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Secreted proteophosphoglycan of Leishmania mexicana amastigotes activates complement by triggering the mannan binding
1Max-Planck-Institut für Biologie, Abteilung Membranbiochemie, Tübingen, Germany.
Abstract:
Cutaneous lesions induced by infection of mice with the protozoan parasite, Leishmania mexicana, contain abundant amounts of a high molecular mass proteophosphoglycan (PPG), which is secreted by the amastigote stage residing in phagolysosomes of macrophages and can then be released into the tissue upon rupture of the infected cells. Amastigote PPG forms sausage-shaped but soluble particles and belongs to a novel class of serine-rich proteins that are extensively O-glycosylated by phosphooligosaccharides capped by mannooligosaccharides. The purified molecule is shown here to efficiently activate complement (C) and deplete hemolytic activity of normal serum and may prevent the opsonization of L. mexicana amastigotes. Complement activation is Ca2+ dependent but does not depend on antibodies or the complement component C1. PPG binds to serum mannan binding protein (MBP), thus activating the MBP-associated serine protease, P100. Subsequently, the C cascade is triggered through C4 leading to covalent modification probably of carbohydrate hydroxyls of PPG by C3 fragments. Thus, PPG is able to activate C via the mannan binding lectin pathway which is unusual for secreted, soluble products of microbial origin. The proteophosphoglycan-induced complement activation is postulated to contribute to the lesion development and pathology caused by the parasite.
Insights
Leishmania mexicana proteophosphoglycan (PPG) activates the complement system via the mannan-binding lectin pathway. This immune response may contribute to lesion development and parasite pathology.
Area of Science:
- Immunology
- Parasitology
- Molecular Biology
Background:
- Leishmania mexicana infection causes cutaneous lesions.
- Amastigote stage secretes proteophosphoglycan (PPG).
- PPG is a novel serine-rich, O-glycosylated molecule.
Purpose of the Study:
- To investigate the role of PPG in complement activation.
- To determine the mechanism of complement activation by PPG.
- To understand PPG's contribution to Leishmania pathogenesis.
Main Methods:
- Purification of PPG from infected mouse lesions.
- Assays for complement activation (hemolytic activity).
- Investigation of complement pathway involvement (Ca2+ dependence, antibody/C1 independence, MBP binding).
Main Results:
- Purified PPG efficiently activates complement (C).
- PPG depletes serum hemolytic activity, potentially inhibiting opsonization.
- PPG binds to mannan-binding protein (MBP), activating the lectin pathway via P100 and C4.
- Complement activation involves C3 fragment modification of PPG.
Conclusions:
- Leishmania mexicana PPG activates complement via the mannan-binding lectin pathway.
- This activation is unusual for secreted microbial products.
- PPG-induced complement activation may contribute to lesion development and pathology.