Monoclonal B-lymphocytosis: reflections and definitions

More than six years have passed since the publication in 2005 that coined the term and proposed a set of criteria for monoclonal B-lymphocytosis (MBL) [1], and 3 years since the resultant revision of the chronic lymphocytic leukemia (CLL) guidelines by the International Workshop on CLL and National Cancer Institute (NCI) [2]. Prior to 2008, the NCI guidelines published by Cheson et al. in 1988 [3] and 1996 [4] served as the definition of CLL, which was based on an absolute lymphocyte count (ALC) of ≥5.0 × 10⁹/L. The proposed MBL criteria used a B-cell count (not ALC) of <5.0 × 10⁹/L, which created overlap with the definition of CLL, and also with small lymphocytic lymphoma (SLL) [5]. Subsequent revision of the CLL and SLL guidelines [2] raised the diagnostic threshold for CLL compared to that in use for the previous 20 years, and eliminated the overlap for CLL [6] although not entirely for SLL. Significant literature has appeared since the original MBL criteria that has expanded and clarified our knowledge of the natural history of small clones [7–12]. Small B-cell clones had been recognized for a number of years, and identified with increasing frequency following the introduction in the 1990s of five-part differential hematology analyzers and flow cytometers.

In this issue of Leukemia and Lymphoma, Molica and colleagues [13] reappraise the clinical implications of MBL. Patients with MBL who present to the hematology clinic typically have a modest lymphocytosis between the upper limit of the normal range of 4.0 × 10⁹/L and the revised MBL/CLL threshold of a B-cell count of ≥ 5.0 × 10⁹/L. When this B-cell count cut-off is used, the ALC is variable, with a range between 5 and 15.0 × 10⁹/L [6]. Most of these patients were defined as having CLL prior to the 2008 revision of the CLL guidelines and its definition. Much has been discussed regarding the psychological impact of a “leukemia” diagnosis, and while this is undoubtedly the case for many, Molica and colleagues [13] also point out that the psychological impact of MBL, especially in the younger patient, may be significant. In this author's experience, the distinction between “early CLL” and “MBL” for many patients appears less consequential than having a clone in the first place, and for some patients, the demarcation between CLL and MBL is a source of additional consternation.

Molica et al. [13] also highlight the evolving issue and implications of assessment of donors for blood product donation, both for transfusion and stem cell transplant. This particular issue considerably blurs the distinction between “clinical” and “screening” MBL, as the latter, if detected, becomes clinically relevant for both donor and potential recipient. It also, to an extent, blurs the distinction between “low-count” and “high-count” MBL. While the former may progress only very rarely, and the latter at a rate of 1–2% per annum [12,14], most donors identified with an MBL clone would no doubt prefer to have no clone rather than a low-count clone. These small CLL-like clones mostly persist indefinitely, although follow-up is still a relatively short 3 years [15]. Recommended monitoring is every 12–18 months. In the setting of blood donation for transfusion, the risk of MBL transmission to immunocompetent recipients is likely to be very low. Many blood donors are aged < 40 years, where the incidence of CLL-like clones is exceptionally low in any event, and transfused cellular products are usually leukodepleted, and sometimes irradiated. Hence, given the very considerable costs of implementation, the implications of large scale detection of MBL in donors (at 5–12% of the population) and the very low risk of adverse clinical outcomes, screening of blood donors is highly unlikely to be appropriate for the foreseeable future. By contrast, in the setting of stem cell transplant (SCT), the significance of risk for both recipient and donor requires ongoing evaluation. Stem cell recipients are heavily immunocompromised, and potentially much more vulnerable to MBL transmission and clinical progression if the donor has a clonal lymphoid population. This becomes even more pertinent in the setting of donor selection for patients with CLL, as siblings are much more likely to be aged > 40 years, and at significantly higher probability (around 15%) of having a (familial) MBL clone present [10,16–18]. There are now reports of MBL transmission, in addition to families identified in research [17,19–22]. There may be individual clinical situations where clinicians and patients form the opinion that risk of transmission of a known low-risk MBL clone is considered acceptable, such as for high-risk, relapsed hematological malignancy. In many settings, however, risk of transmission of a MBL clone will be considered inappropriate. For unrelated donors the situation is more complex, and raises multiple ethical and practical issues for donors and bone marrow transplant registries, as well as potential recipients. Immunophenotyping and flow cytometric evaluation for the presence of small clones do not currently form part of the donor evaluation in most institutions, but the case for this can certainly be made and should be debated. Screening of potential donors would require a more rigorous assessment, explanation and discussion. Donors with a detectable clone, possibly regardless of the clonal level, would generally be excluded from donation, but the level of sensitivity of detection needs consideration and consensus. Nevertheless, even in the relatively small population of registry donors, the implications for more widespread identification of clones would have a significant impact.

While much has been learnt in MBL regarding the prevalence, biology and natural history of small clones [13], there remain some practical elements that, for the general clinician and patient, appear to result in some confusion. One component of this results from the terminology that has arisen in MBL, and how precisely the original and revised definitions differ – a brief glossary of definitions and terms is provided below. Another factor is the variability in ALC when using a B-cell cut-off for diagnosis. In the hematology laboratory, there is an ongoing issue for flow cytometric reporting, as a definitive diagnosis to differentiate CLL, MBL and SLL cannot be made without the full blood count and clinical data. In some laboratories, the hematology analyzers may be under separate supervision or departments, making it difficult to access information regarding cytopenias. Similarly, without a clinical history of adenopathy or splenomegaly, MBL and SLL cannot be differentiated in the laboratory. Another albeit relatively minor issue in the flow cytometry laboratory is that the antigens and antibodies used to define the B-cell count are not specified. CD19 and CD20 are the most commonly used antigens to enumerate the B-cell count. CD20 is often expressed at a lower antigen density in CLL, but in practice there is little difference between CD19 and CD20 when used for B-cell enumeration, although CD19 tends to be the easier to gate on the flow cytometer and the more commonly used [6]. Despite these issues and the ongoing debate regarding the most appropriate cut-off for diagnosis and biological validity, it is important for clinicians, laboratories and researchers to consistently use the current guidelines and definitions [2]. Proposals for change should be concluded by genuine international consensus, and reflect validated biology, historical continuity and common clinical and laboratory practice.

Glossary of definitions and terms related to monoclonal B-lymphocytosis for the general clinician

Absolute lymphocyte count (ALC) The ALC is calculated by the hematology analyzer from the total white cell count and the percentage of lymphocytes. A commonly used reference range is 1.0–4.0 × 10⁹/L. In normal individuals, this comprises T-cells (CD4 and CD8), B-cells (polyclonal kappa and lambda) and natural killer (NK) cells. In CLL and MBL, and often in SLL, there is also the clonal B-cell population.

B-cell count The B-cell count is usually calculated from the percentage of B-cells from the flow cytometer, with the ALC from the hematology analyzer.

Clonal CLL count The clonal population of CLL has a typical phenotype of CD19, CD20, CD5, CD23 and weak monoclonal surface immunoglobulin. The simplest measurement is CD19/CD5 positive cells. Highly sensitive techniques (using eight antibody combinations and multiple sequential gating [23]) are designed for accurate enumeration of minimal residual disease (MRD) in CLL and are not appropriate for diagnostic evaluation of CLL or MBL.

“Old” definitions for CLL and SLL

CLL (1988 and 1996) [3]

This was defined as an ALC > 5.0 × 10⁹/L with the presence of a (CD5+/CD19+) CLL clone ≥ 1.0 × 10⁹/L.

SLL (2001) [5]

“The term SLL, consistent with CLL, is restricted to cases with the tissue morphology and immunophenotype of CLL, but which are non-leukemic.” This definition was retained by the World Health Organization (WHO) classification in 2008 [24].

“New” definitions for CLL, SLL and MBL

MBL (2005) [1]

1. “Detection of a monoclonal B-cell population in the peripheral blood with (i) overall kappa:lambda ratio > 3:1 or < 0.3:1, or (ii) greater than 25% of B-cells lacking or expressing low level surface immunoglobulin, or (iii) a disease-specific immunophenotype.

2. Repeat assessment should demonstrate that the monoclonal B-cell population is stable over a 3-month period.

3. Exclusion criteria: (i) lymphadenopathy and organomegaly, or (ii) associated autoimmune/infectious disease, or (iii) B-lymphocyte count > 5.0 × 10⁹/L, or (iv) any other feature diagnostic of a B-lymphoproliferative disorder. However, a paraprotein may be present or associated with MBL and should be evaluated independently.

4. Subclassification of MBL [1]:

   1. CD5 + 23 + : CLL-like phenotype, the major subcategory.

   2. CD5 + 23 − : moderate level of CD20 and CD79b, called “atypical CLL.”

   3. CD5 − : non-CLL lymphoproliferative disease (LPD) with a “lymphoma”-phenotype.”

Time requirement Some issues arise with the use of a time or duration requirement for the definition of CLL and MBL. The original MBL criteria required the presence of a clone for more than 3 months. Fazi et al. have recently demonstrated that most CLL-like MBL clones persist [15]. The current International Workshop on CLL (iwCLL) guidelines do not contain a time requirement [2], and the requirement for a duration of > 1 month for a diagnosis of CLL in the original NCI 1988 guidelines [3] was removed in the 1996 revision [4].

CLL-like MBL and non-Hodgkin lymphoma-like MBL Division of MBL into two groups, CLL-like and lymphoma-like, may be a simpler and more practical, clinical subclassification for MBL [11]. The group 2 subclassification in the original MBL criteria defined a CD5 +, CD23 − phenotype potentially in keeping with mantle cell lymphoma (MCL). An indolent form of MCL is now documented, and may account for a significant proportion of patients in this group [25,26]. The natural history of non-CLL MBL remains less clear and is probably more heterogeneous than CLL-like MBL, and is an area that requires further study.

CLL (2008) [2]

“The diagnosis of CLL requires the presence of at least 5 × 10⁹ B-lymphocytes/L (5000/μL) in the peripheral blood. The clonality of the circulating B lymphocytes needs to be confirmed by flow cytometry.”

SLL (2008) [2]

“The definition of SLL requires the presence of lymphadenopathy and/or splenomegaly. Moreover, the number of B lymphocytes in the peripheral blood should not exceed 5 × 10⁹/L. In SLL, the diagnosis should be confirmed by histopathologic evaluation of a lymph node biopsy whenever possible.” The 2008 CLL guidelines modified SLL from all patients who were CLL-like but non-leukemic to a requirement for adenopathy and/or splenomegaly. The WHO classification of 2008 [24] retained their 2001 definition and noted the iwCLL definition.

MBL (2008) [2]

“… otherwise healthy adults who have an absolute increase in the clonal B lymphocytes but who have less than 5.0 × 10⁹/L B lymphocytes in the blood. However, in the absence of lymphadenopathy or organomegaly (as defined by physical examination or CT scans), cytopenias, or disease-related symptoms, the presence of fewer than 5.0 × 10⁹ B lymphocytes per liter of blood is defined as ‘monoclonal B-lymphocytosis’”

“General population,” “Low count,” “Screening,” “Research population” These terms refer to individuals detected during the course of population studies [7,8], usually as part of a research project to identify the frequency of MBL, sometimes with highly sensitive flow cytometry. The majority of these clones appear to persist indefinitely but have a very low risk of progression to frank CLL [15]. Most are detected in research projects and therefore will not necessarily present to the hematology clinic. This situation will change dramatically if donors have flow cytometric evaluation for small B-cell clones as part of a routine evaluation.

“Clinical” “high-count” MBL [13] This represents the group of patients who present to the hematology clinic, with the majority having a modest lymphocytosis exceeding the normal ALC reference range (> 4.0 × 10⁹/L). These patients progress at a rate of 1–2% per annum to frank CLL.

“Tissue-involvement,” “Nodal,” “Bone marrow” and other “in situ” MBL [27–29] While the 2008 revised definition of SLL excluded potential overlap with MBL [2], those patients with an “in situ” SLL infiltrate without a detectable peripheral blood clone or adenopathy do not fit clearly into any of the currently defined entities. The revised definition also does not include those patients who have an incidental “in situ” SLL infiltrate such as in the prostate or other organs, when these are biospied in the course of investigation for other disease processes. The frequency with which small clones are present in node, marrow, spleen or other organs without a clone identifiable in the peripheral blood, especially by high sensitivity techniques, remains unclear at present.

Potential conflict of interest

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