Causes and transmission routes
The development of prion diseases has two main etiological causes: a mutation in one's own gene encoding the normal prion protein (this is how hereditary forms arise), or the entry of an exogenous infectious prion into the body.
From an epidemiological perspective, several transmission routes for infectious forms are distinguished:
- Alimentary (foodborne) route. Infection occurs through the consumption of animal-derived products contaminated with prions, as well as through food additives made from raw organs of cattle.
- Artificial (iatrogenic) route. This is infection associated with medical interventions. It can occur via blood transfusion, organ and tissue transplantation, as well as instrumental interventions. Surgical and dental instruments that have undergone inadequate cleaning can become a transmission factor due to the high resistance of prions to standard sterilization.
- Medication-related route. This is a specific case of iatrogenic infection occurring through the administration of immunobiological products of animal or human origin contaminated with prions. A classic example is the development of Creutzfeldt–Jakob disease in children receiving human pituitary somatotropic hormone for the treatment of dwarfism. Transmissions of infection with other hormones, myelopeptides, and enzymes have also been described. In veterinary medicine, a mass infection of sheep using a brain formalin vaccine is known.
Pathogenesis and the role of the lymphoid system
Previously, science was dominated by the hypothesis that prions exclusively affect the central nervous system. However, modern data prove that pathogenesis is a staged process in which lymphoid tissue plays a huge role.
Upon alimentary infection, the pathological protein goes through the following stages:
- Initial stage. Pathological prions enter the intestinal lumen, from where they are transported into the blood and lymphatic system.
- Peripheral replication. Primary reproduction of the pathogen occurs not in the brain, but in peripheral lymphoid organs (tonsils, appendix, spleen). Of key importance here are follicular dendritic cells (FDCs), which specialize in capturing immune complexes. A large amount of normal protein is present on the FDC membrane. The pathological prion is adsorbed onto the cell via complement receptors (upon opsonization) or through direct contact with the normal protein. The accumulation of prions in the spleen directly correlates with the number and activity of FDCs.
- Neuroinvasion. Following the period of accumulation in lymphoid organs (incubation), prions are transported to the CNS. The main pathway of penetration into the brain is via peripheral nerves. There is also an alternative pathway: direct crossing of the blood-brain barrier (BBB).
Pathomorphological changes and clinical presentation
The accumulation of pathological prions in the brain in high concentrations leads to massive and irreversible tissue changes known as transmissible spongiform encephalopathies.
Morphologically, the process is characterized by the following features:
- Spongiform transformation. Vacuoles form in neurons, causing the nervous tissue to acquire a spongy appearance.
- Astrocytosis. Active proliferation of astrocytic neuroglia occurs with hyperproduction of glial fibers.
- Amyloidosis. Extracellular dysproteinosis develops, manifested by the formation of protein aggregates, fibrils, and amyloid deposition.
- Atrophy and sclerosis. Nervous tissue undergoes atrophy followed by the development of sclerosis.
These severe organic lesions of the CNS determine the clinical manifestations of the disease. Patients exhibit profound behavioral changes, gross coordination disturbances, and progressive dementia. The outcome of the disease is always the same: severe wasting and death. A crucial feature of prion infections is that, despite the severe damage to the body, a specific immune response against the pathogen is not formed.
Briefly: answers to topical questions
Which gene encodes the normal cellular prion protein in humans?
The normal cellular prion protein is encoded by the PrNP gene, which is located on the short arm of chromosome 20. This gene is responsible for the synthesis of the normal cellular prion (cellular prion protein), which is a glycoprotein with a molecular weight of 33–35 kDa. The synthesized protein is anchored in the cell membrane and is found in the central nervous system, spleen, lymph nodes, gastrointestinal tract, skin, and follicular dendritic cells. It is sensitive to proteases and performs functions regulating nerve impulse transmission, circadian rhythms, and copper metabolism.
Which sterilization methods are effective for destroying prions on surgical instruments?
For the disinfection of surgical instruments in case of prion contamination risk, rigorous regimens are required, as conventional sterilization modes are ineffective.
The following methods are used:
- Autoclaving — 134 °C for 30 minutes or 121 °C for 4 hours; the source also indicates a regimen of 121 °C for 4 hours with the addition of 1N NaOH.
- Chemical treatment — bleach + 1N NaOH with an exposure time of 1 hour.
To memorize the stages of foodborne infection, use the mnemonic "CLIN-M": Intestine (Gut) → Lymphoid tissue (FDC) → Invasion via nerves → Brain (Spongiform transformation).
Class and exam questions
Can a person contract prions through food?
Yes, the alimentary route is one of the main ones. Infection occurs through the consumption of infected animal products and supplements made from raw livestock organs.
Where does primary prion replication occur during the alimentary route of transmission?
Primary prion replication occurs not in the CNS, but in peripheral lymphoid tissue (spleen, tonsils, appendix) on follicular dendritic cells.
Why do surgical instruments pose a danger in prion transmission?
Pathological prions possess very high resistance to conventional sterilization methods, so standard processing of dental and surgical instruments is insufficient for their destruction.
How does the immune system react to the penetration of prions?
An immune response is not formed in prion infections; the body does not develop defense mechanisms against these agents.
What else to study on this topic
- The mechanism of transformation of the normal prion protein into the pathological one
- Structural and functional features of follicular dendritic cells (FDCs)
- Specific sterilization regimens for prion inactivation
- Creutzfeldt–Jakob disease: forms, clinical course, and diagnosis
- Genetics of prion diseases: hereditary forms and gene mutations
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