A New Agent in the Strategy to Cure AIDS

See page 1913

In recent years the field of immunotherapy has expanded to include therapies never before imagined. Since recombinant DNA (rDNA) technology was first applied to the production of active antigen-specific antibodies more than 30 years ago,¹ there has been a revolution in the development of antibody-based therapies. At present, in the field of cancer alone, 22 monoclonal antibodies are approved for clinical use in the United States.² In this issue of Molecular Therapy, Yang and colleagues³ describe the development of a bispecific reagent that binds both an HIV-1-specific T-cell receptor and an effector T cell, raising the possibility for a new approach to control and perhaps cure HIV/AIDS.

Bispecific antibody (BiAb) binding is not a new concept. Antigen-specific IgG itself has bispecific capability, mediating binding to effector cells or to complement for killing of antigen-expressing cells. The goal in utilizing BiAbs is the redirection of immune cells to an antigen target, and BiAbs were first made by chemical crosslinker or hybrid hybridoma methods more than three decades ago.⁴ The extensive history of BiAb research for T-cell redirection has been reviewed.⁵ New approaches to T-cell redirection include the so-called BiTE reagents and CAR T cells using a single-chain variable fragment (scFv) of antibody to redirect the T cells. The BiTE therapeutics “engage” the T cell using a BiAb that links CD3⁺ T cells to a specific antigen.^6,7 The first BiTE to be approved for clinical use in the United States is blinatumomab (Blincyto, Amgen). CAR T cells are genetically modified ex vivo to express a “chimeric antigen receptor” and are very promising cancer therapies that are still in development.⁸ An advantage of the BiTE and CAR strategies is that they bypass the human leukocyte antigen (HLA) restriction for antigen recognition. Thus, neither the HLA-specific epitope presentation nor the recognition by a native T-cell receptor (TCR) is required.

A next generation of bispecific reagent has linked epitope-specific TCRs to an scFv specific for a cytotoxic cell to effect what has been termed “immune-mobilizing monoclonal TCRs,” called “ImmTACs” when against cancer⁹ and “ImmTAVs” for targeting virus antigens.³ ImmTACs have already entered clinical trials in melanoma patients (ClinicalTrials.gov identifier NCT02570308). The first in vitro use of an anti-HIV-1 ImmTAV is described in the report by Yang et al. as a TCR specific for HIV-1 p17 cloned and genetically linked to an scFv that binds to CD3⁺CD8⁺ T cells (CD8). The reagent has remarkable activity at picomolar concentrations, as has also been shown for ImmTACs.⁹ Yang et al. chose HIV-1 p17 as the target antigen, and these p17-epitope specific ImmTAVs were able to arm CD8 cells for killing of HIV-1-infected CD3⁺CD4⁺ T cells. When peripheral blood mononuclear cells from antiretroviral (ARV)-treated HIV-1-infected donors were activated in vitro to induce HIV-1 replication, the autologous ImmTAV-treated CD8 T cells reduced the number of HIV-1-infected cells by 60–80% compared to the control ImmTAV-treated autologous CD8⁺ cells. Among the CD8⁺ T-cell subsets, the effector and effector memory T cells were more potent in reducing HIV-1 infection than were the central memory and naive CD8⁺ T cells.

To study the in vitro effect of ImmTAVs on reactivated HIV-1, CD4⁺ T cells from HIV-1-infected donors were stimulated with a mitogen, a situation in which HIV-1 antigen could be seen in both the activated CD4 cells and in CD25^–/CD69^–/HLA-DR^–/CD4⁺ “resting” T cells. With CD8-ImmTAV treatment in vitro, both the p17-expressing activated and resting CD4 cells were killed, suggesting that the ImmTAV could be effective in a mixed-cell population as might occur when HIV-1 is being activated from latency in patients. This is important, because any future testing of the reagent in ARV-treated AIDS patients would target the latent virus reservoir, a situation of minimal HIV-1 antigen expression.¹⁰ Furthermore, by targeting an internal HIV-1 gene product, the approach could be effective without regard to surface expression of the HIV-1 envelope proteins, normally targeted by other immunotherapy strategies.

The question, of course, is whether the ImmTAVs could act on latently infected cells from patients. To test this, CD4⁺ T cells from five HIV-1-infected donors receiving ARVs were cultured in vitro with the latency reactivation agents (LRAs) romidepsin and bryostatin. ImmTAV-treated CD8⁺ T cells from a healthy donor prevented HIV-1 reactivation in four of five challenges, but the CD8⁺ T cells treated with control ImmTAVs had no effect. The results are exciting and indicate that HIV-1 reactivation can be controlled by ImmTAVs, but it is a leap of faith to conclude that the ImmTAVs had an effect on the HIV-1 reservoir. It remains to be seen whether the antiviral function in this artificial system will occur when treating patients with chronic infection. Nevertheless, this supports a clinical strategy for testing of ImmTAVs based on use of LRAs.

How might ImmTAVs have a role in the cure for HIV/AIDS? The ultimate goal of cure research is the control of active infection while eradicating the reservoir of latent infection. A strategy for achieving this goal has been proposed.¹¹ In addition to better understanding the various elements of AIDS pathogenesis, the promise of a cure for AIDS will require restoration of the damaged immune system to have effective T cell–specific anti-HIV-1 function. Ironically, as CD4 T-cell counts improve after control of HIV-1 by ARVs, effective HIV-1-specific immunity actually diminishes for reasons related to lack of HIV-1 antigen stimulation, global immune activation, and T-cell exhaustion.^12,13 The attack on the HIV-1 reservoir will require some method for HIV-1 activation, perhaps with LRAs,¹⁴ and then destruction of the virally infected cell by the now-refurbished T-cell immunity.

Several factors should be considered when trying to predict whether the ImmTAV approach will bolster the immune system of an HIV-1-infected person and eradicate infection. First, in natural HIV-1 infection, the virus is able to escape T-cell immunity by several means, but especially by direct mutation of a targeted epitope, by HIV-1 reduction of HLA expression necessary for antigen presentation, and by immune exhaustion.¹⁵ In the ARV-treated patient, in whom HIV-1 replication is minimal, new mutations are unlikely. The repository of HIV-1 variation for certain epitopes of interest can be examined in the proviral DNA of the patient, and so the ImmTAV could be chosen based on one or more epitopes known to be present in the HIV-1 reservoir. Ultimately, a combination therapy with multiple ImmTAVs could be the best way to avoid the mutation-based problem of immune escape. It will be important to better understand the proteins expressed after LRA treatment. But, because of its importance in HIV-1 replication and pathogenesis, HIV-1 p17 is probably a well-chosen target.¹⁶ Yang et al. should be congratulated for being able to target an internal antigen likely to be detected in newly activated HIV-1-infected cells. The investigators have shown not only that ImmTAVs can be effective when used in vitro with LRAs, but also that ARV-induced mitigation of recent infection allows for some level of cytotoxicity of infected cells in vitro. One cannot be sure based on this small experiment that the ImmTAV strategy will be able to search and destroy the entire HIV-1 reservoir, but at least it is active at low HIV-1 antigen expression.

The strategy can be faulted because of its requirement for HLA class I specificity. Any cell that lacks HLA class I antigens should be immune to ImmTAVs. It might be necessary in a cure strategy to use not only LRAs but also agents that activate HLA expression. More importantly, the treatment must be “HLA compatible” for each patient, and, at a practical level, this suggests that easy scalability of the method will be a challenge, not only during clinical trials but also if/when the method is ready for application to a genetically diverse population of AIDS patients.

Second, the treatment must be safe, and this will be of foremost importance knowing that redirected T-cell therapies have profound biological effects that are not for the faint of heart. Cytokine release syndrome (CRS) is a potentially lethal event associated with T-cell activation. Blinatumomab, a CD19:CD3 BiTE, and CD19:CD3 CARs have formidable antileukemia cytotoxic potencies, but both induce the release of cytokines with profound effects.^17,18 CRS can result in fever, headache, fatigue, nausea, hypotension, liver and neurologic abnormalities, and other organ-specific adverse events. With blinatumomab, encephalopathy occurs in as many as 60% of treated patients (cf. FDA blackbox warning). Considering that HIV-1 is an infection not only of the immune system but also of brain, this suggests that a T-cell redirection with ImmTAVs seeking areas of HIV-1 infection could have significant central nervous system toxicity. Given the experience with BiTEs and CARs, it will remain to be seen whether this new strategy for T-cell redirection will be tolerated in the setting of HIV/AIDS.

In summary, a therapeutic method that would both activate HIV-1 from latency and destroy the cellular site of such reactivation is the promise of HIV/AIDS cure strategies.¹⁵ ImmTAVs are likely to enter the fray in the search for an HIV/AIDS cure. Only clinical trials of this new reagent will determine whether they can deliver on such a promise.