HIV-1 viral protein R (Vpr) & host cellular responses

During infection of host cells by HIV-1, active host-pathogen interactions take place. The final balance between these interactions determines the efficiency of viral infection and subsequent disease progression. HIV-infected cells respond to viral invasion with various antiviral strategies such as innate, cellular and humoral immune antiviral defense mechanisms. On the other hand, the virus has also developed tactics to suppress these host cellular responses. Among the many viral offensive strategies, viral protein R (Vpr) plays a particularly active role. Vpr involved in nuclear transport of the viral pre-integration complex, activation of viral transcription, induction of cell cycle G2/M arrest and apoptosis of the host cells. However, specific roles of these Vpr activities in viral pathogenesis and their contribution to disease progression are not fully understood. HIV-1 defective for some or all of these Vpr activities have been associated with slow disease progression in some patients. With regard to the host responses to vpr gene expression, studies show that Vpr is specifically targeted by CDS T-lymphocytes during acute viral infection and that the host innate immune response may also play a crucial role in suppressing the effects of Vpr on various cellular activities. The effect of host cellular responses to vpr gene expression and its roles in nuclear transport, cell cycle G2/M regulation and induction of apoptosis are discussed in this review. Strategies with potential application for future antiviral therapies directed at suppressing Vpr activities are described.

Key words Apoptosis - cell cycle G2/M arrest - disease progression - HIV-1 - host-pathogen interaction - host immune responses nuclear transport - Vpr - viral pathogenesis

Upon infection by human immunodeficiency virus type 1 (HIV-1), host cells react with various innate, cellular and humoral immune responses to counteract the viral invasion. Limited and transient restriction of viral infection is normally achieved. However, HIV ultimately overcomes these antiviral responses resulting in successful viral infection and replication. Expression of several HIV-1 regulatory and accessory genes such as vif, vpu and tat is known to regulate some of the host cell innate immune responses to maximize viral infection or replication1-3. For example, a host innate antiviral response mediated by APOBEC3G was recently found to sabotage HIV reverse transcription through cytidine deamination within minus DNA strand4-7.Targeting viral replication by DNA examination seems to be one of the major host innate responses defending against retroviral infections. On the other hand, HIV-1 accessory protein Vif prevents APOBEC3G from entering the HIV-1 virion during viral assembly, thus ensuring viral replication in target cells8,9.

HIV-1 viral protein R (Vpr) is another accessory protein. It can be found in the 'serum of HIV-1 infected patients and in the cerebrospinal fluid (CSF) of AIDS patients with neurological pathologies10,11. Because Vpr is a highly conserved viral protein, it presents a good target for host antiviral responses. However, little is known at present about Vpr-host cell interactions or whether Vpr is involved in suppressing host antiviral responses as Vif does. Numerous reports certainly indicate a dynamic interaction between Vpr and host cellular responses. We will briefly summarize those findings here and will examine the potential contribution of Vpr to viral pathogenesis and disease progression.

HIV-1 Vpr is a virion-associated viral gene product with an average length of 96 amino acids, and a calculated molecular weight of 12.7 kDa. However, it typically appears as either a 14 kD or 15 kDa band due to post-translational modifications. Vpr is a highly conserved viral protein among HIV, simian immunodeficiency viruses (SIV) and other lentiviruses12,13. Besides lentiviruses, its protein sequence shares no strong homology with any of the known proteins. A tertiary structure of Vpr proposed on the basis of nuclear magnetic resonance (NMR) analysis consists of an a-helix-turn-a-helix domain in the amino-terminal half from amino acids 17 to 46 and a long α-helix from 53 to 78 in the carboxy-terminal half14,15. These three α-helices are folded around a hydrophobic core in a structure which allows interaction of Vpr with different cellular proteins16.

Vpr displays several distinct activities in host cells. These include cytoplasmic-nuclear shuttling17, induction of cell cycle G2 arrest18 and cell killing19. These three Vpr-specific activities were shown to be functionally independent of each other20,21 and have been demonstrated in a wide variety of eukaryotic cells ranging from human to yeast, indicating that Vpr most likely affects highly conserved cellular processes.

In this review, we describe our current understanding of the host-Vpr interactions and the potential roles of Vpr activities in viral pathogenesis and disease progression.

HIV-1 Vpr and host cellular responses

All regulatory and accessory HIV-1 viral proteins are being targeted by HIV-1-specific CDS-positive cytotoxic T-lymphocytes (CTLs)22. However, Vpr is being preferentially targeted by the CDS+ Tlymphocytes as compared with other viral proteins, at least during the acute phase of the viral infection23,24, suggesting a salient role of Vpr during the early phase of infection. Consistently, some of the cellular proteins, such as heat shock proteins (HSPs), respond quickly to viral infection or vpr gene expression, which suggests another level of host innate immune response25,26 (our unpublished data). For example, HSP27 and HSP70 mRNA transcription appeared as early as 3-8 h following HIV infection. We now know that some of the small heat shock proteins, such as HSP27 or HSP70, exert effective protective effect against some or all of the Vpr activities27-29. However, responsive elevations of HSPs to HIV-I infection are transient as the HSP27 and HSP70 mRNA transcripts were significantly downregulated by 24 h after viral infection, concomitant with the first appearance of the full length genomic HIV-1 mRNA30. This observation implies an active interplay between HIV viral proteins and HSP27 or HSP70. Indeed, an active and antagonistic interaction was seen between Vpr and a yeast homologue of HSP2731. Vpr suppresses antigenspecific CDS-mediated CTL and Th1 immune responses32. Consistent with this notion, Rhesus macaques infected with HIV-2 lacking vpr gene had increased antibody titres compared to monkeys infected with the wild-type virus33. Although molecular mechanisms underlying suppression of CTL and antibody production by Vpr are presently unknown, it was surmised that Vpr may prevent antibody production against the virus by inhibiting T-cell clonal expansion through suppressing T-cell proliferation and inducing cell cycle G2/M arrest34. Evidence also suggests that Vpr may suppress host inflammatory responses, which present another level of the host immune responses to viral infections35. Vpr inhibits host inflammatory responses by down regulating proinflammatory cytokines (TNFα and IL-12) and chemokines (RANTES, MIP-1α and MIP-1β) in a manner similar to glucocorticoids36,37; Vpr additionally suppresses host inflammatory response by inhibiting nuclear factor kappa B (NFκB) activity through the induction of IκB37.

Therefore, there appears to be at least two levels of host responses to vpr gene expression; one is the cellular immune response mediated by CD8+ CTLs; and another innate immune responses involving some of the cellular chaperone proteins. Conversely, Vpr counteracts those host immune responses. It prevents T-cell proliferation, suppresses host inflammatory responses including production of cytokines and chemokines. Vpr may also have additional mechanisms to counterbalance some of those innate reactions, such as heat shock proteins, that have specific suppressive activities against Vpr. These specific host responses to Vpr and the counteracting effect by Vpr strongly suggest a very dynamic interaction between vpr gene expression and host reactions. Future studies should reveal to what extent these interactions contribute to the success of viral infection and will determine the best way to exploit those specific host responses to design strategies aimed at suppressing Vpr.