Host Response and Viral Immune Evasion

Interferons are a family of signalling proteins released by virus-infected cells, acting on neighbouring uninfected cells to induce an antiviral state before infection can spread to them. They are the first and most rapidly deployed specifically antiviral component of the innate immune system.

Type I interferons, interferon-alpha and interferon-beta, are produced by most infected cell types and induce expression of numerous antiviral effector proteins within neighbouring cells, including enzymes that degrade viral RNA and inhibit protein synthesis. Together these establish a broad, nonspecific antiviral state.

Type I interferon release is also the direct trigger for much of the systemic symptomatology, fever, myalgia, malaise, characteristic of acute viral illness. This means these classic viral symptoms substantially reflect the host's own interferon-driven response rather than direct tissue damage from the virus itself.

NK cells, natural killer cells, are part of the innate immune system and are capable of recognising and killing virus-infected cells without requiring prior antigen-specific sensitisation, unlike cytotoxic T cells.

NK cells are specifically triggered to kill by the absence or downregulation of MHC class I on a target cell's surface, a "missing self" recognition principle.

This gives NK cells a specific backup surveillance role, capable of detecting and eliminating exactly the cells that have successfully evaded cytotoxic T cell recognition via MHC downregulation.

Pattern recognition receptors, including Toll-like receptors and cytoplasmic sensors such as RIG-I, are host receptors that recognise conserved molecular structures characteristic of viral infection.

The two most important examples are double-stranded RNA, an intermediate produced during replication of many RNA viruses but not normally present in uninfected host cells, and unmethylated CpG DNA motifs, characteristic of many viral, though not host, DNA sequences.

Recognition of either triggers interferon production and broader innate inflammatory signalling.

Fever is a systemic response to infection, mediated substantially by interferon and other pro-inflammatory cytokine release, paralleling, though via a somewhat distinct specific mechanism, the endotoxin-driven fever of Gram negative bacterial sepsis. Modestly elevated temperature is believed to directly impair the replication efficiency of some viruses, in addition to its role as a clinically useful marker of active infection.

Inflammatory responses are the broader coordinated recruitment of innate immune cells and mediators to the site of infection, contributing both to viral containment and to a meaningful proportion of the tissue damage and symptomatic illness associated with viral infection.

Neutralising antibodies are antibodies, predominantly targeting envelope glycoproteins in enveloped viruses or capsid surface proteins in nonenveloped viruses, that bind viral surface proteins and directly block attachment or entry into host cells, preventing infection of new cells. They are the principal correlate of protection for most effective viral vaccines, which is why envelope and capsid glycoproteins are the dominant target of vaccine design across virology as a whole.

Cytotoxic T cells are CD8-positive T lymphocytes that recognise viral peptide fragments presented on infected cells' MHC class I molecules and, upon recognition, directly kill the infected cell before it can complete the viral replication cycle and release progeny virions. This is the principal adaptive mechanism for clearing infection from already-infected cells, a role neutralising antibody, which acts only extracellularly, cannot perform. It is simultaneously the principal source of adaptive immune-mediated tissue damage.

Memory responses develop following resolution of primary infection or vaccination, when a subset of antigen-specific B and T lymphocytes persist long-term as memory cells, capable of mounting a substantially faster and more effective secondary response on re-exposure to the same virus. This is the immunological basis of durable, often lifelong protective immunity against reinfection for many viral infections.

The durability and completeness of this memory response varies considerably by virus, and is directly undermined by whichever immune evasion strategies a given virus is capable of deploying.

Viruses, given their obligate dependence on the host cell and the strong selective pressure exerted by host immunity, have evolved a genuinely diverse range of strategies to blunt, evade, or actively subvert the host response. The principal recurring mechanisms are catalogued here.

Antigenic variation is alteration of surface antigens over time, via genetic drift, shift, or both, allowing a virus to evade neutralising antibody generated against a previous antigenic form. Influenza is the paradigm example, and HIV's exceptionally high mutation rate makes it perhaps the most extreme example of this strategy among all viruses, directly contributing to the persistent difficulty of HIV vaccine development.

Latency, established as a concept means that during transcriptional silence no viral protein is produced, and therefore nothing is available for either antibody or cytotoxic T cell recognition, allowing the virus to persist essentially invisibly to adaptive immunity for extended periods. This is the herpesvirus family's defining evasion strategy.

Downregulation of MHC is a strategy used by several viruses, notably some herpesviruses, which actively reduce host cell surface MHC class I expression, directly preventing cytotoxic T cell recognition of the infected cell. This strategy carries its own vulnerability, since MHC class I downregulation is itself the specific trigger for NK-cell-mediated killing, meaning a virus employing this evasion strategy alone remains vulnerable to a different arm of the innate immune system, a genuine evolutionary trade-off.

Interference with interferons is a strategy in which many viruses encode specific proteins that block interferon production, interferon signalling, or the downstream antiviral effector proteins interferon induces, blunting this first-line innate defence. This strategy is employed to varying degrees across essentially every major virus family, reflecting interferon's central, nonspecific role in early antiviral defence and the correspondingly strong selective pressure on viruses to counteract it.

Infection of immune cells describes viruses that directly infect and impair the very immune cells that would otherwise be responsible for clearing them. The most clinically dominant example is HIV's direct infection and progressive depletion of CD4 T helper cells, which are themselves essential coordinators of both antibody and cytotoxic T cell responses. HIV's evasion strategy therefore simultaneously cripples multiple arms of adaptive immunity at once, rather than blocking a single specific mechanism.

Production of immune modulatory proteins describes viruses that encode proteins directly mimicking, blocking, or otherwise interfering with specific host immune signalling molecules, for example viral proteins that mimic host cytokine receptors, sequestering the cytokine and preventing it from signalling normally, or viral proteins that directly inhibit complement activation. This is a diverse and organism-specific category of evasion strategy.

Poxviruses in particular are noted for encoding an unusually extensive repertoire of such immune modulatory proteins, reflecting their comparatively large genome size and correspondingly greater coding capacity for nonessential, host-interaction-focused genes.