What Are CD24 B Cells and What Is Their Function?

B cells are a part of the immune system responsible for producing antibodies. Scientists identify specific B cells based on molecules on their surface, and one such molecule is CD24. The term “CD24 B cells” refers to B cells at particular stages of development or activation that have this protein. Understanding the role of CD24 provides insight into how immune responses are controlled and how they can malfunction in disease.

Understanding B Cells and the CD24 Protein

B cells are white blood cells that originate in the bone marrow and are a component of the adaptive immune system. Their most recognized function is to become plasma cells, which secrete large quantities of antibodies to neutralize threats. Another subset of B cells becomes memory cells, which “remember” a pathogen for a faster response upon future exposure.

The CD24 protein is a small molecule attached to the outer membrane of various cells, including B cells. It is tethered to the cell surface in a way that allows it to move and cluster in specific areas. This mobility helps it interact with other signaling molecules.

The amount of CD24 on a B cell’s surface is not constant, changing as the cell matures and becomes activated. Immature B cells have high levels of CD24, which decrease as the cells fully mature. This dynamic expression pattern makes CD24 a useful biomarker for identifying B cells at different developmental stages.

Key Functions of CD24 on B Cells

The CD24 protein on B cells helps regulate their activity. Its primary role is inhibitory, controlling the intensity and duration of an immune response. When B cells are activated, CD24 acts as a brake to prevent an excessive reaction that could harm the body’s tissues. This regulation ensures the immune response is strong enough to clear a pathogen without causing unnecessary inflammation.

CD24 exerts its influence by interacting with other proteins on immune cell surfaces. It binds to inhibitory receptors known as Siglecs, specifically Siglec-10 in humans. This interaction sends signals into the cell that dampen activation pathways, slowing the B cell’s proliferation and differentiation. This mechanism is important in a specialized subset of B cells called B regulatory cells (Bregs).

Bregs are B cells whose main purpose is to suppress immune responses, which maintains tolerance and prevents autoimmunity. These cells have high levels of both CD24 and another marker, CD38. The CD24 on these Bregs contributes to their suppressive function, helping to terminate an immune response or prevent the activation of self-reactive cells.

CD24 is also involved in apoptosis, or programmed cell death. This process is a normal part of a B cell’s lifecycle, eliminating unneeded or potentially harmful cells. Engagement of the CD24 molecule can trigger internal signals that lead to the cell’s death, serving as another layer of immune control.

Role of CD24 in B Cell Development

A B cell passes through several developmental stages, and CD24 expression acts as a guidepost for its maturation status. Its levels are high on transitional B cells, which are immature cells that have migrated from the bone marrow to the spleen. In the spleen, these cells complete their development.

The transitional phase is a checkpoint where B cells are tested for their potential to react against the body’s own tissues. This process, called negative selection, eliminates self-reactive B cells to prevent autoimmune diseases. CD24 plays a part in this selection by influencing the survival signals these developing cells receive.

Self-reactive B cells may receive a death signal through their B cell receptor. The high level of CD24 on these cells modulates this signal, contributing to the decision of whether the cell lives or dies. By controlling B cell survival at this checkpoint, CD24 helps ensure that only properly functioning B cells complete their maturation.

CD24 B Cells in Disease

Alterations in the number or function of CD24-expressing B cells are implicated in various diseases. In autoimmune diseases like multiple sclerosis (MS) and systemic lupus erythematosus (SLE), the immune system’s regulatory balance is disturbed. In some of these conditions, the function of suppressive CD24-high B regulatory cells is impaired, contributing to the body’s attack on its own tissues.

This failure of regulation allows self-reactive B cells to survive and produce autoantibodies or promote inflammation. Because of this, the levels of specific CD24-expressing B cell populations can be studied as indicators of disease activity or to understand the immune dysfunction.

The role of CD24 also extends to cancer. Although CD24 is a normal marker on developing B cells, it is also expressed on many cancer cells, including B-cell lymphomas. In the tumor microenvironment, CD24 on cancer cells can interact with the Siglec-10 receptor on immune cells. This interaction delivers a “don’t eat me” signal, allowing cancer cells to evade the immune system.

Future Directions in CD24 B Cell Research

The study of CD24 on B cells is opening new possibilities for diagnosing and treating diseases. Researchers are working to understand the signaling pathways that CD24 controls to harness its potential for therapeutic intervention. For example, understanding how CD24 promotes the function of B regulatory cells could lead to new strategies for treating autoimmune diseases.

A promising area of research is the development of therapies targeting CD24-expressing cells. In autoimmune diseases, treatments could boost the number or function of CD24-high B regulatory cells to restore immune balance. Conversely, in cancer, the goal is to block the inhibitory signals from CD24 on tumor cells, and therapies using monoclonal antibodies are being explored for this purpose.

CD24 may also serve as a biomarker. Monitoring the levels of specific CD24 B cell populations in patients could help diagnose certain diseases, predict their course, or evaluate treatment effectiveness. The manipulation of the CD24 pathway may become a refined tool against immunological disorders and cancers.