The role of induced pluripotent stem cells in research and therapy of primary immunodeficiencies

The advent of reprogramming technology has greatly advanced the field of stem cell biology and nurtured our hope to create patient specific renewable stem cell sources. While the number of reports of disease specific induced pluripotent stem cells is continuously rising, the field becomes increasingly more aware that induced pluripotent stem cells are not as similar to embryonic stem cells as initially assumed. Our state of the art understanding of human induced pluripotent stem cells, their capacity, their limitations and their promise as it pertains to the study and treatment of primary immunodeficiencies, is the content of this review.

Introduction

Primary immunodeficiency diseases (PID) consist of a group of more than 150 mostly monogenic conditions that predispose individuals to different sets of infections, allergy, autoimmunity and cancer [1^•]. The study of human PIDs has allowed identification of genes that play a key role in immune system development and function. In several cases, identification of gene defects in patients with PID has preceded development of corresponding animal models. Furthermore, although humans and mice with defects in ortholog genes often share a similar phenotype, in some cases significant differences have emerged, thus limiting the significance of animal models to dissect the pathophysiology of human PIDs. Therefore, use of human samples remains a fundamental tool to study mechanisms of disease. For example, patient-derived CD34⁺ hematopoietic stem cells (HSCs) can be used to study in vitro the differentiation of T and B lymphocytes and of myeloid cells in patients with various forms of severe combined immunodeficiency (SCID), congenital agammaglobulinemia or severe congenital neutropenia. However, the rarity of these conditions limits access to patient-derived HSCs and thus represents a considerable obstacle for mechanistic studies of disease pathophysiology.

Significant progress has been made in the treatment of human PIDs. For example, SCID is fatal within the first years of life but can be cured by means of hematopoietic cell transplantation (HCT) and – in selected cases – gene or enzyme replacement therapy. Initial gene therapy trials for X-SCID provided proof of principle that the underlying genetic defect could be overcome by transduction of autologous patient CD34⁺ HSCs with a gamma retroviral vector carrying the γc gene, allowing long-term robust immune reconstitution [2]. However, the development of leukemia as a severe adverse effect made clear that the challenge to cure SCID had not yet been conquered. Integration of the vector close to a proto-oncogene, LMO2, led to insertional mutagenesis and clonal expansion of T cells in 5 of 20 patients with X-SCID treated in Paris and London [3, 4]. Similar serious adverse events have been observed also after gene therapy for chronic granulomatous disease [5] and Wiskott–Aldrich syndrome [6]. This prompted development of novel and hopefully safer approaches to gene therapy, based on the use of self-inactivating retroviral and lentiviral vectors in which the LTR have been removed and the transgene is expressed of an internal promoter of lesser potency [7]. Nonetheless, none of these approaches will completely eliminate the risk of insertional mutagenesis. An alternative strategy is represented by locus-specific targeting and gene correction, based on homologous recombination (HR) with use of a correct repair matrix. Although the frequency of HR is low in somatic cells (10⁻⁶), it can be significantly increased by using locus-specific nucleases such as zinc-finger nucleases (ZFN), meganucleases, and Transcription Activator-Like Effector Nucleases (TALENs), that introduce targeted DNA double strand breaks. Ultimately, the preclinical assessment of efficacy of this approach will rely upon use of patient-derived cells and demonstration of gene correction and functional reconstitution. Once again, the rarity of PIDs (and limited access to HSCs in particular) may represent a significant hurdle, unless new tools prove capable of providing viable alternatives.

Induced pluripotent stem cells (iPSCs) are a novel and practical tool for human disease modeling and correction, and in theory could serve as a limitless stem cell source for patient specific cellular therapies.

Initially envisioned as an equivalent to embryonic stem cells (ESCs), iPSC have now been recognized to bear more profound differences to ES cells than originally assumed. Reports of genetic instability of iPSCs have raised concerns about their potential use of in the clinical setting.

Taking a closer look at induced pluripotent stem cells, carefully evaluating their capacity and recognizing their limitations will enable us to accurately judge their potential and prevent us from prematurely dismissing their application in regenerative medicine.