Herpes simplex virus is a masterwork of biological engineering, a pathogen that has co-evolved with humans for so long that it has developed an answer to nearly every defense the immune system throws at it. HSV comes in two types, HSV-1 and HSV-2, and together they infect billions of people worldwide. What makes them unusual among viruses is not just their ability to cause disease but their ability to hide, establishing a lifelong silent infection in nerve cells and periodically reawakening. Understanding how HSV is built, how it breaks into cells, how it goes quiet, and how it dodges immune surveillance reveals one of the most sophisticated survival strategies in virology.
The Anatomy of a Herpes Virus Particle
An HSV particle, called a virion, is built in layers. At its core sits the double-stranded DNA genome, organized into regularly spaced concentric rings inside a protein shell called the capsid, a bit like thread wound on a spool. The spacing between DNA layers is roughly 26 angstroms, a packing arrangement that resembles how some bacterial viruses store their DNA.1PubMed Central. Visualization of tegument-capsid interactions and DNA in intact herpes simplex virus type 1 virions The capsid itself is an icosahedral shell, a roughly spherical shape assembled from repeating protein units. When the capsid is mature and loaded with DNA, the channel at each of its twelve vertices is sealed shut, unlike the open channels seen in immature capsids still inside the cell nucleus.
Surrounding the capsid is a protein layer called the tegument, which is largely disordered. Most of the tegument does not follow the neat geometric pattern of the capsid. However, at each of the capsid’s twelve vertices, five copies of a structure called CATC (capsid-associated tegument complex) form a crown. High-resolution imaging has shown that each CATC copy is a bundle of tegument proteins, including pUL17, pUL25, and pUL36, arranged so their flexible arms can participate in moving the capsid out of the nucleus and along nerve cell projections.2PubMed Central. Structure of the herpes simplex virus 1 capsid with associated tegument protein complexes Outside the tegument sits the envelope, a lipid membrane studded with glycoproteins that handle the critical job of attaching to and fusing with host cells.
How the Virus Enters a Cell
Getting inside a host cell is a multi-step process that requires teamwork among at least four glycoproteins on the viral envelope: gD, gB, gH, and gL.3PubMed Central. Potential nectin-1 binding site on herpes simplex virus glycoprotein d. The process starts when gD latches onto a receptor on the cell surface. HSV can use more than one receptor, but the two most important are nectin-1, a protein found on many cell types including neurons and epithelial cells, and HVEM, a molecule involved in immune signaling. Structural studies have mapped the contact site between gD and nectin-1 in detail, showing that gD grabs onto a specific face of nectin-1’s outermost domain using extensions from its own structure that wrap around the receptor like fingers.4PLoS Pathogens. Structure of Herpes Simplex Virus Glycoprotein D Bound to the Human Receptor Nectin-1
Once gD binds its receptor, it changes shape in a way that signals the other glycoproteins to act. The gH-gL pair and gB then execute the actual membrane fusion, merging the viral envelope with the host cell membrane and dumping the capsid and tegument into the cell’s interior. The exact route of entry varies by cell type. In some cells, the virus fuses directly at the plasma membrane; in others, it is first taken up into an internal compartment before fusing from within.5PubMed Central. Viral entry mechanisms: cellular and viral mediators of herpes simplex virus entry This flexibility in entry route is part of what allows HSV to infect such a wide range of tissues, from skin and mucous membranes to neurons and corneal cells.
Riding the Rails to the Nucleus
Once the capsid is free inside the cell, it needs to reach the nucleus, where the viral DNA will be injected through a nuclear pore and begin directing the cell to make new viruses. In many cell types, the distance from the cell edge to the nucleus is short enough that random diffusion might do the job. But in neurons, the capsid can be centimeters away from the cell body, a vast distance at the molecular scale. HSV solves this problem by hijacking the cell’s own transport system.
Cells maintain a network of protein filaments called microtubules that act like railroad tracks for moving cargo. Motor proteins walk along these tracks carrying cellular freight. After entry, HSV capsids latch onto a motor protein called dynein, which carries cargo toward the minus end of microtubules, meaning toward the cell center and nucleus. Experiments blocking dynein or its helper complex, dynactin, showed that capsid transport to the nucleus stalls without them.6PubMed Central. Function of dynein and dynactin in herpes simplex virus capsid transport In neurons, this same dynein-driven transport carries capsids from the nerve ending all the way back to the cell body in the sensory ganglion, a journey that is essential for the virus to set up its long-term hiding spot.
Establishing Latency in Neurons
Latency is the defining trick of herpesvirus biology. After initial infection of skin or mucosal surfaces, HSV travels up sensory nerve fibers and reaches clusters of nerve cell bodies called ganglia, most commonly the trigeminal ganglia (which serve the face) for HSV-1 and the sacral ganglia (which serve the genital region) for HSV-2. In these neurons, rather than replicating and killing the cell, the virus goes silent. Its DNA persists in the nucleus as a circular molecule, not stitched into the cell’s chromosomes but maintained as a separate loop called an episome.7PubMed Central. Epigenetic regulation of latent HSV-1 gene expression
The silencing of viral genes during latency is controlled by how the DNA is packaged. The viral genome wraps around histone proteins, the same spools that the cell uses to organize its own DNA. Genes needed for active replication get loaded with chemical tags on their histones that signal “keep quiet,” while the one region of the genome that remains active during latency, the latency-associated transcript (LAT), carries different tags that allow it to be read.7PubMed Central. Epigenetic regulation of latent HSV-1 gene expression Neurons themselves contribute to this silencing. A small RNA molecule called miR-9, which is abundant in neurons, has been shown to increase the deposition of repressive histone marks on the promoters of key viral genes, helping to keep the virus locked down.8Nature Communications. Neuronal miR-9 promotes HSV-1 epigenetic silencing and latency by repressing Oct-1 and Onecut family genes
The LAT is not just a bystander in this process. It actively protects the neuron from dying. Neurons infected with HSV would normally trigger their own self-destruction through a process called apoptosis, which is the cell’s way of sacrificing itself to limit viral spread. LAT blocks apoptosis through multiple routes: it encodes a small RNA (a microRNA) that dials down pro-death signals in the TGF-beta pathway,9Nature. Anti-apoptotic function of a microRNA encoded by the HSV-1 latency-associated transcript and it also manipulates components of the interferon defense system to keep the neuron alive.10PubMed Central. The Latency-Associated Transcript Inhibits Apoptosis via Downregulation of Components of the Type I Interferon Pathway during Latent Herpes Simplex Virus 1 Ocular Infection The result is a neuron that survives indefinitely, carrying a dormant viral genome that the immune system cannot easily detect or reach. This is why herpes infection is lifelong.
What Wakes the Virus Up
Reactivation, the shift from silent latency back to active virus production, has long been linked to stress, fever, UV sunlight exposure, and immune suppression. The molecular mechanism connecting these triggers to viral gene activation runs through a stress-signaling pathway in the neuron. During latency, a signaling cascade called the PI3K/Akt pathway keeps viral genes repressed. When that pathway is disrupted, for instance by cellular stress, the JNK stress kinase becomes active. JNK activation is both necessary and sufficient to flip the switch: blocking JNK with chemical inhibitors completely prevents reactivation regardless of the trigger, whether it is a stress hormone like dexamethasone or direct disruption of the PI3K pathway.11PubMed Central. Neuronal Stress Pathway Mediating a Histone Methyl/Phospho Switch Is Required for Herpes Simplex Virus Reactivation
JNK works by creating a “methyl-phospho switch” on viral gene promoters. The repressive methyl marks on histones that keep viral genes quiet do not need to be removed first. Instead, JNK causes a phosphate group to be added to a neighboring position on the same histone, and this phosphorylation overrides the silencing signal, allowing the genes to be read even while the “keep quiet” tag is still technically present.11PubMed Central. Neuronal Stress Pathway Mediating a Histone Methyl/Phospho Switch Is Required for Herpes Simplex Virus Reactivation Glucocorticoid receptor signaling, which is activated by stress hormones like cortisol, also plays a role: it can speed up both viral replication and reactivation from latency.12PubMed Central. Human alpha-herpesvirus 1 (HSV-1) viral replication and reactivation from latency are expedited by the glucocorticoid receptor
Once reactivated, newly assembled viral particles travel back down the nerve fiber to the skin or mucous membrane, a journey that requires a different motor protein, kinesin, which moves cargo in the opposite direction along microtubules. This anterograde transport is what causes the recurrent sores at or near the site of original infection.13PubMed Central. Fast anterograde transport of herpes simplex virus: role for the amyloid precursor protein of alzheimer’s disease The virus essentially retraces its steps, emerging at the body surface to shed and potentially infect new hosts.
A Toolkit for Dodging the Immune System
HSV does not simply hide during latency and hope for the best. It carries an extensive collection of proteins dedicated to actively dismantling immune defenses during active infection. These evasion strategies operate at several levels simultaneously.
Blocking Antigen Presentation
One of the first things the immune system tries to do when a cell is infected is display fragments of viral proteins on the cell surface using molecules called MHC class I. This flags the cell for killing by cytotoxic T cells. HSV produces a protein called ICP47 very early after infection that jams a transporter called TAP, which is responsible for shuttling viral protein fragments into the compartment where they would be loaded onto MHC class I molecules. Without TAP functioning, viral fragments never reach the cell surface, and killer T cells cannot recognize the infected cell.14PubMed Central. Molecular mechanism and species specificity of TAP inhibition by herpes simplex virus ICP47
Shutting Down Host Gene Expression
The virus brings a pre-made weapon in its tegument: a protein called vhs (virion host shutoff), which is an enzyme that chops up messenger RNA. As soon as the tegument is released into the cell, vhs begins degrading the cell’s mRNAs, crippling the cell’s ability to produce proteins, including proteins involved in calling for immune help.15PubMed Central. The herpes simplex virus vhs protein induces endoribonucleolytic cleavage of target RNAs in cell extracts The same vhs protein also degrades double-stranded RNA, which cells produce as a byproduct of infection and which normally triggers potent antiviral alarms. By accelerating the breakdown of these danger signals, vhs keeps the cell from sounding the alarm effectively.16PLoS Pathogens. The herpes simplex virus host shutoff (vhs) RNase limits accumulation of double stranded RNA in infected cells: Evidence for accelerated decay of duplex RNA
Disabling DNA Sensing
Cells have an internal alarm system that detects foreign DNA in the cytoplasm. A sensor protein called cGAS recognizes viral DNA and activates a signaling chain through STING that leads to the production of interferons, powerful antiviral molecules. HSV fights back on multiple fronts. The vhs protein reduces the amount of cGAS available, effectively blinding the cell to the presence of viral DNA.17PubMed Central. Herpes Simplex Virus 1 Abrogates the cGAS/STING-Mediated Cytosolic DNA-Sensing Pathway via Its Virion Host Shutoff Protein, UL41 Another tegument protein, VP11/12 (encoded by UL46), eliminates both STING and another DNA sensor called IFI16, blocking the entire signaling pathway from a different angle.18PubMed Central. Evasion of the STING DNA-Sensing Pathway by VP11/12 of Herpes Simplex Virus 1
Hijacking Antibodies
Even antibodies, often thought of as the immune system’s precision-guided missiles, are neutralized by HSV. The virus expresses a glycoprotein called gE that, together with gI, forms an Fc receptor on the surface of infected cells. In normal immune function, the “Fc” tail of an antibody sticks out after the antibody binds its target, and that exposed tail recruits immune cells and complement proteins to destroy the target. The gE-gI receptor grabs the Fc tail of antibodies that have bound viral proteins on the cell surface, creating a “bipolar bridge” where both ends of the antibody are held down. This blocks complement activation and prevents immune cells from recognizing the antibody-coated cell.19PubMed Central. The herpes simplex virus 1 IgG fc receptor blocks antibody-mediated complement activation and antibody-dependent cellular cytotoxicity in vivo Going further, the gE-gI receptor can actually internalize viral antigens from the cell surface by pulling them inside along with the bridged antibody, stripping the infected cell of the very targets the immune system is trying to attack.20PLoS Pathogens. The Herpes Virus Fc Receptor gE-gI Mediates Antibody Bipolar Bridging to Clear Viral Antigens from the Cell Surface
Blocking Autophagy
Autophagy is a cellular cleanup process that can engulf and destroy pathogens. It also plays a role in loading viral fragments onto immune molecules for presentation to T cells. HSV-1 produces a protein called ICP34.5 that binds directly to Beclin 1, a key autophagy protein, and shuts the process down.21PubMed. HSV-1 ICP34.5 confers neurovirulence by targeting the Beclin 1 autophagy protein This is not just a minor evasion tactic: mutant viruses lacking the Beclin 1-binding region of ICP34.5 are dramatically less virulent in the brain, and they trigger stronger T cell responses because autophagy-dependent antigen presentation can proceed normally.22PubMed Central. Interaction of ICP34.5 with Beclin 1 modulates herpes simplex virus type 1 pathogenesis through control of CD4+ T-cell responses
HSV-1 and HSV-2 Are Close Relatives With Key Differences
The two herpes simplex types share a nearly identical set of 74 protein-coding genes spread across genomes of roughly similar size (HSV-2 strain HG52 is about 154,746 base pairs with a high GC content of around 70%).23PubMed Central. The genome sequence of herpes simplex virus type 2 Their gene sets correspond so closely that sequencing HSV-2 actually helped researchers correct errors in the published HSV-1 sequence. Yet despite this similarity, the two types behave differently in some instructive ways.
One example involves glycoprotein C (gC), the surface protein that in HSV-1 is important for the initial attachment of the virus to cells. When researchers deleted gC from HSV-2, the mutant virus showed no loss in its ability to bind cells or infect them, a stark contrast to gC-deletion mutants in HSV-1, which show reduced binding and slower cell penetration. The main role of gC in HSV-2 appears to be protecting the virus from complement-mediated destruction rather than facilitating entry.24PubMed. Differences in the role of glycoprotein C of HSV-1 and HSV-2 in viral binding may contribute to serotype differences in cell tropism These subtle differences in how shared proteins are used may help explain why the two types prefer different anatomical sites and show different patterns of reactivation frequency.
From an evolutionary standpoint, the two viruses arrived in humans by different routes. HSV-1 appears to have co-evolved with the human lineage over millions of years, diverging alongside our primate ancestors. HSV-2, by contrast, jumped into a human ancestor from the ancestor of modern chimpanzees roughly 1.6 million years ago, a cross-species transmission event that left its mark in the viral genome.25PubMed Central. Evolutionary origins of human herpes simplex viruses 1 and 2
Why Current Drugs Work and Sometimes Fail
The standard treatment for HSV infections, acyclovir and its derivatives, works by targeting the viral enzyme thymidine kinase. The drug is a nucleoside analog that the viral thymidine kinase converts into its active form, which then gets incorporated into the growing viral DNA chain and terminates it. Because human cells lack this particular enzyme, the drug is selectively toxic to virus-infected cells. Resistance arises when mutations in the viral thymidine kinase gene prevent the enzyme from activating acyclovir. Computational studies of specific resistance-conferring mutations have shown that amino acid changes at key positions in the enzyme distort the binding site so that acyclovir can no longer be positioned correctly for the phosphorylation reaction that would activate it.26PubMed. Mechanism of resistance to acyclovir in thymidine kinase mutants from Herpes simplex virus type 1: a computational approach Acyclovir resistance is rare in people with healthy immune systems but is a real clinical problem in immunocompromised patients who take the drug for long periods.
Crucially, acyclovir and related drugs only work against actively replicating virus. They have no effect on the latent genome sitting silently in neurons, which is why they can shorten outbreaks and reduce shedding but cannot cure the infection.
Vaccines and Gene Editing on the Horizon
The evasion strategies described above have directly shaped vaccine design. The most promising experimental vaccines do not target just the entry machinery; they also target the very proteins HSV uses to dodge immunity. A trivalent approach combining glycoproteins gC, gD, and gE, chosen because gD mediates entry while gC blocks complement and gE blocks antibody function, has shown strong results in animal models. Both protein-based and mRNA-based versions of this trivalent vaccine completely prevented genital lesions in mice and guinea pigs.27PubMed Central. Nucleoside-modified mRNA encoding HSV-2 glycoproteins C, D, and E prevents clinical and subclinical genital herpes An HSV-2-based trivalent mRNA vaccine also protected mice against HSV-1 genital infection, with the mRNA formulation producing better viral clearance than the protein version.28PLoS Pathogens. An HSV-2 nucleoside-modified mRNA genital herpes vaccine containing glycoproteins gC, gD, and gE protects mice against HSV-1 genital lesions and latent infection None of these have reached advanced human trials yet, but the logic of targeting immune evasion molecules alongside entry receptors represents a shift in thinking from earlier vaccine candidates that focused on gD alone and failed in clinical trials.
Gene editing offers a fundamentally different approach: rather than boosting the immune response, it aims to destroy the latent viral genome directly. Researchers have used CRISPR-Cas9 delivered via a viral vector to cut the HSV-1 genome at genes essential for reactivation (ICP0 and ICP27). In three-dimensional brain organoids, a model that mimics latent infection in human tissue, the treatment cut viral DNA loads by roughly a thousandfold in the culture fluid and showed evidence of genome excision in the majority of treated samples.29Molecular Therapy – Nucleic Acids. Suppression of HSV-1 infection and viral reactivation by CRISPR-Cas9 gene editing in 2D and 3D culture models This work is still early-stage, and delivering gene-editing tools specifically to latently infected neurons in a living person remains a major hurdle. But it is the only strategy currently being explored that could, in principle, eliminate the latent reservoir rather than just manage outbreaks.
HSV and the Eye
Herpes simplex keratitis, infection of the cornea, is worth understanding separately because the damage it causes is driven as much by the host’s own immune response as by the virus itself. When HSV infects the cornea, the inflammatory cascade triggered by the immune system clearing the virus leads to immune cell infiltration, growth of new blood vessels into tissue that is normally avascular, and loss of corneal nerves.30PubMed Central. Pathogenesis of herpes simplex keratitis: The host cell response and ocular surface sequelae to infection and inflammation Progressive scarring from repeated episodes of reactivation and inflammation is one of the leading infectious causes of blindness worldwide. Treatment involves antivirals to control the virus combined with careful use of anti-inflammatory drugs to limit immune-mediated damage, a balancing act that underscores the paradox at the heart of HSV pathology: the immune response is both the primary defense and a major cause of tissue injury.