What Is Atypical Chronic Lymphocytic Leukemia?

Atypical chronic lymphocytic leukemia (aCLL) is a variant of CLL in which the leukemia cells lack the full set of surface markers that define a classic CLL diagnosis, yet the disease does not fit the diagnostic criteria for any other recognized B-cell cancer. In practice, that means one or more of the expected proteins on the cell surface are missing or expressed at unusual levels, making the case harder to classify and, in many instances, linked to a more aggressive clinical course than standard CLL.

How Classic CLL Gets Diagnosed in the First Place

A typical CLL diagnosis rests on a few well-established checkboxes. A patient needs at least 5,000 clonal B-lymphocytes per microliter of blood, and those cells should co-express a specific cluster of surface proteins: CD19, CD20, CD5, and CD23. At the same time, the cells show restricted light-chain immunoglobulin and weak or absent expression of CD22 and CD79b.¹ When a case ticks all these boxes, classification is straightforward. The trouble starts when it ticks most of them but not all.

What Makes a Case “Atypical”

The “atypical” label can apply in two overlapping ways: the cells look unusual under the microscope, or they carry an unusual surface-marker profile when analyzed by flow cytometry. Morphologically, atypical CLL cells tend to be larger than the small, round lymphocytes seen in classic CLL, and they may include prolymphocytes (bigger cells with a prominent central structure called a nucleolus) or cells with irregular, cleaved shapes.¹ In earlier decades, these visual differences under the microscope were the main way pathologists flagged a case as atypical.

Today, the immunophenotype carries more diagnostic weight than cell shape. Atypical CLL is defined by the absence or weak expression of one or more of the surface proteins expected in classic CLL, most often CD5 or CD23. The crucial caveat is that the case must not meet diagnostic criteria for any other recognized B-cell malignancy; otherwise, it would simply be reclassified as that other disease.¹ That “exclusion of everything else” requirement is what makes aCLL partly a diagnosis of elimination.

The Surface-Marker Profile in Detail

Flow cytometry is the workhorse here. In a study evaluating immunophenotypic markers in aCLL, CD5 was negative in about one in six patients with aCLL, while CD23 was negative in roughly a third.² Missing CD23 is the more common deviation, but some cases show intermediate rather than flatly negative expression, blurring the boundary further. In one cohort of ten aCLL patients, four lacked CD23 entirely, four had intermediate CD23, and two had intermediate CD5, with one case showing unusually strong CD20 on top of the CD23 absence.³

Beyond CD5 and CD23, newer markers help refine the diagnosis. CD200, a protein consistently expressed on CLL cells regardless of their other surface-marker quirks, has emerged as a useful anchor. In the same aCLL cohort study, CD200 was positive in every aCLL case, and CD43 was positive in the vast majority, with both markers co-expressed in over four-fifths of patients.² Because many of CLL’s look-alikes (mantle cell lymphoma, marginal zone lymphoma) tend to be CD200-negative, this marker has practical value for telling them apart.

A modified scoring system that incorporates CD200, along with CD5/CD23 co-expression, FMC7 status, and CD79b intensity, has shown substantially better specificity for distinguishing CLL from non-CLL B-cell cancers than the older Matutes score, without sacrificing sensitivity.⁴ That matters for aCLL because these are exactly the cases where the traditional scoring system falters.

Trisomy 12 and the Genetic Landscape

One chromosomal abnormality stands out in atypical CLL: trisomy 12, an extra copy of chromosome 12. In a large study of over 500 CLL patients, trisomy 12 was the only cytogenetic group with a high rate of atypical morphology, and that frequency jumped even higher when trisomy 12 occurred alongside other chromosomal changes.⁵ The same study found that trisomy 12 cases tended to present at more advanced disease stages and showed stronger expression of surface immunoglobulin and FMC7, both of which are features that push a case toward the “atypical” phenotype.

A more recent analysis of over 270 CLL cases found trisomy 12 in close to 60 percent of immunophenotypically atypical cases, compared to about 20 percent of typical ones. That study also cataloged the specific gene mutations enriched in atypical CLL: BIRC3 was the most frequently mutated gene, and changes in KRAS, NRAS, KMT2D, and SMARCA4 were all significantly more common than in typical CLL.⁶ Other atypical features noted in those cases included increased CD20 expression, decreased or absent CD43 and ROR1, and brighter surface light-chain staining. Cases with higher CD20 expression were more likely to carry mutated IGHV genes, which is interesting because mutated IGHV is generally considered a favorable prognostic marker in CLL.

This creates a nuanced picture. Trisomy 12 and the RAS-pathway mutations are typically associated with worse outcomes, but some individual atypical features (like brighter CD20 and mutated IGHV) point in a more favorable direction. The net effect on any single patient depends on which combination of changes their disease carries.

Prognosis and Clinical Behavior

On balance, atypical CLL tends to behave more aggressively than classic CLL. Patients with aCLL are more likely to carry markers associated with poor prognosis, including trisomy 12, unmutated IGHV genes, and CD38 expression.¹ An analysis comparing aCLL directly to classic CLL and to B-cell prolymphocytic leukemia found that aCLL carried a worse prognosis than standard CLL and showed clinical and laboratory features distinct from B-PLL.⁷

That said, “worse than classic CLL” covers a wide range. Classic CLL itself varies enormously, from patients who never need treatment to those who progress rapidly. Atypical CLL shifts the distribution toward the more aggressive end but does not uniformly predict a grim outcome. The specific genetic and surface-marker features of each case matter enormously, which is part of why the field has not settled on a single, universally accepted definition of aCLL or a separate treatment algorithm for it.

When Atypical CLL Transforms Into Aggressive Lymphoma

Richter transformation, the conversion of CLL into a fast-growing lymphoma (usually diffuse large B-cell lymphoma), is a feared complication in any CLL variant. Case studies suggest that atypical CLL can undergo this transformation, and the genetic underpinnings may differ from what is seen in classic CLL. In one whole-genome sequencing analysis of an aCLL case that transformed to diffuse large B-cell lymphoma, the lymphoma cells carried mutations in genes including COL11A1, MGME1, ZMYM3, ALG6, UBA5, and ATG7. Of these, only ZMYM3 is recurrently mutated in standard CLL, pointing to a molecular background that does not neatly overlap with the typical transformation pathway.⁸ This is a single case, so broad conclusions would be premature, but it underscores that atypical CLL may harbor genetic vulnerabilities that are still poorly mapped.

Separating Atypical CLL From Its Look-Alikes

The main reason aCLL generates diagnostic headaches is that its unusual surface-marker profile overlaps with several other B-cell cancers. Getting the distinction right matters because the diseases differ in treatment and prognosis.

Mantle Cell Lymphoma

Mantle cell lymphoma (MCL) is the most dangerous mimic. Its leukemic phase can look strikingly similar to atypical CLL: both can be CD5-positive and CD23-negative. The critical separator is the chromosomal translocation t(11;14), which fuses the IGH and CCND1 genes and drives overproduction of a protein called cyclin D1. This translocation is the hallmark of MCL but is not part of CLL biology.⁹ However, some studies have identified this translocation in a minority of cases initially classified as atypical CLL. In one series of atypical B-cell leukemias tested by FISH, three out of fifteen cases with the translocation had been classified as atypical CLL, and one of those was reclassified as MCL after spleen tissue was reviewed.¹⁰ The practical lesson is that any case with an MCL-like immunophenotype should be tested for cyclin D1 overexpression or the t(11;14) translocation before the aCLL label sticks.

B-Cell Prolymphocytic Leukemia

B-PLL is defined by the presence of at least 55 percent prolymphocytes in the blood. Historically, cases with 15 to 55 percent prolymphocytes were called CLL/PL and are now grouped under the aCLL umbrella.⁷ The distinction matters because B-PLL is a separate disease with its own management approach. Counting prolymphocytes in the blood film is the straightforward way to separate these two entities, but the boundary at 55 percent is somewhat arbitrary, and edge cases create real classification uncertainty.

Splenic Marginal Zone Lymphoma

Splenic marginal zone lymphoma (SMZL) is another look-alike, especially when it presents in a leukemic phase with high lymphocyte counts and minimal lymph-node enlargement. SMZL can partially express CD23 and CD200, which pushes it toward a CLL-like profile. The absence of strong CD5 expression, positivity for FMC7, and a clinical picture dominated by splenic enlargement favor SMZL over aCLL.¹¹ Careful distinction from both aCLL and MCL is needed to avoid therapeutic missteps.¹²

The Problem of No Standardized Definition

Perhaps the most frustrating aspect of atypical CLL is that no standard or commonly accepted criteria exist for formally distinguishing it from classic CLL.¹ Different laboratories and research groups apply slightly different thresholds for what counts as “absent” versus “dim” expression of CD5 or CD23, and the cutoff for how many atypical surface markers push a case from “unusual CLL” to “aCLL” is not universally agreed upon. One study identifying about 20 percent of CLL cases as immunophenotypically atypical used a combination of increased CD20, absent CD43, absent ROR1, and brighter surface light chains as defining features.⁶ Other groups focus primarily on CD5 and CD23 deviation. The result is that the reported prevalence of aCLL varies depending on who is doing the counting and which markers they emphasize.

This lack of consensus has practical consequences. Without a firmly established diagnostic category, aCLL does not have its own treatment guidelines. Most patients are managed as CLL, with treatment decisions driven by the individual risk markers their disease carries rather than by the “atypical” label itself. Some clinicians argue this is fine because modern CLL therapy is increasingly guided by molecular features like TP53 deletion, IGHV mutation status, and specific gene mutations rather than by broad phenotypic categories. Others worry that lumping aCLL with classic CLL obscures prognostic differences and may delay the development of tailored approaches.

What Happens at the Diagnostic Workup

If you or someone you know is being evaluated for aCLL, the workup typically involves several layers. Blood counts will show elevated lymphocytes, and a blood smear under the microscope may reveal larger or irregularly shaped cells. Flow cytometry then profiles the surface markers. When the profile deviates from the classic CLL pattern, additional tests come into play to rule out the diseases described above: FISH analysis for the t(11;14) translocation (to exclude MCL), cyclin D1 immunohistochemistry, and possibly a bone marrow biopsy or spleen imaging if SMZL is suspected. Molecular testing for IGHV mutation status, TP53 status, and now broader next-generation sequencing panels can further characterize the disease and guide treatment decisions.

The diagnostic process can feel drawn out and uncertain for patients, partly because the boundary between “a CLL case that happens to have an unusual marker or two” and “a genuinely different disease that merely resembles CLL” remains blurry. For some patients, the diagnosis shifts after additional testing, and what started as aCLL turns out to be MCL or SMZL. For others, the aCLL label holds, and the clinical team proceeds to evaluate prognosis and treatment need using the same risk-stratification tools applied to classic CLL.

CD200 as a Diagnostic Anchor

One development worth watching is the growing role of CD200 in sorting out ambiguous cases. CD200 is reliably expressed in CLL regardless of whether the other markers are typical, and it tends to be absent in MCL and many other B-cell cancers. The modified scoring system that swaps surface immunoglobulin intensity for CD200 achieved a specificity of roughly 87 percent, compared to around 54 percent for the older system, in distinguishing CLL from non-CLL B-cell neoplasms.⁴ For patients sitting in the diagnostic gray zone, this kind of improved specificity can be the difference between an accurate diagnosis and a harmful misclassification.

The study of CD200 in aCLL specifically found that it was expressed in every atypical case examined, even when CD5 or CD23 were absent.² That consistency makes CD200 a useful anchor when the rest of the immunophenotype is confusing. It does not tell you whether a CLL case is typical or atypical, but it strongly supports that the disease is CLL (or aCLL) rather than one of its mimics.

Living With Diagnostic Uncertainty

For patients, the atypical CLL label often arrives with more questions than answers. It can feel unsettling to be told that your leukemia does not fully match the textbook version of the disease, especially when treatment guidelines are written for the textbook version. A few practical points are worth keeping in mind. First, the treatments used for classic CLL, including targeted agents like BTK inhibitors and BCL-2 inhibitors, are generally applied to aCLL as well, chosen based on the molecular features of the individual case. Second, the higher frequency of adverse prognostic markers in aCLL means that closer monitoring and earlier treatment discussions may be warranted. Third, the diagnostic picture can evolve over time. Repeat flow cytometry and periodic molecular testing may refine or even change the diagnosis, particularly if the disease transforms or acquires new genetic changes.

Ongoing research is gradually filling in the gaps. Whole-genome sequencing of aCLL cases is beginning to reveal mutational patterns distinct from classic CLL, as illustrated by the Richter transformation case study showing mutations in genes not typically associated with CLL.⁸ Whether these findings eventually lead to a formal reclassification of aCLL as a separate entity, or simply to better risk stratification within the CLL umbrella, remains an open question that the field is actively working to resolve.

1
PubMed Central. Atypical Chronic Lymphocytic Leukemia-The Current Status
2
Journal of Applied Hematology. Evaluation of Immunophenotypic Markers in Diagnosis and Prognosis of Atypical Chronic Lymphocytic Leukemia
3
Blood. Atypical Chronic Lymphocytic Leukemia with CD200 Expression Shares Similar Outcomes to Classical Chronic Lymphocytic Leukemia
4
PubMed. Diagnosis of CLL revisited: increased specificity by a modified five-marker scoring system including CD200
5
PubMed. Trisomy 12 defines a group of CLL with atypical morphology: correlation between cytogenetic, clinical and laboratory features in 544 patients
6
PubMed Central. Immunophenotypic, cytogenetic, and mutational features of chronic lymphocytic leukemia/small lymphocytic lymphoma with atypical immunophenotype
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HemaSphere. PB1896 ATYPICAL CHRONIC LYMPHOCYTIC LEUKEMIA HAS A WORSE PROGNOSIS THAN CLL AND SHOWS DIFFERENT CLINICAL AND LABORATORY FEATURES FROM B‐CELL PROLYMPHOCYTIC LEUKEMIA
8
PubMed Central. Genetic background of Richter transformation of atypical chronic lymphocytic leukemia to diffuse large B-cell lymphoma – a case study
9
Genes, Chromosomes and Cancer. Detection of t(11;14) using interphase molecular cytogenetics in mantle cell lymphoma and atypical chronic lymphocytic leukemia
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PubMed. FISH analysis for BCL-1 rearrangements and trisomy 12 helps the diagnosis of atypical B cell leukaemias
11
Clinical Lymphoma Myeloma and Leukemia. Splenic Marginal Zone Lymphoma Presenting With CLL-Like Features
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Clinical Lymphoma Myeloma and Leukemia. Leukemic-Phase Splenic Marginal Zone Lymphoma Presenting With Marked Lymphocytosis and Minimal Nodal Involvement