CD4 T-cell subsets in autoimmunity

The discovery that functionally heterogeneous CD4⁺ T-cell subsets secrete different cytokines offers an explanation for the ability of certain T cells to mediate a predominant cell-mediated immune response versus a humoral response often accompanied by allergic manifestations. Th1 cells, important for cell-mediated immunity by their production of IL-2, IFN-γ and lymphotoxin, have been implicated in the immunopathology of certain organ-specific autoimmune diseases whereas a role as regulators has been suggested for IL-4 and IL-10 producing Th2 cells. Recent findings, however, beg re-evaluation of the direct role of Th2 cells in the induction or maintenance of tolerance, whereas evidence for the role of a distinct subset of regulatory T cells producing TGF-β to suppress cell-mediated immunopathology is compelling.     

The role of cytokines in T-cell memory in health and disease

Upon stimulation with their cognate antigen, naive T cells undergo proliferation and differentiation into effector cells, followed by apoptosis or survival as precursors of long-lived memory cells. These phases of a T-cell response and the ensuing maintenance of memory T cells are shaped by cytokines, most notably interleukin-2 (IL-2), IL-7, and IL-15 that share the common γ chain (γ_c) cytokine receptor. Steady-state production of IL-7 and IL-15 is necessary for background proliferation and homeostatic survival of CD4⁺ and CD8⁺ memory T cells. During immune responses, augmented levels of IL-2, IL-15, IL-21, IL-12, IL-18, and type-I interferons determine the memory potential of antigen-specific effector CD8⁺ cells, while increased IL-2 and IL-15 cause bystander proliferation of heterologous CD4⁺ and CD8⁺ memory T cells. Limiting availability of γ_c cytokines, reduction in regulatory T cells or IL-10, and persistence of inflammation or cognate antigen can result in memory T cells, which fail to become cytokine-dependent long-lived cells. Conversely, increased IL-7 and IL-15 can expand memory T cells, including pathogenic tissue-resident memory T cells, as seen in lymphopenia and certain chronic-inflammatory disorders and malignancies. These abovementioned factors impact immunotherapy and vaccines directed at memory T cells in cancer and chronic infection.     

Frequency of measles virus-specific CD4+ and CD8+ T cells in subjects seronegative or highly seropositive for measles vaccine

The protective effect of measles immunization is due to humoral and cell-mediated immune responses. Little is known about cell-mediated immunity (CMI) to measles vaccine virus, the relative contribution of CD4(+) and CD8(+) T cells to variability in such immune responses, and the immunologic longevity of the CMI after measles vaccination in humans. Our study characterizes cellular immune response in subjects seronegative or highly seropositive for measles vaccine immunoglobulin G-specific antibody, aged 15 to 25 years, previously immunized with two doses of measles-mumps-rubella II vaccine. We evaluated the ability of subjects to respond to measles vaccine virus by measuring measles virus-specific T-cell proliferation. We examined the frequencies of measles virus-specific memory Th1 and Th2 cells by an ELISPOT assay. Our results demonstrated that proliferation of T cells in seronegative subjects was significantly lower than that for highly seropositive subjects (P = 0.003). Gamma interferon (IFN-gamma) secretion predominated over interleukin 4 (IL-4) secretion in response to measles virus in both groups. The median frequency of measles virus-reactive CD8(+) T cells secreting IFN-gamma was 0.09% in seronegative subjects and 0.43% in highly seropositive subjects (P = 0.04). The median frequency of CD4(+) T cells secreting IL-4 in response to measles virus was 0.03% in seronegative subjects and 0.09% in highly seropositive subjects (P = 0.005). These data confirm the presence of measles virus-specific cellular immune responses post-measles vaccine immunization in humans. The detection of measles virus-induced IFN-gamma and IL-4 production by ELISPOT can be used to identify measles virus-specific low-frequency memory T cells in subjects immunized with measles vaccine. These differences agree in directionality with the observed antibody response phenotype.     

Effect of interleukin 15 and interleukin 2 on anti-CD3-induced T-cell activation and apoptosis in children with common variable immunodeficiency.

BACKGROUND: Defective T-cell function and susceptibility to apoptosis following activation may contribute to the immunodeficiency observed in common variable immunodeficiency (CVID) patients. Interleukin (IL) 2 benefits CVID children by boosting T-cell function. OBJECTIVE: To investigate the effect of another IL-2 common gamma-chain signaling cytokine, IL-15, on T-cell activation and apoptosis of peripheral blood mononuclear cells (PBMCs) from children with CVID compared with IL-2. METHODS: Five children treated at the Chang Gung Children's Hospital, Taoyuan, Taiwan, during 1998 to 2002 who fulfilled World Health Organization criteria for CVID and 8 age-matched, healthy controls were enrolled. The PBMCs were stimulated with anti-CD3 in the presence or absence of IL-2 and IL-15 for 4 days, followed by 24-hour incubation with anti-Fas to induce apoptosis. Surface expression of CD69, CD25, and CD95 (Fas) on CD3+ T cells was evaluated by flow cytometry. The degree of apoptosis was evaluated by propidium iodide-phycoerythrin/annexin V-fluorescein isothiocyanate flow cytometric staining. RESULTS: Following anti-CD3 activation, CD69 and CD25 expression of CVID CD3+ PBMCs was enhanced to levels comparable to controls. Exogenous IL-15 resulted in further enhancement of anti-CD3-induced CD69 expression to a greater extent than that achieved with IL-2. A greater degree of apoptosis was found in CVID patients than controls following anti-CD3 stimulation (P = 0.001). IL-15 but not IL-2 further increased anti-CD3-induced apoptosis in control PBMCs (P = 0.001). In contrast, the degree of anti-CD3-induced apoptosis in CVID PBMCs was unaffected by IL-15 or IL-2. Addition of anti-Fas to cultures prestimulated with anti-CD3 further increased the apoptosis in control PBMCs but had no effect on apoptosis of CVID PBMCs. CONCLUSION: Comparable anti-CD3-induced activation and enhanced apoptosis were observed in PBMCs from children with CVID compared with controls. The degree of apoptosis in CVID PBMCs was not affected by exogenous IL-15 or anti-Fas, suggesting a preactivated status in vivo.     

Failure to reconstitute CD4+ T-cells despite suppression of HIV replication under HAART

HAART in HIV-1-infected individuals has a broad spectrum of clinical outcomes. In the majority of patients, plasma viral load becomes undetectable and CD4+ T-cells increase over time. However, in a number of subjects a discrepancy between plasma viral load and the CD4+ T-cell recovery is observed. CD4+ T-cell count can rise despite persistently detectable plasma viral load (virologic nonresponders), or conversely does not increase despite full plasma viral load suppression (immunologic nonresponder). Defective immune reconstitution may depend on several factors including previous therapeutic failure, duration of antiretroviral therapy, low CD4+ T-cell count at the initiation of HAART, advanced stage of disease, low adherence to HAART, and previous treatment interruption. There is no definitive evidence that age, viral strain/clade, or host genetic factors play a role in these different responses to HAART. The roles of T-cell subsets, thymic function, and cytokines have been investigated. The increased T-cell activation/apoptosis has been associated with a lack of effective immunologic response. Unabated virologic replication in lymphoid tissues, despite undetectable plasma viral load, has been proposed as the underlying mechanism of cellular activation. However, this "paradoxical response" probably can be associated with other events. Insufficient CD4+ T-cell repopulation of lymphoid tissues may be due to a thymus failure or a defect in bone marrow function. Lifelong infection, the toxic effect of antiviral drugs on T- and B-cell precursors, the stage of disease, and the low number of CD4+ T-cells before HAART may also account for thymus exhaustion and insufficient T-cell renewal. Finally, an imbalance in the production of cytokines such as TNF, IL-2 and IL-7 may also be a crucial event for the induction of immune system failure. In patients in which CD4+ T-cells are not increased by HAART, therapeutic strategies aimed at increasing these cells and reducing the risk of infections are needed. IL-2 and/or other cytokines may be of benefit in this setting. Some antiviral drugs may be better than others in immunologic reconstitution. Protease inhibitors may have additional, independent positive effects on the immune system. On the other hand, there may be little rationale for using immunosuppressive agents such as cyclosporine or hydroxyurea in this subgroup of immunologic nonresponder patients, as these molecules may increase T-cell decline and/or favor susceptibility to infections.     

Combinations of IFN-gamma and IL-4 induce distinct profiles of dendritic cell-associated immunoregulatory properties

Interferon gamma (IFN-gamma) and interleukin-4 (IL-4) are not only generated during cell-mediated immunity (CMI) and humoral immunity (HI), but are also generated by innate immune cells in response to pathogenic factors. How these cytokines differentially effect the development of dendritic cell (DC)-associated immunoregulatory properties from progenitor cells during innate immunity is unresolved. To address this we have utilized a homogeneous DC progenitor-like cell line, MTHC-D2, as a model to examine cytokine-induced maturation of DCs. By 6 h IFN-gamma induced genes that are important for antiviral activity and development of CMI, whereas IL-4 induced genes involved in cellular adhesion, uptake of extracellular antigen, suppression of cytotoxic T-cell responses, and that repair the extracellular matrix. By 48 h the cytokine stimulus had induced many properties characteristic of immature DCs; however, these were differentially effected by IFN-gamma and IL-4. IFN-gamma induced the greatest levels of costimulatory/ activation marker expression, and the highest levels of T-cell proliferation, whereas IL-4 induced the greatest levels of phagocytic activity. Stimulation of the cells with CD40 Ab enhanced the levels of costimulatory marker expression and T-cell stimulatory capacity of cells exposed to IFN-gamma, but had little effect on cells exposed to IL-4 in the absence of IFN-gamma.     

Activation, survival and apoptosis of CD45RO+and CD45RO−T cells of human immunodeficiency virus-infected individuals: effects of interleukin-15 and comparison with interleukin-2

HIV infection is associated with increased representation of T cells bearing an activated, memory (CD45RO+) phenotype. Although administration of antiretroviral agents and interleukin-2 (IL-2) augment depleted CD4+ T-cell numbers, such therapies have been preferentially beneficial for CD45RO+ T cells. Interleukin-15 (IL-15) exhibits many biological activities in common with IL-2, including promoting T-cell survival and proliferation. The present study found that these two cytokines differed in their ability to induce proliferation, enhance survival, and control apoptosis of CD45RO+ and CD45RO- T-cell populations of human immunodeficiency- (HIV) infected individuals. When used at equivalent concentrations in vitro, IL-15 was more potent than IL-2 in activating and stimulating proliferation of CD4+CD45RO+, CD8+CD45RO+ and CD8+CD45RO- cells, but failed to be more effective than IL-2 in reducing apoptosis. Poor activation of CD4+CD45RO- cells by IL-15 and to IL-2 appeared to be attributable to low expression of the beta receptor chain utilized by both cytokines. However, IL-15 was more effective than IL-2 in enhancing survival of the CD4+CD45RO- population, suggesting a greater protective effect of IL-15 for naive CD4+ T cells, which are preferentially lost in HIV-infected individuals.     

Dendritic cells generated in the presence of interferon-alpha stimulate allogeneic CD4+ T-cell proliferation: modulation by autocrine IL-10, enhanced T-cell apoptosis and T regulatory type 1 cells

Dendritic cells (DCs) generated in the presence of IFN-alpha (IFN-DCs) exhibit high expression of major histocompatibility and co-stimulatory molecules and a potent ability to stimulate CD8(+) T-cell responses. Here, we found that IFN-DCs were more potent stimulators of bulk and purified CD8(+) T-cell proliferation, as compared with IL-4-DCs. In contrast, IFN-DCs were less efficient than IL-4-DCs in stimulating allogeneic CD4(+) T-cell proliferation, due to a weak induction of naive CD4(+)CD45RO(-) T-cell proliferation by these DCs. However, both DC populations induced similar levels of proliferation of memory CD4(+)CD45RO(+) T cells. IFN-DCs and IL-4-DCs exhibited a similar phenotype and production of IL-10 following maturation induced by CD40 ligation. In contrast, IFN-DCs produced higher levels of IL-10 during the first days of differentiation. In addition, neutralization of IL-10 during the differentiation of DCs increased the expression of DC-LAMP and MHC class II by IFN-DCs, and the ability of IFN-DCs to stimulate allogeneic CD4(+) T-cell proliferation at similar levels, than IL-4-DCs. Independently of IL-10 production, IFN-DCs were found to induce higher levels of CD4(+)T-cell apoptosis, this effect being more sticking on naive T cells. Finally, we demonstrated that IFN-DCs induced a differentiation bias of naive CD4(+) T cells towards Th1 and Tr1 cells, compared to IL-4-DCs. Taken together, these results indicate that, despite the induction of Tr1 cells and enhanced apoptosis of naive CD4(+) T cells, IFN-DCs are potent stimulators of CD8(+) and memory CD4(+) T cells, and induce a strong polarization of naive CD4(+) T cells towards Th1 cells, further supporting their use in immune-based therapy.     

Induction of Mycobacterium tuberculosis-specific primary and secondary T-cell responses in interleukin-15-deficient mice

Several studies have provided evidence that interleukin-15 (IL-15) can enhance protective immune responses against Mycobacterium tuberculosis infection. However, the effects of IL-15 deficiency on the functionality of M. tuberculosis-specific CD4 and CD8 T cells are unknown. In this study, we investigated the generation and maintenance of effector and memory T-cell responses following M. tuberculosis infection of IL-15(-/-) mice. IL-15(-/-) mice had slightly higher bacterial numbers during chronic infection, which were accompanied by an increase in gamma interferon (IFN-gamma)-producing CD4 and CD8 T cells. There was no evidence of increased apoptosis or a defect in proliferation of CD8 effector T cells following M. tuberculosis infection. The induction of cytotoxic and IFN-gamma CD8 T-cell responses was normal in the absence of IL-15 signaling. The infiltration of CD4 and CD8 T cells into the lungs of "immune" IL-15(-/-) mice was delayed in response to M. tuberculosis challenge. These findings demonstrate that efficient effector CD4 and CD8 T cells can be developed following M. tuberculosis infection in the absence of IL-15 but that recall T-cell responses may be impaired.     

Expanding the effector CD4 T-cell repertoire: the Th17 lineage

The Th1/Th2 paradigm has provided the framework for understanding CD4 T-cell biology and the interplay between innate and adaptive immunity for almost two decades. Recent studies have defined a previously unknown arm of the CD4 T-cell effector response--the Th17 lineage--that promises to change our understanding of immune regulation, immune pathogenesis and host defense. The factors that specify differentiation of IL-17-producing effector T-cells from na?ve T-cell precursors are being rapidly discovered and are providing insights into mechanisms by which signals from cells of the innate immune system guide alternative pathways of Th1, Th2 or Th17 development.     

Recombinant cytokines as an approach to immune reconstitution in aging

Aging is characterized by a decreased humoral and cell-mediated immunity to a large variety of exogenous antigens and by an increased propensity to autoantibody production, suggesting an age-related disregulation of the immune system. The decline in immune responsiveness to exogenous antigens has been attributed to thymus involution, consisting of a fall in the capacity to induce intrathymic T-cell growth and differentiation, and also to export mature T cells to the periphery. T-cell activation and secretion of soluble factors have been reported to change with aging, but, as with cytokines, the results are conflicting. We investigated the production of IL-2 and IFN-γ (Th1 type) and IL-4 (Th2 type) cytokines by mitogen-activated spleen cells from young, adult and old mice and their regulation by the addition of a recombinant cytokine (IL-1β, IL-2, IL-3, IL-4, IL-12, IFN-γ) at varying concentrations. The results indicate that cytokine production can be enhanced only when it is deficient, suggesting the possible use of recombinant cytokines as efficient immunomodulators in age-associated immune disorders.     

艾滋病HAART治疗免疫重建炎性综合征的免疫机制初步研究

目的 为探讨我国艾滋病病人(AIDS)启动高效抗反转录病毒治疗(HAART)后,发生免疫重建炎性综合征(IRIS)的免疫学发病机制,在前瞻性研究队列中对启动HAART的AIDS病人部分淋巴细胞亚群、调节性T细胞和部分Th1和Th2细胞因子等进行追踪分析.方法 将接受HAART初始治疗的238例AIDS病人建立前瞻性研究队列,分为在24周内发生IRIS的47例病人(IRIS组)和未发生IRIS的191例病人(非IRIS组).在HAART的0周、12周和24周采集两组血标本,对47例IRIS病例及随机选择的50例非IRIS病例进行免疫机制的研究分析,检测HIV病毒载量和CD4+细胞计数;使用流式细胞术检测部分淋巴细胞亚群和调节性T细胞(CD4+CD25+Foxp3+).在HAART的0周、4周、12周、24周及发生IRIS时分别采集全部238例患者的血标本,使用ELISA法测定血浆细胞因子IL-2、IFN-γ、IL-4、IL-10以及IL-7水平.结果 两组感染者的CD4+、CD8+纯真细胞和记忆细胞比例在0周、12周、24周及发生IRIS时比较差异均无统计学意义,但CD4+记忆细胞和CD8+记忆细胞比例在启动HAART后均明显上升.两组感染者CD4+CD38+活化细胞和CD8+CD38+活化细胞比例在基线时均较正常值明显升高,治疗后均有下降趋势.在0周、12周、24周及发生IRIS时CD4+CD25+Foxp3+调节性T细胞在IRIS组中均较非IRIS组要低.两组IL-2及IFN-γ在HAART后均呈上升趋势,且IRIS组在4周及发生IRIS时更明显高于非IRIS组;两组IL-4及IL-10在HAART后均呈下降趋势,IRIS组中IL-10在4周及发生IRIS时更明显低于非IRIS组.IL-7在两组中基线时均较正常值升高,并随HAART进程逐渐降低,其中IRIS组在各随访点IL-7均要高于非IRIS组.结论 接受HAART治疗者早期即出现记忆T细胞快速上升,在IRIS炎症反应中可能起重要作用.IL-2、IL-10和IFN-γ在发生IRIS时的水平差别,提示IRIS的发生与炎性细胞因子大量增加,炎性抑制因子相对不足有一定关系.IL-7也可能参与到IRIS的发病机制中.

Abstract：

Objective To investigate the immunological pathogenesis of immune reconstitution inflammatory syndrome (IRIS) during highly active antiretroviral therapy( HAART), in this prospective cohort study we analyzed the lymphocyte subsets, lymphocyte activation, changes in regulatory T cells, and levels of Th1 and Th2 cytokines in both IRIS and non-IRIS groups. Methods Two hundred and thirty-eight AIDS patients received HAART and participated prospective research cohort for 24 weeks follow-up. Forty-seven IRIS cases and 191 non-IRIS cases were enrolled in the IRIS group or non-IRIS group respectively. Blood samples were collected in both groups at pre- and post-HAART 12 weeks, 24 weeks. Using flow cytometer to detect the immunophenotypes of lymphocyte subsets (CD4 + CD45RA+ CD62L+, CD8+ CD45RA+ CD62L+naive T cells; CD4+ CD45RO+, CD8+ CD45RO+ memory T cells), activated T lymphocytes (CD4+CD38 +, CD8 + CD38 + cells), and regulatory T cell ( CD4 + CD25 + Foxp3 + ). Blood samples collected at pre-and post-HAART4 weeks, 12 weeks, 24 weeks and used ELISA to detect IL-2, IFN-γ, IL-4, IL-10and IL-7 cytokine serum levels. Results The percentages of CD4 + and CD8 + naive T cells and mlemory T cells exhibited no significant differences at the baseline, 12 weeks, 24 weeks of HAART initiation between both groups, but CD4 + and CD8 + memory T cells were demonstrated a trend towards to increase while compared to baseline during HAART. The percentages of CD4 + and CD8 + activated T cells are significantly higher at the baseline while compared to normal control and demonstrated a downward trend, but between both groups showed no significant difference. The percentages of CD4 + regulatory T cell was lower in IRIS group than non-IRIS group at the baseline, 12 weeks, 24 weeks and the onset of IRIS. Th1 cytokines, IL-2 and IFN-γshowed an upward trend during HAART at the levels of IRIS group had significantly increased at 4 weeks and the onset of IRIS. Th2 cytokines, IL-4 and IL-10 showed a downward trend during HAART,and the levels of IL-10 in IRIS group had significantly decreased at 4 weeks and the onset of IRIS. IL-7 was higher than normal control at the baseline in two groups and showed a downward trend during HAART. The level of IL-7 was higher than non-IRIS group at all follow-up points. Conclusion Memory T cells appear rapid increase in the early stage of HAART and may play a significant role in the inflammatory response of IRIS. CD4 + and CD8 + naive T cells, memory T cells and activated T cells showed no significant difference between IRIS and non-IRIS group within 24 weeks after HAART started. There was a significant reduction in the frequency of regulatory T cells in IRIS group without obvious upward trend during HAART, suggesting that the immune suppression function of regulatory T cells in IRIS was impaired. IL-2 and IFN-γ significantly increased while IL-10 significantly decreased at 4 weeks post-HAART initiation and onset of IRIS in IRISgroup than non-IRIS group, suggested that IRIS was related to cytokines environment disorder. That is, a significant increase in inflammatory cytokines, while the relative lack of non-inflammatory cytokines. The level of IL-7 decreased gradually after HAART started, and it was higher in IRIS group when compared to non-IRIS group in the first 24 weeks after HAART started. Also IL-7 may play a role in the pathogenesis of IRIS.     

Co-inhibitory role of T-cell-associated B7-H1 and B7-DC in the T-cell immune response

B7-H1 and B7-DC expressed on antigen-presenting cells inhibit the T-cell response via the PD-1 counter-receptor on T cells, and co-stimulate T-cell immunity under certain conditions via an unidentified co-stimulatory receptor. However, little is known about the functional consequence of T-cell-associated B7-H1 or B7-DC in the T-cell immune response. Therefore, we evaluated the physiological role of B7-H1 and B7-DC expressed on T cells in terms of cell proliferation and cytokine production by alloreactive T cells. We found that PD-1, B7-H1, and B7-DC were up-regulated in alloreactive CD4(+) and CD8(+) T cells in vitro and in vivo. In the alloreactive T-T model, blockade of the B7-H1:PD-1 or B7-DC:PD-1 pathways significantly increased the proliferation, and IFN-gamma and IL-2 production of alloreactive T cells, although it did not affect the production of other cytokines, including IL-4, IL-10, and IL-12. The data indicate that T-cell-associated B7-H1 and B7-DC negatively regulate the T-cell response via the T-T interaction.     

Costimulation of CD3/TcR complex with either integrin or nonintegrin ligands protects CD4+ allergen-specific T-cell clones from programmed cell death

An optimal stimulation of CD4⁺ cells in an immune response requires not only signals transduced via the TcR/CD3 complex, but also costimulatory signals delivered as a consequence of interactions between T-cell surface-associated costimulatory receptors and their counterparts on antigen-presenting cells ‘APC). The intercellular adhesion molecule-1 ‘ICAM-1, CD54) efficiently costimulates proliferation of resting, but not antigen-specific, T cells. In contrast, CD28 and CD2 support interleukin ‘IL)-2 synthesis and proliferation of antigen-specific T cells more efficiently than those of resting T cells. The molecular basis for this differential costimulation of T cells is poorly understood. Cypress-specific T-cell clones ‘TCC) were generated from four allergic subjects during in vivo seasonal exposure to the allergen. Purified cypress extract was produced directly from fresh collected pollen and incubated with the patients' mononuclear cells. Repeated allergen stimulation was performed in T-cell cultures supplemented with purified extract and autologous APC. The limiting-dilution technique was then adopted to generate allergen-specific TCC, which were also characterized by their cytokine secretion pattern as ThO ‘IL-4 plus interferon-gamma) or Th2 ‘IL-4). Costimulation-induced proliferation or apoptosis was measured by propidium iodide cytofluorometric assay. By cross-linking cypress-specific CD4⁺ and CD8⁺ T-cell clones with either anti-CD3 or anti-CD2, anti-CD28, and anti-CD54 monoclonal antibodies, we demonstrated that CD4+ clones ‘with ThO- or Th2-type cytokine production pattern) undergo programmed cell death only after anti-CD3 stimulation, whereas costimulation with either anti-CD54 or anti-CD28 protects target cells from apoptosis. The costimulation-induced protection from apoptotic death was associated with a significant rise in IL-4 secretion in both Th0 and Th2-type clones. In contrast, cypress-specific Th0 CD8⁺ clones were more susceptible to stimulation-induced apoptosis via either anti-CD3 or anti-CD2, alone or in combination with anti-CD54 or anti-CD28, thus displaying only slight but nonsignificant modifications in the pattern of IL-4 secretion. The death-promoting costimulatory effects were not observed with highly purified normal resting CD4⁺ or CD8⁺ lymphocytes. Taken together, these results suggest that TcR engagement by an allergen in the context of functionally active APC induces activation-dependent cell death of some, perhaps less specific, cells, and this may be an important homeostatic mechanism through which functional expansion of allergen-specific T cells is regulated during an ongoing immune response.     

Persistent apoptosis in HIV-1-infected individuals receiving potent antiretroviral therapy is associated with poor recovery of CD4 T lymphocytes

CD4 T-cell depletion in HIV-1 infection is partly the result of T-cell apoptosis. Spontaneous apoptosis (SA) and apoptosis markers Fas-associated death-domain-like IL-1 beta converting enzyme (FLICE)-like inhibitory protein (FLIP), Bcl-2, TRAIL (tumor necrosis factor-related apoptosis-inducing ligand), TRAIL receptor 1, and Fas were determined in 55 HIV-1 infected persons treated with highly active antiretroviral therapy (HAART) for 48 months. Despite suppressive HAART, SA remained elevated. Increased SA of peripheral blood mononuclear cells (PBMCs) and CD8 T lymphocytes and increased TRAIL receptor 1 expression strongly predicted a poorer recovery of CD4 T-cell count. HAART did not significantly alter anti-or proapoptotic markers in cultured PBMCs and T lymphocytes. The significant relationship between residual T-lymphocyte apoptosis and CD4 T-cell recovery suggests that persistent apoptosis may impede immune restoration.     

Deficient SOCS3 expression in CD4+CD25+FoxP3+ regulatory T cells and SOCS3-mediated suppression of Treg function

Naturally occurring CD4+CD25+FoxP3+ regulatory T cells (Treg) suppress T helper (Th) cell-mediated immune responses. The cytokines IL-2 and IL-6 are known to influence Treg function. However, their relative effects on Th cells versus Treg are not well understood. Stimulation with IL-2, and to a lesser extent, IL-6, enhanced Treg proliferation, FoxP3 and CTLA4 maintenance, and suppressive function. In contrast, when IL-2 or IL-6 were added to Treg/Th cell cocultures, suppression was inhibited. The molecule SOCS3 negatively regulates responses to IL-2 and IL-6. Interestingly, unlike Th cells, Treg were found to be deficient in SOCS3 protein expression. The significance of this finding lies in the need for Treg to rapidly respond to these cytokines to prevent unwarranted immune responses to self-antigens. Overexpression of SOCS3 in Treg decreased their proliferation, FoxP3 and CTLA-4 expression and suppressive function. Thus, up-regulation of SOCS3 expression may be a useful therapeutic approach in diseases where inhibition of Treg is desirable.     

Influence of CD28 co-stimulation on cytokine production is mainly regulated via interleukin-2.

Interaction of CD28 with its ligand B7 plays an important role in the initiation of immune responses. The co-stimulatory signal generated by cross-linking of CD28 molecules results in enhanced T-cell proliferation and augmentation of cytokine production. In particular, mRNA levels of T-helper 1 (Th1)-type cytokines, such as interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) are reported to be strongly increased. We investigated the effect of CD28 co-stimulation on the production of Th2-type cytokines. CD28 mAb induced a strong augmentation of IL-2 secretion in activated T-cell clones. Production of IFN-gamma was also enhanced, but the increase in IL-4 secretion was generally moderate. Augmentation of IL-4 production by CD28 was most pronounced in clones that produced low amounts of IL-2, compared to clones producing high levels of IL-2. It was found that the up-regulation of IL-4 by CD28 co-stimulation was mainly controlled indirectly via an increase of IL-2. Some clones could produce IL-4 in an IL-2-independent manner; in these situations CD28 co-stimulation had no augmenting effect on the production of IL-4. The secretion of IL-4 by peripheral blood CD4+ T cells, that were activated with B7-expressing transfectants, was also found to be dependent on IL-2. Finally, Northern blot analysis confirmed that co-stimulation of CD28 primarily affected IL-2 production, and that inhibition of IL-2/IL-2 receptor interaction abolished the augmenting action of CD28 monoclonal antibody on the production of the Th2-type cytokines IL-4, IL-5 and IL-10 and of the Th1 cytokine IFN-gamma.