Chronic Lymphocytic Leukemia, CLL
Chronic lymphocytic leukemia is more than a buildup of abnormal white blood cells—it is a dynamic disease in which cancer cells and their surrounding environment continually influence one another. This page explores how CLL cells use B-cell receptor signaling and the tumor microenvironment to promote their own survival, resist normal cell death, evade immune defenses, and evolve into different subclones over time. It also explains how modern testing, including flow cytometry and FISH, can reveal the biological characteristics that help guide individualized treatment, and how targeted therapies such as BTK inhibitors work by disrupting the protective signals the leukemia depends on. Understanding this complex ecosystem offers a clearer picture of why CLL behaves so differently from person to person—and how treatment is becoming increasingly precise.
Table of Contents:
Introduction
CLL is primarily a disease of older adults, with the average age being approximately 70. The number of people living with the disease has expanded because therapy has improved, and the treatment of comorbidities, or other associated illnesses, has also been more successful. Overall, the five-year survival has increased from approximately 70% 40 years ago to closer to 90% currently. Males are affected twice as often as females, and the disease predominantly affects Caucasians; it is less common in African American and Hispanic groups and is rare in the Asian population.
There is a relationship within families, and a first-degree relative of a CLL patient has an 8.5-fold elevated risk compared with the general population. However, it is difficult to pinpoint the specific genetic changes involved, as there are over 30 single nucleotide polymorphisms (a variance in a single base of DNA), our genetic blueprint, that may explain its genetic basis. Otherwise, hereditary risk occurs in only about 20% of people.
Chronic lymphocytic leukemia is considered the proliferation, or continuous development, of mature B lymphocytes produced from a distinct clone, usually a CD5 line. This is a surface molecule that helps identify certain lymphocyte populations. The cloning of this B cell implies that the subsequent cells being produced are all the same. However, from clinical experience and cancer science, it has become more apparent, in this specific disease, that having a malignant clone is not synonymous with developing aggressive disease.
Actually, it is recognized that there is a spectrum of disease. Some people have no symptoms and are diagnosed by an elevated lymphocyte count on a complete blood count. Others with an elevated lymphocyte count may have enlarged lymph nodes but few, if any, symptoms. At the other end of the continuum, others present with symptomatic illness requiring multiple lines of therapy and having poor survival.
These cells come from the bone marrow, which is normally its own ecosystem, with hematopoietic stem cells capable of differentiating into any type of blood cell: all white blood cells, including T cells, B cells, and NK cells; red blood cells; and megakaryocytes that produce platelets. There are also progenitor cells that can differentiate into specific target cell types.
Within this marrow terrain, there also exist stromal structural support cells, endothelial cells that form blood vessels, extracellular matrix, other lymphocytes such as T cells, other immune cells such as macrophages, and cytokines, which act in intercommunication between cells.
Understanding the biology and science of the disease has evolved. Just a few decades ago, it was believed that B cells simply built up and did not die. Today, it is recognized that the disease comprises both an epigenetic or genetic expression contribution and a tumor microenvironment (TME) that promotes survival and proliferation.
Support of the TME involves a significant CLL cell driver of disease, which is the surface signaling protein, or B-cell receptor (BCR). These abnormal B cells are capable of altering the TME, and the cellular components of the TME—stromal cells, T cells, macrophages, extracellular matrix, and chemokines—then send signals back to the CLL cells. It is bidirectional communication designed for the CLL to continue proliferating, resist apoptosis, and migrate.
In CLL, B cells accumulate in the bone marrow and act to disrupt its normal balance. This situation can result in fewer red blood cells, causing anemia; thrombocytopenia, or low platelets, which allows bruising and/or bleeding; and low neutrophils, reducing immune protection from infection.
So this is more than crowding out the other cells. Rather, the CLL B lymphocytes manipulate and alter the microenvironment to support survival, at the expense of normal blood cell production.
Normal B cells produce immunoglobulins, or antibodies, that are needed for immune defense against pathogens and some toxicants. These CLL B cells do not function in a similar way, resulting in low levels of the immune antibodies IgA, IgM, and IgG.
As a result, CLL patients must be careful to avoid exposure to infections, especially in the respiratory system, and remain proactive through preventive vaccines for such diseases as RSV, shingles, influenza, and COVID.
The BCR Receptor Complex
B-cell receptor (BCR) is a sensor on the B-cell surface and is present on all B cells. Normally, when encountering an antigen, the antigen binds to the BCR, activating it. CD4 T cells then activate the specific needed B-cell clone, resulting in multiple copies. With this cascade, the B cells then transform into plasma cells and produce antibodies. This response ends when the stimulus, pathogen, or toxin is resolved.
This sequence of activation requires multiple steps that include the binding of the antigen, followed by the stimulation of Bruton tyrosine kinase (BTK), which allows the B cell to proliferate. This then signals PLCγ2 proteins, which convert the signal to the chemical messenger Ca+/PKC protein. This then relays this information to increase cell proliferation and activates NF-κB, increasing DNA replication and expression, ultimately resulting in the conversion of normal B cells to plasma cells, which produce antibodies.
The normal process is unidirectional: B-cell stimulation creates antibodies to an antigen. In CLL, it becomes bidirectional!
The CLL cells send signals that alter the TME, and the altered TME then sends signals back through the BCR to support its proliferation and survival. The TME produces CXCL12 and CXCL13, which recruit B cells; B-cell activating factor (BAFF), to promote survival; and CD40, to increase B-cell activation.
These interactions, which include other TME pathways, encourage survival, proliferation, resistance to apoptosis, and spread to areas such as the lymph nodes and spleen, our largest lymphoid organ.
Initially, a CLL patient can start with one original clone, but with ongoing divisions, additional genetic changes can occur, with a transition into different subclones over time. Some may be slow-growing, others have strong survival, and still others may not be removed when DNA damage occurs. Therapy, as well, has the potential to change the predominant form to a different form.
However, ultimately, it is the microenvironment that selects the subclones that will survive, influencing treatment options and length of survival.
Diagnostic Testing
Flow Cytometry
A specific microscopic cancer diagnosis reveals the type of cells but few of their individual characteristics.
Flow cytometry is a technological method, using blood or tissue, to support both diagnosis and analysis of cancer cell characteristics. This assists in monitoring disease aggressiveness in solid tumors and is the standard for diagnosing blood and bone marrow cancers, the leukemias and lymphomas.
It can also assess specific immune cell populations such as T cells, B cells, and marrow cells, and their subsets, based on their surface markers. The information provided can reveal immune strength and health and their functional responses of activation, proliferation, and cytokine production.
By identifying cancer-specific cell types and their biologic characteristics, oncologists can be guided toward therapies that will be effective and individualized to each patient. This data can also be used for comparison of pre- and post-treatment effectiveness and can help determine how well treatment is working or whether there is a need for change.
Flow cytometry testing allows thousands of cells to be placed in a fluid, collected, sorted rapidly, and analyzed. They are then labeled with fluorescent dyes or proteins that attach to cell-marker antigens on the cell surface. The cells are then subjected to optical lasers, causing the signal emissions that can be detected, measured, and processed by computer software to determine multiple cancer-specific cellular properties simultaneously.
Cytometry also allows the identification of the immune components present, the physical and chemical characteristics of each cell subtype, levels of immune activity, growth and replication, and functional qualities such as cytokine activity and response to infection.
Fluorescence in Situ Hybridization (FISH Test)
Alterations in the cellular DNA of cancer cells, whether genetic or from alterations in DNA expression, often caused by toxic exposures, can change cellular pathways, creating abnormal protein production that affects cancer growth and replication.
FISH is genetic testing that can recognize chromosomal DNA abnormalities by allowing direct visualization of the genes. It maps genetic material, identifying mutations, missing or extra gene copies, or rearrangements of different genetic areas.
To perform the test, tissue or blood samples are obtained, to which a fluorescent compound is bound to the cancer cell’s DNA sequences. The compound absorbs light at one wavelength but, when activated, emits a longer wavelength of light that, when viewed against a dark background, allows structures to be visualized by a pathologist under an optical fluorescent microscope.
Identifying specific gene mutations or chromosome abnormalities can assist in cancer diagnosis and treatment. It is used in various cancers, including breast, chronic lymphocytic leukemia (CLL), multiple myeloma, glioma, bladder cancer, acute and chronic myeloid leukemia, and melanoma. In breast cancer, it assesses HER2 gene status, and in CLL, TP53 mutations, both of which can provide direction for more effective treatment options.
For prognosis, FISH can identify genetic patterns that can predict how cancer is expected to behave, as well as provide information as to how a given tumor will respond to specific therapies or whether the cells might be resistant.
Treatments
BTK inhibitors are targeted drugs that block an important signaling enzyme called Bruton tyrosine kinase (BTK). BTK is an integral part of the B-cell receptor (BCR), which messages the B cell to proliferate, spread, and survive.
Inhibiting BTK
Affect CLL cells, moving them from their tumor environment in lymph nodes and the bone marrow, where they are protected and can manipulate immune responses. Moving them into the peripheral blood removes this shield of its supportive environment, removing the signaling that allows their continued growth and spread.
If the CLL cells have a CD20 receptor on their surface, moving them into the bloodstream allows recognition by therapies using monoclonal antibodies that can recognize the cell as abnormal and allow the immune system to target it for removal.
The drugs currently available for therapy are:
Disruption of BTK signaling
Zanubrutinib (Brukinsa) disrupts BCR signaling.
Acalabrutinib (Calquence) disrupts BCR signaling and mobilizes cells from lymphoid tissue.
Ibrutinib (Imbruvica) disrupts stromal/T-cell interaction in the microenvironment and interferes with spread by affecting migration of cells between the bone marrow and blood.
Pirtobrutinib (Jaypirca) blocks BTK signaling.
Venetoclax (Venclyxto), when cells are removed from their protected environment, helps overcome apoptosis resistance.
Common Side Effects
These drugs do differ in their side-effect profiles, but uniformly they can cause fatigue, increased risk of infection, GI side effects, bruising, and musculoskeletal problems.
Other Therapies
Rituximab (Rituxan)/Obinutuzumab (Gazyva) target the CD20 receptor and help the immune system eliminate the abnormal cells.
Side effects include increased risk of upper respiratory infections, infusion reactions, reduction in blood counts, and allergic reactions.
PI3K inhibitors interfere with PI3K signaling, which is part of the PI3K/AKT/mTOR pathway that regulates cell growth, proliferation, and survival. These drugs are less frequently used due to toxicity and the success of BTK drugs.