show similar phenotypic/genetic features to chronic lymphocytic leukemia cells-CLL-like MBL-but little is known about non-CLL-like MBL.

METHODS

PB samples from 639 healthy individuals (46% men/54% women) >40 years old (62 ± 13 years) with normal lymphocyte counts (2.1 ± 0.7 × 10(9)/L) were immunophenotyped using high-sensitive flow cytometry, based on 8-color stainings and the screening for >5 × 10(6) total PB leukocytes.

RESULTS

Thirteen subjects (2.0%; 9 males/4 females, aged 73 ± 10 years; absolute lymphocyte count: 2.4 ± 0.8 × 10(9)/L) showed a non-CLL-like clonal B-cell population, whose frequency clearly increased with age: 0.4%, 3%, and 5.4% of subjects aged 40-59, 60-79, and ≥80 years, respectively. One single B-cell clone was detected in 9/13 cases, while two B-cell clones were found in 4/13 (n = 17 MBL populations). Nine MBL cell populations showed a CD5(-) phenotype (usually overlapping with marginal zone-derived (MZL) or lymphoplasmacytic (LPL) non-Hodgkin lymphoma (NHL) B-cells, or an unclassifiable NHL), but CD5(-/+d) (n = 3) and CD5(+) (n = 3 non-CLL-like MBL, consistent with a mantle-cell lymphoma (MCL)-like phenotype, and n = 2 CLL-like) MBL were also identified; iFISH supported the diagnosis in most cases. No preferential IGHV usage of B-cell receptor could be found. Twelve cases reevaluated at month +12 showed circulating clonal B-cells, at mean levels significantly higher than those initially detected.

CONCLUSIONS

Non-CLL-like MBL cases frequently show biclonality, in association with MZL-, LPL-, MCL-like, or unclassifiable phenotypic profiles. As with CLL-like MBL, the frequency of non-CLL-like MBL increases with age, with a clear predominance of males.

Prevalence, clinical aspects, and natural history of IgM MGUS

BACKGROUND

Waldenström macroglobulinemia (WM) and chronic lymphocytic leukemia (CLL) are related B-cell cancers that share several clinical and biological features. Both WM and CLL have associated precursor conditions: monoclonal gammopathy of undetermined significance (MGUS) of immunoglobulin M (IgM) type and monoclonal B-cell lymphocytosis (MBL), respectively. Recently, a case of MBL with an IgM MGUS was reported, suggesting a close biological relationship between these entities. While much is known about MGUS overall, investigations of IgM MGUS specifically have been fragmentary.

METHODS

In this article, we review data on the prevalence, clinical aspects and natural history of IgM MGUS, and focus on identifying gaps in our understanding of the complex relationships among B-cell malignancies and their precursors.

RESULTS

There appears to be marked heterogeneity in the prevalence of IgM MGUS across populations. However, studies have varied in definition, design, laboratory methods, and endpoints. IgM MGUS differs from non-IgM MGUS in certain respects, including prevalence across racial groups, rate of progression, and pattern of malignant outcomes. There are limited data regarding the coincident occurrence of IgM MGUS and MBL.

CONCLUSIONS

Future studies incorporating both protein electrophoresis and flow cytometry are needed to define the underlying spectrum and causes of precursor development, risk factors for progression, and markers that distinguish low- and high-risk precursor patients.

A computer simulation for exploring the detection of monoclonal B-cell lymphocytosis by flow cytometry

BACKGROUND

Monoclonal B-cell lymphocytosis (MBL) is defined by the presence of monoclonal B-cells in peripheral blood in the absence of hematologic disease. MBL is detected by flow cytometry with increasing frequency as the number of B-cells acquired increases.

METHODS

Computer simulations in R language were used to examine the impact of increasing the number of B-cells acquired on the sensitivity of detecting MBL and to explore the possibility of detecting distinct B-cell clones among polyclonal B-cell populations.

RESULTS

With simulated populations containing 0.1%-1.0% monoclonal B-cells, the number of clonal B-cells detected showed a normal distribution in the upper range of clonal cells acquired and more nearly log-normal as the distributions became bounded by 0. The distributions peaked around the clonal prevalence. The detection of MBL increased sharply with a small increase in the total number of B-cells acquired when the number of clonal cells acquired was near the MBL cutoff point. MBL could be detected in log-normally distributed polyclonal B-cell populations.

CONCLUSIONS

Sampling variability in detecting monoclonal B-cells can be investigated through simulation. The observed population prevalence of MBL can be approximated with reasonable assumptions about the distribution of clonotypes in the circulating B-cell compartment.

Monoclonal B cell lymphocytosis: clinical and population perspectives

Monoclonal B Cell Lymphocytosis (MBL) refers to clones of CLL-like cells that exhibit CLL characteristics that fall short of the numbers required for CLL diagnosis. Data from large CLL kindreds document increased prevalence of MBL suggesting a genetic contribution to its etiology. The molecular features that favor progression of MBL to CLL are poorly understood but an elevated B-cell count is a risk factor for progression. An important consideration when evaluating volunteers from CLL families who are willing to donate bone marrow is that MBL be ruled out since the MBL donor clone could result in a second CLL in the recipient. Further studies of MBL are needed to identify the molecular features and how they evolve during progression.

Different biology and clinical outcome according to the absolute numbers of clonal B-cells in monoclonal B-cell lymphocytosis (MBL)

The biological and clinical relationship between Chronic Lymphocytic Leukaemia (CLL) and Monoclonal B-cell Lymphocytosis (MBL) has now been reported in some detail. This review investigates associations between biology and disease activity as they relate to the absolute numbers of abnormal cells. The clonal B-cells in CLL-type MBL are indistinguishable from CLL with respect to surface phenotype and the presence of chromosomal abnormalities. However, the majority of CLL-type MBL cases in the general population have very low numbers of clonal B-cells, typically in the range 0.1-10 per μL, and such cases use different IGHV genes than higher-count CLL-type MBL cases and often show intraclonal heterogeneity. Cases with higher counts are biologically similar to CLL although there is a relationship between the CLL cell count at presentation and the likelihood of further clonal expansion. Individuals presenting with CLL cell counts above 2,000 per μL are more likely to have gradually increasing B-cell counts over time and although the risk of requiring treatment for progressive CLL remains low there may be impaired normal B-cell activity.

Prevalence of monoclonal B-cell lymphocytosis: a systematic review

BACKGROUND

Individuals with monoclonal B-cell lymphocytosis (MBL) have been identified in clinic outpatients, in unaffected relatives of patients with chronic lymphocytic leukemia (CLL), and in general populations. MBL and its relationship with CLL have been actively investigated over the last decade. This report systematically reviews the prevalence of MBL in the context of the populations studied and the evolution of laboratory methods used to define MBL.

METHODS

To identify published studies that have assessed the prevalence of MBL, we systematically searched the MEDLINE databases and consulted with members of the International MBL Study Group. We reviewed the 10 articles that were identified by this process. We abstracted information on study populations, laboratory tests, criteria for designating MBL, and the reported frequencies.

RESULTS

Three of the ten studies were published in 2009, three between 2007 and 2008, and four between 2002 and 2004. Reported prevalences varied widely, ranging from 0.12 to 18.2%. This variability was clearly associated with both the laboratory methods and the populations studied. MBL was more common among older individuals and kindred of persons with CLL. The most common MBL subtype was CLL-like MBL.

CONCLUSIONS

Large population-based studies of MBL that employ standardized laboratory methods with a consensus case definition are needed to assess prevalence and establish risk factors. These studies should include prospective follow-up of MBL cases to determine the relationship between MBL and CLL. Data from original studies should be reported in sufficient detail to allow future synthesis of information from multiple studies, such as meta-analysis.

Common occurrence of monoclonal B-cell lymphocytosis among members of high-risk CLL families

Monoclonal B-cell lymphocytosis (MBL) is an asymptomatic haematological condition characterized by low absolute levels of B-cell clones with a surface immunophenotype similar to that of chronic lymphocytic leukaemia (CLL). In the general population, MBL increases with age with a prevalence of 5-9% in individuals over age 60 years. It has been reported to be higher among first-degree relatives from CLL families. We report results of multi-parameter flow cytometry among 505 first-degree relatives with no personal history of lymphoproliferative disease from 140 families having at least two cases of CLL. Seventeen percent of relatives had MBL. Age was the most important determinant where the probability for developing MBL by age 90 years was 61%. MBL clustered in certain families but clustering was independent of the number of known CLL cases in a family. As is the case with CLL, males had a significantly higher risk for MBL than did females (P = 0·04). MBL patients had significantly higher mean absolute lymphocyte counts (2·4 × 10(9) /l) and B-cell counts (0·53 × 10(9) /l) than those with a normal B-cell immuno-phenotype. Our findings show that MBL occurs at a very high rate in high risk CLL families. Both the age and gender distribution of MBL are parallel to CLL, implying a shared inherited risk.

CLL-like monoclonal B-cell lymphocytosis: are we all bound to have it?

CLL-like monoclonal B-cell lymphocytosis (MBL) shares a unique immunophenotype with chronic lymphocytic leukemia (CLL), and represents the vast majority of clonal B-cell expansions found in the peripheral blood of otherwise healthy subjects. Along with the improvement of laboratory techniques and the widespread availability of multiparameter flow cytometry, the finding of tiny aberrant B-cell populations became more frequent, prompting the need for clinical and biological definition of the nature of this condition and its relationship with leukemia development. MBL seems to be a melting-pot containing several entities, identical in terms of phenotype but with extremely different risks of leukemia development (from low to none) that seem to correlate with the number of B lymphocytes. CLL-like MBL observed in the clinical setting ("Clinical MBL"), usually being characterized by lymphocytosis, demonstrated a sizeable, even if low (1.1-1.4% per year), risk of leukemic progression, but represents a minority of all MBL cases. The vast majority of CLL-like MBL are detected in general population screenings and do not likely have a risk of CLL that is substantially higher than that of unaffected individuals. Interestingly, MBL frequency increases with age, being virtually undetectable under 40 years of age but being present in 50-75% of the people older than 90 years. It has been proposed that MBL could be interpreted as an epiphenomenon of a chronic and persistent antigenic stimulation. The (rare) possibility to evolve into a frank leukemia might then depend on biological and molecular factors insofar unknown that may modify the modality of cell reaction as well as the potential to acquire further genetic abnormalities. Therefore, the real challenge of the next years in the MBL research field is not to increase the sensitivity of detection, neither to implement screening protocols to be applied to the general population, rather to unravel the biologic features that, at individual level, will identify those (few) cases that are at risk of developing a progressive disease.

Monoclonal B-cell lymphocytosis (MBL): biology, natural history and clinical management

Chronic lymphocytic leukemia (CLL) and the other low-grade non-Hodgkin lymphomas are among the most common lymphoid malignancies. Recent studies suggest that more than 4% of the general population over age 40 harbor a population of clonal B cells with the phenotype of either CLL or another B-cell malignancy, a condition now designated monoclonal B-cell lymphocytosis (MBL). Although all cases of CLL appear to be preceded by MBL, the majority of individuals with MBL will not develop a hematologic malignancy. The biologic characteristics and clinical implications of MBL appear to differ based on whether it is identified during the diagnostic evaluation of lymphocytosis or incidentally discovered through screening of individuals with normal lymphocyte counts as part of research studies using highly sensitive detection methods. In this paper, we provide a state of the art review on the prevalence, nomenclature, biology, natural history and clinical management of MBL.

Monoclonal B-cell lymphocytosis in first-degree relatives of patients with sporadic (non-familial) chronic lymphocytic leukaemia

Although biological similarities have been described among monoclonal B-cell lymphocytosis (MBL) and chronic lymphocytic leukaemia (CLL), the relationships between these two conditions are not fully understood, and new epidemiological studies in different populations and different countries continue to be reported. Here, we investigated 167 first-degree relatives from 42 families of patients with non-familial (sporadic) CLL, using four-colour flow cytometry. MBL was found in seven of 167 subjects (4.1%). Monoclonality was detected in all cases either by light-chain restriction or by polymerase chain reaction. Fluorescence in situ hybridization did not show any chromosomal abnormality. The prevalence of MBL according to age was 0 (0/54) in individuals aged less than 40 years, 2.5% (2/81) between 40 and 60 years, and 15.6% (5/32) in individuals over 60 years. The prevalence of MBL cases in individuals over 60 years was similar to that found in familial CLL relatives at the same age group. This suggests that in older first-degree relatives of patients with sporadic CLL, the risk of MBL detection is as high as in older first-degree relatives from CLL families, which could render these individuals belonging to 'sporadic CLL families' as susceptible as individuals from 'familial CLL' to the development of clinical CLL.

Monoclonal B-cell lymphocytosis: definitions and natural history

The diagnostic term monoclonal B-cell lymphocytosis (MBL) is used to characterise individuals with a circulating population of clonal B-cells, a total B-cell count of <5 x 10(9)/L, and no other features of a B-cell lymphoproliferative disorder. The clinical implications of MBL may differ depending on whether an individual with a normal lymphocyte count is identified via a screening assay (screening MBL) or identified through clinical evaluation of lymphocytosis (clinical MBL). The B-cell count used to distinguish between clinical MBL (<5 x 10(9)/L) and CLL (> or = 5 x 10(9)/L) was selected largely based on tradition and technological advances and it is unknown whether the natural history of 'clinical MBL' differs from that of patients with Rai stage 0 CLL. Since, a diagnosis of 'leukemia' may lead to profound psychologic distress for patients, we believe the diagnosis of CLL should be based on an individual's risk of developing symptoms, requiring chemotherapeutic treatment and/or dying of disease. Additional studies are needed to determine whether the clinical outcome of patients with MBL differs from that of patients with Rai stage 0 CLL and to identify what B-cell threshold optimally distinguishes between these conditions.

Brief report: natural history of individuals with clinically recognized monoclonal B-cell lymphocytosis compared with patients with Rai 0 chronic lymphocytic leukemia

PURPOSE

The diagnosis of monoclonal B-cell lymphocytosis (MBL) is used to characterize patients with a circulating population of clonal B cells, a total B-cell count of less than 5 x 10(9)/L, and no other features of a B-cell lymphoproliferative disorder including lymphadenopathy/organomegaly. The natural history of clinically identified MBL is unclear. The goal of this study was to explore the outcome of patients with MBL relative to that of individuals with Rai stage 0 chronic lymphocytic leukemia (CLL).

PATIENTS AND METHODS

We used hematopathology records to identify a cohort of 631 patients with newly diagnosed MBL or Rai stage 0 CLL. Within this cohort, 302 patients had MBL (B-cell counts of 0.02 to 4.99 x 10(9)/L); 94 patients had Rai stage 0 CLL with an absolute lymphocyte count (ALC) < or = 10 x 10(9)/L; and 219 patients had Rai stage 0 CLL with an ALC more than 10 x 10(9)/L. Data on clinical outcome were abstracted from medical records.

RESULTS

The percentage of MBL patients free of treatment at 1, 2, and 5 years was 99%, 98%, and 93%, respectively. B-cell count as a continuous variable (hazard ratio [HR] = 2.9, P = .04) and CD38 status (HR = 10.8, P = .006) predicted time to treatment (TTT) among MBL patients. The likelihood of treatment for MBL patients was lower (HR = 0.32, P = .04) than that of both Rai stage 0 CLL patients with an ALC less than 10 x 10(9)/L (n = 94) and Rai stage 0 CLL patients with an ALC more than 10 x 10(9)/L (n = 219; P = .0003).

CONCLUSION

Individuals with MBL identified in clinical practice have a low risk for progression at 5 years. Because B-cell count seems to relate to TTT as a continuous variable, additional studies are needed to determine what B-cell count should be used to distinguish between MBL and CLL.

The prognosis of clinical monoclonal B cell lymphocytosis differs from prognosis of Rai 0 chronic lymphocytic leukaemia and is recapitulated by biological risk factors

Monoclonal B-cell lymphocytosis (MBL) is an asymptomatic monoclonal expansion of <5.0 x 10(9)/l circulating CLL-phenotype B-cells. The relationship between MBL and Rai 0 CLL, as well as the impact of biological risk factors on MBL prognosis, are unknown. Out of 460 B-cell expansions with CLL-phenotype, 123 clinical MBL (cMBL) were compared to 154 Rai 0 CLL according to clinical and biological profile and outcome. cMBL had better humoral immune capacity and lower infection risk, lower prevalence of del11q22-q23/del17p13 and TP53 mutations, slower lymphocyte doubling time, and longer treatment-free survival. Also, cMBL diagnosis was a protective factor for treatment risk. Despite these favourable features, all cMBL were projected to progress, and lymphocytes <1.2 x 10(9)/l and >3.7 x 10(9)/l were the best thresholds predicting the lowest and highest risk of progression to CLL. Although IGHV status, CD38 and CD49d expression, and fluorescence in situ hybridization (FISH) karyotype individually predicted treatment-free survival, multivariate analysis identified the presence of +12 or del17p13 as the sole independent predictor of treatment requirement in cMBL (Hazard ratio: 5.39, 95% confidence interval 1.98-14.44, P = 0.001). Overall, these data showed that cMBL has a more favourable clinical course than Rai 0 CLL. Given that the biological profile can predict treatment requirement, stratification based on biological prognosticators may be helpful for cMBL management.

Monoclonal B-cell lymphocytosis

The incidence and presenting features of chronic lymphocytic leukemia (CLL) have changed significantly over the last century. Routine diagnostic techniques can now detect very low levels of CLL phenotype cells. Monoclonal B-cell lymphocytosis (MBL) is a relatively recent diagnostic category encapsulating individuals with an abnormal B-cell population but not meeting the diagnostic criteria for a B-cell malignancy. This review focuses on CLL-type MBL, which represents the majority of MBL cases identified in diagnostic laboratories. CLL-type MBL has a phenotype identical to CLL and shares the same chromosomal abnormalities even at the lowest levels detectable. Recent evidence suggests that the immunoglobulin gene usage plays a key role in whether the abnormal cells will develop in significant numbers. In most cases, CLL-type MBL is a stable condition with only 1% per year among those presenting for clinical attention developing progressive disease requiring treatment, although suppressed immune function may have a more significant impact on outcome.

The immunoglobulin gene repertoire of low-count chronic lymphocytic leukemia (CLL)-like monoclonal B lymphocytosis is different from CLL: diagnostic implications for clinical monitoring

In the revised National Cancer Institute Working Group (NCI-WG)/International Workshop on Chronic Lymphocytic Leukemia (IWCLL) guidelines for CLL, CLL-like monoclonal B lymphocytosis (MBL) is defined as the presence of less than 5 x 10(9)/L B lymphocytes in the peripheral blood. However, the concentration of MBL in the blood is extremely variable. MBL in subjects with lymphocytosis require treatment at a rate of 1.1% per year and present immunoglobulin (IG) gene features and similar to good prognosis CLL. Little is known about low-count MBL cases, accidentally found in the general population. We analyzed IGHV-D-J rearrangements in 51 CLL-like MBL cases from healthy individuals, characterized by few clonal B cells. Seventy percent of the IGHV genes were mutated. The most frequent IGHV gene was IGHV4-59/61, rarely used in CLL, whereas the IGHV1-69 gene was lacking and the IGHV4-34 gene was infrequent. Only 2 of 51 (3.9%) MBL cases expressed a CLL-specific stereotyped HCDR3. Therefore, the IG gene repertoire in low-count MBL differs from both mutated and unmutated CLL, suggesting that the detection of MBL in an otherwise healthy subject is not always equivalent to a preleukemic state. Detailed IG analysis of individual MBL may help to identify cases that necessitate continuous clinical monitoring to anticipate disease progression.

Multiple myeloma, chronic lymphocytic leukaemia and associated precursor diseases

Multiple myeloma and chronic lymphocytic leukaemia share common biological and clinical features including the presence of defined precursor conditions (monoclonal gammopathy of undetermined significance and monoclonal B-cell lymphocytosis respectively). Here, we discuss evidence from the literature on the potential aetiological roles for genetic and chronic immune stimulatory factors on the pathway from precursor to malignancy. Also, we speculate on the relationship between precursor and malignancy and talk about future directions and gaps in the literature.

Waldenström macroglobulinaemia and IgM monoclonal gammopathy of undetermined significance: emerging understanding of a potential precursor condition

Previously thought to be best described as a plasma cell disorder, Waldenström macroglobulinaemia (WM) is now understood to be a distinct clinicobiological entity. WM shares B-cell origin and certain other features with both chronic lymphocytic leukaemia (CLL) and multiple myeloma (MM). WM and CLL arise from B-cells at discrete stages in their maturation process, and MM arises from B-cells that have fully differentiated into plasma cells. While MM has a well-known precursor condition, monoclonal gammopathy of undetermined significance (MGUS), CLL and WM may also have associated precursor states, monoclonal B-cell lymphocytosis (MBL) and IgM MGUS, respectively. This review explores the features that link or distinguish these haematolymphoid malignancies, with special attention to emerging data regarding IgM MGUS and its unique relationship to WM, and identifies important gaps in our understanding of the putative precursor conditions, MBL and IgM MGUS.

Bioethical considerations of monoclonal B-cell lymphocytosis: donor transfer after haematopoietic stem cell transplantation

Monoclonal B-cell lymphocytosis (MBL) is a recently described laboratory finding in otherwise healthy individuals. In MBL, a light chain-restricted, clonal B-cell population, often with a chronic lymphocytic leukaemia (CLL) phenotype, is identified by flow cytometry. Although the prognostic significance remains unclear, there is an increased incidence in ageing populations and those with a family history of CLL. During the past decade of MBL study, three families have come to our attention in which prospective sibling haematopoietic stem cell donors were found to have an MBL. These families raise complex bioethical issues with regard to disclosure of research data, eligibility for clinical trials and potential donor transfer of MBL. These issues are explored in this report. Identification of MBL among prospective sibling transplant donors will become a common occurrence in transplant practice as transplantation is increasingly offered to older individuals and those with CLL.

Overview of monoclonal gammopathies of undetermined significance

Among the B-cell lymphoproliferative disorders, monoclonal gammopathy of undetermined significance (MGUS) is the humoral counterpart to monoclonal B-cell lymphocytosis. This review introduces the papers from the section devoted to MGUS at the International Workshop entitled 'Monoclonal B-cell lymphocytosis and chronic lymphocytic leukaemia: environmental and genetic risk factors.'

B cells behaving badly

The pathogenesis of B-cell lymphoproliferative disorders in general and B-cell chronic lymphocytic leukaemia in particular appears to involve dysfunctional regulation of humoral and cellular immunity with the subsequent development of genetic aberrations in B cells. In theory, either component may arise de novo or may be influenced by environmental exposures including infectious agents, antigens, genotoxic chemicals, or radiation. As an intermediary within the exposure-disease continuum, monoclonal B-cell lymphocytosis may be a helpful biomarker for teasing out these various contributions to risk. This article introduces a series of papers that resulted from an International Workshop held in May 2007 entitled 'Monoclonal B-cell Lymphocytosis and Chronic Lymphocytic Leukemia: Environmental and Genetic Risk Factors'. Research efforts, such as those described in this issue, should lead to improved interventions, more predictive biomarkers, more effective treatments, and a greater appreciation of how the immune system functions over the entire human lifespan.

Overview of monoclonal B-cell lymphocytosis

Monoclonal B-cell lymphocytosis (MBL) has been the subject of more intensive investigation for the last 10 years. The increased presence of MBL in unaffected, first-degree relatives with familial chronic lymphocytic leukaemia (CLL) suggest that it is surrogate marker for early disease. In normal population studies, MBL is found to be increased in ageing subjects. Consensus criteria for the diagnosis of MBL have been proposed. The differential diagnosis has been further clarified and the prevalence of MBL is most prominent in the elderly. The aetiology of MBL is unknown but probably involves immune mechanism of senescence or altered response. Environmental health studies suggest that exposure to certain toxins may lead to MBL but further work is needed. MBL is a precursor to CLL but may also regress, remain stable or progress to clinical CLL.

Monoclonal B-cell lymphocytosis as a biomarker in environmental health studies

The first studies of monoclonal B-cell lymphocytosis (MBL) in the general population were conducted as part of environmental health investigations that began in 1991. MBL was observed as an unexpected finding when blood samples were immunophenotyped by two-colour flow cytometric methods in common use at that time. The initial observations led to a workshop in 1995, at which case definitions were considered and medical follow-up investigations were recommended. Medical follow-ups were conducted in 1997 and 2003. A total of eight cases of confirmed MBL and three cases of presumptive MBL were identified. This review summarizes the findings from those investigations and discusses the issues related to using MBL as a biomarker in environmental health research and population-based studies.

Chronic lymphocytic leukaemia: an overview of aetiology in light of recent developments in classification and pathogenesis

This overview of the epidemiology of chronic lymphocytic leukaemia (CLL) summarizes the evolution of classification and coding systems and describes the intersection of pathogenesis and aetiology. The role of the putative precursor to CLL, monoclonal B-cell lymphocytosis (MBL), is considered, and ideas for future investigations of the MBL-CLL relationship are outlined. We discuss the epidemiology of CLL, focusing on descriptive patterns and methodological considerations. Postulated risk factors are reviewed including the role of ionizing and non-ionizing radiation, occupational and environmental chemical exposures, medical conditions and treatments, and lifestyle and genetic factors. We conclude by raising key questions that need to be addressed to advance our understanding of CLL aetiology. Recommendations for future epidemiological studies are given, including the standardization of reporting of CLL across cancer registries, the clarification of the natural history of MBL, and the circumvention of the methodological shortcomings of prior epidemiological investigations in relation to radiation, chemical exposures and infectious agents.

Murine models of chronic lymphocytic leukaemia: role of microRNA-16 in the New Zealand Black mouse model

Mouse models are valuable tools in the study of human chronic lymphocytic leukaemia (CLL). The New Zealand Black (NZB) strain is a naturally occurring model of late-onset CLL characterized by B-cell hyperproliferation and autoimmunity early in life, followed by progression to CLL. Other genetically engineered models of CLL that have been developed include (NZB x NZW) F1 mice engineered to express IL5, mice expressing human TCL1A, and mice overexpressing both BCL2 and a tumour necrosis factor receptor-associated factor. The applicability to human CLL varies with each model, suggesting that CLL is a multifactorial disease. Our work with the de novo NZB model has revealed many similarities to the human situation, particularly familial CLL. In NZB, the malignant clones express CD5, zap-70, and have chromosomal instability and germline Ig sequence. We also identified a point mutation in the 3'-flanking sequence of Mirn16-1, which resulted in decreased levels of the microRNA, miR-16 in lymphoid tissue. Exogenous restoration of miR-16 to an NZB malignant B-1 cell line resulted in cell cycle alterations, suggesting that the altered expression of Mirn15a/16-1 is an important molecular lesion in CLL. Future studies utilizing the NZB mouse could ascertain the role of environmental triggers, such as low dose radiation and organic chemicals in the augmentation of a pre-existing propensity to develop CLL.

Cytometry in monoclonal B-cell lymphocytosis and chronic lymphocytic leukaemia--the Hunting of the Snark?

Cytometry has become important in the detection and determination of risk of monoclonal B-cell lymphocytosis; methodology has changed, and will continue to change, as cytometric technology changes.