Anelloviruses are a unique group of small, circular, single-stranded DNA viruses. They are characterized by their compact genetic material, which ranges from approximately 2.0 to 3.9 kilobases in length. These viruses are considered a major component of the human virome, representing the vast collection of viruses that coexist with the human body. The first anellovirus, initially named Torque teno virus (TTV), was identified in 1997 in a Japanese patient experiencing hepatitis of unknown origin. While initially discovered in the context of illness, subsequent research has shown anelloviruses are widespread and generally not associated with disease in healthy individuals.
Section 2: The Ubiquitous Nature of Anelloviruses
Anelloviruses are widespread, infecting almost the entire human population. Most individuals acquire these viruses early in life, often within the first months, and carry them persistently throughout their lives. Their presence means they are considered a normal part of the human microbiome.
Transmission of anelloviruses occurs through various routes. Common pathways include exposure to bodily fluids such as saliva and respiratory droplets. Evidence also suggests transmission through blood, including via transfusions, and potentially through fecal-oral contact. The high prevalence in children from a young age further supports efficient person-to-person spread, likely through everyday contact and shared environments.
Section 3: Anelloviruses and the Immune System
Anelloviruses are non-pathogenic in healthy individuals. Their interaction with the human body offers a unique perspective on immune system activity. These viruses are kept in check by the host’s immune response, and their quantity, or “viral load,” in the bloodstream can serve as an indicator of immune function.
Higher levels of anelloviruses correlate with a less effective immune system. When the immune system is robust, it maintains a lower anellovirus load. Conversely, if the immune system is compromised or weakened, it may struggle to control viral replication, leading to an increase in anellovirus DNA levels. This makes anelloviruses a biomarker for assessing the overall state of a person’s immune response.
Section 4: Role in Health and Disease States
Anellovirus levels can change in various health and disease states, reflecting the status of the immune system. This makes them valuable as an indicator of an underlying immune issue. Researchers have observed correlations between anellovirus levels and specific clinical situations, including outcomes in transplant recipients, certain autoimmune conditions, and severe infections.
In organ transplant patients, who receive immunosuppressive medications to prevent rejection, anellovirus levels increase substantially after transplantation. A higher anellovirus load suggests a greater degree of immunosuppression, which is a desired effect of the medication. Conversely, lower anellovirus levels can signal insufficient immunosuppression, potentially increasing the risk of organ rejection. Monitoring anellovirus levels helps clinicians fine-tune medication dosages, aiming for a balance that prevents rejection while minimizing infection risk.
Beyond transplantation, altered anellovirus levels have been noted in individuals with autoimmune diseases and during severe infections. For instance, a decrease in anellovirus load has been observed during acute immune activation, with levels returning to baseline as the immune response normalizes. Anelloviruses serve as a dynamic marker, fluctuating in response to the body’s ongoing immune activity.
Section 5: Detection and Classification
Scientists primarily use molecular techniques, such as Polymerase Chain Reaction (PCR), to detect and quantify anelloviruses in biological samples. PCR methods amplify specific segments of the viral DNA, allowing for sensitive and accurate identification and measurement of the viral load. These techniques are refined to differentiate between the various types of anelloviruses present in a sample.
Anelloviruses exhibit extensive genetic diversity, leading to their classification into many species and strains. They are broadly grouped into several genera, with Torque teno virus (TTV) being the most studied type, belonging to the genus Alphatorquevirus. Other human-infecting genera include Betatorquevirus (Torque teno mini virus or TTMV) and Gammatorquevirus (Torque teno midi virus or TTMDV), each characterized by slight differences in genome size.