An unusual syncytia-inducing human immunodeficiency virus type 1 primary isolate from the central nervous system that is restricted to CXCR4, replicates efficiently in macrophages, and induces neuronal apoptosis.

Macrophage/microglia cells are the principal targets for human immunodeficiency virus type 1 (HIV-1) in the central nervous system (CNS). Prototype HIV-1 isolates from the CNS are macrophage (M)-tropic, non-syncytia-inducing (NSI), and use CCR5 for entry (R5 strains), but whether syncytia-inducing (SI) CXCR4-using X4 strains might play a role in macrophage/microglia infection and neuronal injury is unknown. To explore the range of features among HIV-1 primary isolates from the CNS, the authors analyzed an HIV-1 strain (TYBE) from cerebrospinal fluid of an individual with acquired immunodeficiency syndrome (AIDS) that was unusual because it was SI. Like other CNS isolates, HIV-1/TYBE replicated to high level in primary human macrophages, but, in contrast to CNS prototypes, TYBE used CXCR4 exclusively to infect macrophages. A functional TYBE env clone confirmed the X4 phenotype and displayed a highly charged V3 sequence typical of X4 strains. Supernatant from TYBE-infected primary human macrophages induced apoptosis of neurons. Thus, TYBE represents a novel type of CNS-derived HIV-1 isolate that is CXCR4-restricted yet replicates efficiently in macrophages and induce neuronal injury. These results demonstrate that HIV-1 variants in the CNS may possess a broader range of biological characteristics than generally appreciated, raise the possibility that X4 strains may participate in AIDS neuropathogenesis, and provide a prototype clade B HIV-1 strain that replicates efficiently in primary macrophages through the exclusive use of CXCR4 as a coreceptor.

Development of a human neuronal cell model for human immunodeficiency virus (HIV)-infected macrophage-induced neurotoxicity: apoptosis induced by HIV type 1 primary isolates and evidence for involvement of the Bcl-2/Bcl-xL-sensitive intrinsic apoptosis pathway.

Neuronal apoptosis within the central nervous system (CNS) is a characteristic feature of AIDS dementia, and it represents a common mechanism of neuronal death induced by neurotoxins (e.g., glutamate) released from human immunodeficiency virus (HIV)-infected macrophages (HIV/macrophage-induced neurotoxicity). Neuronal apoptosis may result from activation of the intrinsic (mitochondrial/bcl-2 regulated) or extrinsic (death receptor) pathways, although which pathway predominates in CNS HIV infection is unknown. Apoptosis initiated by the intrinsic pathway is typically blocked by antiapoptosis Bcl-2 family proteins, such as Bcl-2 and Bcl-xL, but whether these can block HIV/macrophage-induced neuronal apoptosis is unknown. To determine the potential role of the Bcl-2 family in HIV/macrophage-induced neuronal apoptosis, we developed a unique in vitro model, utilizing the NT2 neuronal cell line, primary astrocytes and macrophages, and primary CNS HIV type 1 (HIV-1) isolates. We validated our model by demonstrating that NT2.N neurons are protected against HIV-infected macrophages by N-methyl-D-aspartate (NMDA) glutamate receptor antagonists, similar to effects seen in primary neurons. We then established stable NT2.N neuronal lines that overexpress Bcl-2 or Bcl-xL (NT2.N/bcl-2 and NT2.N/bcl-xL, respectively) and determined their sensitivity to macrophages infected with primary R5, X4, and R5/X4 HIV-1 isolates. We found that NT2.N/bcl-2 and NT2.N/bcl-xL neurons were resistant to apoptosis induced by either R5, X4, or R5/X4 isolates and that resistance was abrogated by a Bcl-2 antagonist. Thus, the NMDA receptor/bcl-2-regulated apoptotic pathway contributes significantly to HIV/macrophage-induced neuronal apoptosis, and Bcl-2 family proteins protect neurons against the spectrum of primary HIV-1 isolates. Modulation of bcl-2 gene expression may therefore offer adjunctive neuroprotection against development of AIDS dementia.