The Epstein-Barr virus (EBV) infects a majority of the global population, often without symptoms. Histology, the microscopic examination of tissue, allows pathologists to identify the characteristic changes EBV causes within cells. This analysis is used to understand how the virus interacts with the body, from temporary infections to its involvement in certain cancers. The visual evidence in tissue architecture and cell appearance reveals the virus’s presence and its effects.
Cellular Targets of the Epstein-Barr Virus
The Epstein-Barr virus primarily infects two main cell types: B-lymphocytes and epithelial cells. Epithelial cells, which line surfaces like the back of the throat and salivary glands, are where the infection often begins. The virus is actively produced in these cells and shed in saliva, allowing for transmission between individuals. From there, EBV infects B-lymphocytes (B-cells), a component of the immune system, often in nearby lymphoid tissues like the tonsils. This allows the virus to establish a persistent presence and spread throughout the body.
Once inside B-cells, EBV can enter a state of latency, remaining dormant within the cell for the lifetime of the individual. During latency, the virus hides from the immune system but can reactivate under certain conditions. This ability to establish a lifelong, persistent infection is central to the various conditions associated with EBV.
General Histological Features of EBV Infection
During an active EBV infection like infectious mononucleosis, a peripheral blood smear will show atypical lymphocytes. These are not the infected B-cells but are reactive cytotoxic T-cells activated to fight the virus. These reactive cells, sometimes called Downey cells, are larger than normal lymphocytes, with abundant cytoplasm and a large, irregularly shaped nucleus.
This cellular reaction also occurs in lymphoid tissues like lymph nodes and the spleen, leading to lymphoid hyperplasia. This is a significant increase in the number of lymphocytes as the immune system mounts its response. The normal architecture of the lymph node can be partially obscured by this proliferation of immune cells.
In EBV-associated hepatitis, the histology involves a characteristic infiltration of lymphocytes into the liver. These cells can be seen arranged in a single-file line within the liver’s small blood vessels, a feature described as a “string of beads” pattern. This arrangement, along with a collection of lymphocytes in the portal tracts, is a hallmark of the liver’s reaction to the virus.
Histological Patterns in EBV-Associated Malignancies
In classical Hodgkin lymphoma, the diagnostic cell is the Hodgkin and Reed-Sternberg (HRS) cell. These are large, cancerous B-cells that stand out against a background of non-cancerous inflammatory cells. Classic HRS cells have an “owl-eye” appearance due to having at least two nuclei, each containing a large nucleolus. Variants include “mummified” cells with condensed cytoplasm and lacunar cells that appear to sit in an empty space.
Burkitt lymphoma presents a characteristic histological image known as the “starry sky” pattern. This appearance is created by a dense, uniform sheet of dark-staining, medium-sized cancer cells, representing the “night sky.” Scattered throughout this sheet are pale-staining benign macrophages that have engulfed dead tumor cell debris. These pale macrophages appear as the “stars” in the dark sky, creating a recognizable pattern.
Nasopharyngeal carcinoma (NPC) is characterized by large cancerous epithelial cells with indistinct borders that grow in connected sheets, a syncytial growth pattern. A defining feature of NPC histology is that these cancer cell sheets are heavily infiltrated by a dense population of non-cancerous lymphocytes. This mix of large, pale cancer cells and small, dark lymphocytes creates a two-toned appearance.
In individuals with compromised immune systems, EBV can be linked to smooth muscle tumors. Histologically, these tumors are composed of spindle-shaped cells that are characteristic of smooth muscle. Confirming the virus’s presence within these tumor cells is part of the diagnosis in the context of immunodeficiency.
Confirmatory Staining Techniques in Histology
To confirm the presence of Epstein-Barr virus within tumor cells, pathologists use specialized staining techniques. The most widely used method is in situ hybridization (ISH), which looks for EBV-encoded small RNAs (EBERs). Because EBERs are abundantly present in the nucleus of every latently infected cell, they serve as an excellent marker. The EBER-ISH test uses a labeled probe that binds to these RNA molecules, causing the nucleus of infected cells to appear dark blue or black.
Another method is immunohistochemistry (IHC), which detects specific viral proteins like Latent Membrane Protein 1 (LMP1). This protein is produced by the virus and embedded in the membrane of the infected cell. The IHC technique uses antibodies that bind to the LMP1 protein, producing a brown stain that highlights the cells expressing this oncoprotein.
While EBER-ISH is considered the most sensitive test, IHC for proteins like LMP1 provides complementary information on viral activity. Pathologists often use these techniques together to build a complete picture. This visual confirmation is a standard part of the diagnostic process for EBV-associated malignancies.