Inclusion body

Definition

A discrete, intracellular aggregate of viral proteins, nucleic acids, or other macromolecules that becomes visible under light microscopy, often serving as a diagnostic sign of viral infection in host cells. In arthropods, inclusion bodies typically form in the cytoplasm or nucleus of infected cells during replication of baculoviruses, iridoviruses, or entomopoxviruses, and may be categorized as occlusion bodies (polyhedra, capsules) or non-occlusion bodies depending on whether they contain infectious virions embedded in a crystalline protein matrix.

Etymology

Latin inclusio 'a shutting in, enclosure' + body, referring to the enclosed aggregate within the cell.

Example

Nuclear polyhedrosis viruses (NPVs) in lepidopteran larvae produce large, polyhedral occlusion bodies in infected cell nuclei; these refractile inclusions are readily visible in wet-mount preparations of larval fat body tissue and are used to diagnose baculovirus infections in field-collected caterpillars.

Synonyms

• cell inclusion
• intracellular inclusion

Related Terms

• occlusion body
• polyhedron
• capsule
• virogenic stroma
• Fat body
• baculovirus
• cytopathic effect
• Histopathology

Usage Notes

In entomological pathology, 'inclusion body' often specifically denotes viral factory structures, whereas 'occlusion body' refers to the environmentally resistant protein crystals that package virions for horizontal transmission. Not all inclusion bodies are occlusion bodies: non-occluded viruses such as many iflaviruses produce inclusion bodies without protective polyhedrin or spheroidin matrices. In muscle pathology, the unrelated term 'inclusion body myositis' refers to protein aggregates in human muscle fibers and should not be confused with arthropod viral inclusions.