Brigitte Santner-Nanan

Expression of interleukin (IL)-2 and IL-7 receptors discriminates between human regulatory and activated T cells

Abnormalities in CD4+CD25+Foxp3+ regulatory T (T reg) cells have been implicated in susceptibility to allergic, autoimmune, and immunoinflammatory conditions. However, phenotypic and functional assessment of human T reg cells has been hampered by difficulty in distinguishing between CD25-expressing activated and regulatory T cells. Here, we show that expression of CD127, the α chain of the interleukin-7 receptor, allows an unambiguous flow cytometry–based distinction to be made between CD127lo T reg cells and CD127hi conventional T cells within the CD25+CD45RO+RA− effector/memory and CD45RA+RO− naive compartments in peripheral blood and lymph node. In healthy volunteers, peripheral blood CD25+CD127lo cells comprised 6.35 ± 0.26% of CD4+ T cells, of which 2.05 ± 0.14% expressed the naive subset marker CD45RA. Expression of FoxP3 protein and the CD127lo phenotype were highly correlated within the CD4+CD25+ population. Moreover, both effector/memory and naive CD25+CD127lo cells manifested suppressive activity in vitro, whereas CD25+CD127hi cells did not. Cell surface expression of CD127 therefore allows accurate estimation of T reg cell numbers and isolation of pure populations for in vitro studies and should contribute to our understanding of regulatory abnormalities in immunopathic diseases.

Systemic Increase in the Ratio between Foxp3+ and IL-17-Producing CD4+ T Cells in Healthy Pregnancy but Not in Preeclampsia

Preeclampsia is the leading cause of morbidity and mortality in pregnancy. Although the etiology of preeclampsia is still unclear, it is believed to involve rejection of the fetus, possibly due to an imbalance between regulatory (Treg) and effector T cells. To test this, we compared the frequencies of circulating CD4+ T cells expressing Foxp3, IFN-γ, IL-10, or IL-17 at the end of the third trimester of healthy and preeclamptic pregnancies. The size of the Treg cell compartment, defined by the frequency of CD4+CD25high, CD4+CD127lowCD25+, and CD4+Foxp3+ cells was significantly higher in normal compared with preeclamptic pregnancies. CD4+CD25high and CD4+CD127lowCD25+ populations in preeclampsia were not significantly different from those in nonpregnant controls, whereas CD4+Foxp3+ cells numbersre slightly lower in preeclampsia. The suppressive activity of ex vivo-sorted CD4+CD127lowCD25+ Treg cells was not significantly different between the three study groups. The percentage of CD4+IL-17-producing T cells decreased significantly in healthy compared with preeclamptic pregnancies and nonpregnant controls, whereas CD4+IL-10- and CD4+IFN-γ-producing cells remained unchanged. Consequently, the ratio of Foxp3+ Treg to IL-17-expressing CD4+ T cells was significantly increased in healthy but not in preeclamptic pregnancies. Thus, preeclampsia is associated with the absence of normal systemic skewing away from IL-17 production toward Foxp3+ expression. Finally, preeclamptic women had significantly higher levels of soluble endoglin, an inhibitor of TGF-β receptor signaling, which may bias toward IL-17 production. These results suggest that homeostasis between regulatory and proinflammatory CD4+ T cells might be pivotal for the semiallogeneic fetus to be tolerated within the maternal environment.

Early commitment of naïve human CD4 + T cells to the T follicular helper (T FH ) cell lineage is induced by IL-12

T follicular helper (TFH) cells are a specialized subset of CD4+ T cells that localize to B‐cell follicles, where they are positioned to provide help for the induction of optimal humoral immune responses. Key features of TFH cells are the expressions of CXCR5, ICOS, interleukin (IL)‐21 and BCL‐6. The requirements for human TFH cell development are unknown. Here we show that IL‐6, IL‐12, IL‐21 and IL‐23 are capable of inducing IL‐21 expression in naïve CD4+ T cells isolated from human tonsils, peripheral blood and cord blood. However, only IL‐12 induced sustained expressions of CXCR5 and ICOS on these activated naïve CD4+ T cells, and endowed them with the ability to provide increased help to B cells for their differentiation into immunoglobulin‐secreting cells. The effects of IL‐12 were independent of interferon‐γ and T‐bet, and associated with upregulation of BCL‐6 expression. Thus, these cytokines, particularly IL‐12, are likely to act at an early stage during dendritic cell‐mediated priming of naïve CD4+ T cells into a TFH cell fate, and thus underpin antibody‐mediated immunity.

Autoregulation of NFATc1/A Expression Facilitates Effector T Cells to Escape from Rapid Apoptosis

Threshold levels of individual NFAT factors appear to be critical for apoptosis induction in effector T cells. In these cells, the short isoform A of NFATc1 is induced to high levels due to the autoregulation of the NFATc1 promoter P1 by NFATs. P1 is located within a CpG island in front of exon 1, represents a DNase I hypersensitive chromatin site, and harbors several sites for binding of inducible transcription factors, including a tandemly arranged NFAT site. A second promoter, P2, before exon 2, is not controlled by NFATs and directs synthesis of the longer NFATc1/B+C isoforms. Contrary to other NFATs, NFATc1/A is unable to promote apoptosis, suggesting that NFATc1/A enhances effector functions without promoting apoptosis of effector T cells.

Proliferation of weakly suppressive regulatory CD4+ T cells is associated with over-active CD4+ T-cell responses in HIV-positive patients with mycobacterial immune restoration disease.

The role of Treg in patients with late‐stage HIV disease, who commence combination antiretroviral therapy (cART) and develop pathogen‐specific immunopathology manifesting as immune restoration disease (IRD) remains unclear. We hypothesised that Treg could be defective in either numbers and/or function and therefore unable to ensure the physiological equilibrium of the immune system in patients with IRD. Phenotypic and functional CD4+ T‐cell subsets of eight late‐stage HIV patients with nadir CD4 count <50 cells/μL, who developed mycobacterial IRD upon commencing cART were compared with six therapy naive HIV+ patients (nadir CD4 count <50 cells/μL), who did not develop an IRD after cART. Mycobacterium‐avium‐specific CD4+ T cells from IRD patients produced high levels of IFN‐γ and IL‐2 compared with controls (p<0.001). Surprisingly, we found a significant expansion of CD127loFoxp3+CD25+ Treg in IRD patients and a higher ratio of Treg to effector/memory subsets (p<0.001). In vitro suppression assays demonstrated reduced functional capacity of suppressor cells and diminished IL‐10 secretion in IRD patients. Plasma levels of IL‐7 were increased in patients and, interestingly, exogenous IL‐7 and other cytokines strongly inhibited Treg suppression. These data suggest that despite substantial Treg expansion in IRD, their ability to induce suppression, and thereby downregulate aberrant immune responses, is compromised.

IL-2 and autoimmune disease.

A decade after the first description of IL-2-deficient mice, the redundancy of IL-2 as a T cell growth factor is well accepted and the focus of research has shifted to the unexpected multiorgan autoimmunity and inflammation observed in mice lacking components of the IL-2/IL-2R system. So far, a set of defects at the levels of repertoire selection, the generation of suppressive regulatory T cells, T cell homing and clonal contraction via activation induced cell death (AICD) have been documented. We propose that these individual defects jointly contribute to the severe disturbance of T cell homeostasis and self-tolerance underlying the immunopathology of the IL-2 deficiency syndrome.

STAT3 is required for IL-21–induced secretion of IgE from human naive B cells

The production of immunoglobulin E (IgE) is tightly regulated. This is evidenced by the fact that it comprises less than 0.0001% of serum Ig, and aberrant production causes atopic conditions, including allergy, rhinitis, and anaphylaxis. Interleukin-4 (IL-4) is a well-characterized inducer of IgE by human and murine B cells, whereas interferon-gamma can antagonize this effect. IL-21 has also been recognized for its ability to suppress IL-4-induced IgE production by murine B cells. Here, we identified IL-21 as an inducer of IgE production by CD40L-stimulated human naive B cells. Furthermore, there was a striking synergy between IL-4 and IL-21 on inducing IgE secretion by CD40L-stimulated human B cells, such that the levels detected under these conditions exceeded those induced by IL-4 or IL-21 alone by more than 10-fold. IL-21 induced activation of STAT3 and analysis of B cells from patients with loss-of-function STAT3 mutations revealed that the ability of IL-21 to induce IgE secretion, and augment that driven by IL-4, was STAT3-dependent. These findings highlight a fundamental difference between the regulation of IgE production by human and murine B cells and have implications for the dysregulated production of IgE in conditions characterized by extremely high levels of serum IgE.

Differential expression of CD21 identifies developmentally and functionally distinct subsets of human transitional B cells

The transitional stage of B-cell development represents an important step where autoreactive cells are deleted, allowing the generation of a mature functional B-cell repertoire. In mice, 3 subsets of transitional B cells have been identified. In contrast, most studies of human transitional B cells have focused on a single subset defined as CD24(hi)CD38(hi) B cells. Here, we have identified 2 subsets of human transitional B cells based on the differential expression of CD21. CD21(hi) transitional cells displayed higher expression of CD23, CD44, and IgD, and exhibited greater proliferation and Ig secretion in vitro than CD21(lo) transitional B cells. In contrast, the CD21(lo) subset expressed elevated levels of LEF1, a transcription factor highly expressed by immature lymphocytes, and produced higher amounts of autoreactive Ab. These phenotypic, functional, and molecular features suggest that CD21(lo) transitional B cells are less mature than the CD21(hi) subset. This was confirmed by analyzing X-linked agammaglobulinemia patients and the kinetics of B-cell reconstitution after stem cell transplantation, which revealed that the development of CD21(lo) transitional B cells preceded that of CD21(hi) transitional cells. These findings provide important insights into the process of human B-cell development and have implications for understanding the processes underlying perturbed B-cell maturation in autoimmune and immunodeficient conditions.

Altered decidual DC-SIGN+ antigen-presenting cells and impaired regulatory T-cell induction in preeclampsia.

Regulatory T (Treg) cell expansion is required for tolerance of the semi-allogeneic fetus in healthy pregnancy and impaired in preeclampsia in humans. However, the reasons remain unknown. Herein, we show that expansion of CD4(+)Helios(-)Foxp3(+) adaptive Treg (iTreg) cells, rather than CD4(+)Helios(+)Foxp3(+) natural Treg cells, accounts for this expansion in healthy pregnancy. This expansion is even more pronounced in the decidua, where there is an overrepresentation of iTreg cells. In preeclampsia, however, there is impaired systemic iTreg cell expansion, associated with a lack of iTreg cell overrepresentation in the decidua. Because decidual antigen-presenting cells (APCs) may be important for iTreg cell induction, we studied decidual CD14(+) APCs using immunohistochemistry and flow cytometry. We show that decidual CD14(+)DC-SIGN(+) APCs are closely associated with Foxp3(+) Treg cells. Furthermore, CD14(+)DC-SIGN(+) cells display a distinct phenotype compared with their CD14(+)DC-SIGN(-) counterparts. In particular, they have increased expression of tolerogenic molecules, HLA-G, and immunoglobulin-like transcript 4. In vitro, CD14(+)DC-SIGN(+) APCs from healthy pregnant women induced iTreg cells significantly more efficiently than CD14(+)DC-SIGN(-) APCs. Conversely, in preeclampsia, both CD14(+)DC-SIGN(+) and CD14(+)DC-SIGN(-) APCs induced iTreg cells poorly. These results suggest that decidual CD14(+)DC-SIGN(+) APCs may play important roles in iTreg cell induction, a process that is defective in preeclampsia and likely contributes to its pathogenesis.

IL-10 Potentiates Differentiation of Human Induced Regulatory T Cells via STAT3 and Foxo1

Foxp3+ regulatory T cells (Tregs) play essential roles in maintaining the immune balance. Although the majority of Tregs are formed in the thymus, increasing evidence suggests that induced Tregs (iTregs) may be generated in the periphery from naive cells. However, unlike in the murine system, significant controversy exists regarding the suppressive capacity of these iTregs in humans, especially those generated in vitro in the presence of TGF-β. Although it is well known that IL-10 is an important mediator of Treg suppression, the action of IL-10 on Tregs themselves is less well characterized. In this article, we show that the presence of IL-10, in addition to TGF-β, leads to increased expansion of Foxp3+ iTregs with enhanced CTLA-4 expression and suppressive capability, comparable to that of natural Tregs. This process is dependent on IL-10R–mediated STAT3 signaling, as supported by the lack of an IL-10 effect in patients with IL-10R deficiency and dominant-negative STAT3 mutation. Additionally, IL-10–induced inhibition of Akt phosphorylation and subsequent preservation of Foxo1 function are critical. These results highlight a previously unrecognized function of IL-10 in human iTreg generation, with potential therapeutic implications for the treatment of immune diseases, such as autoimmunity and allergy.