Perforin-dependent apoptosis functionally compensates Fas deficiency in activation-induced cell death of human T lymphocytes

Activation-induced cell death (AICD) is involved in peripheral tolerance by controlling the expansion of repeatedly stimulated T cells via an apoptotic Fas (CD95; APO-1)-dependent pathway. The TNFRSF-6 gene encoding Fas is mutated in children suffering from autoimmune lymphoproliferative syndrome (ALPS), which is characterized by lymphoproliferation and autoimmunity. We examined AICD in Fas-deficient T cells from ALPS patients. We showed that primary activated Fas-deficient T cells die by apoptosis after repeated T cell antigen receptor (TCR) stimulation despite resistance to Fas-mediated cell death. This Fas-independent AICD was found to be mediated through a cytotoxic granules-dependent pathway. Cytotoxic granules-mediated AICD was also detected in normal T lymphocytes though to a lesser extent. As expected, the cytotoxic granules-dependent AICD was abolished in T cells from Rab27a- or perforin-deficient patients who exhibited defective granules-dependent cytotoxicity. Supporting an in vivo relevance of the cytotoxic granules-dependent AICD in ALPS patients, we detected an increased number of circulating T lymphocytes expressing granzymes A and B. Altogether, these data indicated that the cytotoxic granules-dependent cell death in ALPS may compensate for Fas deficiency in T lymphocytes. Furthermore, they identified a novel AICD pathway as a unique alternative to Fas apoptosis in human peripheral T lymphocytes.

Cernunnos, a novel nonhomologous end-joining factor, is mutated in human immunodeficiency with microcephaly

DNA double-strand breaks (DSBs) occur at random upon genotoxic stresses and represent obligatory intermediates during physiological DNA rearrangement events such as the V(D)J recombination in the immune system. DSBs, which are among the most toxic DNA lesions, are preferentially repaired by the nonhomologous end-joining (NHEJ) pathway in higher eukaryotes. Failure to properly repair DSBs results in genetic instability, developmental delay, and various forms of immunodeficiency. Here we describe five patients with growth retardation, microcephaly, and immunodeficiency characterized by a profound T+B lymphocytopenia. An increased cellular sensitivity to ionizing radiation, a defective V(D)J recombination, and an impaired DNA-end ligation process both in vivo and in vitro are indicative of a general DNA repair defect in these patients. All five patients carry mutations in the Cernunnos gene, which was identified through cDNA functional complementation cloning. Cernunnos/XLF represents a novel DNA repair factor essential for the NHEJ pathway.

Severe combined immunodeficiency and microcephaly in siblings with hypomorphic mutations in DNA ligase IV

DNA double-strand breaks (dsb) during V(D)J recombination of T and B lymphocyte receptor genes are resolved by the non-homologous DNA end joining pathway (NHEJ) including at least six factors: Ku70, Ku80, DNA-PK(cs), Artemis, Xrcc4, and DNA ligase IV (Lig4). Artemis and Lig4 are the only known V(D)J/NHEJ factors found deficient in human genetic disorders. Null mutations of the Artemis gene result in a complete absence of T and B lymphocytes and increased cellular sensitivity to ionizing radiations, causing radiosensitive-SCID. Mutations of Lig4 are exclusively hypomorphic and have only been described in six patients, four exhibiting mild immunodeficiency associated with microcephaly and developmental delay, while two patient had leukemia. Here we report a SCID associated with microcephaly caused by compound heterozygous hypomorphic mutations in Lig4. Residual activity of Lig4 in these patients is underscored by a normal pattern of TCR-alpha and -beta junctions in the T cells of the patients and a moderate impairment of V(D)J recombination as tested in vitro. These observations contrast with the severity of the clinical immunodeficiency, suggesting that Lig4 may have additional critical roles in lymphocyte survival beyond V(D)J recombination.

Failure of SCID-X1 gene therapy in older patients

Gene therapy has been shown to be a highly effective treatment for infants with typical X-linked severe combined immunodeficiency (SCID-X1, γc-deficiency). For patients in whom previous allogeneic transplantation has failed, and others with attenuated disease who may present later in life, the optimal treatment strategy in the absence of human leukocyte antigen (HLA)–matched donors is unclear. Here we report the failure of gene therapy in 2 such patients, despite effective gene transfer to bone marrow CD34+ cells, suggesting that there are intrinsic host-dependent restrictions to efficacy. In particular, there is likely to be a limitation to initiation of normal thymopoiesis, and we therefore suggest that intervention for these patients should be considered as early as possible.

The Repair of DNA Damages/Modifications During the Maturation of the Immune System: Lessons from Human Primary Immunodeficiency Disorders and Animal Models

The immune system is the site of various genotoxic stresses that occur during its maturation as well as during immune responses. These DNA lesions/modifications are primarily the consequences of specific physiological processes such as the V(D)J recombination, the immunoglobulin class switch recombination (CSR), and the generation of somatic hypermutations (SHMs) within Ig variable domains. The DNA lesions can be introduced either by specific factors (RAG1 and RAG2 in the case of V(D)J recombination and AID in the case of CSR and SHM) or during the various phases of cellular proliferation and cellular activation. All these DNA lesions are taken care of by the diverse DNA repair machineries of the cell. Several animal models as well as human conditions have established the critical importance of these DNA lesions/modifications and their repair in the physiology of the immune system. Indeed their defects have consequences ranging from immune deficiency to development of immune malignancy. The survey of human pathology has been highly instrumental in the past in identifying key factors involved in the generation of DNA modifications (AID for the Ig CSR and generation of SHM) or the repair of specific DNA damages (Artemis for V(D)J recombination). Defects in factors involved in the cell cycle checkpoints following DNA damage also have deleterious consequences on the immune system. The continuous survey of human diseases characterized by primary immunodeficiency associated with increased sensitivity to ionizing radiation should help identify other important DNA repair factors essential for the development and maintenance of the immune system.

Gene therapy of human severe combined immunodeficiency (SCID)-X1 disease

Severe combined immunodeficiency-X1 (SCID-X1) is an X-linked inherited disorder characterized by an early block in T and natural killer (NK) lymphocyte differentiation. This block is caused by mutations of the gene encoding the gammac cytokine receptor subunit of interleukin-2, -4, -7, -9, and -15 receptors, which participates in the delivery of growth, survival, and differentiation signals to early lymphoid progenitors. After preclinical studies, a gene therapy trial for SCID-X1 was initiated, based on the use of complementary DNA containing a defective gammac Moloney retrovirus-derived vector and ex vivo infection of CD34+ cells. After a 10-month follow-up period, gammac transgene-expressing T and NK cells were detected in two patients. T, B, and NK cell counts and function, including antigen-specific responses, were comparable to those of age-matched controls. Thus, gene therapy was able to provide full correction of disease phenotype and, hence, clinical benefit.

Hypoglycaemia induced by antibodies to insulin receptor following a bone marrow transplantation in an immunodeficient child

Severe hypoglycaemia developed seven months after a bone marrow transplantation in a child with severe combined immunodeficiency. His serum exerted potent insulin-like activity: (a) it stimulated insulin receptor autophosphorylation and kinase activity in cell-free systems, this effect being additive to insulin; (b) it increased glucose transport in isolated soleus muscle. These insulin-like effects were due to immunoglobulins against the insulin receptor. Indeed, the patient serum immunoprecipitated human or murine insulin receptors from different tissues and inhibited insulin binding to receptor on human IM-9 lymphocytes. After corticoids and immunosuppressive therapy by azathioprine, the patient hypoglycaemic episodes disappeared, and concomitantly, the antibodies to insulin receptor were no longer detected, as judged by both immunoprecipitation of insulin receptor and stimulation of glucose transport.