Residual brain infection in relapsing-fever borreliosis

Background. Neurological involvement is common in the spirochetal infection relapsing fever ( RF) in both humans and experimental animals. RF is best known for antigenic variation caused by the sequential expression of variable outer membrane lipoproteins of 2 sizes, variable small ( Vsp) and variable large ( Vlp) proteins. Less understood is the persistence of RF borreliae in the brain after they are cleared from the blood, referred to as residual brain infection ( RBI). Our goal was to investigate the phenomenon of RBI in RF. Methods. We studied RBI in immunocompetent mice by culturing blood and perfused brain samples 1 month after intraperitoneal inoculation with Borrelia turicatae serotype 1 ( Bt1). Mice deficient in Toll-like receptor 2 ( TLR2(-/-)) or in B and T cells ( scid) were included for comparison. Results. All scid mice had persistent infection in blood and brain. RBI was found in 3 ( 19%) of 16 immunocompetent and TLR2(-/-) mice. RBI was caused by either persistence of the original serotype ( Bt1) or newly emerged Vsp (n = 1, renamed Bt3) or Vlp serotypes. The Vsp of Bt1 ( Vsp1) and Bt3 ( Vsp3) were 75% identical. Conclusions. RBI in RF is relatively frequent and can occur by persistence of the original or newly emerged serotypes.