Immune Response to HIV-1 Infection and Vaccine Development

Abstract

Human immunodeficiency virus (HIV)-1 infection represents an ongoing challenging public health epidemic. This is in part because of the socioeconomic burden on low-income countries, lack of access to highly active antiretroviral therapy and other medical treatment, and progression to acquired immunodeficiency syndrome (AIDS) over the course of years. To control or eradicate this virus, a prophylactic vaccine must be generated. Despite several decades of research, development, and clinical trials, there is not yet an effective immunization. This chapter focuses on unique aspects of the immune response to this infection, challenges of vaccine development, key clinical trials, and promising vaccine strategies.

1. Introduction

Currently, the UNAIDS estimates that there are 38.4 million people with human immunodeficiency virus (HIV)-1 infections worldwide as of 2021. Of these 38.4 million people, approximately a quarter of people (25.2% or 9.7 million) with HIV-1 infections are not accessing the standard of care, antiretroviral therapy (ART) [1].

ART is the only known treatment to slow the progression to acquire immunodeficiency syndrome (AIDS) and to prevent the spread of infection, but there are several shortcomings with this therapy, as discussed in further detail in the next section.

HIV-1 infection and its progression to AIDS is a global health crisis and disproportionally affects low-income countries, particularly those in Sub-Saharan Africa. Though there is 12% of the global population in Sub-Saharan Africa, this region has 71% of the world’s population of people with HIV infections [2]. HIV infection is the highest in young men and women in child-bearing age [3], with women having up to eightfold higher risk for acquiring HIV than their male cohorts [2]. These women with ages between 15 and 24 are also more likely to acquire the infection five to seven years before men in the same age group [2]. Given the great number of women impacted by HIV-1 infections, this leads to pediatric infections from perinatal transmission or transmissions from breast milk. Notably, pediatric infections are very different than adult infections and detailed in Section 5.

It is widely accepted that immunization is the most cost-effective, scalable, and lasting public health intervention method to end the HIV epidemic [4]. Though a prophylactic HIV-1 vaccine has been researched and developed for several decades, there is yet to be a licensed vaccine on the market. The main obstacles preventing the generation of a successful vaccine are as follows: (i) HIV-1 has a high rate of mutation and viral replication, with extraordinary worldwide genetic diversity [4], as discussed in the following paragraph and “immune response” section, (ii) immune response behind an HIV-1 infection is never completely eradicated, leading to an incomplete understanding of correlates of immune protection [4], (iii) HIV-1 creates a latent reservoir unlike other viral infections, as reviewed in Section 3, (iv) there is no appropriate animal model [4], and (v) there are funding issues associated with vaccine development [4, 5].

2. Limitations of ART

Despite its significant role in the prevention of HIV-1 infection transmission, there are several social, economic, and technical obstacles associated with ART therapy.

Besides coping with social stigma for ART [6], there is the social and economic challenge of the limited availability of ART therapy in low-income countries [7]. Given that the countries most affected by HIV are resource poor, only a small proportion of people of the global population with HIV have benefited ART [3]. Despite scaling up ART in several of these countries of sub-Saharan Africa, there are still countries where less than 25% of the adult population has access to ART [2]. Additionally, there is limited screening, so people may be unaware of their positive HIV status and never enroll in ART therapy [2].

The technical shortcomings of ART include the following: (i) limited impact on latent reservoir, (ii) strict, daily adherence to drug regimen, (iii) requirement of lifelong treatment, (iv) negative effect on immune cells, (v) drug adverse effects, (vi) drug-drug interactions, and (vii) drug resistance [7, 8].

Notably, if ART is started promptly in initial stages of infection, the latent reservoir is significantly reduced in size, but this reservoir is never completely eradicated [8, 9]. Regardless of the initiation of ART, the latent reservoir of HIV-1 infected cells is a momentous challenge in terms of limited immune response and potential for drug or vaccine treatment.

Lifelong treatment is necessary for continued viral load suppression and for the prevention of transmission to sexual partners, but there are some negative inevitable health consequences. Chronic inflammation and altered tissue architecture in lymph nodes and gastrointestinal tract occur on ART. This has significant repercussions on the functionality and survival of immune cells [8]. Additionally, long-term use is also associated with cardiovascular diseases, cancer, liver disease, long-term peripheral and central nervous complications, renal and metabolic disorders, and osteoporosis [4].

The adverse effects of ART drugs include insulin resistance, lipodystrophy, and dyslipidemia [8] and may lead to patient noncompliance. Additionally, given the barriers to accessibility, it may cause patients to be noncompliant and lead to issues with drug resistance [7].

Because of the difficulties of medication compliance, other ART drug deliveries are being tested in clinical trials, like long-acting injectable antiretrovirals, implantable devices, and vaginal rings [6]. Currently, there are long-acting injectable ART delivered by injections every two months being studied. The injection regimen increased patient adherence as compared to daily oral pills but has yet to be fully complete drug approval and implementation processes. Further research is aimed to extend the duration between injections to once yearly or twice a year [10].

3. Immune response

HIV-1 infection has a unique pathogenesis and degree of biological complexity unparalleled by other viruses. This biological complexity involves a wide range of immune cells and mechanisms, with the initial CD4 and CD8 T cell response, viral entry, and establishment of the latent reservoir reviewed in this section. Additionally, autologous neutralizing antibodies, broadly neutralizing antibodies (BnAbs), and extensive B cell dysfunction are important aspects to the immune response, but details are given in Sections 3.1–3.3.

In the initial stage after HIV-1 exposure but before infection, there are competing “HIV viral quasi-species” found in the donor secretions, vaginal mucosa of the recipient, and systemic component [11]. In the majority of infections (>75%), one variant referred to as the transmitted founder invades past the mucosal lumen to the stroma and travels to the local lymph nodes. (In 20% of infections, there are multiple variants involved in establishing systemic infection.) This transmitted founder has little genetic diversification. Here, at the lymph node, the CD4 T cells disperse the infection and exponential viral amplification, and systemic spread occurs [11].

Once systemic spread from the CD4 T cells is established, the CD8 T cells partially suppress the peak viral load after 30 days of infection. HIV rapidly evades the immune system and mutates to alter the epitope-human leukocyte antigen (HLA) binding. Over the next few weeks to months, the one viral variant becomes more like quasi-species version after exposure and diversifies its viral genome, due to the error-prone reverse transcriptase machinery adding different genetic codes to each infected cell [11, 12]. These viral variants may differ up to 40% of amino acids [13].

Though HIV-1 infection progresses in this pattern discussed in the last two paragraphs, understanding the specific mechanism of the virus entry is important aspect of the immune response. This mechanism begins with HIV envelope (Env) protein, gp120, binding CD4, and a cell surface protein receptor present on multiple types of immune cells. This gp120/CD4 interaction induces a conformational change, allowing gp120 to then bind to a coreceptor, CCR5, or CXCR4 [14, 15, 16]. Once coreceptor has been activated, gp41, another HIV-1 envelope protein, changes configuration to form a trimer-of-hairpins and fuses HIV-1 to the cell membrane, finalizing the entry [14].

The coreceptor of CCR5 or CXCR4 is of particular importance because this determines their susceptibility to infection. HIV-1 virus has two main “tropic” species, macrophage-tropic or M-tropic, and T-cell-tropic or T-tropic viruses [15, 16]. M-tropic strains are noted to be more preferentially transferred and utilize the CCR5 coreceptor, while T-tropic strains target the CXCR4 coreceptor [16]. Interestingly, there are some unique intermediate viruses referred to as “dual-tropic,” meaning they can infect cells with either CXCR4 or CCR5 [15, 16]. Some drugs in ART regimens and in clinical trials involve mechanisms to antagonize the CCR5 or CXCR4 coreceptors and inhibit viral fusion inhibitors [14, 16]. Though drugs are effective, there are several associated problems, including multidrug resistance, adverse effects, and high cost [14]. Further research into understanding the mechanisms of HIV-1 entry and potential steps to terminate the infection may prove to be advantageous.

One of the major challenges of this virus is the early formation and somewhat permanent establishment of latent reservoir of infected CD4+ T cells. HIV-1 preferentially targets CD4+ T cells and then quickly reduces the memory CD4+ T cells in gut-associated lymphoid tissue in the first 4–10 days [17]. There are some studies that suggest that the initial prime target of the virus is epithelial Langerhans cells and dendritic cells during HIV-1 vaginal transmission, instead of the CD4 lymphocyte [11].

Regardless of the HIV-1 initial target, the impact of memory CD4+ T cell infection is significant because this establishes the latent reservoir and allows the virus to persist even with ART [18]. Utilizing nonhuman primate (NHP) simian immunodeficiency virus (SIV) studies, the latent reservoir is established within the first three days of infection, even before the virus is detectable [11].

Additionally, besides the establishment of the latent reservoir, there is another deleterious effect affecting CD4+ T lymphocytes in gastrointestinal mucosa. The Th17 subset of CD4+ T cells has an established role for maintaining mucosal integrity in the gastrointestinal tract. When these cells are infected, they no longer maintain a tight barrier or prevent gut bacterial products from invading the bowel wall [19]. This leads to microbial translocations, chronic immune activation, and shifts in microbiome [19, 20, 21, 22].

3.1 Autologous neutralizing antibodies

Though the B cell response is less than optimal, as discussed in more detail in Section 6, the immune system is eventually able to produce autologous strain-specific neutralizing antibodies to the transmitted founder virus and the viral escape mutants. This occurs approximately three months to a year after infection [11]. These neutralizing antibodies are estimated to neutralize approximately 50% of the diverse strains of HIV-1 and only occur in half of patients with HIV-1 infections [12]. While these antibodies are ultimately overwhelmed by the continued viral evolution over the subsequent years, it is an important stepping stone for some individuals to progress generating broadly neutralizing antibodies [11], as reviewed in the next section.

3.2 Broadly neutralizing antibodies

Broadly neutralizing antibodies (BnAbs) are defined as antibodies with the ability to neutralize diverse isolates of HIV-1 and represent a potential avenue for prophylaxis and therapy [23]. This class of antibodies have high levels of mutations in rare, low-affinity naive B cell receptors [24], and these B-cell receptor (BCR) display atypical structural and binding characteristics that the immune system usually negatively selects against during B cell development [24, 25]. Some of the features of BnAbs are similar to those of autoreactive antibodies, indicating that they may evade immune tolerance mechanisms [26, 27]. Through the chronic viral replication, BnAbs are generated due to the extensive affinity maturation in germinal centers [27, 28].

BnAbs are an interesting and important phenomenon of the HIV-1 immune response, because only 10–20% of people with HIV-1 infections produce these antibodies after many years of infection [24, 25]. These individuals usually take approximately two to three years to produce BnAbs and are sometimes referred to as “elite neutralizers” [29]. While the BnAbs are able to neutralize many strains of HIV-1 variants, they ultimately are futile at controlling the host’s infection because of years of sustained viremia [24].

Progress in the identification of BnAbs as well as the study of structural properties has significantly advanced the field, due to its application as potential therapy. To identify BnAbs, researchers can isolate HIV-1 Env-reactive memory B cells from multiple sources including antigen-specific B cell sorts, from plasma cell sorts, and from clonal memory B cell cultures [25].

The structural studies of BnAbs contribute to our understanding of the immune response to HIV-1 infection and its progression. Generally speaking, BnAbs perforate the glycan shield of the HIV Env trimer in five regions, likely dismantling Env function [23]. This massive glycan shield of the HIV Env trimer concealing the antigenic target is a major obstacle for the humoral immune system to produce antibodies that can neutralize HIV-1 variants [30].

Of these regions in the HIV Env trimer, the V2 apex is one of the most important because of its role in maintaining the metastability of the Env spike, which influences the CD4/CCR5 conformational changes. The CD4 binding site (CD4bs) is the primary receptor for HIV and exposes CCR5 after activation. Interfering with the V2 apex may prevent viral penetration. BnAbs targeting the V2 apex are considered a potent antibody but limited in breadth (less than 70%) and display incomplete neutralization (<100%) [23]. Further research is needed to elucidate this field.

3.3 B cell dysfunction

During acute HIV infection, multiple immune components lead to extreme B cell dysfunction. This is namely due to of polyclonal activation, hypergammaglobulinemia, nonspecific plasmablast surge, impaired memory and naïve B cells, and B cell exhaustion [28, 31].

Through polyclonal activation, B cells terminally differentiate into plasmablasts (cells rapidly produced in early antibody response to generate antibodies [32]) and plasma cells approximately a week after infection [11]. Polyclonal activation has been studied and shown to be elicited via multiple pathways directly from serum cytokines and indirectly from HIV Nef protein [28]. The polyclonal activation leads to a state of hypergammaglobulinemia [31].

Interestingly, though there is a state of hypergammaglobulinemia, the plasmablasts increase to comprise only up to 13% of circulating B cells. This response is not pathogen-specific since only 1.5% or less are HIV-specific and unable to neutralize the virus. In other viral infections like RSV and dengue virus, plasmablasts increase to comprise 30% of total lymphocytes, with the majority being pathogen specific [11, 33].

During HIV-1 infection, there is an increase in circulating antibodies and increase in activity of B cells, but a disruption in the microgenerative environment impacting memory and naïve B cell subsets [28]. This decline in circulating memory B cells is of particular importance and could be classified as a marker of disease progression, as they are linked to CD4+ T cell population numbers [34]. The low levels of memory B cells lead to the characteristic opportunistic infections, namely Pneumocystis carinii and Cryptococcus neoformans [34].

Among the B cell dysfunction category is B cell exhaustion, only recently described in 2008 in HIV. B cell exhaustion refers to the decreased ability to proliferate in response to de novo stimuli. HIV infections cause this exhaustion in a specific subset of B cells that are tissue-like memory B cells. This subset of cells has increased expression of multiple inhibitory receptors (CD22, CD72, and LAIR-1) as compared to normal resting memory and terminally differentiated B cells. Other unique inhibitory receptors to B cell exhaustion are also being investigated. Overall, B cell exhaustion is similar to CD4 and CD8 T cell exhaustion, but B cell exhaustion has proven to be more challenging to study since the experimental science is less direct as compared to T cell assays [31].

4. Variety of immune responses: progressors, immunological nonresponders, long-term nonprogressors, and elite controllers

One challenging aspect of HIV-1 infections is that there are a wide range of potential immune responses. There are four categories used to describe an individual’s immune response to HIV-1: HIV-1 natural progressors, immunological nonresponders (INR), elite controllers, and long-term nonprogressors. Each category is described in detail below.

HIV-1 natural progressors refer to a typical response to an HIV-1 infection. The timeline for this response is not clearly defined but most likely reflects an intermediate progression where AIDS develops 3–10 years after seroconversion [35]. If these patients were to receive ART, the progression to AIDS would be less likely or potentially very slow and gradual.

Immunological nonresponders (INRs) have not been universally defined, but the most general accepted classification of INR is a patient who does not meet a specific CD4+ T cell count level or a specific percentage CD4+ T cell increase over baseline after a certain length of ART. The literature values for these specific levels and percentages vary widely with the CD4+ T cell count range of 200–500 and percentages from over 5–30%. The length of ART also changes from 6 to 144 months. Though the CD4+ T cell does not increase as it should, the viral load is suppressed. Given the inconsistent definition across studies, this subset is approximated at 10–40% of people with HIV-1 infections. This subset of people with HIV-1 infections is more likely to have morbidity and mortality from AIDs and non-AIDs conditions, because their immune systems are significantly dysfunctional [22].

Elite controllers and long-term nonprogressors represent those with immune responses that suppress HIV-1 viral load naturally without ART. These categories are differentiated by the degree of viral load suppression [36]. Elite controllers suppress the HIV-RNA values to less than 50 copies per milliliter, while long-term nonprogressors suppress the HIV-RNA to less than 5000 copies per milliliter [35].

Elite controllers are rare, estimated to be 0.1–1% of all people with HIV-1 infections but represent a functional cure. Understanding this population may lead to greater understanding of successful immune responses against HIV-1 [18]. As described in Loucif paper, elite controllers maintain suppressed viral loads most likely due to a combination of the following factors: high-quality and polyfunctional CD8+ T cells, memory B cell responses, preserved memory and pTfh CD4+ T cells, lack of natural killer (NK) activation, preserved plasmacytoid dendritic cell counts, and preservation of gut mucosal immunity [8].

The polyfunctionality of CD8+ T cells is a key differentiating factor between elite controllers and progressors. It has been found that the CD8+ T cells in elite controllers are able to degranulate properly and release perforin, granzyme, and cytokines (interferon-gamma, tumor necrosis factor-alpha, interleukin-2, and macrophage inflammatory protein-1beta) [36]. It is believed that the functional CD8+ T cell response is directly linked to disease progression [36].

Long-term progressors display similar characteristics as elite controllers, primarily the polyfunctional T cells [17]. They maintain high levels of CD4+ and CD8+ T cells without ART therapy. Approximately 5% of the total HIV population are long-term nonprogressors [21, 35].

4.1 Disadvantages for elite controllers and long-term nonprogressors

While the immune responses of elite controllers and long-term nonprogressors control the viral load in general, there are some downsides that these groups face. These patients can decline at any time, despite having long periods of naturally suppressed viral loads. While it is challenging to estimate the number of regressions in a small population of those with HIV-1 infections, it is believed that about 25% of elite controllers decline [37].

Researchers compared elite controllers who lost the ability to suppress the virus against the “persistent” elite controllers. The “persistent” elite controllers had low viral diversity, low HIV-DNA concentrations, overall lower inflammation, decreased immune activation, and proinflammatory cytokine concentrations. It was also found that the high Gag-specific T-cell polyfunctionality was no longer present in the individuals who lost viral control [37].

Though elite controllers represent a functional cure to HIV-1 infections, it is worth noting that this subset of patients is susceptible for hospitalizations of all causes (as compared to patients with HIV-1 infections on ART). The majority of these hospitalizations were due to cardiovascular and psychiatric diseases [36]. It is believed that a subset of elite controllers has this increased risk of hospitalizations and adverse effects because there is a persistent immune dysfunction driving the pathology [19].

5. Pediatric immune response

In addition to the various categories of immune responses, there is also a subset of young patients who acquire the infection perinatally or from breastfeeding from an HIV-1 positive mother. This occurs in part due to the large number of reproductive age women with limited access to ART and birth control in low-income countries. These patients have a different timeline than the standard adult, in terms of both immune response and overall disease course, and represent a significant public health crisis in low-income countries.

The timeline of the immune response in a pediatric patient begins with an infant with HIV-1 with high titers of passively transferred maternal neutralizing antibodies until three months of age. After this point, the neutralizing antibodies decrease but increase at 12 months, meaning that the infant is able to produce this antibody type. Some of these young patients then produce broadly neutralizing antibodies much earlier in infection, with diverse epitope specificities, and higher breadth and potency than that of adult patients [29]. One broadly neutralizing antibody (BF520.1) studied was noted to have limited somatic hypermutation and an absence of insertions and deletions, unlike the studies performed on adult antibodies. Given these core differences, the pediatric BnAbs are thought to be derived from different pathways than those produced in adults [29].

The overall disease course in a pediatric patient is unique because of a faster progression to AIDS [29] and a higher risk for neurocognitive deficits [38]. Without ART, children progress to AIDS within a year, as compared to adults taking a decade to progress [29]. These patients are more likely to have neurodevelopmental and neurocognitive disorders as compared to patients who acquired HIV-1 infection as adults. Given the rapid growth of nervous system in early infancy and childhood, understanding how HIV-1 infections impact childhood development is important. Pediatric patients are more likely to have physical brain damage from the inflammation and multinucleated giant cells in the cerebral cortex. The main manifestation of this impaired neurocognition is limitations in language function [38]. The studies of neurodevelopmental and neurocognitive effects on HIV-1 infection in pediatric patients are limited but represent a growing field of interest given the number of young patients in low-income countries [38].

Notably, 53% of untreated children with HIV-1 die by two years of age in sub-Saharan Africa. Before three years of age, this statistic changes to 75% children with HIV-1 dying [9]. Because of two modes of vertical transmission with HIV-1 infection occurring perinatally (in utero or intrapartum) and through breastfeeding, these groups of children have been assessed separately and identified that the children infected perinatally are at higher risk of death (60%) as compared to children infected through breast milk (36%) [9].

Given pediatric patients’ immature immune systems and progression to AIDS, more research regarding acquisition prevention in these patients as well as funding is needed to combat this public health crisis in low-income countries.

6. Select clinical trials

Despite several decades of research, vaccine development, and clinical trials, currently, there is not any effective vaccine to prevent acquisition of HIV-1 infection. When HIV-1 was initially identified as the causative agent for AIDS in 1983–1984, researchers believed that a prophylactic vaccine would be generated within two years. This two-year estimate drastically underestimated the challenges and biological complexity of HIV-1 and illustrated the fact that HIV-1 is unlike any other viral disease that has a vaccine [4].

Though most clinical trials have found no efficacy, one clinical trial referred to as RV144 had unexpected success. This trial was controversial before it even began because it was believed to have a high likelihood of failure, given the early-phase clinical trials assessing the immunogens used in this trial. The initial data found the vaccine components were poorly immunogenic in isolation. Regardless of whether they were administered alone or in combination, there was only modest T-cell and humoral responses with no virus neutralization. However, the phase 3 trial proceeded in part to study a heterologous prime-boost strategy [39].

RV144 was the first trial that showed that any vaccine could induce protection against HIV-1 infection. Despite all odds, this vaccine had 60.5% efficacy in the first year [11] and decreased to 31.2% efficacy at 42-month post-vaccination [4]. Though the efficacy decreased significantly by three years, simulated studies believe that even if the vaccine had 50% efficacy for two years, it would have a significant impact in high prevalence areas [24].

As the only trial in humans with any efficacy, researchers had great interest in investigating what immune correlates were associated with protection against infection. The immune correlates for HIV-1 infection are unique inherently, given that the virus is never cleared naturally, but there is some type of protection against infection not yet understood. The immune correlates were identified as formation of non-neutralizing IgG against the V1/V2 region of HIV-1 Env, antibody-dependent cellular cytotoxicity in patients with low IgA, and Env-specific polyfunctional CD4+ T cells [4, 11, 24]. The mechanistic rationale behind how HIV-specific non-neutralizing antibodies protected against HIV-1 acquisition is not well understood and controversial [40].

In an effort to replicate RV144’s success, a similar trial referred to as Uhambo or HVTN 702 was designed. There were clear differences between the two trials: RV144 had been conducted in Thailand in 2009, testing a recombinant Canarypox vector prime followed by two injections of a recombinant gp120 boost. HVTN 702 was conducted in South Africa with the same vector prime, similar protein boost but slightly different adjuvant, and different envelope sequences [24]. Investigators chose to change the envelope sequences to reflect the locally circulating HIV-1 variants in South Africa [41].

Ultimately, HVTN 702 was unsuccessful and terminated due to lack of efficacy in 2020 [41]. There was no significant production of the V2 loop antibodies deemed to be the critical immune correlate in RV144. This trial did result in high levels of binding antibodies, antibody-dependent cellular phagocytosis, and antibody-dependent cellular cytotoxicity activity, but overall no efficacy [42]. Perhaps, this lack of efficacy is due to the vast genetic diversity of the Sub-Saharan African with clade C, the difference in host genetic factors, or other indeterminate factors due to clinical trial differences as discussed by Gray et al. [41].

As previously discussed, the immune response to HIV-1 infection is intricate and complex with multiple stages of infection and potential responses.

7. Broadly neutralizing antibodies in passive immunization trials

Given the intricate method, the immune system forms BnAbs; immunization to induce BnAbs is proving to be exceedingly difficult. As previously reviewed in Section 5, the natural production is several years into infection and not clear why only a subset of people with HIV-1 infections generate these antibodies. The mechanism behind their evolution is also yet to be fully understood [25].

Before reviewing the vaccine strategies for BnAb induction, it is important to note that there have been clinical trials with passive immunization using VRC01, an IgG1 BnAb against the Env CD4 binding site. In vitro studies revealed that this BnAb has wide coverage against HIV-1 subtypes B and C. These trials were HVTN 704/HPTN 085 in the US, Peru, Brazil, and Switzerland and HVTN 703/HPTN 081 in South Africa, Zimbabwe, Malawi, Botswana, Kenya, Mozambique, and Tanzania, and ran from 2016 to 2018. The ultimate goal of these trials was to investigate if VRC01 is capable to preventing HIV-1 acquisition [43].

The results published in 2021 indicated that this BnAb was unable to prevent overall HIV-1 acquisition, but that in VRC01-sensitive HIV-1 isolates, BnAb prophylaxis was effective [43]. The VRC01-sensitive HIV-1 isolates were only ~30% of the strains in circulation [44]. These results suggest that the VRC01 suppressed early circulating strains, but the immune system eventually lost to evolving resistant viral variants. It is likely that a combination of BnAbs is necessary to prevent viral escape [43].

Additionally, “bispecific” and “trispecific” BnAbs were developed and tested in phase 1 clinical trials [43]. Bispecific or trispecific BnAbs have two or three different specificities in a single molecule. This unique class of BnAbs may lead to increased neutralization breadth and limit viral escape [45].

While studying how efficacious are BnAbs, it is important to assess their behavior in vitro, the administration of BnAbs via intravenous therapy is not a feasible drug delivery system for large populations. It may also have limited use if administered in combination of BnAbs in high-risk groups, but overall this is not feasible or sustainable method for HIV-1 prevention [44].

8. Promising vaccine strategies: general broadly neutralizing antibody vaccine

With the intricate and complex immune response to HIV-1 infection, it is no surprise that multiple vaccine strategies exist. Some promising vaccine strategies include SOSIP trimers, eOD-GT8 60mer and gene therapy, HIVcons immunogens, and mosaic immunogens. All these strategies are discussed in detail in Sections 9–12.

A feasible long-term solution or “the holy grail of HIV-1 vaccine development” is a vaccine that induces the production of BnAbs [25]. The current belief is that a prophylactic vaccine must induce BnAbs with a wide neutralization breath and/or HIV-1-specific antibodies to mediate antibody-dependent cellular cytotoxicity or other effector functions [39].

Multiple different concepts of BnAb-based vaccines are being investigated, including germline-targeted and lineage-based designs as well as SOSIP trimers detailed in Section 13 [27, 39]. In an effort to produce a germline-based vaccine, there are ongoing studies following HIV-1 isolates in people with active HIV-1 infections and BnAb [39]. A germline-targeted vaccine assesses an unmutated common ancestor of a BnAb, then approximates the critical BnAb precursor features, and designs the immunogens based on the structural and immunological information [27]. The goal of this germline approach is to preferentially activate B cells that are BnAb precursors and eventually produce memory B cells capable of BnAb production [46].

A lineage-based vaccine is focused on the immunological pathways forming antibody lineages that generate BnAb [27]. Regardless of which design is used, both germline-based and lineage-based designs use the “reverse vaccinology” to drive the production of the BnAb [44, 47].

These concepts are further complicated by the strategy of prime-boost vaccination models, where a priming vector is administered followed by a series of protein boosts. For this strategy to lead to BnAb production, the prime would be a precursor naive B cell, and the boosts would then select for neutralizing members of antibody lineage [27, 47].

9. Promising vaccine strategies: env glycoprotein trimers/SOSIP trimers

Another promising vaccine strategy that may result in BnAb production is immunogens based off the recombinant stabilized HIV envelope (Env) glycoprotein trimers, specifically the SOSIP trimers [47, 48]. SOSIP trimers are proteins that adopt a native-like confirmation and strongly approximate the features of HIV-1 virion [46]. Currently, the SOSIP trimers induce neutralizing antibodies against “relatively neutralization resistant (tier 2) autologous viruses” but there are multiple studies suggesting their neutralization breadth [39, 47, 49], as well as improvements in their production [39, 50], stabilization [46], and immunogenicity as future vaccine candidates [51].

To study the neutralization breadth, one study was completed through computational modeling that assessed different combination and sequential vaccine series and then administered to groups of rabbits. While this experiment did not induce BnAb formation, it proved that the neutralizing antibody response can be cross-boosted when Env trimers from different clades were administered sequentially [49]. Further research is needed to identify a SOSIP trimer that induces BnAb production.

One study determined the adjuvants compatible with SOSIP trimers, since the adjuvant is a key component to boost the immune response against the vaccine antigens. This study found that different adjuvant classes did not interfere with the integrity of the SOSIP trimer, with the exception of aluminum sulfate formulations [51].

Additionally, to assess how feasible and scalable SOSIP production was, Dey et al. completed a trial that resulted 3.52 grams of fully purified trimers. The overall production was performed while abiding by current Good Manufacturing Practice guidelines. The quality of SOSIP composition was compared to samples generated from a research laboratory and reported to be the same in terms of antigenicity, disulfide bond patterns, and glycan composition. This trial also helped assess how stable the trimers were at different temperatures and storage conditions [50].

10. Promising vaccine strategies: eOD-GT8 60mer and gene therapy

Additional BnAb-based vaccines currently being assessed are eOD-GT8 60 mer (protein nanoparticle derived from the VRC01 CD4-binding site) and gene therapy through a recombinant adeno-associated virus-mediated (rAAV) construct containing HIV BnAb genes [44].

This eOD-GT8 60mer delivered with AS01B adjuvant was tested in 2018 in an early clinical trial. This nanoparticle was originally studied in genetically modified knock-in mice and animal studies and designed to activate B-cell precursors to form BnAb. The results showed that 97% subjects produced precursor VRC01 IgG B cells, according to the preliminary data in early 2021 at the 2021 R4P conference. This is an important finding, because it provides some evidence that stimulation of rare B-cell precursors is possible. Further study is needed to validate this approach and determine that it works consistently [44].

A gene therapy approach to delivery BnAb genes utilizes the rAAV vector and selects genes like VRC01, b12, 4E10, 2F5, 2G12, 3BNC117, 10–1074, and 10E8. This strategy was assessed in rhesus macaques and led to undetectable viremia for three years. While this was incredibly positive, the animals developed antidrug antibody (ADA) responses. Further testing with nonhuman primates (NHP) yielded similar results, with a decline of a BnAb corresponding to ADA response. In the NHP study, these animals did have mucosal protection against the simian version of HIV but for a shorter duration (less than a month). This may have been due to only receiving one BnAb gene in the rAAV vector. The ADA response remains the main hurdle in this approach, provided that the BnAb gene combinations are validated [44].

These initial data from both the eOD-GT8 60 mer and rAAV vector with BnAb gene immunization strategies are encouraging. More research and trials are necessary to fully understand their potential to generate a prophylactic HIV-1 immunization.

11. Promising vaccine strategies: HIVcons immunogen-based vaccine

A promising concept that may result in a prophylactic HIV-1 vaccine is one with a HIVcons immunogen basis. This design is a 778 amino acid insert created analyzing all clades through bioinformatics. The insert specifically focused on sequence conservation and did not use epitopes as a “string of beads” [13]. String of beads epitope design refers to a technique where short sequences of amino acids or spacers are added between epitopes in an attempt to allow the epitopes to be cleaved correctly [52]. Researchers creating HIVcons immunogen elected to not use the string of beads construct, because it could create irrelevant epitopes not present on HIV-1-infected cells. Additionally, this construct could skew the design toward frequent Caucasian HLA alleles [13].

While the HIVcons insert was successfully tested in multiple early clinical trials in two different vectors in both healthy volunteers and patients with HIV-1 infections, researchers had to redesign to create a “second-generation conserved mosaic tHIVconsvX” vaccine, utilizing the CdAdOx1 vector from simian adenovirus Y25. This was due to licensure issues with the previous vectors, but believed to be advantageous overall since the second-generation design was improved, as discussed by Tomas Hanke [13].

12. Promising vaccine strategies: mosaic immunogens

Mosaic immunogens is another vaccine design that is showing promising results in clinical trials. These immunogens are bioinformatically optimized bivalent antigens derived from different clades of HIV-1 group M strain [53]. These sequences were then placed into MVA or Ad26 vectors, followed by a clade C gp140 protein boost. This was evaluated in the APPROACH study in East Africa, South Africa, Thailand, and the USA. The results indicated that the Ad26/Ad26+ high dose gp140 vaccinations produced Env-specific antibody responses and T cell responses in the vast majority of recipients (100%, 83% for each of these responses) [54].

Building off the APPROACH study data as well as TRAVERSE and ASCENT studies, two studies, Imbokodo (HVTN 705) and Mosaico (HVTN 706), were designed. Imbokodo was phase 2b efficacy clinical trial for women in Southern Africa evaluating a heterologous prime/boost of Ad26.Mos4.HIV and adjuvanted aluminum phosphate Clade C gp140 [6, 44]. In total, it has four mosaic antigens within the vector aimed at inducing an immune response. Preliminary results of Imbokodo showed no significant efficacy (25.2% efficacious with 95% confidence interval between −10.5% and 49.3%), but these data are yet to be published in a peer-reviewed journal [55].

Though Imbokodo was completed, Mosaico, the similar trial, is not yet finished and has some differences in vaccine components and patient population [53]. This vaccine has gp140 immunogens from different HIV strains, rather than clade C specific as in Imbokodo [6, 44]. Mosaico enrolled gay men and other men who have sex with men as well as transgender women. This study is still ongoing in hopes that the slight differences between immunogens and transmission mode will result in some vaccine efficacy [55, 56].

13. Conclusion

There is great need for a prophylactic HIV-1 vaccination to end the HIV/AIDS epidemic. HIV-1 immunology and vaccine development remain a uniquely challenging field. This is for multiple factors: complex immune mechanisms, rapid mutation of the viral variants, establishment of the latent reservoir early in infection, no predictive animal models or natural immune correlates, and only one efficacious clinical vaccine trial with RV144 in 2009 with numerous failed trials. Despite these factors, the field has made great strides, particularly through the study of categorization of immune responses and characterization of broadly neutralizing antibodies (BnAbs). This progress has led to multiple promising vaccine candidates in various stages of development, namely, SOSIP trimers, eOD-GT8 60mer and gene therapy, HIVcons immunogens, and mosaic immunogens.

Conflict of interest

The authors declare no conflict of interest.