Peter Reichardt

Signaling signatures and functional properties of anti-human CD28 superagonistic antibodies.

Superagonistic CD28 antibodies (CD28SAs) activate T lymphocytes without concomitant perturbation of the TCR/CD3-complex. In rodents these reagents induce the preferential expansion of regulatory T cells and can be used for the treatment of autoimmune diseases. Unexpectedly, the humanized CD28 superagonist TGN1412 caused severe and life threatening adverse effects during a recently conducted phase I clinical trail. The underlying molecular mechanisms are as yet unclear. We show that TGN1412 as well as the commercially available CD28 superagonist ANC28.1 induce a delayed but extremely sustained calcium response in human naïve and memory CD4+ T cells but not in cynomolgus T lymphocytes. The sustained Ca++-signal was associated with the activation of multiple intracellular signaling pathways and together these events culminated in the rapid de novo synthesis of high amounts of pro-inflammatory cytokines, most notably IFN-γ and TNF-α. Importantly, sustained transmembranous calcium flux, activation of Src-kinases as well as activation of PI3K were found to be absolutely required for CD28SA-mediated production of IFN-γ and IL-2. Collectively, our data suggest a molecular basis for the severe side effects caused by TGN1412 and impinge upon the relevance of non-human primates as preclinical models for reagents that are supposed to modify the function of human T cells.

Very-late-antigen-4 (VLA-4)-mediated brain invasion by neutrophils leads to interactions with microglia, increased ischemic injury and impaired behavior in experimental stroke

AbstractNeuronal injury from ischemic stroke is aggravated by invading peripheral immune cells. Early infiltrates of neutrophil granulocytes and T-cells influence the outcome of stroke. So far, however, neither the timing nor the cellular dynamics of neutrophil entry, its consequences for the invaded brain area, or the relative importance of T-cells has been extensively studied in an intravital setting. Here, we have used intravital two-photon microscopy to document neutrophils and brain-resident microglia in mice after induction of experimental stroke. We demonstrated that neutrophils immediately rolled, firmly adhered, and transmigrated at sites of endothelial activation in stroke-affected brain areas. The ensuing neutrophil invasion was associated with local blood–brain barrier breakdown and infarct formation. Brain-resident microglia recognized both endothelial damage and neutrophil invasion. In a cooperative manner, they formed cytoplasmic processes to physically shield activated endothelia and trap infiltrating neutrophils. Interestingly, the systemic blockade of very-late-antigen-4 immediately and very effectively inhibited the endothelial interaction and brain entry of neutrophils. This treatment thereby strongly reduced the ischemic tissue injury and effectively protected the mice from stroke-associated behavioral impairment. Behavioral preservation was also equally well achieved with the antibody-mediated depletion of myeloid cells or specifically neutrophils. In contrast, T-cell depletion more effectively reduced the infarct volume without improving the behavioral performance. Thus, neutrophil invasion of the ischemic brain is rapid, massive, and a key mediator of functional impairment, while peripheral T-cells promote brain damage. Acutely depleting T-cells and inhibiting brain infiltration of neutrophils might, therefore, be a powerful early stroke treatment.

The molecular makeup and function of regulatory and effector synapses

Summary:  Physical interactions between T cells and antigen‐presenting cells (APCs) form the basis of any specific immune response. Upon cognate contacts, a multimolecular assembly of receptors and adhesion molecules on both cells is created, termed the immunological synapse (IS). Very diverse structures of ISs have been described, yet the functional importance for T‐cell differentiation is largely unclear. Here we discuss the principal structure and function of ISs. We then focus on two characteristic T‐cell–APC pairs, namely T cells contacting dendritic cells (DCs) or naive B cells, for which extremely different patterns of the IS have been observed as well as fundamentally different effects on the function of the activated T cells. We provide a model on how differences in signaling and the involvement of adhesion molecules might lead to diverse interaction kinetics and, eventually, diverse T‐cell differentiation. We hypothesize that the preferred activation of the adhesion molecule leukocyte function‐associated antigen‐1 (LFA‐1) and of the negative regulator for T‐cell activation, cytotoxic T‐lymphocyte antigen‐4 (CTLA‐4), through contact with naive B cells, lead to prolonged cell–cell contacts and the generation of T cells with regulatory capacity. In contrast, DCs might have evolved mechanisms to avoid LFA‐1 overactivation and CTLA‐4 triggering, thereby promoting more dynamic contacts that lead to the preferential generation of effector cells.

Human regulatory T cells rapidly suppress T cell receptor-induced Ca(2+), NF-κB, and NFAT signaling in conventional T cells.

Inhibition of calcium signaling is critical for the suppression of T cell responses by regulatory T cells. Suppressing Calcium Suppresses T Cells Regulatory T cells (Tregs) are required to keep conventional T cells in check, and disruption of the generation or function of Tregs leads to autoimmunity. Conversely, Tregs can have a deleterious effect by dampening antitumor responses of T cells. Thus, improved understanding of the mechanisms by which Tregs inhibit T cell receptor (TCR)–induced responses in conventional T cells would help to develop better therapies against autoimmune disorders and cancer. Schmidt et al. found that TCR-induced, Ca2+-dependent signaling in human conventional T cells that were incubated with Tregs was inhibited compared to that in nonsuppressed T cells, which led to defective activation of the transcription factors NFAT and NF-κB. In contrast, Ca2+-independent signaling was unaffected. Suppressed signaling persisted after the Tregs were removed from cocultures. Increasing the intracellular concentration of Ca2+ in conventional T cells reversed the inhibitory effects of Tregs. Together, these data suggest that inhibition of Ca2+ signaling is critical for the suppressive effects of Tregs. CD4+CD25hiFoxp3+ regulatory T cells (Tregs) are critical mediators of self-tolerance, which is crucial for the prevention of autoimmune disease, but Tregs can also inhibit antitumor immunity. Tregs inhibit the proliferation of CD4+CD25− conventional T cells (Tcons), as well as the ability of these cells to produce effector cytokines; however, the molecular mechanism of suppression remains unclear. Here, we showed that human Tregs rapidly suppressed the release of calcium ions (Ca2+) from intracellular stores in response to T cell receptor (TCR) activation in Tcons. The inhibition of Ca2+ signaling resulted in decreased dephosphorylation, and thus decreased activation, of the transcription factor nuclear factor of activated T cells 1 (NFAT1) and reduced the activation of nuclear factor κB (NF-κB). In contrast, Ca2+-independent events in Tcons, such as TCR-proximal signaling and activation of the transcription factor activator protein 1 (AP-1), were not affected during coculture with Tregs. Despite suppressing intracellular Ca2+ mobilization, coculture with Tregs did not block the generation of inositol 1,4,5-trisphosphate in TCR-stimulated Tcons. The Treg-induced suppression of the activity of NFAT and NF-κB and of the expression of the gene encoding the cytokine interleukin-2 was reversed in Tcons by increasing the concentration of intracellular Ca2+. Our results elucidate a previously unrecognized and rapid mechanism of Treg-mediated suppression. This increased understanding of Treg function may be exploited to generate possible therapies for the treatment of autoimmune diseases and cancer.

Control of Uterine Microenvironment by Foxp3+ Cells Facilitates Embryo Implantation

Implantation of the fertilized egg into the maternal uterus depends on the fine balance between inflammatory and anti-inflammatory processes. Whilst regulatory T cells (Tregs) are reportedly involved in protection of allogeneic fetuses against rejection by the maternal immune system, their role for pregnancy to establish, e.g., blastocyst implantation, is not clear. By using 2-photon imaging we show that Foxp3+ cells accumulated in the mouse uterus during the receptive phase of the estrus cycle. Seminal fluid further fostered Treg expansion. Depletion of Tregs in two Foxp3.DTR-based models prior to pairing drastically impaired implantation and resulted in infiltration of activated T effector cells as well as in uterine inflammation and fibrosis in both allogeneic and syngeneic mating combinations. Genetic deletion of the homing receptor CCR7 interfered with accumulation of Tregs in the uterus and implantation indicating that homing of Tregs to the uterus was mediated by CCR7. Our results demonstrate that Tregs play a critical role in embryo implantation by preventing the development of a hostile uterine microenvironment.

Guanine Nucleotide-Binding Proteins of the G12 Family Shape Immune Functions by Controlling CD4+ T Cell Adhesiveness and Motility

Integrin-mediated adhesion plays a central role in T cell trafficking and activation. Genetic inactivation of the guanine nucleotide-binding (G) protein alpha-subunits Galpha(12) and Galpha(13) resulted in an increased activity of integrin leukocyte-function-antigen-1 in murine CD4(+) T cells. The interaction with allogeneic dendritic cells was enhanced, leading to an abnormal proliferative response in vitro. In vivo, T cell-specific inactivation of Galpha(12) and Galpha(13) caused lymphadenopathy due to increased lymph node entry and enhanced T cell proliferation, and the susceptibility toward T cell-mediated diseases was enhanced. Mechanistically, we show that in the absence of Galpha(12) and Galpha(13) the activity of the small GTPases Rac1 and Rap1 was increased, whereas signaling of the small GTPase RhoA was strongly reduced. Our data indicate that locally produced mediators signal through Galpha(12)- and Galpha(13)-coupled receptors to negatively regulate cell polarization and adhesiveness, thereby fine-tuning T cell trafficking, proliferation, and susceptibility toward T cell-mediated diseases.

T Cell Activation Results in Conformational Changes in the Src Family Kinase Lck to Induce Its Activation

T cell activation involves the conformational activation of the tyrosine kinase Lck. Conformational Kinase Activation Lck is a tyrosine kinase that is critical for T cell activation, and its activity is induced by the T cell receptor (TCR). Phosphorylation of Lck at various residues either promotes or inhibits its activity, and Lck exists in various phosphorylated states in a T cell. With fluorescence lifetime imaging microscopic analysis of live human T cells and biochemical analyses, Stirnweiss et al. found that TCR activation produced a conformational change in Lck. In vitro studies showed that this “open” conformation of Lck exhibited enhanced kinase activity. Thus, phosphorylation, location, and conformation all potentially contribute to the regulation of Lck activity. The lymphocyte-specific Src family protein tyrosine kinase p56Lck (Lck) is essential for T cell development and activation and, hence, for adaptive immune responses. The mechanism by which Lck activity is directed toward specific substrates in response to T cell receptor (TCR) activation remains elusive. We used fluorescence lifetime imaging microscopy to assess the activation-dependent spatiotemporal changes in the conformation of Lck in live human T cells. Kinetic analysis of the fluorescence lifetime of Lck biosensors enabled the direct visualization of the dynamic local opening of 20% of the total amount of Lck proteins after activation of T cells with antibody against CD3 or by superantigen-loaded antigen-presenting cells. Parallel biochemical analysis of TCR complexes revealed that the conformational changes in Lck correlated with the induction of Lck enzymatic activity. These data show the dynamic, local activation through conformational change of Lck at sites of TCR engagement.

Sphingosine 1-Phosphate–Induced Motility and Endocytosis of Dendritic Cells Is Regulated by SWAP-70 through RhoA

The phospholipid mediator sphingosine 1-phosphate (S1P) enhances motility and endocytosis of mature dendritic cells (DCs). We show that in vitro migration of Swap-70−/− bone marrow-derived DCs (BMDCs) in response to S1P and S1P-induced upregulation of endocytosis are significantly reduced. S1P-stimulated movement of Swap-70−/− BMDCs, specifically retraction of their trailing edge, in a collagen three-dimensional environment is impaired. These in vitro observations correlate with delayed entry into lymphatic vessels and migration to lymph nodes of skin DCs in Swap-70−/− mice. Expression of S1P receptors (S1P1–3) by wild-type and Swap-70−/− BMDCs is similar, but Swap-70−/− BMDCs fail to activate RhoA and to localize Rac1 and RhoA into areas of actin polymerization after S1P stimulus. The Rho-activating G protein Gαi interacts with SWAP-70, which also supports the localization of Gα13 to membrane rafts in BMDCs. LPS-matured Swap-70−/− BMDCs contain significantly more active RhoA than wild-type DCs. Preinhibition of Rho activation restored migration to S1P, S1P-induced upregulation of endocytosis in mature Swap-70−/− BMDCs, and localization of Gα13 to membrane rafts. These data demonstrate SWAP-70 as a novel regulator of S1P signaling necessary for DC motility and endocytosis.

Src Homology 2-Domain Containing Leukocyte-Specific Phosphoprotein of 76 kDa Is Mandatory for TCR-Mediated Inside-Out Signaling, but Dispensable for CXCR4-Mediated LFA-1 Activation, Adhesion, and Migration of T Cells

Engagement of the TCR or of chemokine receptors such as CXCR4 induces adhesion and migration of T cells via so-called inside-out signaling pathways. The molecular processes underlying inside-out signaling events are as yet not completely understood. In this study, we show that TCR- and CXCR4-mediated activation of integrins critically depends on the membrane recruitment of the adhesion- and degranulation-promoting adapter protein (ADAP)/Src kinase-associated phosphoprotein of 55 kDa (SKAP55)/Rap1-interacting adapter protein (RIAM)/Rap1 module. We further demonstrate that the Src homology 2 domain containing leukocyte-specific phosphoprotein of 76 kDa (SLP76) is crucial for TCR-mediated inside-out signaling and T cell/APC interaction. Besides facilitating membrane recruitment of ADAP, SKAP55, and RIAM, SLP76 regulates TCR-mediated inside-out signaling by controlling the activation of Rap1 as well as Rac-mediated actin polymerization. Surprisingly, however, SLP76 is not mandatory for CXCR4-mediated inside-out signaling. Indeed, both CXCR4-induced T cell adhesion and migration are not affected by loss of SLP76. Moreover, after CXCR4 stimulation, the ADAP/SKAP55/RIAM/Rap1 module is recruited to the plasma membrane independently of SLP76. Collectively, our data indicate a differential requirement for SLP76 in TCR- vs CXCR4-mediated inside-out signaling pathways regulating T cell adhesion and migration.

A role for LFA-1 in delaying T-lymphocyte egress from lymph nodes

Lymphocytes use the integrin leukocyte function‐associated antigen‐1 (LFA‐1) to cross the vasculature into lymph nodes (LNs), but it has been uncertain whether their migration within LN is also LFA‐1 dependent. We show that LFA‐1 mediates prolonged LN residence as LFA‐1−/− CD4 T cells have significantly decreased dwell times compared with LFA‐1+/+ T cells, a distinction lost in hosts lacking the major LFA‐1 ligand ICAM‐1. Intra‐vital two‐photon microscopy revealed that LFA‐1+/+ and LFA‐1−/− T cells reacted differently when probing the ICAM‐1‐expressing lymphatic network. While LFA‐1+/+ T cells returned to the LN parenchyma with greater frequency, LFA‐1−/− T cells egressed promptly. This difference in exit behaviour was a feature of egress through all assessed lymphatic exit sites. We show that use of LFA‐1 as an adhesion receptor amplifies the number of T cells returning to the LN parenchyma that can lead to increased effectiveness of T‐cell response to antigen. Thus, we identify a novel function for LFA‐1 in guiding T cells at the critical point of LN egress when they either exit or return into the LN for further interactions.