Human Immunodeficiency Virus (HIV) is a retrovirus that attacks CD4+ T cells, crucial for fighting infections. Despite advancements, HIV establishes a hidden, inactive state (latency) within the body. This allows the virus to persist without actively replicating, posing a major challenge to eradication. The central question is whether this hidden virus can be detected, a topic with profound implications for developing a cure.
Understanding Latent HIV
Latent HIV refers to the virus that has integrated its proviral DNA into the DNA of resting memory CD4+ T cells. In this inactive state, the virus does not actively produce new particles, unlike active HIV replication where it continuously replicates. These inactive cells form a “latent reservoir” within the body. Cells in this reservoir do not produce viral proteins or particles, making them undetectable by the immune system and unaffected by current antiretroviral therapies (ART). The reservoir is composed mainly of long-lived resting memory CD4+ T cells, established early in infection. These cells can remain dormant for years, acting as a persistent viral source. If ART is stopped, these latently infected cells can reactivate and begin producing new virus, leading to a rapid rebound in viral load.
Detecting Active HIV
Detecting active HIV relies on identifying viral components or the body’s immune response. Common methods include antibody tests, antigen/antibody tests, and nucleic acid tests (NATs). Antibody tests look for antibodies the immune system produces in response to HIV, typically detectable 23 to 90 days after exposure. Most rapid tests and self-tests are antibody tests.
Antigen/antibody tests detect both HIV antibodies and a viral protein called p24 antigen, which appears earlier than antibodies, usually within 18 to 45 days after exposure. These tests are widely used in clinical settings. NATs (viral load tests) directly detect HIV genetic material (RNA) in the blood. NATs can identify HIV as early as 10 to 33 days after exposure and are highly sensitive, capable of detecting as few as 20 copies of HIV RNA per milliliter of blood. These tests are crucial for diagnosing acute infection and monitoring ART effectiveness by measuring active virus in the blood.
Challenges in Detecting Latent HIV
Detecting latent HIV presents significant challenges because these cells are “invisible” to standard diagnostic methods. Unlike active HIV, latent HIV integrates its DNA into the host cell’s genome but remains transcriptionally silent. This silence means latently infected cells do not actively produce new virus particles or products, rendering them undetectable by tests for active viral components. Standard tests for active HIV, such as antigen or viral load tests, are therefore ineffective at identifying these dormant cells.
One major difficulty stems from the extremely low number of latently infected cells (1 in 100,000 to 1 in 1,000,000 CD4+ T cells). This scarcity makes it challenging to isolate and identify them from the vast population of healthy cells. Furthermore, latently infected cells outwardly resemble healthy cells, lacking unique surface markers that could distinguish them from their uninfected counterparts. This absence of distinct identifiers means the immune system does not recognize and destroy them, allowing them to persist despite ART. The virus can persist in these cells for years, forming a stable reservoir in various body tissues, further complicating detection.
Research Efforts to Detect Latent HIV
Researchers are exploring various strategies to overcome the challenges of detecting the latent HIV reservoir. One approach involves viral outgrowth assays (VOAs), considered the gold standard for measuring replication-competent latent virus. In a VOA, resting CD4+ T cells are isolated and stimulated to reactivate latent virus. If the virus reactivates and replicates, it can be detected, indicating the presence of a replication-competent latent reservoir. While effective, VOAs are time-consuming, expensive, and require large blood samples.
Molecular assays represent another avenue, focusing on detecting HIV DNA or RNA within cells, even when the virus is inactive. Techniques like qPCR and ddPCR measure total or integrated HIV DNA within cells, estimating reservoir size. However, these methods can overestimate the functional reservoir as they detect all viral DNA, including defective proviruses that cannot reactivate. Researchers also investigate “shock and kill” and “block and lock” strategies, aiming to reactivate latent virus for elimination or permanently silence it.
The search for specific biomarkers on or within latently infected cells is an active area of research. Identifying unique markers would enable scientists to isolate and study these rare cells without reactivating the virus. Several cell surface molecules, including PD-1, CD2, CD20, and CD32a, are suggested as potential indicators of latently infected cells. While some markers show promise in enriching for HIV DNA, consistent and reliable markers distinguishing all latently infected cells from healthy ones remain elusive.
The Role of Latent HIV Detection in Eradication Strategies
Accurately detecting and quantifying the latent HIV reservoir is crucial for achieving a functional cure or eradication. It is the primary barrier to curing HIV, allowing persistence despite antiretroviral therapy (ART). Measuring the reservoir is necessary for monitoring new cure interventions in clinical trials. Precise tools are needed to determine if experimental therapies successfully reduce or eliminate these long-lived infected cells. Without reliable detection, confirming a cure before safely stopping ART is difficult. Mapping the reservoir’s locations and cellular makeup provides insights for targeted therapies aimed at eliminating or permanently silencing these persistent viral hiding places.