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Expression of major surface protein 2 antigenic variants during acute Anaplasma marginale rickettsemia
G Eid1, D M French, A M Lundgren
1Department of Infectious Diseases, University of Florida, Gainesville, USA.
Abstract:
Antigenic variants of Anaplasma marginale major surface protein 2 (MSP-2), a target of protective immune responses, have been detected by use of copy-specific monoclonal antibodies reactive with some, but not all, organisms during acute rickettsemia. The presence of polymorphic msp-2 genes was confirmed by cloning and sequencing two gene copies, 11.2 and DF5, each of which encodes a full-length MSP-2 with a unique amino acid sequence. Transcription of msp-2 genes during acute rickettsemia was analyzed by use of cDNA cloning of hybrid-selected msp-2 mRNA. Sequencing of cDNA clones, designated AR1 to AR14, indicated that DF5 msp-2 was transcribed during acute rickettsemia. Two classes of variant msp-2 genes were also transcribed during acute rickettsemia. The first class of variant transcripts, typified by clones AR3, AR4, AR7, and AR14, each encoded a single or small number of amino acid substitutions relative to DF5. The second type, AR5, encoded a large region of amino acid polymorphism, including additions, deletions, and substitutions, as compared to DF5. Specific antibody directed against the AR5 polymorphic region bound a unique MSP-2 expressed on A. marginale that was not recognized by antibody generated against DF5. Similarly, anti-AR5 peptide antibody reacted with a different MSP-2 that was not bound by anti-DF5 antibody. This expression confirmed that variant msp-2 transcripts encode structurally distinct MSP-2 molecules which bear unique B-cell epitopes. These results support the hypothesis that the large msp-2 gene family, which constitutes a minimum of 1% of the genome, encodes antigenic variants critical to evasion of protective immune response directed against surface MSP-2 epitopes.
Insights
Anaplasma marginale major surface protein 2 (MSP-2) exhibits antigenic variation through polymorphic genes. These variants enable the pathogen to evade the host immune response during infection.
Area of Science:
- Veterinary Microbiology
- Molecular Biology
- Immunology
Background:
- Anaplasma marginale causes significant economic losses in cattle.
- Major surface protein 2 (MSP-2) is a key target of protective immunity against A. marginale.
- Antigenic variation in MSP-2 complicates vaccine development and disease control.
Purpose of the Study:
- To investigate the genetic polymorphism and transcription of Anaplasma marginale msp-2 genes.
- To understand the molecular basis of MSP-2 antigenic variation during rickettsemia.
- To determine if variant msp-2 transcripts encode distinct surface proteins.
Main Methods:
- Cloning and sequencing of msp-2 gene copies (11.2 and DF5).
- cDNA cloning and sequencing of hybrid-selected msp-2 mRNA from acute rickettsemia.
- Characterization of variant transcripts (AR1-AR14).
- Immunological assays using specific antibodies against MSP-2 variants.
Main Results:
- Two polymorphic msp-2 genes (11.2 and DF5) with unique sequences were identified.
- Transcription of DF5 and two classes of variant msp-2 genes occurred during acute rickettsemia.
- Variant transcripts, particularly AR5, encoded structurally distinct MSP-2 molecules with unique epitopes.
- Specific antibodies recognized these variant MSP-2 proteins, confirming their expression on the bacterial surface.
Conclusions:
- The Anaplasma marginale msp-2 gene family encodes antigenically distinct variants.
- Transcriptional switching and expression of these variants contribute to immune evasion.
- Understanding MSP-2 variation is crucial for developing effective control strategies against anaplasmosis.