Tracking the Emergence of Host-Specific Simian Immunodeficiency Virus
env
and
nef
Populations Reveals
nef
Early Adaptation and Convergent Evolution in Brain of Naturally Progressing Rhesus Macaques
Authors: Lamers Susanna L. , Nolan David J. , Rife Brittany D. , Fogel Gary B. , McGrath Michael S. , Burdo Tricia H. , Autissier Patrick , Williams Kenneth C. , Goodenow Maureen M. , Salemi Marco 
Published: June 2015

While a clear understanding of the events leading to successful establishment of host-specific viral populations and productive infection in the central nervous system (CNS) has not yet been reached, the simian immunodeficiency virus (SIV)-infected rhesus macaque provides a powerful model for the study of human immunodeficiency virus (HIV) intrahost evolution and neuropathogenesis. The evolution of the gp120 and
nef
genes, which encode two key proteins required for the establishment and maintenance of infection, was assessed in macaques that were intravenously inoculated with the same viral swarm and allowed to naturally progress to simian AIDS and potential SIV-associated encephalitis (SIVE). Longitudinal plasma samples and immune markers were monitored until terminal illness. Single-genome sequencing was employed to amplify full-length
env
through
nef
transcripts from plasma over time and from brain tissues at necropsy.
nef
sequences diverged from the founder virus faster than gp120 diverged. Host-specific sequence populations were detected in
nef
(∼92 days) before they were detected in gp120 (∼182 days). At necropsy, similar brain
nef
sequences were found in different macaques, indicating convergent evolution, while gp120 brain sequences remained largely host specific. Molecular clock and selection analyses showed weaker clock-like behavior and stronger selection pressure in
nef
than in gp120, with the strongest
nef
selection in the macaque with SIVE. Rapid
nef
diversification, occurring prior to gp120 diversification, indicates that early adaptation of
nef
in the new host is essential for successful infection. Moreover, the convergent evolution of
nef
sequences in the CNS suggests a significant role for
nef
in establishing neurotropic strains.

IMPORTANCE
The SIV-infected rhesus macaque model closely resembles HIV-1 immunopathogenesis, neuropathogenesis, and disease progression in humans. Macaques were intravenously infected with identical viral swarms to investigate evolutionary patterns in the gp120 and
nef
genes leading to the emergence of host-specific viral populations and potentially linked to disease progression. Although each macaque exhibited unique immune profiles, macaque-specific
nef
sequences evolving under selection were consistently detected in plasma samples at 3 months postinfection, significantly earlier than in gp120 macaque-specific sequences. On the other hand,
nef
sequences in brain tissues, collected at necropsy of two animals with detectable infection in the central nervous system (CNS), revealed convergent evolution. The results not only indicate that early adaptation of
nef
in the new host may be essential for successful infection, but also suggest that specific
nef
variants may be required for SIV to efficiently invade CNS macrophages and/or enhance macrophage migration, resulting in HIV neuropathology.