Mechanisms of HIV persistence during HAART

Mechanisms of HIV persistence during HAART

HIV Virus: Replication Cycle and Pathogenesis in absence of HAART

The human immunodeficiency virus (HIV) is a formidable pathogen that has posed a significant global health challenge for decades. Known for its complex replication cycle and ability to evade the immune system, HIV leads to acquired immunodeficiency syndrome (AIDS) if left untreated. Highly Active Antiretroviral Therapy (HAART) has revolutionized the management of HIV, significantly improving the quality of life and life expectancy of individuals living with the virus. However, to fully appreciate the impact of HAART, it's essential to delve into the intricacies of the HIV replication cycle and its pathogenesis in the absence of treatment.

HIV, the human immunodeficiency virus, exhibits a remarkable ability to infiltrate a variety of host cells, including T-cells, macrophages, dendritic cells, hematopoietic stem cells, and even astrocytes. This broad cell tropism contributes to the complexity of its pathogenesis. The interaction between the viral envelope and host cell membrane triggers a series of events that culminate in the viral replication cycle.

The initial contact between the viral and host membranes instigates the formation of a fusion pore, a critical conduit for the delivery of viral contents into the host cell's cytoplasm. Within this cellular environment, the viral RNA genome undergoes reverse transcription, converting it into DNA. This viral DNA is then translocated into the cell nucleus, where it is seamlessly integrated into the host genome. This integration, facilitated by the viral enzyme integrase, ensures the long-term persistence of viral genetic material within the host cell. Upon the activation of host cells, viral genes are expressed and subsequently translocated into the cytoplasm. Within this cytoplasmic realm, viral proteins are synthesized and meticulously assembled, ultimately giving rise to immature viral particles. The culmination of this process leads to the release of these particles from the host cell through a budding process. Central to the maturation of these viral particles is the viral protease enzyme. It plays a pivotal role in cleaving viral polyproteins into their functional components, thereby transforming the immature virion into an infectious form. This transformation is essential for the virus's ability to further infiltrate and infect other cells.

The course of HIV infection unfolds across three distinct phases, each marked by varying dynamics influenced by both viral replication and host immune responses. The first phase, an acute symptomatic stage, is characterized by a surge in plasma virus load, a precipitous decline in CD4+ T-cell counts, the establishment of viral reservoirs, and the emergence of anti-HIV-specific immune responses. Subsequently, viral load drops, and there is a partial and temporary resurgence in CD4+ T-cell counts, signaling the onset of a prolonged asymptomatic chronic phase. During this phase, the decline in virus production and CD4+ T-cell count occurs at a gradual pace. As the CD4+ T-cell count falls below the critical threshold of 200 cells per microliter, the final phase of AIDS commences. Throughout the course of infection, various forms of HIV can be detected, including plasma HIV RNA, total-integrated-unintegrated HIV DNA, and HIV RNA present in mononuclear peripheral cells (PBMC).

HIV Replication Cycle

HIV persistence during HAART: a terrifying reality

The human immunodeficiency virus (HIV) stands as a relentless adversary, its intricate replication cycle and immune-evading tactics challenging medical research and treatment efforts for decades. The advent of Highly Active Antiretroviral Therapy (HAART) in the field of HIV treatment fueled hope for a potential cure. In 1997, a breakthrough study by Perelson and colleagues underscored the initial promise of HAART, as they demonstrated a staggering 99% reduction in HIV RNA concentration within the plasma during the first two weeks of treatment. This astonishing reduction fueled speculation that with approximately 3.1 years of sustained and suppressive HAART, the complete elimination of HIV might be achieved.

Amid the rising optimism, 1997 also brought a sobering revelation. The work of Chun TW and associates unveiled the existence of a latent HIV reservoir nestled within resting memory CD4 T-cells. This discovery revealed a hidden dimension of the virus's persistence strategy that was previously unknown. It shattered the notion that HAART alone might lead to a swift eradication of the virus.

Further investigations into the latent HIV reservoir introduced a paradigm shift in understanding HIV pathogenesis and persistence. Researchers unveiled a model that illuminated the establishment of a stable reservoir during the acute phase of infection. Remarkably, infected activated CD4 T-cells, rather than succumbing to HIV infection, entered a state of dormancy, preserving integrated HIV DNA within their genetic makeup. This unique cellular population forged an environment conducive to viral persistence. A specific gene expression pattern emerged, supporting long-term cell survival and responsiveness to antigenic stimulation.

Under these strategic conditions, HIV assumed a transcriptionally silent state, granting it refuge from both the immune responses of the host and the effects of HAART. The virus could endure in this latent form for extensive durations, evading detection and maintaining a covert presence. Even in the face of prolonged periods of effective and suppressive HAART, traces of HIV persistence manifested as detectable RNA and cellular HIV DNA in the bloodstream.

The juxtaposition of Perelson's initial optimism and Chun TW's subsequent revelation marked a pivotal moment in HIV research and treatment. It illuminated the complex battle between medical intervention and the virus's tenacity, underscoring the challenges of achieving complete eradication. As we embark on this exploration into the HIV virus's replication cycle and pathogenesis in the absence of HAART, it is with a newfound appreciation for the intricacies of viral persistence and the ongoing pursuit of innovative strategies to overcome its resistance.

Impact of HAART on HIV-1 reservoirs (Deeks et al., 2012)

Mechanisms of persistence

In the realm of HIV research, the monitoring of plasma HIV RNA, intracellular HIV RNA, and DNA forms has emerged as a crucial avenue for understanding the dynamics of virus persistence and replication, especially in individuals receiving effective Highly Active Antiretroviral Therapy (HAART). However, the question of whether the detection of these components equates to ongoing virus replication remains a subject of ongoing investigation.

The quantification of plasma HIV RNA levels has long served as a marker for assessing the viral load in HIV-infected individuals. Effective HAART is designed to suppress plasma HIV RNA levels, thereby inhibiting active virus replication and reducing the risk of disease progression. However, the presence of detectable plasma HIV RNA in some HAART-treated patients has raised intriguing questions. While this phenomenon might suggest ongoing viral replication, alternative explanations, such as the release of viral particles from previously infected cells, have also been proposed.

Within the intricate landscape of the immune system, memory T-cells stand as key players in both immune defense and the persistence of HIV. These cells house integrated HIV DNA, and their proliferation acts as a potent mechanism for expanding the viral reservoir without the necessity for active replication. Infected cells that undergo antigen-stimulation become activated, potentially generating new viral particles. Therefore, even with effective HAART, the intricacies of intracellular HIV RNA and DNA forms underscore the persistence of the virus within the host.

Remarkably, recent studies have unveiled a subset of HAART-treated patients with detectable virus production in lymphoid tissues. In regions where drug penetration or activity is suboptimal, HIV replication can persist in abundance. This phenomenon raises concerns about the potential disruption of the lymphoid architecture, potentially favoring the formation of sanctuary sites of viral reservoirs. The complex interplay between viral replication, drug penetration, and tissue-specific dynamics sheds light on the nuanced challenges faced in HIV treatment. HIV's invasive nature is not limited to systemic compartments. It also penetrates the central nervous system (CNS), where it can establish a foothold. Even in the presence of HAART, HIV RNA levels persist in the cerebrospinal fluid (CSF). This persistence highlights the unique challenges in achieving complete viral suppression in sanctuary sites like the CNS.

The introduction of integrase inhibitor drugs as part of HAART regimens presents an innovative approach to combating HIV replication. These drugs target HIV integration following infection, blocking the virus from incorporating its genetic material into the host cell's DNA. Interestingly, studies have shown an increase in intracellular unintegrated HIV DNA in peripheral blood when integrase inhibitors are added to HAART. This observation indirectly points to rounds of virus replication, despite the overall effectiveness of treatment. Intriguingly, HIV itself has been implicated in promoting cell proliferation. Pioneering research has unveiled a link between HIV integration and oncogenic genes that regulate cell proliferation. This discovery underscores the intricate relationship between viral infection and cellular mechanisms, potentially contributing to the maintenance of the viral pool.

Immune activation and HIV persistence: Friends or Foes?

HIV infection initiates a cascade of events that extend far beyond its immediate impact on the immune system. One of the most significant outcomes is the induction of chronic immune activation and inflammation. Even with the advent of Highly Active Antiretroviral Therapy (HAART), this underlying state of activation often persists, though it may show some improvement. This phenomenon is of paramount importance in understanding the intricate relationship between HIV, the immune system, and the body's overall health.

A key element in this process is the gut tissues, which sustain considerable damage during the course of HIV infection. This damage creates a vulnerability: the translocation of microbial products from the gastrointestinal tract into the bloodstream. These microbial products then act as triggers for immune activation, setting up a self-perpetuating cycle. Despite the introduction of HAART, this cycle is not easily disrupted, contributing to the persistence of immune activation and inflammation even in those with otherwise suppressed viral loads.

The relationship between markers of immune activation and the persistence of HIV during long-term suppressive therapy remains an area of active investigation. While researchers have made strides in understanding this connection, the precise mechanisms are far from straightforward. The existing data paints a nuanced and often inconsistent picture, reflecting the intricate bilateral interactions between the virus and the immune system.

In some studies involving macaques, a tantalizing possibility emerged: HIV-specific immune responses appeared to correlate with a reduction in the size of the viral reservoir. This finding raised hopes for a functional cure, where the virus could be effectively controlled without the need for continuous treatment. However, not all data supports this conclusion. Other studies have indicated a positive association between increased levels of immune activation and higher levels of intracellular HIV DNA in peripheral blood. This suggests that caution is necessary when considering strategies that involve intense T-cell activation, as they may inadvertently exacerbate viral persistence.

Recent evidence has unveiled unexpected layers in the relationship between immune activation, inflammation, and HIV persistence. Surprisingly, some studies have failed to establish a clear association between the levels of immune activation or inflammation and the load of persistent HIV within the body. This revelation underscores the existence of other critical factors at play, influencing the dynamics of HIV persistence that extend beyond simple markers of immune response.

In essence, the landscape of HIV persistence and its interaction with the immune system is far more intricate than previously thought. As researchers strive to unearth the underlying mechanisms, these findings reinforce the notion that achieving a comprehensive understanding of HIV pathogenesis and persistence requires a multifaceted approach. Only through continued investigation, fueled by collaboration and cutting-edge research, can we hope to design interventions that address the complexities of immune activation, inflammation, and HIV persistence in a holistic manner.

(A) Acute HIV infection (B)Chronic activation of the immune system (Klatt et al., 2013)

Future direction

Current Highly Active Antiretroviral Therapy (HAART) has made significant strides in suppressing HIV replication, yet complete eradication of the virus remains elusive. Despite potent antiretroviral regimens, HIV persists within lymphoid organs, the nervous system, and peripheral blood. The intricate mechanisms sustaining this persistence continue to be a subject of scientific debate. These mechanisms encompass a delicate interplay between cellular proliferation and residual low-level virus replication. A complex array of factors, including immune responses, contributes to virus persistence, though the interpretations of recent studies diverge on this matter. Ongoing research endeavors seek to expand our comprehension of HIV's tenacious persistence, with the ultimate goal of refining strategies for future eradication efforts. This pursuit of knowledge holds promise for informing the design of targeted approaches aimed at mitigating the enduring impact of HIV.

Written by Pragna Krishnapur

Pragna Krishnapur completed her bachelor degree in Biotechnology Engineering in Visvesvaraya Technological University before completing her masters in Biotechnology at University College Dublin.

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15th Aug 2023 Pragna Krishnapur, MSc

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