HIV-1 Infection of the brain: Which pathogenic mechanisms are relevant for tissue damage?

Analysis of ENV V3 sequences from HIV-1-infected brain indicates restrained virus expression throughout the disease

The isolation of human immunodeficiency virus type 1 (HIV-1) from the cerebrospinal fluid (CSF) of asymptomatic virus carriers suggests that the viral infection spreading to the brain occurs early during infection. The aim of the present study was to investigate whether HIV-1 infection of the brain parenchyma also occurs during the early phase of infection. We also wished to compare the degree of replication of the virus in the brain at different clinical stages associated with HIV-1 infection. With the use of polymerase chain reaction (PCR), the viral genomes present in seven of eight brain specimens obtained from two asymptomatic HIV-1 carriers and six AIDS patients were amplified. Thereafter, the number of viral copies present in each brain specimen was quantified, the third variable region (V3) of the gp 120 glycoprotein was sequenced and these results compared with the histopathological findings in the tissue. The HIV-1 DNA genome was amplified from seven of the eight brain tissues, including the specimens obtained from the two asymptomatic carriers. An increased number of viral copies in the brain was found in association with histopathological findings of HIV-1 encephalitis. The analysis of the V3 sequences, however, revealed the presence of a homogeneous virus population in the brain at every clinical stage of the disease. These results suggest that, although entry of the virus in the parenchyma may occur early during infection, HIV-1 replication in the brain is constrained until the terminal phase of AIDS encephalitis.

HIV type 1 V3 sequences and the development of dementia during AIDS

The most frequent neurological complication of AIDS is a dementia-like syndrome. Power and collaborators (J Virol 1994; 68:4643-4649) have reported an association between the clinical signs of AIDS dementia and the amino acid composition of two positions (305 and 329) within the V3 region of HIV-1 strains amplified from brain tissue. Similarly, we analyzed position 305 in the V3 region of HIV-1 present in the brain or cerebrospinal fluid of 25 nondemented subjects at different clinical stages of HIV-1 infection. Our results are, however, at variance with the findings presented by Power and colleagues. Histidine, found to be common among sequences derived from demented patients, was also present in the majority (16 of 25) of nondemented patients analyzed by us. In the hands of Power and colleagues, sequences derived from nondemented patients contained proline at position 305. None of our patients had proline in this position. We also asked the question whether the presence of a specific amino acid at position 305 of the V3 loop is linked to an increased capacity of HIV-1 isolates to infect primary microglial cells, the major target cell for HIV-1 infection in the brain. Primary HIV-1 isolates derived from blood and cerebrospinal fluid of five patients, two asymptomatic and three AIDS patients, were used to infect microglia cell cultures. Infection was monitored by syncytium formation and by p24 antigen release in the culture supernatant. All but one of the paired blood/CSF isolates replicated in human brain cultures. Replication occurred independently from the amino acid present at position 305 of the V3 region of the viral envelope. Our results indicate that the majority of HIV-1 isolates, even derived during the asymptomatic stage, have the capacity to infect microglial cells. The relevance of viral envelope sequences in determining tropism for microglial cells and development of neurological symptoms remains an open question.

Neuropathology of early HIV-1 infection

Early HIV-1 invasion of the central nervous system has been demonstrated by many cerebrospinal fluid studies; however, most HIV-1 carriers remain neurologically unimpaired during the so called "asymptomatic" period lasting from seroconversion to symptomatic AIDS. Therefore, neuropathological studies in the early pre-AIDS stages are very few, and the natural history of central nervous system changes in HIV-1 infection remains poorly understood. Examination of brains of asymptomatic HIV-1 positive individuals who died accidentally and of rare cases with acute fatal encephalopathy revealing HIV infection, and comparison with experimental simian immunodeficiency virus and feline immunodeficiency virus infections suggest that, invasion of the CNS by HIV-1 occurs at the time of primary infection and induces an immunological process in the central nervous system. This includes an inflammatory T-cell reaction with vasculitis and leptomeningitis, and immune activation of brain parenchyma with increased number of microglial cells, upregulation of major histocompatibility complex class II antigens and local production of cytokines. Myelin pallor and gliosis of the white matter are usually found and are likely to be the consequence of opening of the blood brain barrier due to vasculitis; direct damage to oligodendrocytes by cytokines may also interfere. These white matter changes may explain, at least partly, the early cerebral atrophy observed, by magnetic resonance imaging, in asymptomatic HIV-1 carriers. In contrast, cortical damage seems to be a late event in the course of HIV-1 infection. There is no significant neuronal loss at the early stages of the disease, no accompanying increase in glial fibrillary acid protein staining in the cortex, and only exceptional neuronal apoptosis. Although HIV-1 proviral DNA may be demonstrated in a number of brains, viral replication remains very low during the asymptomatic stage of HIV-1 infection. This makes it likely that, although opening of the blood brain barrier may facilitate viral entry into the brain, specific immune responses including both neutralising antibodies and cytotoxic T-lymphocytes, continuously inhibits viral replication at that stage.