Toward gene therapy for human CD3 deficiencies

The role of the different CD3γ domains in TCR expression and signaling

The CD3 subunits of the T-cell antigen receptor (TCR) play a central role in regulation of surface TCR expression levels. Humans who lack CD3γ (γ^—) show reduced surface TCR expression levels and abolished phorbol ester (PMA)-induced TCR down-regulation. The response to PMA is mediated by a double leucine motif in the intracellular (IC) domain of CD3γ. However, the molecular cause of the reduced TCR surface expression in γ^— lymphocytes is still not known. We used retroviral vectors carrying wild type CD3γ or CD3δ or the following chimeras (EC-extracellular, TM-transmembrane and IC): δ_ECγ_TMγ_IC (δγγ for short), γγδ, γδδ and γγ-. Expression of γγγ, γγδ, γδδ or γγ- in the γ^— T cell line JGN, which lacks surface TCR, demonstrated that cell surface TCR levels in JGN were dependent on the EC domain of CD3γ and could not be replaced by the one of CD3δ. In JGN and primary γ^— patient T cells, the tested chimeras confirmed that the response to PMA maps to the IC domain of CD3γ. Since protein homology explains these results better than domain structure, we conclude that CD3γ contributes conformational cues that improve surface TCR expression, likely at the assembly or membrane transport steps. In JGN cells all chimeric TCRs were signalling competent. However, an IC domain at CD3γ was required for TCR-induced IL-2 and TNF-α production and CD69 expression, indicating that a TCR without a CD3γ IC domain has altered signalling capabilities.

Gene therapy for primary immune deficiencies: a Canadian perspective

The use of gene therapy (GT) for the treatment of primary immune deficiencies (PID) including severe combined immune deficiency (SCID) has progressed significantly in the recent years. In particular, long-term studies have shown that adenosine deaminase (ADA) gene delivery into ADA-deficient hematopoietic stem cells that are then transplanted into the patients corrects the abnormal function of the ADA enzyme, which leads to immune reconstitution. In contrast, the outcome was disappointing for patients with X-linked SCID, Wiskott-Aldrich syndrome and chronic granulomatous disease who received GT followed by autologous gene corrected transplantations, as many developed hematological malignancies. The malignancies were attributed to the predilection of the viruses used for gene delivery to integrated at oncogenic areas. The availability of safer and more efficient self-inactivating lentiviruses for gene delivery has reignited the interest in GT for many PID that are now in various stages of pre-clinical studies and clinical trials. Moreover, advances in early diagnosis of PID and gene editing technology coupled with enhanced abilities to generate and manipulate stem cells ex vivo are expected to further contribute to the benefit of GT for PID. Here we review the past, the present and the future of GT for PID, with particular emphasis on the Canadian perspective.

A change in CD3γ, CD3δ, CD3ϵ, and CD3ζ gene expression in T-lymphocytes from benzene-exposed and benzene-poisoned workers

Benzene is known to be highly toxic to a variety of cell types, including lymphocytes. A previous study showed that T-lymphocyte immune function disorder might be related to benzene exposure. To elucidate characteristics of TCR signal transduction in benzene-exposed workers, expression levels of CD3γ, CD3δ, CD3ϵ, and CD3ζ genes in peripheral blood mononuclear cells (PBMC) were analyzed. Real-time RT-PCR using SYBR Green I was used to detect CD3 gene expression levels in PBMC from 20 benzene-exposed workers, seven workers with chronic mild benzene poisoning, five workers with chronic severe benzene poisoning and 14 healthy individuals (controls). The relative mRNA expression level was analyzed by the 2(-Δct) × 100% method. In benzene-exposed worker cells, significantly higher CD3δ, CD3ϵ, and CD3ζ, expression levels were observed as compared with values for cells from the healthy controls. In the workers with chronic severe benzene poisoning, lymphocyte CD3γ, CD3ϵ, and CD3ζ gene expression levels were significantly lower than in control cells. Lymphocytes from chronic mildly benzene- poisoned workers evinced two different gene expression patterns, i.e., CD3γ and CD3ϵ levels were similar to those in the benzene-exposed worker cells, but CD3δ and CD3ζ expression levels were significantly lower relative to those in cells from chronic severely-benzene-poisoned counterparts. It remained to be determined if these reductions in expression of these genes presage or are indicative of deficiencies in the activities of T-lymphocytes in these workers. For now, it is hoped that this study may contribute to a better understanding of the disorders in cellular immunity frequently found with benzene-exposed workers.

CD3γ-independent pathways in TCR-mediated signaling in mature T and iNKT lymphocytes

Antigen recognition by T-lymphocytes through the T-cell antigen receptor, TCR-CD3, is a central event in the initiation of an immune response. CD3 proteins may have redundant as well as specific contributions to the intracellular propagation of TCR-mediated signals. However, to date, the relative role that each CD3 chain plays in signaling is controversial. In order to examine the roles of CD3γ chain in TCR signaling, we analyzed proximal and distal signaling events in human CD3γ(-/-) primary and Herpesvirus saimiri (HVS)-transformed T cells. Following TCR-CD3 engagement, certain early TCR signaling pathways (ZAP-70, ERK, p38 and mTORC2 phosphorylation, and actin polymerization) were comparable with control HVS-transformed T cells. However, other signaling pathways were affected, such TCRζ phosphorylation, indicating that the CD3γ chain contributes to improve TCR signaling efficiency and survival. On the other hand, CD3γ(-/-) primary invariant NKT cells (iNKT cells) showed a normal expansion in response to alpha-galactosylceramide (α-GalCer) and TCRVβ11(bright) iNKT cells were preferentially selected in this in vitro culture system, perhaps as a consequence of selective events in the thymus. Our results collectively indicate that a TCR lacking CD3γ can propagate a number of signals through the remaining invariant chains, likely the homologous CD3δ chain, which replaces it at the mutant TCR.

Gene therapy for immunodeficiency diseases

Primary immunodeficiency diseases represent good targets for hematopoietic stem cell-targeted gene therapy. Severe combined immunodeficiencies (SCID) have been the first examples of successful gene therapy based on the ex vivo usage of retroviral vectors. New advances in the technology of gene transfer should further promote gene therapy as a safe and effective therapeutic strategy of immunodeficiency diseases.