Protective role of the virus-specific immune response for development of severe neurologic signs in simian immunodeficiency virus-infected macaques

Abstract

The pathogenesis of human immunodeficiency virus-associated motor and cognitive disorders is poorly understood. In this context both a protective and a harmful role of the immune system has been discussed. This question was addressed in the present study by correlating the occurrence of neurologic disease in simian immunodeficiency virus (SIV)-infected macaques with disease progression and the humoral and cellular intrathecal antiviral immune response. Overt neurologic signs consisting of ataxia and apathy were observed at a much higher frequency in rapid progressor animals (6 of 12) than in slow progressors (1 of 7). Whereas slow progressors mounted a strong antiviral antibody (Ab) response as evidenced by enzyme-linked immunosorbent and immunospot assays, neither virus-specific Ab titers nor Ab-secreting cells could be found in the cerebrospinal fluid (CSF) or brain parenchyma of rapid progressors. Similarly, increased infiltration of CD8(+) T cells and cytotoxic T lymphocytes specific for viral antigens were detected only in the CSF of slow progressors. The finding that neurologic signs develop frequently in SIV-infected macaques in the absence of an antiviral immune response demonstrates that the immune system does not contribute to the development of motor disorders in these animals. Moreover, the lower incidence of neurologic symptoms in slow progressors with a strong intrathecal immune response suggests a protective role of the virus-specific immunity in immunodeficiency virus-induced central nervous system disease.

FIG. 1

Integrity of the BBB and intrathecal IgG synthesis. Albumin and IgG concentrations in plasma and CSF were determined. (A) Q_Alb was calculated. The values for each time point are shown as percentages of the mean preinfection values. (B) Q_IgG is compared with the limQ_IgG for intrathecal IgG synthesis. Values of >0 are indicative of intrathecal antibody production. Representative data of two rapid progressors and two slow progressors are depicted.

FIG. 2

Virus specific antibodies and viral antigen in plasma and CSF. (A) Env-specific titers in plasma and CSF. Representative data of two rapid progressors and two slow progressors are shown. (B) Intrathecal synthesis of Env-specific antibodies. Values of >2 are indicative of intrathecal synthesis of antigen-specific antibodies. Representative data of two slow progressors are shown. (C) Representative levels of viral antigen in plasma and CSF of a rapid progressor and a slow progressor. Microglial cells isolated from the brains of SIVmac251 MPBMC-infected animals were cultured for 20 h, and the supernatant was assayed for p27 production. Overall levels of viral antigen produced by microglial cells isolated from SIVmac251 MPBMC-infected animals with AIDS were 9 ± 4 and 8,574 ± 4,155 pg/ml (mean ± SEM) for slow progressors (n = 8) and rapid progressors (n = 6), respectively.

FIG. 3

Antigen specificity of Abs in plasma and CSF. Paired plasma (P) and CSF (C) samples of two rapid progressors (RP) and two slow progressors (SP) were subjected to Western blot analysis at 1:50 dilutions (plasma) and 1:2 dilutions (CSF). Positive (+) and negative (−) controls are shown in the first two lanes.

FIG. 4

Kinetics of Env-specific ASC in blood and brain samples of SIV-infected animals. Numbers of total plasma cells and Env-specific ASC were determined by ELISPOT. (A) Percent Env-specific ASC among total plasma cells in blood and brain samples. (B) Absolute numbers of Env-specific ASC per brain. Each point represents ASC isolated from one sacrificed animal. In addition, the line of best fit is shown for the values of the brain. p.i., postinfection.

FIG. 5

Kinetics of CD8⁺ T cells in CSF and brain tissue. CSF cells and gradient-purified brain cells were counted, and the number of CD8⁺ T cells was calculated according to the percentage among total cells as determined by flow cytometry. Representative data of CSF from two slow progressors (A) and two rapid progressors (B) are shown. (C) The number of CD8⁺ T cells per gram of brain tissue of individual SIV-infected slow progressors (□) is shown. In addition, the mean values for uninfected (○ and shaded area) and infected (⧫) animals sacrificed between 2 and 4 wpi, 5 and 15 wpi, 16 and 31 wpi, and after 32 wpi are depicted. Error bars, standard errors of the means.

FIG. 6

Virus-specific cytotoxicity in blood and CSF. Blood (A, B, and C) and CSF (D) cells were polyclonally stimulated, and cytotoxic capacities against autologous target cells expressing the SIV antigens Gag, Env, Pol, or HIV-1 reverse transcriptase (RT) were determined in a chromium release assay. Representative data of a rapid progressor (C) and slow progressors (A, B, and D) are shown.