Human Herpesvirus Family Testing

2. HHV Life Cycle - Latent Infection

The infection process of HHV typically includes several stages.

• Initially, the virus enters the human body through various routes, such as mucosal surfaces, broken skin, or blood transfusion. Once inside, it rapidly infects nearby cells and utilizes the host cell's mechanisms to replicate. During replication, the virus releases its genome, which is recognized by the host cell's pattern recognition receptors (PRRs) such as Toll-like receptors (TLRs), C-type lectin receptors (CLRs), NOD-like receptors (NLRs), and RIG-I-like receptors (RLRs). These receptors identify pathogen-associated molecular patterns (PAMPs) of the virus, triggering an innate immune response. This immune response leads to the release of various cytokines and inflammatory mediators, limiting viral spread and initiating adaptive immunity.
• The latency and reactivation properties of HHV allow it to persist in the host body long-term, posing an ongoing challenge to the immune system. The virus does not remain active continuously but instead establishes latent infection in specific cells, where its genome exists freely in the nucleus, and viral protein expression is suppressed to evade immune surveillance. When the host's immune function declines or under certain stimuli, the virus reactivates, initiating another round of replication and dissemination. During reactivation, the virus once again triggers a strong immune response, but the immune system often fails to completely eliminate the virus, leading to persistent infection.
• HHV's immune evasion strategies play a crucial role in its successful infection and long-term persistence in the host. Studies have shown that HHV can interfere with host immune responses through various mechanisms, such as encoding proteins that inhibit cytokine production or immune signaling pathways, producing miRNA to regulate host gene expression, and suppressing immune cell activation and proliferation. These complex immune evasion mechanisms enable HHV to survive in the host for extended periods and reactivate under favorable conditions, causing disease.

4. Focus on HHV Vaccines/Therapies

Varicella-Zoster Virus (VZV)

• The WHO recommends routine childhood varicella vaccination in countries where the disease is a significant public health and socioeconomic concern and where high (85-90%) and sustained vaccine coverage can be achieved.
• In the U.S., VZV-related morbidity and mortality rates have declined following vaccine implementation. The current recommendation is routine vaccination at 12-15 months and for all individuals aged 13 years and older without evidence of immunity.
• In Europe, varicella vaccination policies vary, with most countries not following the WHO routine vaccination recommendation.

Cytomegalovirus (CMV) Vaccine

The goal is to prevent congenital CMV infection and its complications. CMV vaccine development has been a high priority since 2001. A Phase II trial of CMV glycoprotein B vaccine with MF59 adjuvant showed reduced maternal and congenital infections but requires further validation for safety and efficacy.