Aberrant T-cell antigen receptor-mediated responses in autoimmune lymphoproliferative syndrome

Exogenous immunoglobulin downregulates T-cell receptor signaling and cytokine production

Intravenous immune globulin (IVIG), a polyvalent solution of pooled human immunoglobulin, is a potent immunomodulating agent. It is currently approved to treat autoimmune diseases, including idiopathic thrombocytopenia purpura, autoimmune hemolytic anemia, and Kawasaki disease. The basis of IVIG's immunomodulatory properties is not entirely understood. Proposed mechanisms include Fc receptor blockade, interference with cytokine network, and provision of anti-idiotypic antibodies. IVIG has also been shown to affect T-lymphocyte function, although a direct effect has been difficult to reconcile given the lack of immunoglobulin or Fc-receptors on T cells. Experiments were thus carried out to determine whether IVIG was acting on a specific T-cell subset and at the level of the T-cell receptor (TCR), and whether cytokine expression patterns were modulated. T lymphocytes obtained from adult peripheral blood and umbilical cord blood were cultured over a 1-wk time course in the presence of pharmacological IVIG concentrations (Gamunex(®) , 0-2.0 mg/ml). Cells were exposed to various stimulation conditions, and TCR signaling competence was assessed by quantifying activation-induced upregulation of CD25 and CD69, as well as production of specific T-cell cytokines. IVIG was found to significantly decrease T-lymphocyte proliferation, in a dose and time-dependent manner, in both cord and adult blood. IVIG markedly reduced phytohemagglutinin and anti-CD3-induced upregulation of CD25 and CD69 in both CD4 and CD8 T-cell subsets, although phorbol ester-induced responses were intact, suggesting a defect in the CD3/TCR signaling pathway. IVIG also inhibited anti-CD3-induced cytokine production of IL-10, IL-2, and IFN-γ in a dose-dependent manner. These data suggest that IVIG may have direct T-cell immunomodulatory properties by dampening TCR responses. Further studies are needed to more precisely define the molecular targets of IVIG.

Impaired T-cell receptor activation in IL-1 receptor-associated kinase-4-deficient patients

BACKGROUND

IL-1 receptor-associated kinase 4 (IRAK-4) is an effector of the Toll-like receptor and IL-1 receptor pathways that plays a critical role in innate immune responses. The role of IRAK-4 in adaptive immune functions in human subjects is incompletely understood.

OBJECTIVE

We sought to evaluate T-cell function in IRAK-4 deficient patients.

METHODS

We compared upregulation of CD25 and CD69 on T cells and production of IL-2, IL-6, and IFN-gamma after stimulation of PBMCs from 4 IRAK-4-deficient patients and healthy control subjects with anti-CD3 and anti-CD28.

RESULTS

Upregulation of CD25 and CD69 on T cells and production of IL-6 and IFN-gamma, but not IL-2, was significantly reduced in IRAK-4-deficient patients.

CONCLUSIONS

IRAK-4-deficient patients have defects in T-cell activation.

Genetic defects of apoptosis and primary immunodeficiency

Programmed cell death is important for maintaining lymphocyte homeostasis. Several human-inherited diseases with impaired apoptosis have been identified at the genetic level: autoimmune lymphoproliferative syndrome, caspase-8 deficiency state, and X-linked lymphoproliferative syndrome. These diseases feature excess lymphocyte accumulation, autoimmunity, or immunodeficiency. Elucidating their molecular pathogenesis has also provided new insights into the signaling mechanisms regulating apoptosis and lymphocyte activation.

Intact T cell responses in ataxia telangiectasia

Ataxia telangiectasia (A-T) is an autosomal recessive multisystem disorder associated with a variable immune deficiency. The mechanism for this remains unclear. Qualitative and quantitative defects of cellular immunity have been previously reported. However, despite laboratory evidence of significant immune abnormalities, opportunistic infections are uncommon. To address this discrepancy, we analyzed cytokine production by quantitative real-time PCR and T cell function at the single cell level by flow cytometry in four A-T patients. CD4 and CD8 T cell subsets from these patients displayed intact signaling in response to anti-CD3 stimulation, similar to controls. Stimulated T cells from A-T patients also produced normal to increased levels of Th1 (IL-2, IFN-gamma) and Th2 (IL-10, IL-4) cytokines, relative to control values. Our results suggest that T cells from A-T patients may be more functionally intact than previously observed. This helps to explain the paucity of opportunistic infections encountered, unlike that encountered in other primary immunodeficiencies.

Autoimmune lymphoproliferative syndrome: molecular basis of disease and clinical phenotype

Autoimmune lymphoproliferative syndrome (ALPS) is a variable clinical condition manifest by lymphoproliferative disease, autoimmune cytopenias and susceptibility to malignancy. Central to the cellular pathogenesis is defective FAS-induced apoptosis, which in turn leads to dysregulation of lymphocyte homeostasis. The majority of patients have heterozygous mutations in the FAS (TNFRSF6) gene, but the condition is genetically heterogeneous and mutations in FAS ligand and caspase-8 and caspase-10, all of which are involved in Fas mediated signalling, have also been identified. This review provides a detailed insight into the pathophysiology of lymphocyte apoptosis and how this relates to the variable and complex clinical manifestations of ALPS.

Histologic features of sinus histiocytosis with massive lymphadenopathy in patients with autoimmune lymphoproliferative syndrome

Autoimmune lymphoproliferative syndrome (ALPS) is an inherited disorder associated with defects in Fas-mediated apoptosis, characterized most often by childhood onset of lymphadenopathy, splenomegaly, hypergammaglobulinemia, and autoimmune phenomena. Children with sinus histiocytosis with massive lymphadenopathy (SHML) have a somewhat similar clinical phenotype in which prominent adenopathy also is associated with hypergammaglobulinemia, and autoimmune phenomena are reported in 10-15% of cases. We observed histopathological features of SHML in the lymph nodes of some of our ALPS patients, further suggesting an association between these two disorders. We, thus, reviewed the lymph nodes from 44 patients ALPS type Ia, all of whom were confirmed to have germline mutations in the TNFRSF6 gene encoding Fas (CD95/Apo-1). Eighteen of 44 (41%) patients had a histiocytic proliferation resembling SHML. The affected patients included 15 males and 3 females ranging in age from 11 months to 30 years at the time of the LN biopsy. The lymph nodes contained S-100+ histiocytes with characteristic nuclear features of SHML, and showed evidence of emperipolesis in both hematoxylin and eosin (H and E) and immunostained sections. The extent of the histiocytic proliferation was variable, being confluent in 2 cases, multifocal in 13, and only evident as isolated SHML-type histiocytes in 3. In lymph nodes without confluent SHML changes, increased numbers of CD3+CD4-CD8+ (double negative) alphabeta T-cells, also negative for CD45RO, a feature of ALPS, could be identified in the paracortex. Furthermore, because SHML shares many clinical features with ALPS, we sought evidence of ALPS in sporadic SHML. We attempted to sequence TNFRSF6 DNA from archived tissue of 14 cases of Rosai-Dorfman disease. Full sequencing of the gene was successful in 4 of the cases; no mutations were identified. Nevertheless, our observations suggest that histologic features of SHML are part of the pathologic spectrum of ALPS type Ia. It remains to be determined if some cases of apparently sporadic SHML may be associated with heritable defects in Fas-mediated apoptosis.

Stimulation of Fas agonistic antibody-mediated apoptosis by heparin-like agents suppresses Hsp27 but not Bcl-2 protective activity

We report that in Jurkat T cells or freshly isolated T lymphocytes, physiological concentrations of high-molecular weight sulfated polysaccharides such as heparin, heparan sulfate, and dextran sulfate significantly increased the percentage of cell death induced by Fas IgM agonistic antibody. The phenomenon was caspase dependent and P53 independent and correlated with an increased accessibility of cell surface Fas receptors. We also observed that the Fas IgM agonistic antibody-dependent formation of sodium dodecyl sulfate (SDS)-resistant large structures containing Fas receptor was decreased in the presence of heparin-like agents. In contrast, the different agents had no effect when cell death was triggered by FasL, the natural ligand of Fas that does not generate SDS-resistant forms of Fas. Interestingly, the synergistic effect of heparin-like agents toward Fas IgM agonistic antibody-mediated cell death abolished Hsp27 antiapoptotic activity but did not alter much the protection generated by Bcl-2 expression.

Autoimmune lymphoproliferative syndrome

PURPOSE OF REVIEW

The autoimmune lymphoproliferative syndrome is a recently identified human disorder of lymphocyte apoptosis that has provided important information about Fas-mediated lymphocyte apoptosis. In this review we summarize current information regarding the diagnosis, management and underlying molecular basis of the syndrome.

RECENT FINDINGS

The genetic basis of autoimmune lymphoproliferative syndrome has continued to expand with the recently identified defects in caspase-8 and caspase-10 along with the more frequent defect in Fas and unusual Fas ligand deficiency. Genotype-phenotype links and differences continue to be assessed while the variation in penetrance remains to be fully defined. An increased risk for lymphoreticular malignancy has clearly been established in those autoimmune lymphoproliferative syndrome patients with defects in the gene encoding for the death domain of Fas. Therapy remains directed at managing acute problems although a preliminary report suggests sulphadoxine-pyrimethamine treatment may be successful in patients with the syndrome or autoimmune lymphoproliferative syndrome-like disease and this approach is presently being studied in a controlled trial.

SUMMARY

Defects in multiple molecules within the Fas apoptotic pathway may result in autoimmune lymphoproliferative syndrome and, despite recent advances, a number of patients remain with unidentified genetic defects. There is also clear need for improved understanding of mechanisms underlying the development of autoimmunity in this disorder and to provide early evidence for development of malignancy. This syndrome is the first human disorder linked to a germline defect in lymphocyte apoptosis and it continues to be an area of productive research and new information regarding this process of lymphocyte homeostasis and its role in human disease.

Association of immune abnormalities with telomere shortening in autosomal-dominant dyskeratosis congenita

Dyskeratosis congenita (DC) is an inherited bone marrow failure disorder characterized by abnormal skin pigmentation and nail dystrophy. We have recently described, in 10 members of a large 3-generation family, an autosomal-dominant form of DC (AD DC) that is due to a mutation in the gene-encoding human telomerase RNA (TERC), resulting in telomere shortening. In studying the immunologic consequences of TERC mutations, severe B lymphopenia and decreased immunoglobulin M (IgM) levels were noted. T cells were found to overexpress senescent markers, including CD57 and Fas receptor, and were moderately reduced in cell number. To determine whether these in vivo findings were related to cellular replicative defects, short-term cultures of AD DC lymphocytes were established to measure proliferation, mitoses, and apoptosis. AD DC lymphocytes displayed a markedly reduced proliferative capacity and increased basal apoptotic rate. Finally, telomere shortening was most prominent in third-generation subjects, and there appeared to be a correlation between telomere length and in vivo and in vitro immune findings. In summary, the observed lymphopenia and hypogammaglobulinemia in AD DC is likely a consequence of replicative failure and premature senescence of lymphocytes, supporting a role of telomerase activity in immune homeostasis.

Ageing, autoimmunity and arthritis: Perturbations of TCR signal transduction pathways with ageing - a biochemical paradigm for the ageing immune system

It is widely accepted that cell-mediated immune functions decline with age, rendering an individual more susceptible to infection and possibly cancer, as well as to age-associated autoimmune diseases. The exact causes of T-cell functional decline are not known. One possible cause could be the development of defects in the transduction of mitogenic signals following TCR stimulation. This T-cell hyporesponsiveness due to defects of signalling through the TCR either from healthy elderly subjects or from individuals with autoimmune diseases such as rheumatoid arthritis or systemic lupus erythematosus results in an impaired ability to mount efficient immune responses and to maintain responsiveness to foreign antigens. This implies that a high proportion of autoreactive T cells might accumulate either intrathymically or in the periphery. T-cell anergy and differential TCR signalling could thus also be key players in the disruption of tolerance and the onset of autoimmune diseases. The increasing number of the elderly may lead to an increase of clinically important autoimmune diseases. We will review the signal transduction changes through the TCR-CD3 complex in T lymphocytes from healthy elderly subjects, which result in a modification of the activation of transcription factors involved in IL-2 gene expression leading to decreased IL-2 production. The putative contribution of altered T-cell signalling with ageing in the development of autoimmune diseases will be also discussed.

Autoimmune lymphoproliferative syndrome

Autoimmune lymphoproliferative syndrome arises early in childhood in people who inherit mutations in genes that mediate lymphocyte apoptosis, or programmed cell death. In the immune system, antigen-induced lymphocyte apoptosis maintains immune homeostasis by limiting lymphocyte accumulation and minimizing reactions against self-antigens. In autoimmune lymphoproliferative syndrome, defective lymphocyte apoptosis manifests as chronic, nonmalignant adenopathy and splenomegaly; the expansion of an unusual population of CD4-CD8- T cells; and the development of autoimmune disease. Most cases of autoimmune lymphoproliferative syndrome involve heterozygous mutations in the lymphocyte surface protein Fas (CD95, Apo1) that impair a major apoptotic pathway. Prospective evaluations of patients and their families have revealed an ever-expanding spectrum of autoimmune lymphoproliferative syndrome and its major complications.

The lysophospholipid receptor G2A activates a specific combination of G proteins and promotes apoptosis

G2A, a G protein-coupled receptor for which lysophosphatidylcholine (LPC) is a high affinity ligand, belongs to a newly defined lysophospholipid receptor subfamily. Expression of G2A is transcriptionally up-regulated by stress-inducing and cell-damaging agents, and ectopic expression of G2A leads to growth inhibition. However, the G proteins that functionally couple to G2A have not been elucidated in detail. We report here that G2A ligand independently stimulates the accumulation of both inositol phosphates and cAMP. LPC does not further enhance inositol phosphate accumulation but dose-dependently augments intracellular cAMP concentration. Expression of G alpha(q) and G alpha(13) with G2A potentiates G2A-mediated activation of a NF-kappa B-luciferase reporter. These results demonstrate that G2A differentially couples to multiple G proteins including G alpha(s), G alpha(q), and G alpha(13), depending on whether it is bound to ligand. G2A-transfected HeLa cells display apoptotic signs including membrane blebbing, nuclear condensation, and reduction of mitochondrial membrane potential. Furthermore, G2A-induced apoptosis can be rescued by the caspase inhibitors, z-vad-fmk and CrmA. Although apoptosis occurs without LPC stimulation, LPC further enhances G2A-mediated apoptosis and correlates with its ability to induce cAMP elevation in both HeLa cells and primary lymphocytes. Rescue from G2A-induced apoptosis was achieved by co-expression of a G alpha(12/13)-specific inhibitor, p115RGS (regulator of G protein signaling), in combination with 2',5'-dideoxyadenosine treatment. These results demonstrate the ability of G2A to activate a specific combination of G proteins, and that G2A/LPC-induced apoptosis involves both G alpha(13)- and G alpha(s)-mediated pathways.