CD68 is a specific protein found in the human body, acting as a distinguishing marker on certain cells. It is a transmembrane glycoprotein that belongs to the Cluster of Differentiation (CD) nomenclature system. This system classifies different types of white blood cells based on unique markers on their cell surfaces, much like a unique uniform identifies a particular type of worker. Understanding this marker assists scientists and medical professionals in identifying and studying the presence and distribution of these cells throughout various tissues, offering valuable insights into health and disease processes.
Which Cells Have the CD68 Marker?
The CD68 marker is predominantly found on monocytes and macrophages, which are specialized immune cells. These cells originate in the bone marrow, circulate in the bloodstream as monocytes, and then migrate into various tissues where they mature into diverse macrophage populations. Macrophages are often described as the body’s “scavenger cells” or “cleanup crew,” engulfing and digesting cellular debris, dead cells, foreign particles, and invading pathogens. Their widespread presence allows them to perform specialized roles in organs such as the spleen, liver (Kupffer cells), brain (microglia), and lungs (alveolar macrophages).
While macrophages are the primary expressers of CD68, the marker is also found, to a lesser extent, on other immune cells. These include dendritic cells, which are important for initiating adaptive immune responses by presenting antigens to T cells. Additionally, some peripheral blood granulocytes, such as neutrophils and basophils, can show some expression of CD68.
The Biological Function of CD68
The CD68 protein is involved in the function of lysosomes and endosomes. Lysosomes are cellular organelles that serve as “recycling and waste disposal centers,” breaking down various macromolecules, cellular debris, and foreign substances. CD68 is primarily localized within these lysosomal and endosomal compartments inside monocytes and macrophages, supporting the efficient processing and degradation of materials taken up by the cell. A smaller fraction of the protein also circulates to the cell surface, allowing for extracellular interactions.
The protein plays a direct and significant role in phagocytosis, the process by which macrophages engulf and consume particles. CD68 is also recognized as a member of the scavenger receptor family, which binds and internalizes a wide range of molecules, including oxidized low-density lipoproteins and apoptotic cells. These receptors contribute to functions like clearing cellular debris, promoting the engulfment of foreign substances, and mediating the recruitment and activation of macrophages to sites of inflammation or injury.
CD68 as a Diagnostic Tool
Scientists and pathologists frequently use the CD68 marker to identify macrophages in tissue samples, providing insights into various health conditions. This is often achieved through immunohistochemistry (IHC), a laboratory technique where specific antibodies bind to the CD68 protein in thin tissue sections. These antibodies are then visualized, typically through a color reaction, making the CD68-positive cells apparent under a microscope, allowing for precise localization. A “CD68-positive” result confirms the presence of cells expressing this marker, indicating macrophage infiltration.
This diagnostic application is valuable in identifying conditions characterized by significant inflammation, where macrophages accumulate. For example, it helps diagnose chronic inflammatory diseases or certain infections. CD68 staining can also identify early signs of conditions like atherosclerosis, where CD68-positive cells can serve as an early marker of graft occlusion. In neurological contexts, CD68 upregulation in microglia has been observed in amyotrophic lateral sclerosis brains. In oncology, CD68 serves as a marker for identifying tumor-associated macrophages (TAMs) within cancerous growths. Their presence can offer insights into the tumor microenvironment and assist in classifying certain types of lymphomas or histiocytic disorders, such as aggressive non-Hodgkin lymphoma.
What CD68 Levels Indicate About Disease
Beyond simply identifying macrophages, the quantity or density of CD68-positive cells within a tissue can provide information about a disease’s characteristics or likely progression, often referred to as its prognosis. In some cancers, such as Hodgkin lymphoma, a higher number of tumor-associated macrophages (TAMs), identified by CD68 staining, has been linked to a less favorable patient outlook. High CD68 expression has also been associated with glioma progression and can be used with other markers to predict early outcomes in classical Hodgkin lymphoma. This suggests that a dense infiltration of these macrophages may contribute to tumor growth, metastasis, or resistance to treatment.
However, interpreting the meaning of CD68 levels is complex and requires careful consideration by a pathologist within the specific clinical context. For example, in soft tissue sarcomas, increased CD68 expression correlated with a higher malignancy grade, but this did not consistently correlate with shorter disease-specific survival. This highlights that while macrophages are often associated with inflammation and disease progression, their role can be multifaceted; they can sometimes contribute to an anti-tumor immune response. Therefore, the prognostic significance of CD68 can vary depending on the type of cancer, its stage, the organ involved, and the overall immune environment, making a nuanced interpretation by experts necessary, often in conjunction with other markers.