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Peptide libraries define the fine specificity of anti-polysaccharide antibodies to Cryptococcus neoformans
P Valadon1, G Nussbaum, L F Boyd
1Department of Cell Biology, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
Abstract:
Cryptococcus neoformans is an encapsulated fungus that causes a life-threatening meningoencephalitis in patients with AIDS. Monoclonal antibodies to the capsular glucuronoxylomannan can modulate the infection in mice, but the epitopes on this complex polysaccharide recognized by protective and non-protective antibodies have not been defined. We have used 2H1, one of our most protective antibodies, to screen phage display peptide libraries for peptide mimotopes that would allow us to explore the fine specificity of anti-cryptococcal polysaccharide antibodies. Hexa- and decapeptides have been identified with sequence homologies that define four motifs: 1, (E)TPXWM/LM/L; 2, W/YXWM/ LYE; 3, DWXDW; and 4, (Ar)WDGQ(Ar). Peptides representing these motifs compete with each other for a shared binding site that overlaps the polysaccharide binding site. Motifs 1 and 2 confer high affinity binding, and PA1, which displays a motif 1 peptide with the sequence LQYTPSWMLV, binds to 2H1 with a Kd of 295 nM. Analysis of the interaction between the 2H1 binding peptides and 24 structurally related anti-polysaccharide antibodies reveals a complex pattern of reactivity that strongly suggests binding to or close to the complementary determining regions. Furthermore, those antibodies that have been shown to have different specificity, and in some cases different protective potential, do not bind any of the peptides selected by the protective 2H1 antibody. This study shows that peptide mimotopes for a complex microbial polysaccharide can be identified by screening phage peptide libraries and demonstrates the usefulness of such peptides in analyzing closely related interactive sites of proteins in general and of antibodies in particular.
Insights
Researchers identified peptide mimotopes for Cryptococcus neoformans glucuronoxylomannan using phage display. These peptides help define antibody binding sites, aiding the development of targeted therapies for fungal infections like meningoencephalitis.
Area of Science:
- Immunology
- Microbiology
- Structural Biology
Background:
- Cryptococcus neoformans causes life-threatening meningoencephalitis in AIDS patients.
- Monoclonal antibodies targeting the fungal capsule can modulate infection, but specific epitopes are undefined.
- Understanding antibody-epitope interactions is crucial for developing effective antifungal therapies.
Purpose of the Study:
- To identify peptide mimotopes of the Cryptococcus neoformans glucuronoxylomannan (GXM) capsule.
- To explore the fine specificity of anti-GXM antibodies using these mimotopes.
- To investigate the binding interactions between protective antibodies and peptide mimotopes.
Main Methods:
- Screening of phage display peptide libraries with a protective monoclonal antibody (2H1).
- Identification and characterization of four peptide motifs (TPXWM/LM/L, W/YXWM/LYE, DWXDW, WDGQ).
- Analysis of peptide-antibody binding affinities and competition assays.
Main Results:
- Identified four distinct peptide motifs that mimic GXM epitopes.
- Demonstrated that these peptides bind to a shared site overlapping the polysaccharide-binding site.
- Showed that protective antibody 2H1 binds a high-affinity motif 1 peptide (PA1) with Kd of 295 nM.
- Observed complex binding patterns suggesting interaction near antibody complementary determining regions.
Conclusions:
- Phage display is effective for identifying mimotopes of complex microbial polysaccharides.
- Peptide mimotopes are valuable tools for analyzing antibody specificity and binding sites.
- This approach can differentiate between protective and non-protective antibodies, informing therapeutic strategies.