Regulation of T-cell tolerance by calcium/NFAT signaling

Layers of regulation in induction of mixed chimerism by anti-CD40L

The pathways regulating immunological tolerance are complex and overlapping. Here, we comment on our findings that the PD-1, CTLA-4, LAG-3 and TGFβ inhibitory molecules are all involved in mediating peripheral CD8 T‑cell tolerance induced by anti-CD40L and allogeneic bone marrow transplantation in mice. These observations suggest the possibility of targeted manipulation of these pathways for induction of mixed hematopoietic chimerism for donor-specific transplantation tolerance.

ORAI1 deficiency impairs activated T cell death and enhances T cell survival

ORAI1 is a pore subunit of Ca(2+) release-activated Ca(2+) channels that mediate TCR stimulation-induced Ca(2+) entry. A point mutation in ORAI1 (ORAI1(R91W)) causes SCID in human patients that is recapitulated in Orai1(-/-) mice, emphasizing its important role in the immune cells. In this study, we have characterized a novel function of ORAI1 in T cell death. CD4(+) T cells from Orai1(-/-) mice showed robust proliferation with repetitive stimulations and strong resistance to stimulation-induced cell death due to reduced mitochondrial Ca(2+) uptake and altered gene expression of proapoptotic and antiapoptotic molecules (e.g., Fas ligand, Noxa, and Mcl-1). Nuclear accumulation of NFAT was severely reduced in ORAI1-deficient T cells, and expression of ORAI1 and a constitutively active mutant of NFAT recovered cell death. These results indicate NFAT-mediated cell death pathway as one of the major downstream targets of ORAI1-induced Ca(2+) entry. By expressing various mutants of ORAI1 in wild-type and Orai1(-/-) T cells to generate different levels of intracellular Ca(2+), we have shown that activation-induced cell death is directly proportional to the intracellular Ca(2+) concentration levels. Consistent with the in vitro results, Orai1(-/-) mice showed strong resistance to T cell depletion induced by injection of anti-CD3 Ab. Furthermore, ORAI1-deficient T cells showed enhanced survival after adoptive transfer into immunocompromised hosts. Thus, our results demonstrate a crucial role of the ORAI1-NFAT pathway in T cell death and highlight the important role of ORAI1 as a major route of Ca(2+) entry during activated T cell death.

GRAIL: a unique mediator of CD4 T-lymphocyte unresponsiveness

GRAIL (gene related to anergy in lymphocytes, also known as RNF128), an ubiquitin-protein ligase (E3), utilizes a unique single transmembrane protein with a split-function motif, and is an important gatekeeper of T-cell unresponsiveness. Although it may play a role in other CD4 T-cell functions including activation, survival and differentiation, GRAIL is most well characterized as a negative regulator of T-cell receptor responsiveness and cytokine production. Here, we review the recent literature on this remarkable E3 in the regulation of human and mouse CD4 T-cell unresponsiveness.

Transcriptional regulation of Foxp3 in regulatory T cells

Regulatory T (Treg) cells constitute a unique T-cell lineage that plays a pivotal role in the maintenance of the peripheral tolerance. The transcription factor Foxp3 (Forkhead box P3) was identified as a master regulator for the development and function of Treg cells. It is well defined that Foxp3 expression is critical to program CD4+CD25+ Treg cell development and function; however, the molecular mechanisms that are involved in the regulation of the Foxp3 expression remain unclear. Recent studies have showed an indication that this process is influenced by a number of transcription factors. In this review, we summarize the current knowledge of how Foxp3 expression is controlled at molecular level by focusing on these factors.

Role of antigen persistence and dose for CD4+ T-cell exhaustion and recovery

It is currently not understood how some chronic infections exhaust antigen-specific T cells over time and which pathogen components contribute to exhaustion. Here, we dissected the behavior of primed CD4(+) T cells exposed to persistent antigen using an inducible transgenic mouse system that allowed us to control antigen presentation as the only experimental variable, independent of the persistent inflammation and disease progression that complicate infectious models. Moreover, this system restricted antigen presentation to dendritic cells (DCs) and avoided confounding B, CD8(+) T, or innate cell responses. When antigen presentation was extended beyond the expansion phase, primed CD4(+) T cells survived, but exhibited reduced memory functionality in terms of their proliferative capacity and cytokine expression potential. The effect was antigen dose and time dependent, not associated with increased PD-1 expression or reduced calcium influx, but impaired Jun phosphorylation in response to TCR engagement. Upon antigen removal, the cells regained the ability to proliferate, but remained unable to produce high levels of IL-2 and TNF-α. These data show that persistent antigen by itself rapidly induces a dysfunctional state in CD4(+) T cells that is only partially reversible upon antigen removal. These findings have implications for vaccine optimization and for the possible reinvigoration of CD4(+) T cells during chronic infection.

NFAT, immunity and cancer: a transcription factor comes of age

Nuclear factor of activated T cells (NFAT) was first identified more than two decades ago as a major stimulation-responsive DNA-binding factor and transcriptional regulator in T cells. It is now clear that NFAT proteins have important functions in other cells of the immune system and regulate numerous developmental programmes in vertebrates. Dysregulation of these programmes can lead to malignant growth and cancer. This Review focuses on recent advances in our understanding of the transcriptional functions of NFAT proteins in the immune system and provides new insights into their potential roles in cancer development.

Low-strength T-cell activation promotes Th17 responses

We show that the strength of T-cell stimulation determines the capability of human CD4(+) T cells to become interleukin-17 (IL-17) producers. CD4(+) T cells received either high- (THi) or low (TLo)-strength stimulation via anti-CD3/CD28 beads or dendritic cells pulsed with superantigen in the presence of pro-Th17 cytokines IL-1β, transforming growth factor β, and IL-23. We found that TLo, but not THi, stimulation profoundly promoted Th17 responses by enhancing both the relative proportion and total number of Th17 cells. Titration of anti-CD3 revealed that low TCR signaling promoted Th17 cells, but only in the presence of anti-CD28. Impaired IL-17 production in THi cells could not be explained by high levels of Foxp3 or transforming growth factor β-latency-associated peptide expressed by THi cells. Nuclear factor of activated T cells was translocated to the nucleus in both THi and TLo cells, but only bound to the proximal region of the IL-17 promoter in TLo cells. The addition of a Ca(2+) ionophore under TLo conditions reversed the pro-Th17 effect, suggesting that high Ca(2+) signaling impairs Th17 development. Although our data do not distinguish between priming of naive T cells versus expansion/differentiation of memory T cells, our results clearly establish an important role for the strength of T-cell activation in regulating Th17 responses.

Up-regulation of heme oxygenase-1 expression through CaMKII-ERK1/2-Nrf2 signaling mediates the anti-inflammatory effect of bisdemethoxycurcumin in LPS-stimulated macrophages

We have identified a novel anti-inflammatory signaling pathway that leads to the expression of heme oxygenase-1 (HO-1) in response to bisdemethoxycurcumin (BDMC), an analog of curcumin. Treatment with BDMC suppressed inducible nitric oxide synthase expression and nitric oxide (NO) production by down-regulating NF-kappaB activity in lipopolysaccharide (LPS)-stimulated RAW264.7 macrophages. These effects were reversed by blocking HO-1 activity or expression. The signaling pathway involved in BDMC-mediated HO-1 induction included Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and extracellular signal-regulated kinase 1/2 (ERK1/2). BDMC induced phosphorylation of ERK1/2 in a CaMKII-dependent manner. Pretreatment with the mitogen-activated protein kinase kinase 1/2 (MEK1/2) inhibitor, U0126, inhibited CaMKII-induced stimulation of HO-1 promoter activity, suggesting that ERK1/2 is a downstream mediator of CaMKII in BDMC signaling to HO-1 expression. Furthermore, the CaMKII-ERK1/2 cascade targets the transcription factor, NF-E2-related factor-2 (Nrf2). Finally, inhibition of the Ca(2+)-CaMKII-ERK1/2-linked cascade attenuated significantly suppression by BDMC of LPS-induced iNOS expression and subsequent NO production. Collectively, our findings identify a Ca(2+)/calmodulin-CaMKII-ERK1/2-Nrf2 cascade as a novel anti-inflammatory pathway mediating BDMC signaling to HO-1 expression in macrophages.