Introduction

The human immunodeficiency virus (HIV) is a retrovirus with a size of approximately 120 nm [1] and belongs to the group of lentiviruses. It contains a genome with two RNA strands, the enzymes integrase, reverse transcriptase, and protease, and the TAT protein. The viral genomic RNA comprises nine genes (gag, pol, env, tat, rev, nef, vif, vpr, and vpu) with a total size of less than 10 kb. It is capable of initiating the production of 15 proteins needed for replication in the host cell.

There are two species of virus, HIV-1 and HIV-2. Both have their origin in monkeys in Africa, HIV-1 in chimpanzees, and HIV-2 in the sooty mangabey. Probably at the beginning of the 20th century, the virus jumped species from animal to man via at least three stations, leading to the three groups of the virus, M, N, and O [2]. HIV-2 is less aggressive in terms of virulence and infectivity and is therefore more or less confined to West Africa, whereas HIV-1 has spread globally. In the following, the term HIV is used—if not explicitly otherwise stated—for HIV-1 for which the majority of research is applied. The reason for the worldwide spread of HIV lies in the fact that the virus only leads to clear symptoms years after infection, in some cases, early on to flu-like symptoms, and thereby is able to spread to quite a number of new hosts before treatment is initiated, resulting in exponential distribution. This behavior makes the difference to other virus infections, such as Ebola, which kill their hosts within days after infection and thereby prevent global spread.

HIV is sexually transmitted via semen or by the transfer of blood or breast milk of feeding mothers. Other body fluids, such as tears, saliva, or sweat, are not involved. Untreated, HIV infection finally leads to acquired immunodeficiency syndrome (AIDS). Life expectancy is approximately 11 years for untreated patients [3]. AIDS was first identified in 1981, and its connection to HIV was established in 1983 [4, 5].

HIV infection/AIDS proceeds in three phases. Following infection, flu-like symptoms, fever, or swelling of lymph nodes can occur and might last for one or two weeks. The next phase is symptomless and might last on average 11 years [6] but can reach up to 20 years [7]. The last phase is the AIDS phase and is characterized by opportunistic infections of any of the organ systems. The impact of HIV/AIDS has been tremendous regarding both society as a whole and individuals [8, 9].

HIV mechanism of infection

Upon entry of the host’s body, the virus starts infecting immune cells, in particular T cells (CD4⁺ T cells), macrophages, and dendritic cells (DCs), and begins to replicate at an extremely high speed, leading to plasma levels of viruses in the millions per mL [10]. In parallel, the number of CD4⁺ T cells declines, and CD8⁺ T cells are activated and start killing viruses and producing antibodies directed against the virus. This leads to a decline in viral plasma levels after an initial peak. In the long run, the levels of CD4⁺ T cells become too low to be able to control opportunistic infections, and the patient inevitably develops AIDS, which finally leads to death. The AIDS phase begins when the levels of immune cells have decreased to approximately 200 lymphocytes per mm³. At that time point, the levels of virus had increased again to approximately 500 copies per mL.

HIV is diagnosed via blood tests, which are based on antibody or antibody + p24 antigen or PCR testing. They are possible starting 23–90 days after infection (antibody), 18–45 days after infection (antibody + p24), or 10–33 days after infection (PCR) [11].

HIV disseminates throughout the human body within 1–2 weeks [12]. Attachment of the virus to the host cell proceeds via the HIV envelope (Env) glycoprotein complex, which is built from three gp41 and three gp120 proteins that are formed following post-translational cleavage of the gp160 polypeptide [13]. The proteins are heavily protected using glycosylation against attacks by the host’s immune system [14] (Figure 1).

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Figure 1. Composition of the Env glycoprotein complex of HIV. The Env ectodomain has a median of 30 N-linked glycosylation sites per protomer [15], which strongly limits accessibility to the protein surface by host defense systems. This and the enormous genetic variability due to the mutation of HIV make it nearly impossible for the host to generate antibodies that can neutralize HIV [16]. The same issue holds for the production of vaccines for HIV treatment, which will be discussed later.

The attachment procedure runs via HIV-mediated CD4 receptor and C-X-C chemokine receptor type 4 (CXCR4) or C-C chemokine receptor type 5 (CCR5) coreceptor interaction, leading to a gp120 conformation change, which is necessary for coreceptor coupling [17]. For the fusion of the virus and host cell membranes, gp41 is responsible [15]. CXCR4 primarily facilitates infection in CD4⁺ T cells, while CCR5 is mainly associated with macrophage infection [18]. Following attachment and fusion, the cell content of the virus (genome, reverse transcriptase, integrase, protease) is released into the host cell’s cytoplasm. The viral RNA is then converted into viral DNA with the help of reverse transcriptase, transported into the nucleus of the host cell through the nuclear pore, and embedded in chromatin, followed by integration into the host genome using the enzyme integrase. The integrated viral DNA is called provirus. It remains dormant, forming latent reservoirs unless the host cell activates it. Multiple copies of viral RNA are then created by the transcription of the provirus by the host cell RNA polymerase II enzyme [19] and transported into the cytoplasm. The other building blocks of the virus are created by the host’s ribosomes according to the plan laid down in the HIV RNA and inserted into the host’s genome. At the host cell membrane, the viral RNA genome and proteins are assembled to form multiple immature virions. After formation of an Env, the immature virions emerge from the host cell membrane [20, 21] and, after maturation, move on to infect further host cells. A summary of the procedure is provided in Figure 2. Productive infection is initially established in a pool of phenotypically and clonotypically distinct T cells, and latently infected cells are generated simultaneously [12].

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Figure 2. The HIV replication cycle of a single virus leads to multiple copies. NRTI: nucleoside reverse transcriptase inhibitors; NNRTI: non-nucleoside reverse transcriptase inhibitors; INSTI: integrase strand transfer inhibitors; PI: protease inhibitors; FI: fusion inhibitors; CCR5: C-C chemokine receptor type 5; AI: attachment inhibitors: bind to the gp120 protein of the virus; PAI: post-attachment inhibitors (block CD4 receptors on immune cells); CI: capsid inhibitors.

The HIV-1 genome contains 9,173 nucleotides and is primarily a coding RNA with nine open reading frames, which is able to produce 15 proteins [22]. The matrix of the virus, capsid and nucleocapsid, and p6 proteins are produced by the proteolytically processed Gag polyprotein precursor. Protease, reverse transcriptase, and integrase are compiled by the Gag-Pol polyprotein. The proteins gp120 and gp41 are encoded in the env gene. Virions contain genomic RNA as a non-covalent dimer, which is 5' capped and 3' polyadenylated.

HIV/AIDS symptoms

According to WHO [23], HIV infection is diagnosed by antibody testing based on two tests with different antibodies and/or by two positive virological tests for HIV or its components (HIV-RNA or HIV-DNA or ultrasensitive HIV p24 antigen). The immune status of a patient is assessed by measuring the absolute number (per mm³) or percentage of CD4⁺ T cells. Increasing depletion of CD4⁺ T cells is a clear sign of progression coupled with an increased likelihood of opportunistic infections and other clinical events, leading, finally, if untreated, to death.

Clinical criteria for diagnosis of advanced HIV in adults and children with confirmed HIV infection [23]:

• Presumptive or definitive diagnosis of any stage 3 or stage 4 condition and/or;

Immunological criteria for diagnosing advanced HIV in adults and children five years or older with confirmed HIV infection:

• CD4 count less than 350 per mm³ of blood in an HIV-infected adult or child and/or;

Immunological criteria for diagnosing advanced HIV in a child younger than five years of age with confirmed HIV infection:

• %CD4⁺ < 30 among those younger than 12 months;

• %CD4⁺ < 25 among those aged 12–35 months;

• %CD4⁺ < 20 among those aged 36–59 months;

HIV/AIDS develops in three stages: acute infection, clinical latency, and AIDS [23]. Primary infection usually presents as an acute febrile illness 2–4 weeks post-exposure, often with lymphadenopathy, pharyngitis, maculopapular rash, orogenital ulcers, and meningoencephalitis. Clinical staging of established HIV infection comprises four stages: asymptomatic (1), mild (2, e.g., moderate weight loss), advanced (3, severe weight loss), and severe symptoms (4, substantial weight loss). More details are available in [23].

Treatment of HIV infection

HIV infection is still an uncurable disease necessitating lifelong treatment. Start of ART (antiretroviral therapy, formerly called highly active antiretroviral therapy, HAART) is recommended as soon as the diagnosis has been made [14, 24] and irrespective of baseline CD4 count. It may even be provided the same day [17]. Based on the different parts of the mechanism of HIV replication, there are currently nine classes and more than 30 drugs described on the National Institute of Health (NIH) website [25], ranging from single-drug to four-drug tablets, as outlined in Tables 1 and 2.

Due to rapid viral reproduction, peak levels of RNA can reach up to 10⁷ copies per mL of plasma [26]. T cells infected before ART decline rapidly, with an estimated half-life of 1–2 days [27, 28]. ART leads to a decay of viral RNA in the plasma of patients down to 0.6 copies/mL. Adherence to continuous drug intake according to prescription is the most important issue of HIV medication. It allows a nearly normal life of the patient with viral plasma levels below the detection limit. As a further result, the patient is no longer transmittable regarding the virus. According to Matsui et al. [29], “undetectable = untransmittable” (U = U). Based on the same U = U principle, lenacapavir was developed for the prophylaxis of HIV infection [30, 31]. Additionally, it is active against highly resistant treatment of HIV [32]. Lenacapavir is a capsid inhibitor that is prophylactically injected every 6 months.

Cessation of drug intake on ART inevitably results in recurrence of the virus in the blood or viral rebound, which occurs within several weeks, with a mean of two weeks, in the majority of patients [33, 34]. The cause of viral rebound is that a small fraction of infected circulating CD4⁺ T cells with intact viral genomes (below 5–10%) transition into a state of latency and only regain activity as quickly as 14 days after ART is discontinued [35]. That is the main reason why ART is a lifelong treatment.

However, approx. 0.2–0.5% of HIV patients (“elite controllers”) are able to sustain viral load suppression and sufficiently high CD4⁺ T cell counts after discontinuing ART [17, 36]. Elite controllers are characterized by broad, specific, and superior T-cell, innate cell, and natural killer (NK) cell effector functions. Detailed analyses for previously untreated elite controllers have been reported by Turk et al. [37] and by Jiang et al. [38]. They conclude that these elite controllers seem to exemplify attributes of a “block and lock” mechanism of viral control, defined by silencing of pro-viral gene expression through chromosomal integration into repressive chromatin locations caused by cell-mediated immune selection forces that preferentially eliminate pro-viral sequences more permissive to viral transcription, in a process that they suggest referring to as the “autologous shock and kill” mechanism.

Latent HIV reservoir formation (latency)

Latent HIV reservoirs are the major hurdle to HIV cure. The HIV reservoir is shaped by viral transcriptional suppression and by clonal expansion [39]. Genetic diversity of the virus increases rapidly during infection, due to errors in reverse transcription [40] and high rates of viral recombination and turnover [21, 27]. Latency is established very early in the course of infection, typically within the first days or weeks following exposure [41] and cannot be blocked by early ART [42]. However, early treatment does have an effect on the time of viral rebound [43]. Currently, a clinical trial (NCT02140255) is recruiting infants with HIV using early intensive treatment to achieve HIV remission. The ART regimen consists of three to four components out of nucleoside reverse transcriptase inhibitors (NRTIs), nevirapine, raltegravir, dolutegravir, VRC07-523LS, and VRC01 administered within 48 h of birth. In a simian immunodeficiency virus (SIV) monkey model, latency was observed as early as three days after infection [44]. HIV integrates into the host genome as a translationally inactive provirus, resulting in a long-lived (latent) reservoir of infected cells. The potential cell targets range from T cells to macrophages, myeloid cells, mast cells, NK cells, microglia, and DCs [45–48]. Most commonly, resting memory CD4⁺ T cells are involved [38]. Intact HIV proviruses are predominantly integrated in heterochromatin [49]. Transcriptional silence might only occur in a subset of proviruses integrated in repressive heterochromatin locations, such as repetitive satellite DNA or zinc finger (ZNF) genes that are loaded with inhibitory histone modifications deposited by the human silencing hub (HUSH) complex [50].

The affected body parts include lymphoid tissues, which cover most of the HIV reservoir [49, 51, 52], in particular the gut mucosa and gut-associated lymphoid tissue (GALT) [53], the central nervous system [36, 54] (where astrocytes [55], pericytes [56] and myeloid-resident cells, such as microglia and perivascular macrophages [57] can be infected), reproductive organs, lung, heart, liver, kidneys, thymus, and bone marrow [58]. These are body regions, some of which cannot easily be reached by ART. But even if they could be reached, it would not help, since the proviruses are protected by their translational inactivity.

HIV latency in T cells is maintained by blocking transcriptional elongation, completion, and splicing [59, 60], keeping the phenotype of the cells unchanged [61–64]. More than 50% of all latently infected cells after ART treatment result from some type of clonal expansion [41]. Latency can be induced by two mechanisms: the loss or inhibition of host post-transcriptional regulatory systems and the scarcity of several RNA exporters, such as polypyrimidine tract-binding protein (PTB) or matrin3 (MATR3), which causes nucleus retention of HIV RNAs in resting CD4⁺ T cells [65, 66]. Various studies have shown that the half-life of the HIV reservoir can range somewhere between 44 months and 13 years, and in some cohorts, no decay was observed at all [39].

Approaches to curing HIV

Before going into detail, we need to define what “HIV cure” means. It should comprise the following characteristics:

• No further treatment

• Permanent suppression of HIV transcription

• No disease progression

• Undetectable virus replication

• No viral transmission

In a consensus meeting, a Target Product Profile for a procedure or drug that achieves HIV cure has been agreed upon, including the steps to come up with a single drug that cures HIV in one administration [67, 68].

The currently approved drugs used in ART are characterized by the fact that they are not able to kill the virus but, instead, inhibit viral replication. However, any drug able to actually kill the virus, additionally, would need the ability to reach it, wherever it is hiding, which is a second, and hardly achievable goal. In principle, the human immune system should be the first line of defense. Unfortunately, this is actually the major point of attack of the virus, shutting down the immune system, not completely, but long enough to allow for widespread distribution of the virus within the population. Shutting the immune system down, however, is not enough for the virus; disabling any attack by the immune system comes on top. Furthermore, constant change of the viral surface makes it nearly impossible to use antibodies for the treatment directed against the virus. Any vaccination efforts have therefore so far been unsuccessful.

Attempts to cure HIV can be characterized into the following categories, which—except for the last one—need combinations of more than one type:

• Kill the virus

• Unlock the latent reservoirs

• Permanently close down the reservoirs

• Modify the surface of the T cells (CCR5Δ32 mutation)

We will start with the last category, which has been performed by stem cell transplantation and which has provided a cure for HIV to a small number of patients.

Kill approaches

Combination approaches

Currently available ART uses combinations of different drugs that, in the majority, aim to inhibit the reproduction of the virus by either attacking early in the process, such as attachment to the host cell, or late, for example, by inhibiting capsid formation. Combinations of other mechanisms are the next step in the search for an HIV cure. Various approaches have been investigated to diminish viral rebound by either reactivating viral reservoirs and subsequently killing the virus (named “shock and kill” or “kick and kill” or “shock and clear”), by suppressing the reservoirs “forever” so that reactivation of the dormant virus is blocked (“block and lock”) or by locking the virus and killing it in the infected cell (“block and clear”). Figure 3 provides an illustration of the three approaches.

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Figure 3. Illustration of the “shock and kill”, “block and lock”, and “block and clear” approaches. LRA: latency reversing agent; LPA: latency promoting agent; Akt: protein kinase B; Akt-I: Akt inhibitor.

In Table 8, some examples of the different approaches are listed.

Table 8. Some examples of the different combination approaches.

Method	Components	Remarks	Reference(s)
Shock and kill	OKT3 + IL-2	Clinically not effective	[168]
	ART + vorinostat (LRA) + AGS-004 (DC vaccine)	NCT02707900 Safe but not effective	[169, 170]
	ART + vorinostat (LRA) + VRC07-523LS (bNab)	NCT03803605	[171]
	ART + vorinostat (LRA) + disulfiram	NCT03198559 Evidence of latency reversal, but not safe	[172]
	Venetoclax (pro-apoptotic) + anti-CD3 plus anti-CD28 T cell stimulation	Bcl-2 antagonist	[173]
	CRISPRa + gRNA-V	In vitro, an effective all-in-one approach	[174]
	Vorinostat + acitretin	RIG-I inducer Reactivates latent HIV and induces apoptosis	[175]
	LEDGINs + JQ1	Inhibits the HIV-1 integrase and LEDGF/p75 interaction	[167]
	YSE028 + JQ1	PKC activator + BET bromodomain inhibitor	[176]
Block and lock	dCA + ART	Tat inhibitor	[177]
	LEDGINs + ZL0580	BRD4 modulator	[167, 178]
	Triplitode + ART (Tenofovir + lamivudine + Lopinavir / ritonavir + raltegravir)	Immune modulator	A
	ZL0580 + ART	BRD4 modulator	[179]
Block and clear	LPA + Capivasertib	Akt inhibitor	[180]

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CRISPRa: CRISPR activated; RIG-I: retinoic acid-inducible gene I; dCA: didehydro-cortistatin A; LRA: latency reversing agent; Akt: protein kinase B; A: NCT02219672 (not published yet).

Conclusions

AIDS was first identified in 1981, and its connection to HIV was established in 1983. And, still in 2025, there is no cure for the disease available except for the few cases with CCR5Δ32 donor stem cell transplantations. The reasons for the lack of a general cure are the latent reservoirs where the virus can hide in the form of a provirus until ART is stopped, the high mutation rate, and the avoidance of any attacks by the patient’s immune system, which is either no longer functioning or cannot detect and fight the virus. These characteristics are very similar to cancer, in particular regarding oncogenes or cancer stem cells.

However, in contrast to many types of cancer, HIV infection is no longer a destination of death, at least in those parts of the world that can afford treatment. ART has made HIV a life-long treatment with relatively high safety. Measured at this standard, any HIV cure needs to provide significantly greater benefits and/or diminished side effects in comparison to lifelong ART, because with improved treatment, HIV has become a chronic but manageable condition.

Huge efforts are still being undertaken in the search for a cure. The steps necessary for achieving a cure have been identified and described by a Working Group in 2021 [67]. Most probably, the process would comprise three generations of drugs until the final goal of “one drug one shot”. The first generation includes combinations of compounds with different mechanisms, excluding ART, such as, for example, the “shock and kill” approach, which is currently considered by most researchers as the preferred procedure to move on. But also, bNabs and vaccines fall into that category. The second generation includes modifications of cells, such as CAR T cells, NK cells, or B cells, without the need for prior myeloablation. And the third generation comprises in vivo modifications of the patient’s cells using gene therapies or other methods available at that time.

In the current first generation, combinations of drugs have not been able to completely deplete the latent reservoirs since the LRAs are either not strong enough or not able to reach all the reservoirs. Likewise, the kill components of the “shock and kill” approach have not been able to clear all newly produced viruses. In the kill phase after HIV-1 reactivation, it is necessary to inhibit replication of the reactivated virus and prevent new infection. This could be done, for example, by specific cytolytic T cells, but so far this procedure has not been effective [174]. Sealing the reservoirs, as described in the “block and lock” approach, did not lead to a permanent lockdown of the reservoirs. Combinations of LRAs with drugs exhibiting kill power, nevertheless, should be further evaluated.

Vaccines have not fulfilled the expectations that had been put into this procedure, although much progress has been made in B-cell lineage immunogen design, germline-targeting, immunofocusing, and structure-based immunogen design [100]. However, the learnings from the COVID-19 pandemic and the huge success of mRNA vaccine technology should be applied to HIV in a broad effort. The advantages of the mRNA approach are multifold, ranging from simultaneous use of many different components, scalability, the use of the LNP technology, and the large safety margin. The COVID-19 vaccines were injected intramuscularly and nevertheless were able to reach the lungs in sufficient concentrations to be effective. This should provide hope that, in the case of HIV, also hard-to-reach reservoirs could be a target. Another aspect of the vaccine technology is the use of adjuvants, which have made great progress, but further efforts are still needed.

Gene editing might be the ultimate goal because it could offer a permanent solution through either completely excising the HIV provirus or by modifying susceptible cells to resist infection. However, it must reach all reservoirs and must avoid off-target effects.

In summary, the road to a cure for HIV is heavily populated, but until the final goal is reached, there are still quite a number of stations on the way. My personal favorite to achieve a cure, defined as the eradication of HIV, is mRNA technology. On the way to a cure, I would also like to see a monkey study with alemtuzumab, aiming at two objectives. First, is treatment right after infection effective in avoiding the formation of latent reservoirs? And second, is the treatment with alemtuzumab at the point of rebound able to kill all newly infected cells and simultaneously deprive the virus of its site of survival, namely any cells in which it could hide?