A20 is a dual-function ubiquitin-editing enzyme that maintains immune homeostasis by restraining inflammation. Although A20 serves a similar negative feedback function for T-cell receptor (TCR) signaling, the molecular mechanisms utilized and their ultimate impact on human T-cell function remain unclear. TCR engagement triggers the assembly of the CARD11-BCL10-MALT1 (CBM) protein complex, a signaling platform that governs the activation of downstream transcription factors including NF-κB and c-Jun/AP-1. Utilizing WT and A20 knockout Jurkat T cells, we found that A20 is required to negatively regulate NF-κB and JNK. Utilizing a novel set of A20 mutants in NF-κB and AP-1-driven reporter systems, we discovered the ZnF7 domain is crucial for negative regulatory capacity, while deubiquitinase activity is dispensable. Successful inactivation of A20 in human primary effector T cells congruently conferred sustained NF-κB and JNK signaling, including enhanced upregulation of activation markers, and increased secretion of several cytokines including IL-9. Finally, loss of A20 in primary human T cells resulted in decreased sensitivity to restimulation-induced cell death and increased sensitivity to cytokine withdrawal-induced death. These findings demonstrate the importance of A20 in maintaining T-cell homeostasis via negative regulation of both NF-κB and JNK signaling.

For effective adaptive immunity, T lymphocytes must rapidly expand and contract in an antigen-specific manner to effectively control invading pathogens and preserve immunological memory, without sustaining excessive collateral damage to host tissues. Starting from initial antigen encounter, carefully calibrated programmed cell death pathways are critical for maintaining homeostasis over distinct phases of the T cell response. Restimulation-induced cell death (RICD), a self-regulatory apoptosis pathway triggered by re-engagement of the T cell receptor (TCR), is particularly important for constraining effector T cell expansion to preclude overt immunopathology; indeed, genetic disorders affecting key molecules involved in RICD execution can manifest in excessive lymphoproliferation, malignancy, and autoimmunity. Herein we review our current knowledge of how RICD sensitivity is ultimately regulated over the course of an immune response, including recent revelations on molecules that tune RICD by enforcing resistance or promoting susceptibility in expanding versus mature effector T cells, respectively. Detailed dissection of the molecular and temporal control of RICD also illuminates novel therapeutic strategies for correcting abnormal T cell responses noted in various immune disorders by ultimately tuning RICD sensitivity.

With the recent advent of several generations of targeted DNA nucleases, most recently CRISPR/Cas9, genome editing has become broadly accessible across the biomedical community. Importantly, the capacity of these nucleases to modify specific genomic loci associated with human disease could render new classes of genetic disease, including autosomal dominant or even idiopathic disease, accessible to gene therapy. In parallel, the emergence of adeno-associated virus (AAV) as a clinically important vector raises the possibility of integrating these two technologies towards the development of gene editing therapies. Though clear challenges exist, numerous proof-of-concept studies in preclinical models offer exciting promise for the future of gene therapy.

In addition to their broad potential for therapeutic gene delivery, adeno-associated virus (AAV) vectors possess the innate ability to stimulate homologous recombination in mammalian cells at high efficiencies. This process—referred to as AAV-mediated gene targeting—has enabled the introduction of a diverse array of genomic modifications both in vitro and in vivo. With the recent emergence of targeted nucleases, AAV-mediated genome engineering is poised for clinical translation. Here, we review key properties of AAV vectors that underscore its unique utility in genome editing. We highlight the broad range of genome engineering applications facilitated by this technology and discuss the strong potential for unifying AAV with targeted nucleases for next-generation gene therapy.

Significant strides in nuclease engineering have enabled a broad range of biomedical applications. However, numerous challenges remain, and in particular the long term expression mediated by AAV delivery of nuclease-encoding constructs to post-mitotic cells raises concerns with specificity and immunogenicity. That is, off-target nuclease activity can induce genotoxicity, and expression of an exogenous nuclease has the potential to elicit an immune response against transduced cells. Thus, it would be advantageous to limit the duration of nuclease expression following delivery.

Background Kallikrein 6 (KLK6) is a newly identified member of the kallikrein family of secreted serine proteases that prior studies indicate is elevated at sites of central nervous system (CNS) inflammation and which shows regulated expression with T cell activation. Notably, KLK6 is also elevated in the serum of multiple sclerosis (MS) patients however its potential roles in immune function are unknown. Herein we specifically examine whether KLK6 alters immune cell survival and the possible mechanism by which this may occur. Methodology/Principal Findings Using murine whole splenocyte preparations and the human Jurkat T cell line we demonstrate that KLK6 robustly supports cell survival across a range of cell death paradigms. Recombinant KLK6 was shown to significantly reduce cell death under resting conditions and in response to camptothecin, dexamethasone, staurosporine and Fas-ligand. Moreover, KLK6-over expression in Jurkat T cells was shown to generate parallel pro-survival effects. In mixed splenocyte populations the vigorous immune cell survival promoting effects of KLK6 were shown to include both T and B lymphocytes, to occur with as little as 5 minutes of treatment, and to involve up regulation of the pro-survival protein B-cell lymphoma-extra large (Bcl-XL), and inhibition of the pro-apoptotic protein Bcl-2-interacting mediator of cell death (Bim). The ability of KLK6 to promote survival of splenic T cells was also shown to be absent in cell preparations derived from PAR1 deficient mice. Conclusion/Significance KLK6 promotes lymphocyte survival by a mechanism that depends in part on activation of PAR1. These findings point to a novel molecular mechanism regulating lymphocyte survival that is likely to have relevance to a range of immunological responses that depend on apoptosis for immune clearance and maintenance of homeostasis.