T-cell immunity is especially important for chronically infectious pathogens, such as the human immunodeficiency virus (HIV) or herpes simplex virus (HSV), as a key part of their life cycle is the long-term avoidance of the immune system. These pathogens maintain chronic persistent infections by hiding from, inactivating, or avoiding the numerous and varied components and cells of the immune system. In terms of T-cell immunity this is achieved by limiting T-cell maturation and/or refinement or inhibiting activation in the first place. By doing this they can persist in an individual for a lifetime.
Controlling and eliminating these viruses requires a strong and directed T-cell response. This is demonstrated by their correlates of protection established during natural infection and control (Table 1). Therefore, a great research interest and need exists for new innovative and novel approaches to develop effective means of generating this type of immunity, while refining and improving on older methods to maximise their effects.
It is, however, unfortunate that current methods have proven largely inadequate at achieving this goal.
There has been some success in terms of the HIV-1 RV144 vaccine trial, which showed a modest ~31%
immunity in its participants by its end point, which was 31% higher than all previous trials of HIV vaccines (Rerks-Ngarm et al., 2009). It does however give hope to the field that an effective vaccine can be developed. Many believe that a more effective HIV vaccine will require both strong T-cell and antibody-mediated immune responses (Hsu and O’Connell, 2017; Stephenson et al., 2016). Methods for generating antibodies are numerous and great progress has been made in this field, however, attempts at developing efficient and effective methods of generating T-cell immunity have not been unsuccessful. The only highly effective T-cell vaccine vector available (a recombinant Adenovirus 5 (rAD5) vector), showed problematic results in the widely discussed Phambili / STEP trials that were
28 stopped prematurely due to ineffectiveness and demonstrated a possible increased likelihood of contracting HIV by some patients who had AD5 immunity (Buchbinder et al., 2008; Sekaly, 2008).
The high seroprevalence of AD5 antibodies and immunity among humans proved to be a deciding factor in the problems faced in this trial, which also determined that AD5 is probably not a suitable T-cell vaccine vector for HIV (Buchbinder et al., 2008; Fitzgerald et al., 2011).
It is worth discussing T-cell immunogenicity and HIV in greater depth, as HIV has a multitude of mechanisms that help it evade the immune system. It has arguably the most comprehensive and complete attack and evasion strategy of any virus studied to date, with specific strategies and methods of evading every part of the immune system. This is also why it may require every part of the immune system to combat it.
In terms of T-cell evasion, HIV uses its Nef protein to initially downregulate expression of MHC class I proteins, allowing it to hide from and evade T-cell immunity early on, while later it ultimately kills T- cells, effectively eliminating their threat and their ability to mature into a form that could threaten the virus, resulting in acquired immunodeficiency syndrome (AIDS) (Blagoveshchenskaya et al., 2002).
Because HIV has a defence strategy against every part of the immune system, it becomes relevant as to how effective each of these defence mechanisms are. Antigen presentation by MHC-I and subsequent destruction of infected cells by CD8+ cytotoxic T lymphocytes (CTL) is a powerful mechanism used by the immune system to detect intracellular pathogens and clean up the remnants of viral infections.
This mechanism forms a crucial part of both the innate and adaptive immune systems, which makes it an ideal target for HIV to target early on, as it begins to cripple the immune system. Nearly every step in the assembly and trafficking of MHC-I class proteins can be a potential target for immune evasion.
Poxviruses, for example, encode multiple inhibitors that target MHCI presentation mechanisms for downregulation or ablation (Ploegh, 1998). HIV, on the other hand, only has one gene, Nef, that it uses to achieve this goal. HIV’s Nef works by accelerating the process of MHC-I endocytosis and sequestration to the trans-Golgi apparatus. Data generated on this process has indicated that Nef is not particularly adept at achieving this effect. This inefficiency is more than made up for, however, by strengths in other areas that also attack the CTL network, not least of which is the viral targeting of this network itself, preventing the generation of mature CD8+ cells (Blagoveshchenskaya et al., 2002;
Collings et al., 1999; Tähtinen et al., 2001).
HIV elite controllers are a subset of <1% of infected individuals who naturally control HIV to extremely low levels that are undetectable by conventional assay (<50 RNA copies per ml). Studies on elite controllers have shown that they commonly exhibit strong enrichment of particular types of human leukocyte antigen (HLA) class I genes (which are responsible for antigen presentation via MHCI complexes), as well as immune presentation and CTL training through MHCII presentation on antigen presenting cells (APC), which present antigens to CD4+ T cells via MHCII (Deeks et al., 2015; Fellay
29 et al., 2007; Jia et al., 2011). This strong association between elite controllers and MHC / HLA genes emphasises the importance and capability that T-cell immunity has to combat and control HIV infections. Moreover, the observations that this mechanism is a strong correlate of protection for elite controllers is powerful evidence that a vaccine will need to include a strong T-cell immunogenicity.
Having a particular set of HLA genes does not necessarily mean only the lucky individuals with the right genes will ever be able to be elite controllers: it more realistically means that people without those genes will simply take a substantially longer period of time and a greater number of T-cell maturation rounds to get there. Once HIV has been contracted, the number of T-cell maturation rounds is limited by the virus-destroying T-cells, preventing a sufficiently long enough life time for this to ever develop (Parham, 2014; Sompayrac, 2019). That said, a preventative vaccine will have the necessary time and lack of inhibition frame for this process to occur.
It is important to note however that HIV does not entirely eliminate MHCI complexes, as this results in a process called ‘missing self-regulation’ which results in immediate destruction by natural killer cells, which have inhibitory “do not kill” receptors that recognise MHCI complexes. These receptors are polymorphic in nature and have their own entire screening process during natural killer (NK) cell development. Only NK cells that have demonstrated their ability to recognise self-cells are given
‘licence to kill’, and any cell that fails this highly important self-recognition test whereby MHCI complexes cause the stimulation of the ‘do not kill’ signal to the NK “licence to kill” receptors, is eliminated (Sompayrac, 2019). HIV evasion of T-cell immunogenicity is two-phased and highly sophisticated, as the virus uses Nef to limit, but not eliminate, self-exposure by downregulating MHCI exposure, delaying the development of a suitably specific and mature T-cell response, and then eliminate T-helper cells from living long enough to ever be capable of developing a fully mature response. In this sense, the generation of a fully mature T-cell response might only be possible through the development of an effective T-cell vaccine. However, once a full complement of mature memory CD-8 T-cells has been generated they are no longer dependent on the presence of a function of CD4+
helper T-cell population, which is required for their genesis (Parham, 2014; Sompayrac, 2019). The limiting of MHCI exposure has also been identified in cancer research as a mechanism by which cancerous cells also evade attack by CTLs, as such vaccines capable of generating enhanced and mature T-cell immunogenicity are of high interest for combating and treating a variety of cancers as well (Angell et al., 2014; Motwani et al., 2019).
HIV with its RNA genome of ~10 kb effectively also avoids many of the subcellular innate immune defences discussed above, by being an RNA virus and integrating a DNA copy of itself into the human genome with integrase protein. By performing much of its replication as RNA after the genetic material has been reverse transcribed into DNA and incorporated into the genome where it is safe, it effectively avoids most of the pathogenic DNA replication detection mechanisms and receptors. This greatly decreases its likelihood of eliciting a strong T-cell response (Rathinam and Fitzgerald, 2011). While
30 HIV is highly effective at avoiding the creation of T-cell immunogenicity, it still only down-regulates MHCI, as it cannot eliminate it. Any T-cell vaccine that is highly effective at creating a powerful and focussed CTL-mediated immunogenicity will therefore pose a significant threat to HIV. If this were not the case, HIV would not need such an advanced strategy for avoiding CTLs. While the retroviral life cycle strategy is highly effective at minimising exposure to T-cell activation processes, other viral replication cycles are not. Using these to generate this type of immunity is therefore a viable design strategy for the development of novel rational vaccine design (Jia et al., 2011; Williamson and Swanstrom, 2015).