Pathogenesis of neuroborreliosis - Lessons from a monkey model

The diagnosis of human LNB can be difficult, because its major clinical man ifestations - meningitis, facial palsy, radiculitis, and neuritis - are non -specific and the characteristic skin lesion is usually absent at the time of neurological involvement. Thus, CSF assays are often used in diagnosis. Culture of CSF is rarely performed because it has a low yield and requires special culture medium. PCR of the CSF identified spirochetal DNA in clinic al specimens with greater sensitivity, but it suffers from a number of disa dvantages. Measurement of specific antibody in the CSF also has its limitat ions. The role of available assays for LNB has not been studied carefully i n a comparative investigation. The recent development of the non-humane pri mate (NHP) model of LNB allowes us to address this need in a faithful model of human LNB. We compared PCR and culture in their ability to detect spirochetal presence in the CSF and brain tissue of infected NHPs, and related these measures o f infection to the development of anti-B. burgdorferi antibody. We also tes ted a bioassay, the mouse infectivity test (MIT), in this model. Using thes e four assays (PCR, culture, MIT, and CSF Ab) at least one assay for spiroc hetal presence in CSFs from NHPs was positive in 87% of CSFs tested during early infection in the CNS. Detection of spirochetal presence by PCR, MIT, and culture in the CSF was inversely related to the concomitant presence of anti-B. burgdorferi antibody intrathecally. The performance of any particu lar test was associated with the strength of the host immune response. In e arly CNS infection, when anti-B. burgdorferi antibody had not yet appeared, or in immunocompromised hosts, the MIT compared favorably to culture and P CR in infected NHPs; antibody in the CSF was the most useful assay in immun ocompetent NHPs. This is the first study demonstrating that a bioassay using inoculation of mice, the mouse infectivity test (MIT), has potential as a useful adjunct i n the diagnosis of LNB. The MIT for LNB was modeled after the rabbit infect ivity test or RIT, which is considered the "gold standard" for the diagnosi s of the related CNS infection, neurosyphilis, and felt to be very sensitiv e and specific. The presence of specific anti-B. burgdorferi antibody in the CSF is the mos t widely used assay for Lyme neuroborreliosis. In the immunocompetent NHPs in our study it was a very successful assay for detection of CNS invasion. However, it is frequently false-negative, especially early in the course of the infection, or if there is transient immunosuppression. Transient suppr ession of the anti-B. burgdorferi immune response in the human could occur in instances of co-infection, i.e. simultaneous transmission via the tick o f another pathogen other than B. burgdorferi. Thus, mild immunosuppression as accomplished in our NHPs with corticosteroids was designed to mimic cond itions in the human host which allow B. burgdorferi in the natural state to gain a firm foothold in the central nervous system in the 10-15% of B. bur gdorferi-infected patients who develop clinically symptomatic nervous syste m disease. This study is the first to compare utility of available diagnostic techniqu es in LNB in which necropsy proved presence of infection in the CNS. None o f the assays was ideal for all conditions, and the utility of the assay was associated with the host immune status. The differences in the responses o f immunocompromised and immunocompetent NHPs in this study were striking. I n immunocompetent NHPs the window of opportunity for CNS invasion prior to the development of CSF antibody was brief, and the chance of detection of s pirochete low by any of the three techniques used (i.e. culture, PCR, or MI T); in this group, measurement of CSF antibody was generally diagnostic. In immunocompromised NHPs, intrathecal antibody production was delayed, and t his helpful diagnostic assay was false-negative; diagnosis required more la bor-intensive assays such as PCR, culture, and MIT during weeks 3.5 to 9.5 after infection. It is likely that had the experiment been allowed to proce ed longer in the immunosuppressed NHPs, antibody would have eventually been produced intrathecally. The clinical relevance of the data on comparison of diagnostic assays is cl ear. The appearance of anti-B. burgdorferi antibody in the CSF may be delay ed especially when there is interference with the anti-B. burgdorferi immun e response. In these circumstances, or for a short time early in CNS invasi on in immunocompetent individuals, the measurement of anti-B. burgdorferi a nti body in the CSF may be negative; under these circumstances the likeliho od of detecting spirochete by PCR, culture, or MIT is at its highest. Conve rsely, detecting spirochetal presence by culture, PCR, or MIT will be least likely to be successful when anti-B. burgdorferi antibody is present.