A calculated balance between Virus Replication, Cell Proliferation and Host Immune System
HIV Virus: Replication Cycle and Pathogenesis in absence of HAART
Human immunodeficiency virus (HIV) is transmitted through body fluids (blood, sexual fluids, and breast milk) and causes acquired immunodeficiency syndrome (AIDS). Current highly active antiretroviral therapy (HAART) effectively suppresses virus replication preventing disease progression and mortality. Virus eradication has still to be achieved and current efforts are working to design functional cures.
HIV infects T-cell, macrophages, dendritic cells, hematopoetic stem cells, and astrocytes (Iordanskiy et al., 2013). The contact between the viral and host membrane triggers the formation of a fusion pore responsible for the delivery of viral content into the cellular cytoplasm (Jiao et al., 2015), where the genomic HIV RNA is reverse transcribed into DNA, translocated into the nucleous and integrated into the host genome (Melikyan et al., 2000). Upon cells activation, viral genes are expressed and translocated into the cytoplasm where viral proteins are synthesized and assembled together to compose the immature viral particle (Freed, 2015). During and after release of the virus from the cells (budding), the viral protease enzyme promotes the maturation of the viral particle to become an infectious virion (Fig.1). Figure 1 HIV-1 cell cycle adapted from Pasternak AO et al 2013.
The course of HIV infection develops in three phases (Fig.2) and it is highly variable from patient to patient reflecting a complex interplay between virus replication and host immune responses. A first symptomatic acute phase is characterized by high plasma virus load, a sharp drop in CD4T-cell counts, establishment of virus reservoirs, and the development of the anti HIV specific immune response (Douek et al., 2003). This is followed by a drop in viral load and a partial and temporary increase in CD4 T-cell numbers that marks the beginning of a long asymptomatic chronic phase during which the virus production and the CD4 T-cell number decline is slow. When CD4 T-cell count is less than 200 cells/L the last phase AIDS begins. Over the course of infection different HIV forms can be detected: plasma HIV RNA, total-integrated-unintegrated HIV DNA and HIV RNA in mononuclear peripheral cells (PBMC).
Figure 2; Natural course of HIV-1 infection and markers for HIV-1 detection. Red indicates markers used to detect HIV-1. Image adapted from https://www.slideshare.net/Positive_Force/life-cy... Nature Reviews Immunology.
HIV persistence during HAART: a terrifying reality
The introduction of HAART encouraged HIV cure hopes. In 1997, Perelson and colleagues demonstrated that HIV RNA concentration in plasma dropped by 99% in the first two weeks of HAART (Perelson et al., 1997) (Fig.3) and they estimated that 3.1 years of suppressive HAART would eliminate HIV.
Figure 3: Impact of HAART on HIV-1 reservoirs. Taken and adapted from The International AIDS society scientifc working group on HIV cure.
Sadly, in the same year, Chun TW (Chun et al., 1997) demonstrated the presence of a latent HIV reservoir in resting memory CD4 T-cells. Further studies generated a model by which a stable reservoir is established during the acute phase when infected activated CD4 T-cells rather than die as a result of HIV infection return to a resting state, maintaining HIV DNA integrated in their genome (Finzi et al., 1999). This cell population creates a favourable environment for viral persistence, including a pattern of gene expression that enables long-term survival, and the ability to respond to antigenic stimulation (Siliciano and Greene, 2011). Under these conditions, HIV can persist for long periods in a transcriptionally silent state that protects it from both immune responses and HAART. HIV persistence can be detected as RNA and cellular HIV DNA in blood even after long period of effective and suppressive HAART (Palmer et al., 2008).
Mechanisms of persistence
Cell Proliferation vs Ongoing Virus Replication
Plasma HIV RNA, intracellular HIV RNA and DNA forms can be measured in subjects receiving effective HAART (Ruggiero A. et al. 2017 https://www.futuremedicine.com/doi/abs/10.2217/fv... Whether detection equals ongoing virus replication is not still understood. Memory T-cells carry integrated HIV DNA and their proliferation provide effective expansion of the viral reservoir without replication (Bosque et al., 2011). Infected cells that undergo antigen-stimulation become activated and their proliferation can produce new viruses. When drugs are efficient, HIV replication should be immediately suppressed. Interestingly, two recent studies described that a subset of HAART-patients has virus production in lymphoid tissue where drug penetration or activity are suboptimal (Fletcher et al., 2014, Lorenzo-Redondo et al., 2016). In these body compartments, HIV proliferation is abundant in the absence of HAART and induces severe disruption of the lymphoid architecture which may favour the formation of sanctuary sites of viral reservoir (Schacker et al., 2006). HIV also invades the central nervous system (CNS) and HIV RNA levels remain detectable in the cerebrospinal fluid (CSF) despite HAART. Moreover, in studies where integrase inhibitor drugs were introduced in HAART, an increase of intracellular unintegrated HIV DNA in peripheral blood was observed (Buzon et al., 2010). These drugs block HIV integration following infection and the detection of unintegrated HIV DNA is an indirectly evidence of rounds of virus replication.
Of note, HIV itself can promote cell proliferation with consequent maintenance of the virus pool. Two independent studies demonstrated that HIV integration preferentially occurs within oncogenic genes that regulate cell proliferation (Maldarelli et al., 2014, Wagner et al., 2014).
Immune activation and HIV persistence: Friends or Foes?
HIV infection promotes chronic immune activation and inflammation, which improve but rarely resolve with HAART (Klatt et al., 2013). During this process, gut tissues are damaged enabling the translocation of gastrointestinal microbial products into the systemic circulation that in turn induce immune activation, establishing a self-perpetuating cycle (Shan and Siliciano, 2014).
How markers of immune activation correlate with parameters of HIV persistence during long-term suppressive therapy is poorly defined. Existing data is inconsistent reflecting complex bilateral interactions (Hatano et al., 2013, Chun et al., 2011, Cockerham et al., 2014, Ruggiero et al., 2015). Some studies in macaques showed that HIV-specific responses could reduce the size of the viral reservoir (Miller et al., 2005, Paiardini et al., 2005, Ploquin et al., 2016) providing a rational for a functional cure. Other data indicated that increased levels of immune activation are positively associated with levels of intracellular HIV DNA in peripheral blood (Ruggiero et al., 2015, Hatano et al., 2013, Cockerham et al., 2014) suggesting that T-cell activation in eradication strategies should be used with care. Furthermore, recent evidence showed lack of association between immune activation/inflammation and HIV persistence load (Gandhi et al., 2017) indicating the presence of other crucial players acting in this context.