Abstract
It has become a now long-standing tradition that we launch a Human Gene Therapy special issue in conjunction with the annual congress of the European Society of Gene and Cell Therapy (ESGCT). At the last congress in Barcelona in October 2019, which attracted >1,900 participants, presentations not only centered on latest developments in the gene and cell therapy space related to monogenic diseases and cancer, but also opened the stage for strengthening interaction between gene therapy and infectious diseases with a session dedicated to this topic.
Driven by this spirit, we launched a special issue on “Gene and Cell Therapy for Inherited and Acquired Immune Deficiency” to cover contributions that highlight current knowledge and technologies being developed in the field of inherited primary immune deficiencies (PIDs) and applications to prevention and treatment of infectious diseases. In the context of the latter, we focused on human immunodeficiency virus (HIV) infection, an epidemic that has to date caused ∼33 million deaths (
Gene therapies—in particular when looking ahead and considering genome editing and in vivo strategies—possess potential to achieving cures as well as effective protection against a number of infectious diseases. We have recently experienced a glimpse of what is possible if efforts are combined, when the potential of gene- or nucleic acid-based strategies is recognized and advanced in a target-oriented manner and both logistical and financial support are provided. An enveloped RNA virus not larger than 120 nm in diameter, which is now known as severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2), has taught us this lesson. With first reports of a new viral outbreak coming at the end of 2019, this new coronavirus had emerged by end of January 2020 as the cause of a new pandemic, COVID-19, with close to 82 million cases of SARS-CoV-2 infections, including at least 1,808,041 deaths worldwide by January 1, 2021 (homepage World Health Organization [WHO]). The sorrow and pain that is the reality for so many behind these numbers is unimaginable, and we can only take a moment of silence to remember those who have suffered or lost a loved one.
With the first vaccines now becoming available—hopefully soon also to resource-limited parts of the world—let us hope that this pandemic will be coming to an end. Most of the COVID-19 vaccines, both those under development and those having already received regulatory approval, use technologies that have been developed for gene therapy, such as lipid nanoparticle-formulated RNA or human and nonhuman adenovirus-based vectors. However, the potential of gene and cell therapy in infectious disease goes far beyond being a vaccine development platform. This special issue, which was initially intended to become the ESGCT 2020 congress issue, the 28th annual congress of the ESGCT organized together with British Society for Gene and Cell Therapy (BSGCT) in the wonderful city of Edinburgh, will highlight some of these options and engages with the challenges that are lying ahead.
In May 2020, The National Institutes of Health and the Bill & Melinda Gates Foundation organized an expert scientific roundtable meeting to review the current state of the field and discuss hurdles involved in moving toward safe and effective in vivo targeting and gene editing in hematopoietic stem cells (HSCs) (this issue, Cannon et al. 1 ). Short summaries of the presentations are covering the topics (1) therapeutic potential of modified HSCs, (2) leveraging HSC biology and differentiation, and (3) in vivo targeting of HSCs.
In her commentary entitled “Engaging Cell and Gene Therapists in HIV Cure” (this issue, Johnston 2 ), Rowena Johnston of The Foundation for AIDS Research (amfAR) reminds us that “for 38 million people living with HIV (PWH), the best option remains daily ART, yet only two-thirds are accessing it. Enthusiasm among PWH for a cure is high, and willingness to participate in clinical research, including CGT [Cell and Gene Therapy] approaches, is largely encouraging.” Her network analysis revealed that collaboration between “HIV cure research” and “cell and gene therapy HIV cure research” ought to be further expanded, and she calls for “an influx of fresh ideas and energy from CGT researchers keen to embrace a new challenge for an old virus.”
To this end, Joseph M. McCune and colleagues of the Bill & Melinda Gates Foundation comment on the potential of gene therapy in the context of HIV and discuss why in vivo gene therapy holds the greatest promise as a strategy to achieve a cure for HIV, in particular, in resource-limited parts of the world (this issue, McCune et al. 3 ). However, McCune and colleagues also highlight the current limitations and the fact that knowledge on the HIV latent reservoir is lacking, and that we might be missing essential information of host cell–HIV interactions, as our current understanding is mainly based on CD4 cells in the peripheral blood.
Like Johnston, McCune and colleagues mention the “Berlin patient,” Timothy Ray Brown, who sadly passed away in 2020, as an example that an HIV cure is possible. Timothy Brown received two allogeneic bone marrow transplantations with HSCs deficient for the HIV coreceptor CCR5 when he was treated for cancer. Because HIV remained undetectable after ART withdrawal, he was reported to be cured. Ten years thereafter (this issue, Johnston 2 ), a second patient (the London patient) was reported to be cured also after transplantation with CCR5-deficient HSCs. Although in both cases HSCs from donors with a natural occurring CCR5 deficiency were available, finding an human leukocyte antigen (HLA)-matching donor with the CCR5Δ32/Δ32 genotype is not an easy task.
Genome editing, which Matthew Porteus (Stanford University) defines in his ESGCT eSchool lecture as a more precise form of gene therapy, offers the possibility to modify HSCs to overcome the current challenge of identifying a CCR5Δ32/Δ32 HLA-matching donor. Further strategies to control HIV include broadly neutralizing anti-HIV antibodies, CCR5-deleted T cells, chimeric antigen receptor (CAR) T cells, or designer nuclease or recombinase-based strategies to excise HIV proviruses (this issue, Cornu et al., 4 Johnston, 2 and McCune et al. 3 ). As impressively demonstrated in 2020 in the context of the SARS-CoV-2 vaccine development effort, funding is key to move novel strategies forward to the clinics. McCune and colleagues here report on the HIV Frontiers Program, initiated by the Bill & Melinda Gates Foundation in 2019 , aiming to develop “an effective, durable, safe, and affordable single-shot HIV cure that could be used anywhere in the world.”
As highlighted in the commentary of McCune and colleagues and as a topic of roundtable meeting, in vivo gene therapy represents a promising approach for PIDs. Rajawat and colleagues report on promising results in this regard with lentiviral vectors (LVs) (this issue, Rajawat et al. 5 ). SCID-X1 neonatal canine pups were treated with cocal-enveloped pseudotyped lentiviral vectors (cocal LVs) delivering the interleukin-2 receptor gamma chain after mobilization with no prior conditioning, resulting in therapeutic levels of gene-corrected CD3 T cells for at least 16 months. Since cocal LVs are less susceptible to serum inactivation than the commonly used vesicular stomatitis virus glycoprotein envelope while manufactured with similar efficiencies, cocal LVs may be suitable candidates to move from ex vivo to in vivo gene therapy.
In the context of inherited immune deficiencies, adenosine deaminase-deficient severe combined immune deficiency (ADA-SCID) was the target of the first human gene therapy clinical trial, whereas SCID-X1, the X-linked severe combined immune deficiency, was the first disease for which a functional cure through gene therapy was reported. The latter was due to seminal contributions of Marina Cavazzana and Alain Fischer at Hôpital Necker (Paris) and Adrian Thrasher and colleagues at Great Ormond Street Hospital (London) for the London trial. However, SCID-X1 was unfortunately also the first PID with reported cases of leukemia as consequences of gamma retroviral vector-mediated genotoxicity. In response, safer vector systems were developed, with LVs being the current delivery system of choice for ex vivo gene therapies. So far, conventional gene therapy strategies have been followed, and phase I/II human clinical trials for various PIDs are underway (this issue, Rai et al. 6 ). Rai and colleagues summarize the current state of developing genome editing approaches as an alternative to conventional gene strategies to treat PIDs (this issue, Rai et al. 6 ).
In contrast to conventional gene therapies that rely on the addition of a correct version of a defective or missing gene to provide a functional cure, genome editing sets the stage for a real cure of monogenic diseases through repairing or correcting the underlying disease-causing defect. Of the two main endogenous repair pathways that are activated by designer nuclease created DNA double-strand breaks (DSBs), homology-directed repair (HDR)-mediated genome editing rather than nonhomologous end joining (NHEJ) is the preferred repair pathway to correct loss-of-function mutations (this issue, Rai et al. 6 ). Exchange of the entire gene cassette rather than repair of patient-specific mutations is the more attractive strategy since it may preserve physiological control of gene expression while broadening applicability. As examples, Rai and colleagues present approaches for SCID-X1, chronic granulomatous disease, Wiskott–Aldrich syndrome, X-linked hyper-IgM syndrome, and immune dysregulation, polyendocrinopathy, enteropathy, X-linked. However, a number of challenges need to be addressed before using genome editing for routine clinical application (this issue, Rai et al. 6 ). In this regard, Rai and colleagues listed (1) delivery of the genome editing components in a safe and nontoxic manner, (2) increasing the frequency of HDR-mediated integration in HSCs or hematopoietic stem and progenitor cells (HSPCs), which use NHEJ as the preferential strategy to correct DSBs, (3) efficacy, that is, reaching therapeutically relevant levels of expression, (4) preservation of engraftment and long-term repopulating ability of genome-edited HSPCs, and (5) specificity of designer nucleases (this issue, Rai et al. 6 ).
As a prime candidate to delivery of HDR templates for gene editing of HSPCs, adeno-associated viral serotype 6 (AAV6) vectors appear to be effective (this issue, Rai et al. 6 ), adding a further area of application for the AAV vector system, which has gained popularity as a gene delivery tool for conventional in vivo gene therapy. This special issue features an exclusive interview with Jude Samulski, who cloned the first AAV in 1978 7 and since then has developed these Dependoparvoviruses as safe and efficient vectors for gene and cell therapy. This endeavor includes seminal contributions to deciphering AAV infection biology.
Owing to its tropism for primary human CD4 T lymphocytes, AAV6 has garnered interest in the context of anti-HIV therapeutic strategies (this issue, Stone et al. 8 ). However, pre-existing immunity against AAVs might pose a challenge in this regard. Stone and colleagues, therefore, investigated whether rapamycin administration would allow in vivo delivery of AAV6 and a capsid-modified version of AAV6 in seropositive rhesus macaques. Indeed, gene expression in muscle was obtained, which suggests that combining rapamycin and subcutaneous AAV vector administration could allow for expression of broadly neutralizing anti-HIV antibodies from muscle in AAV seropositive individuals.
In addition to muscle, cells of the hematopoietic system are targets for several gene therapy-based anti-HIV approaches. The comprehensive review of Cornu and colleagues provides an excellent summary on the current state of the field, in particular, highlighting genome editing-related strategies for developing a cure for HIV (this issue, Cornu et al. 4 ).
Chanut and colleagues review changes induced by the most frequently applied conditioning regimens, using alkylating agents and total body irradiation in murine models of human diseases (this issue, Chanut et al. 9 ). Studies such as those discussed by Chanut and colleagues are important to distinguish conditioning-induced effects including severe side effects from those caused by the transplantation. However, samples from patients with rare diseases, including PID patients, are often limited. To overcome these limitations, Sens and colleagues generated a set of IL-10 signaling pathway-related knockout induced pluripotent stem cells clones that were subsequently differentiated into macrophages (this issue, Sens et al. 10 ). In addition to gaining valuable insights into the pathomechanism, Sens and colleagues used these novel disease models to test therapeutic LV-based and small molecule-based approaches.
As already mentioned, this special issue was planned for the 28th annual congress of the ESGCT organized jointly with the BSGCT. The COVID-19 pandemic forced a change of plans for the congress,* but did not stop us from preparing this special issue. We focused on inherited and acquired immune deficiencies as an example to discuss the current state of cell and gene therapy and to show that the potential of cell and gene therapy goes far beyond monogenic diseases and is opening new avenues for developing prevention from and cure of infectious diseases.