Background:  HIV replication in the central nervous system occurs during acute infection and advanced AIDS, but may not occur in some asymptomatic individuals with chronic, untreated HIV infection. HIV-specific T cells partially control systemic HIV replication, but their role in the central nervous system is uncertain. We characterized anti-HIV CD8⁺ T cells in cerebrospinal fluid (CSF) and peripheral blood mononuclear cells (PBMC) among individuals with immune control of systemic HIV replication without ART.

Methods:  We collected CSF mononuclear cells and PBMC from 3 asymptomatic adults with chronic HIV infection, not receiving ART, and without CSF pleocytosis (≤3 cells/μL). CSF was collected on ice, spun at 200 X g for 10 minutes at 4 ^°C, and cells were resuspended in RPMI medium. Approximately 80% of CSF cells were expanded ex vivo for 10 to 14 days with an anti-CD3 monoclonal antibody. Epitope recognition by expanded cells was assessed by ELISpot and intracellular cytokine staining. We evaluated responses to individual epitopic peptides (8 to 11 amino acids in length), and to HIV peptide pools derived from Gag, Pol, Nef, and Env (18 amino acids overlapping by 11). To facilitate comparisons, CSF and PBMC cells were processed identically, including ex vivo expansion from the same number of PBMC as CSF cells.

Results:  Peripheral CD4 cell counts in the 3 subjects were >800 cells/mm³, and plasma HIV-1 RNA was <3000 copies/mL. Each 40-mL CSF specimen provided 80,000 to 130,000 mononuclear cells, which were readily expanded ex vivo to >5 x 10⁶ cells after 10 to 14 days in culture. ELISpot using 10 to 25 peptides representing known CD8⁺ T-cell epitopes readily detected CD8⁺ T cell responses from expanded CSF T cells, the repertoire of epitope recognition being qualitatively similar to PBMC. In 2 subjects, response magnitudes were greater in CSF; 2 subjects possessed an HLA B57 allele (often associated with slower disease progression). Cytotoxic T lymphocytes (CTL) in CSF and blood from these subjects recognized KF11, a common HLA B57-restricted epitope and responses were confirmed by intracellular cytokine staining. In 1 subject the percentage of expanded CD8⁺ T cells specific for KF11 was 24.2% from CSF vs.\ 2.7% from PBMC.

Conclusions:  Epitope-specific CD8⁺ T cells can be readily expanded from CSF of asymptomatic HIV-infected individuals, even without CSF pleocytosis. Anti-HIV CTL responses may play an important role in controlling HIV replication in the central nervous system among individuals with immunologic control of plasma viremia without ART.