- Email: [email protected]
HIV-1 Latency by Transition
Immunity
Previews HIV-1 Latency by Transition Boris Julg1,2 and Dan H. Barouch1,2,* 1Center
for Virology and Vaccine Research, Beth Israel Deaconess Medical Center, Boston, MA 02115, USA Institute of Massachusetts General Hospital, Massachusetts Institute of Technology, and Harvard University, Cambridge, MA 02139, USA *Correspondence: [email protected] https://doi.org/10.1016/j.immuni.2017.09.019 2Ragon
The latent HIV-1 reservoir represents the major barrier for the development of an HIV-1 cure. In this issue of Immunity, Shan et al. (2017) highlight that effector-to-memory transitioning (EMT) CD4+ T cells are particularly permissive for the establishment of latent HIV-1 infection. Antiretroviral therapy (ART) has revolutionized HIV-1 treatment but does not cure HIV-1 infection, and plasma viremia rebounds in the majority of HIV-1-infected individuals when ART is stopped. HIV-1 establishes a pool of long-lived memory CD4+ T cells in which replication-competent virus persists as integrated proviral DNA. These latently infected cells are essentially invisible to the immune system and cannot be eliminated by ART but are nevertheless capable of reigniting viral replication in the absence of suppressive ART. A growing body of literature has characterized these latently infected CD4+ T cells and has described mechanisms for inhibition of HIV-1 gene expression in the viral reservoir, but it remains unclear how latent infection is initially established. In particular, the cell populations responsible for the establishment of latent infection and the determinants of susceptibility for latent infection remain to be determined. In this issue of Immunity, Shan et al. (2017) demonstrate that the establishment of HIV-1 latency occurs most efficiently in CD4+ T cells that are transitioning from the effector back to resting memory state (Figure 1). HIV-1 appears to hijack this natural cellular transition to establish latency and then persist for the lifetime of the individual. The viral reservoir was first described approximately two decades ago with the observation of a latent but inducible viral reservoir in individuals on ART with undetectable viremia (Finzi et al., 1997). Data from nonhuman primates suggest that seeding of the viral reservoir can occur very early after infection, potentially even before acute viremia is detected (Whitney et al., 2014). These observations suggest
important challenges for both the prevention and the elimination of the viral reservoir. The HIV-1 reservoir is found primarily in stable, long-lived memory CD4+ T cells (Siliciano et al., 2003), which are believed to represent cells that have transitioned back to a resting state with transcriptionally silent virus. Homeostatic proliferation of latently infected CD4+ T cells and low levels of viral replication may contribute to reservoir persistence (Chomont et al., 2009). The location of viral integration sites in the genome may also influence the expansion and persistence of latently infected CD4+ T cells (reviewed in Lusic and Siliciano, 2017). However, despite an emerging understanding of the mechanisms of viral persistence, little is known about how latency is initially established. In this issue of Immunity, Shan et al. (2017) investigate which subsets of CD4+ T cells optimally support the initial establishment of latency. The authors focused on C-C chemokine receptor type 5 (CCR5)-tropic viruses, as they found that the majority of reservoirderived replication-competent viral isolates in a random sample of ART-treated individuals were CCR5-tropic. Using CCR5-tropic, green fluorescent protein (GFP)-encoding reporter viruses, they infected cells in a primary latency model in which CD4+ T cells from healthy donors are transduced with the anti-apoptotic gene Bcl-2 to promote survival and to mimic the in vivo quiescent state of resting CD4+ T cells. While both activated and resting CD4+ T cells became readily and productively infected with virus, minimal to no latently infected cells were detected following re-stimulation, raising the question of how CCR5-tropic viruses readily establish latency in vivo. The authors infected
CCR5+ primary resting memory CD4+ T cells with the reporter virus but again did not detect efficient establishment of latent infection. Although resting memory CD4+ T cells harbor a significant proportion of the viral reservoir, they are relatively resistant to infection due to blocks of both reverse transcription (Baldauf et al., 2012) and integration (Bukrinsky et al., 1992). Activated CD4+ T cells represent the principal target for HIV-1, but they are often eliminated by activation-induced cell death, virus-induced cytopathic effects, and host CD8+ cytotoxic T cells. Shan et al. (2017) report that CCR5 expression on CD4+ T cells increases markedly between 6 and 12 days after activation as these cells revert from an activated state back to a resting state (Figure 1). These ‘‘effector-to-memorytransitioning’’ (EMT) CD4+ T cells were particularly permissive to latent infection. Moreover, these EMT CD4+ T cells exhibited a global decrease in gene transcription, including reduced levels of NFkB, which is a major transcription factor that regulates HIV-1 gene transcription. The combination of high CCR5 expression that promotes viral entry and low levels of transcription factors that are required for HIV-1 transcription may provide the ‘‘perfect storm’’ that promotes the establishment of viral latency in EMT CD4+ T cells. Although most of the CCR5-expressing EMT CD4+ T cells turn off CCR5 expression once they enter quiescent memory stage, the authors found that the frequency of HIV-1 DNA was 10- to 100-fold higher in CCR5-expressing resting memory CD4+ T cells than in CCR5-negative memory or naive cells. Moreover, these CCR5-expressing cells harbored more
Immunity 47, October 17, 2017 ª 2017 Elsevier Inc. 611
Immunity
Previews
In conclusion, Shan et al. (2017) define EMT CD4+ T cells as a transient cell population several days after activation with upregulated CCR5 expression and decreased overall gene transcription. CD4+ T cells that are transitioning from effector to memory states may therefore be particularly susceptible to latent HIV-1 infection. These insights have important implications for the development of strategies to prevent and to cure HIV-1 infection. REFERENCES Baldauf, H.M., Pan, X., Erikson, E., Schmidt, S., Daddacha, W., Burggraf, M., Schenkova, K., Ambiel, I., Wabnitz, G., Gramberg, T., et al. (2012). Nat. Med. 18, 1682–1687.
Figure 1. Establishment of the Latent HIV-1 Reservoir Effector-to-memory transitioning (EMT) CD4+ T cells exhibit upregulated CCR5 expression and decreased overall gene transcription, rendering them particularly permissive to latent HIV-1 infection.
replication-competent virus as determined by quantitative viral outgrowth assays. The authors concluded that resting memory CD4+ T cells that express CCR5 contribute significantly to the viral reservoir in memory CD4+ T cells, suggesting CCR5 as a potential biomarker that can identify latently infected cells. This manuscript defines key early events and CD4+ T cells subpopulations that favor the establishment of HIV-1 latency in vitro. The conclusion that EMT CD4+ T cells are particularly permissive for latent HIV-1 infection is conceptually appealing and represents an important advance in our understanding of the viral reservoir. These findings should be validated in vivo in animal models and humans, e.g., by using CCR5 blockade to prevent viral entry in EMT CD4+ T cells. These concepts may lead to new HIV-1 prevention and cure strategies. For example, the authors show that virusspecific CD8+ T cells can inhibit the
612 Immunity 47, October 17, 2017
establishment of latently infected CD4+ T cells in vitro, which has clear implications for the development of prophylactic and therapeutic vaccines. Although vaccine-elicited CD8+ T cells have not previously been shown to block the acquisition of simian immunodeficiency virus (SIV) infection in nonhuman primates, they are likely responsible for virologic control and clearance following SIV infection in a prophylactic study in rhesus monkeys immunized with a cytomegalovirus (CMV) vector-based SIV vaccine (Hansen et al., 2013). T cell responses enhanced by therapeutic vaccination with an adenovirus serotype 26 and modified vaccinia Ankara (Ad26/MVA)vector based vaccine have also been correlated with delayed viral rebound and post-rebound virologic control following ART discontinuation in SIVinfected rhesus monkeys, suggesting an effect on the viral reservoir (Borducchi et al., 2016).
Borducchi, E.N., Cabral, C., Stephenson, K.E., Liu, J., Abbink, P., Ng’ang’a, D., Nkolola, J.P., Brinkman, A.L., Peter, L., Lee, B.C., et al. (2016). Nature 540, 284–287. Bukrinsky, M.I., Sharova, N., Dempsey, M.P., Stanwick, T.L., Bukrinskaya, A.G., Haggerty, S., and Stevenson, M. (1992). Proc. Natl. Acad. Sci. USA 89, 6580–6584. Chomont, N., El-Far, M., Ancuta, P., Trautmann, L., Procopio, F.A., Yassine-Diab, B., Boucher, G., Boulassel, M.R., Ghattas, G., Brenchley, J.M., et al. (2009). Nat. Med. 15, 893–900. Finzi, D., Hermankova, M., Pierson, T., Carruth, L.M., Buck, C., Chaisson, R.E., Quinn, T.C., Chadwick, K., Margolick, J., Brookmeyer, R., et al. (1997). Science 278, 1295–1300. Hansen, S.G., Piatak, M., Jr., Ventura, A.B., Hughes, C.M., Gilbride, R.M., Ford, J.C., Oswald, K., Shoemaker, R., Li, Y., Lewis, M.S., et al. (2013). Nature 502, 100–104. Lusic, M., and Siliciano, R.F. (2017). Nat. Rev. Microbiol. 15, 69–82. Shan, L., Deng, K., Gao, H., Xing, S., Capoferri, A.A., Durand, C.M., Rabi, S.A., Laird, G.M., Kim, M., Hosmane, N.N., et al. (2017). infection. Immunity 47, this issue, 766–775. Siliciano, J.D., Kajdas, J., Finzi, D., Quinn, T.C., Chadwick, K., Margolick, J.B., Kovacs, C., Gange, S.J., and Siliciano, R.F. (2003). Nat. Med. 9, 727–728. Whitney, J.B., Hill, A.L., Sanisetty, S., PenalozaMacMaster, P., Liu, J., Shetty, M., Parenteau, L., Cabral, C., Shields, J., Blackmore, S., et al. (2014). Nature 512, 74–77.
Previews HIV-1 Latency by Transition Boris Julg1,2 and Dan H. Barouch1,2,* 1Center
for Virology and Vaccine Research, Beth Israel Deaconess Medical Center, Boston, MA 02115, USA Institute of Massachusetts General Hospital, Massachusetts Institute of Technology, and Harvard University, Cambridge, MA 02139, USA *Correspondence: [email protected] https://doi.org/10.1016/j.immuni.2017.09.019 2Ragon
The latent HIV-1 reservoir represents the major barrier for the development of an HIV-1 cure. In this issue of Immunity, Shan et al. (2017) highlight that effector-to-memory transitioning (EMT) CD4+ T cells are particularly permissive for the establishment of latent HIV-1 infection. Antiretroviral therapy (ART) has revolutionized HIV-1 treatment but does not cure HIV-1 infection, and plasma viremia rebounds in the majority of HIV-1-infected individuals when ART is stopped. HIV-1 establishes a pool of long-lived memory CD4+ T cells in which replication-competent virus persists as integrated proviral DNA. These latently infected cells are essentially invisible to the immune system and cannot be eliminated by ART but are nevertheless capable of reigniting viral replication in the absence of suppressive ART. A growing body of literature has characterized these latently infected CD4+ T cells and has described mechanisms for inhibition of HIV-1 gene expression in the viral reservoir, but it remains unclear how latent infection is initially established. In particular, the cell populations responsible for the establishment of latent infection and the determinants of susceptibility for latent infection remain to be determined. In this issue of Immunity, Shan et al. (2017) demonstrate that the establishment of HIV-1 latency occurs most efficiently in CD4+ T cells that are transitioning from the effector back to resting memory state (Figure 1). HIV-1 appears to hijack this natural cellular transition to establish latency and then persist for the lifetime of the individual. The viral reservoir was first described approximately two decades ago with the observation of a latent but inducible viral reservoir in individuals on ART with undetectable viremia (Finzi et al., 1997). Data from nonhuman primates suggest that seeding of the viral reservoir can occur very early after infection, potentially even before acute viremia is detected (Whitney et al., 2014). These observations suggest
important challenges for both the prevention and the elimination of the viral reservoir. The HIV-1 reservoir is found primarily in stable, long-lived memory CD4+ T cells (Siliciano et al., 2003), which are believed to represent cells that have transitioned back to a resting state with transcriptionally silent virus. Homeostatic proliferation of latently infected CD4+ T cells and low levels of viral replication may contribute to reservoir persistence (Chomont et al., 2009). The location of viral integration sites in the genome may also influence the expansion and persistence of latently infected CD4+ T cells (reviewed in Lusic and Siliciano, 2017). However, despite an emerging understanding of the mechanisms of viral persistence, little is known about how latency is initially established. In this issue of Immunity, Shan et al. (2017) investigate which subsets of CD4+ T cells optimally support the initial establishment of latency. The authors focused on C-C chemokine receptor type 5 (CCR5)-tropic viruses, as they found that the majority of reservoirderived replication-competent viral isolates in a random sample of ART-treated individuals were CCR5-tropic. Using CCR5-tropic, green fluorescent protein (GFP)-encoding reporter viruses, they infected cells in a primary latency model in which CD4+ T cells from healthy donors are transduced with the anti-apoptotic gene Bcl-2 to promote survival and to mimic the in vivo quiescent state of resting CD4+ T cells. While both activated and resting CD4+ T cells became readily and productively infected with virus, minimal to no latently infected cells were detected following re-stimulation, raising the question of how CCR5-tropic viruses readily establish latency in vivo. The authors infected
CCR5+ primary resting memory CD4+ T cells with the reporter virus but again did not detect efficient establishment of latent infection. Although resting memory CD4+ T cells harbor a significant proportion of the viral reservoir, they are relatively resistant to infection due to blocks of both reverse transcription (Baldauf et al., 2012) and integration (Bukrinsky et al., 1992). Activated CD4+ T cells represent the principal target for HIV-1, but they are often eliminated by activation-induced cell death, virus-induced cytopathic effects, and host CD8+ cytotoxic T cells. Shan et al. (2017) report that CCR5 expression on CD4+ T cells increases markedly between 6 and 12 days after activation as these cells revert from an activated state back to a resting state (Figure 1). These ‘‘effector-to-memorytransitioning’’ (EMT) CD4+ T cells were particularly permissive to latent infection. Moreover, these EMT CD4+ T cells exhibited a global decrease in gene transcription, including reduced levels of NFkB, which is a major transcription factor that regulates HIV-1 gene transcription. The combination of high CCR5 expression that promotes viral entry and low levels of transcription factors that are required for HIV-1 transcription may provide the ‘‘perfect storm’’ that promotes the establishment of viral latency in EMT CD4+ T cells. Although most of the CCR5-expressing EMT CD4+ T cells turn off CCR5 expression once they enter quiescent memory stage, the authors found that the frequency of HIV-1 DNA was 10- to 100-fold higher in CCR5-expressing resting memory CD4+ T cells than in CCR5-negative memory or naive cells. Moreover, these CCR5-expressing cells harbored more
Immunity 47, October 17, 2017 ª 2017 Elsevier Inc. 611
Immunity
Previews
In conclusion, Shan et al. (2017) define EMT CD4+ T cells as a transient cell population several days after activation with upregulated CCR5 expression and decreased overall gene transcription. CD4+ T cells that are transitioning from effector to memory states may therefore be particularly susceptible to latent HIV-1 infection. These insights have important implications for the development of strategies to prevent and to cure HIV-1 infection. REFERENCES Baldauf, H.M., Pan, X., Erikson, E., Schmidt, S., Daddacha, W., Burggraf, M., Schenkova, K., Ambiel, I., Wabnitz, G., Gramberg, T., et al. (2012). Nat. Med. 18, 1682–1687.
Figure 1. Establishment of the Latent HIV-1 Reservoir Effector-to-memory transitioning (EMT) CD4+ T cells exhibit upregulated CCR5 expression and decreased overall gene transcription, rendering them particularly permissive to latent HIV-1 infection.
replication-competent virus as determined by quantitative viral outgrowth assays. The authors concluded that resting memory CD4+ T cells that express CCR5 contribute significantly to the viral reservoir in memory CD4+ T cells, suggesting CCR5 as a potential biomarker that can identify latently infected cells. This manuscript defines key early events and CD4+ T cells subpopulations that favor the establishment of HIV-1 latency in vitro. The conclusion that EMT CD4+ T cells are particularly permissive for latent HIV-1 infection is conceptually appealing and represents an important advance in our understanding of the viral reservoir. These findings should be validated in vivo in animal models and humans, e.g., by using CCR5 blockade to prevent viral entry in EMT CD4+ T cells. These concepts may lead to new HIV-1 prevention and cure strategies. For example, the authors show that virusspecific CD8+ T cells can inhibit the
612 Immunity 47, October 17, 2017
establishment of latently infected CD4+ T cells in vitro, which has clear implications for the development of prophylactic and therapeutic vaccines. Although vaccine-elicited CD8+ T cells have not previously been shown to block the acquisition of simian immunodeficiency virus (SIV) infection in nonhuman primates, they are likely responsible for virologic control and clearance following SIV infection in a prophylactic study in rhesus monkeys immunized with a cytomegalovirus (CMV) vector-based SIV vaccine (Hansen et al., 2013). T cell responses enhanced by therapeutic vaccination with an adenovirus serotype 26 and modified vaccinia Ankara (Ad26/MVA)vector based vaccine have also been correlated with delayed viral rebound and post-rebound virologic control following ART discontinuation in SIVinfected rhesus monkeys, suggesting an effect on the viral reservoir (Borducchi et al., 2016).
Borducchi, E.N., Cabral, C., Stephenson, K.E., Liu, J., Abbink, P., Ng’ang’a, D., Nkolola, J.P., Brinkman, A.L., Peter, L., Lee, B.C., et al. (2016). Nature 540, 284–287. Bukrinsky, M.I., Sharova, N., Dempsey, M.P., Stanwick, T.L., Bukrinskaya, A.G., Haggerty, S., and Stevenson, M. (1992). Proc. Natl. Acad. Sci. USA 89, 6580–6584. Chomont, N., El-Far, M., Ancuta, P., Trautmann, L., Procopio, F.A., Yassine-Diab, B., Boucher, G., Boulassel, M.R., Ghattas, G., Brenchley, J.M., et al. (2009). Nat. Med. 15, 893–900. Finzi, D., Hermankova, M., Pierson, T., Carruth, L.M., Buck, C., Chaisson, R.E., Quinn, T.C., Chadwick, K., Margolick, J., Brookmeyer, R., et al. (1997). Science 278, 1295–1300. Hansen, S.G., Piatak, M., Jr., Ventura, A.B., Hughes, C.M., Gilbride, R.M., Ford, J.C., Oswald, K., Shoemaker, R., Li, Y., Lewis, M.S., et al. (2013). Nature 502, 100–104. Lusic, M., and Siliciano, R.F. (2017). Nat. Rev. Microbiol. 15, 69–82. Shan, L., Deng, K., Gao, H., Xing, S., Capoferri, A.A., Durand, C.M., Rabi, S.A., Laird, G.M., Kim, M., Hosmane, N.N., et al. (2017). infection. Immunity 47, this issue, 766–775. Siliciano, J.D., Kajdas, J., Finzi, D., Quinn, T.C., Chadwick, K., Margolick, J.B., Kovacs, C., Gange, S.J., and Siliciano, R.F. (2003). Nat. Med. 9, 727–728. Whitney, J.B., Hill, A.L., Sanisetty, S., PenalozaMacMaster, P., Liu, J., Shetty, M., Parenteau, L., Cabral, C., Shields, J., Blackmore, S., et al. (2014). Nature 512, 74–77.