Mechanism of immune evasion
The main feature of infection caused by Trypanosoma brucei is the parasite's ability to constantly alter its antigenic profile. This phenomenon is known as antigenic variation (or antigenic transformation). It forms the basis of the disease's pathogenesis, causing it to take a chronic and wave-like course.
Each new wave of pathogen replication in the host's bloodstream is accompanied by the emergence of a completely novel antigenic variant that is unrecognized by existing defense mechanisms. The entire cycle takes just a few days and repeats over and over, denying the body a chance to completely clear the infection.
Cycle of antigenic transformation
The interaction between the parasite and the host immune system is a continuous race that proceeds in several strict stages:
- Proliferation: Active growth of the parasite population occurs in the host's bloodstream.
- Immune response: High antigen concentrations stimulate a robust production of specific circulating antibodies.
- Elimination: The generated antibodies successfully attack and destroy the overwhelming majority (over 99%) of the pathogen population.
- Mutant survival: Only a tiny fraction of parasites (less than 1%) survives, having undergone antigenic transformation. They become completely invulnerable to the current pool of circulating antibodies.
- Relapse: This new variant, resistant to previous antibodies, begins to actively multiply. In response, the immune system is forced to produce a new titer of specific antibodies, and the cycle repeats.
Molecular basis and consequences for the host
The substrate for such striking changes is provided by specific molecules on the parasite's coat — variant surface glycoproteins (VSG). The genetic apparatus of Trypanosoma brucei contains a colossal reservoir of information: the genome harbors over a thousand genes capable of transcribing more than a thousand unique variants of these antigens.
This survival strategy has catastrophic consequences for the host. Constant, relentless activation of the immune system in an attempt to keep up with the parasite's new antigenic variants leads to its exhaustion. Ultimately, this causes a progressive yet relentless decline in the immune system's ability to mount an adequate response, which determines the severity of trypanosomiasis.
Briefly: answers to topical questions
What specific diseases do African trypanosomes cause in humans?
African trypanosomes cause African trypanosomiasis in humans, also known as sleeping sickness. Depending on the subspecies of the pathogen, two forms of the disease are distinguished: the Gambian form (anthroponosis) and the Rhodesian form (zoonosis).
Which insect is the specific vector for Trypanosoma brucei?
The specific vector of Trypanosoma brucei is the tsetse fly (Glossina sp.). Specifically, these include riverine tsetse fly species (Glossina palpalis) and savanna species (Glossina morsitans).
Which subspecies of Trypanosoma brucei are pathogenic to humans?
The following subspecies of Trypanosoma brucei are pathogenic to humans:
- Gambian trypanosome (Trypanosoma brucei gambiense) — the causative agent of the Gambian form of sleeping sickness (anthroponosis).
- Rhodesian trypanosome (Trypanosoma brucei rhodesiense) — the causative agent of the Rhodesian form of African trypanosomiasis (zoonosis).
What are the clinical stages of African trypanosomiasis?
The disease proceeds in stages, with two main phases distinguished:
- Early stage: characterized by the formation of a trypanosomal chancre at the site of the bite, irregular fever, lymphadenopathy, hepatosplenomegaly, and progressive weakness. In this stage, parasites are localized in the blood plasma and lymph.
- Late stage (neurological stage): associated with the penetration of trypanosomes into the cerebrospinal fluid. It manifests as neuropsychiatric disorders, progressive somnolence, increasing muscle weakness, emaciation, and mental depression. Without treatment, death ensues.
Remember the interaction cycle with the acronym "R-A-E-T": Replication (multiplication in blood) — Antibodies (production in response) — Elimination (destruction of 99%) — Transformation (survival of 1% with new antigens).
Class and exam questions
Which parasite structures undergo antigenic transformation?
The surface variant glycoproteins (VSGs) located on the pathogen's membrane undergo alterations.
How does the parasite avoid destruction by circulating antibodies?
Less than 1% of the population changes its surface antigens, becoming invulnerable to previously generated antibodies.
What are the consequences for the host of the pathogen's constant antigen switching?
This causes continuous activation of the immune system, gradually leading to a decrease in its protective response capacity (exhaustion).
What else to study on this topic
- Structure and functions of variant surface glycoproteins (VSGs)
- Mechanisms of transcription of genes encoding new antigenic variants
- Dynamics of specific antibody production in response to new antigen pools
- Pathophysiological consequences of chronic host immune activation
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