Antigen polymorphism in Borrelia hermsii, a clonal pathogenic bacterium

Area of Science:

• Microbiology
• Immunology
• Evolutionary Biology

Background:

• Relapsing fever spirochetes like Borrelia hermsii evade immune responses by altering surface lipoprotein expression.
• This immune evasion mechanism involves duplicative transposition of surface lipoprotein-encoding genes into a single expression site.
• A large gene family encodes these lipoproteins, with some diversity predating major Borrelia species divergences.

Purpose of the Study:

• To investigate the evolutionary mechanisms driving diversification within the vsp (variable surface protein) gene subfamily of Borrelia hermsii.
• To understand how recombination and immune selection contribute to the antigenic diversity of vsp alleles.

Main Methods:

• Sequencing of over 90% of vsp alleles within the Borrelia hermsii strain HS1.
• Analysis of allelic mosaicism to infer recombination events.
• Assessment of amino acid differentiation to quantify serotype variation.

Main Results:

• A high degree of allelic mosaicism was observed within the vsp gene family.
• Intragenic recombination, influenced by host immune selection, appears to be the primary driver of vsp allele diversification.
• Recombinational diversification resulted in significant amino acid changes (30-40%), sufficient to overcome antibody cross-reactivity.

Conclusions:

• The evolution of the vsp gene family in Borrelia hermsii is characterized by punctuated events of allelic differentiation.
• This diversification strategy allows the spirochete to effectively evade host immune responses.
• Evolutionary diversification is driven by recombination rather than gradual accumulation of point mutations.