Non-pathogenic Infection by Attenuated SIV Correlates With Lack of Productive Infection of NK Cells

A.S. von Gegerfelt,¹ A. Valentin,² C. Alicea,¹ A.S. Zolotukhin,¹ D. Michalowski,¹ J. Bear,¹ S.V. Smulevitch,¹ G.N. Pavlakis², and B.K. Felber¹; ¹Human Retrovirus Pathogenesis Section and ²Human Retrovirus Section, BRL, CCR, National Cancer Institute, Frederick, MD 21702 (USA)

Background: Replacement of the viral Rev-RRE mechanism generated attenuated strains of HIV and SIV. We found that the Rev-independent SIV induces chronic infection but is not pathogenic upon infection of neonatal and juvenile rhesus macaques for more than 5 years. Animals infected with this attenuated virus lack detectable levels of virus production in their plasma. However, they have a potent immune response able to control pathogenic challenge. Therefore, Rev regulation is essential for SIV pathogenicity. Replacement of Rev-RRE by the CTE provides a different approach to dramatically lower the virulence of pathogenic lentiviruses. The mechanism leading to lack of pathogenicity of this virus strain is under investigation. Material and Methods: Primary cells were isolated by immunomagnetic beads and analyzed by FACS. PBMC-grown virus stocks were used for infection of primary cells and cell lines.  In vitro binding assays were used to identify cellular proteins binding to the negative-acting RNA elements (INS) located in gag and env of HIV-1. Results: A panel of cell lines and purified subsets of primary cells were infected in vitro by the Rev-independent HIV and SIV and were monitored for virus production. We identified several human cell lines as well as primary NK cells that are non-permissive for propagation of the Rev-independent HIV, whereas they are permissive for wild type virus. This data suggest that cells exist where the cellular milieu can negatively affect virus expression. Search of factors affecting levels of virus expression identified the cellular PSF protein as a candidate. PSF binds specifically to gag and env mRNA and inhibits their expression. Conclusions: We postulate that the lack of propagation of the Rev-independent virus in some primary cells may play an important role in pathogenesis. PSF has properties of a key factor mediating the posttranscriptional regulation of HIV-1 and is likely part of a novel mRNA regulatory mechanism hijacked by the virus. The role of PSF and other factors for virus down-regulation and latency in different cell types in under investigation.