Photoswitching alters fluorescence readout of jGCaMP8 Ca2+ indicators tethered to Orai1 channels.

We used electrophysiology and Ca2+ channel tethering to evaluate the performance of jGCaMP8 genetically encoded Ca2+ indicators (GECIs). Orai1 Ca2+ channel-jGCaMP8 fusions were transfected into HEK 293A cells and jGCaMP8 fluorescence responses recorded by simultaneous total internal reflection fluorescence microscopy and whole-cell patch clamp electrophysiology. Noninactivating currents from the Orai1 Y80E mutant provided a steady flux of Ca2+ controlled on a millisecond time scale by step changes in membrane potential. Test pulses to -100 mV produced Orai1 Y80E-jGCaMP8f fluorescence traces that unexpectedly declined by ~50% over 100 ms before reaching a stable plateau. Testing of Orai1-jGCaMP8f using unroofed cells further demonstrated that rapid and partial fluorescence inactivation is a property of the indicator itself, rather than channel function. Photoinactivation spontaneously recovered over 5 min in the dark, and recovery was accelerated in the absence of Ca2+. Mutational analysis of residues near the tripeptide fluorophore of jGCaMP8f pointed to a mechanism: Q69M/C70V greatly increased (~90%) photoinactivation, reminiscent of fluorescent protein fluorophore cis-trans photoswitching. Indeed, 405-nm illumination of jGCaMP8f or 8m/8s/6f led to immediate photorecovery, and simultaneous illumination with 405 and 488-nm light blocked photoinactivation. Subsequent mutagenesis produced a variant, V203Y, that lacks photoinactivation but largely preserves the desirable properties of jGCaMP8f. Our results point to caution in interpreting rapidly changing Ca2+ signals using jGCaMP8 and earlier series GECIs, suggest strategies to avoid photoswitching, and serve as a starting point to produce more photostable, and thus more accurate, GECI derivatives.

Regulatory T cells suppress Th17 cell Ca2+ signaling in the spinal cord during murine autoimmune neuroinflammation.

T lymphocyte motility and interaction dynamics with other immune cells are vital determinants of immune responses. Regulatory T (Treg) cells prevent autoimmune disorders by suppressing excessive lymphocyte activity, but how interstitial motility patterns of Treg cells limit neuroinflammation is not well understood. We used two-photon microscopy to elucidate the spatial organization, motility characteristics, and interactions of endogenous Treg and Th17 cells together with antigen-presenting cells (APCs) within the spinal cord leptomeninges in experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. Th17 cells arrive before the onset of clinical symptoms, distribute uniformly during the peak, and decline in numbers during later stages of EAE. In contrast, Treg cells arrive after Th17 cells and persist during the chronic phase. Th17 cells meander widely, interact with APCs, and exhibit cytosolic Ca2+ transients and elevated basal Ca2+ levels before the arrival of Treg cells. In contrast, Treg cells adopt a confined, repetitive-scanning motility while contacting APCs. These locally confined but highly motile Treg cells limit Th17 cells from accessing APCs and suppress Th17 cell Ca2+ signaling by a mechanism that is upstream of store-operated Ca2+ entry. Finally, Treg cell depletion increases APC numbers in the spinal cord and exaggerates ongoing neuroinflammation. Our results point to fundamental differences in motility characteristics between Th17 and Treg cells in the inflamed spinal cord and reveal three potential cellular mechanisms by which Treg cells regulate Th17 cell effector functions: reduction of APC density, limiting access of Th17 cells to APCs, and suppression of Th17 Ca2+ signaling.

The P522R protective variant of PLCG2 promotes the expression of antigen presentation genes by human microglia in an Alzheimer's disease mouse model.

The P522R variant of PLCG2, expressed by microglia, is associated with reduced risk of Alzheimer's disease (AD). Yet, the impact of this protective mutation on microglial responses to AD pathology remains unknown. Chimeric AD and wild-type mice were generated by transplanting PLCG2-P522R or isogenic wild-type human induced pluripotent stem cell microglia. At 7 months of age, single-cell and bulk RNA sequencing, and histological analyses were performed. The PLCG2-P522R variant induced a significant increase in microglial human leukocyte antigen (HLA) expression and the induction of antigen presentation, chemokine signaling, and T cell proliferation pathways. Examination of immune-intact AD mice further demonstrated that the PLCG2-P522R variant promotes the recruitment of CD8+ T cells to the brain. These data provide the first evidence that the PLCG2-P522R variant increases the capacity of microglia to recruit T cells and present antigens, promoting a microglial transcriptional state that has recently been shown to be reduced in AD patient brains.

Regulatory T cells promote remyelination in the murine experimental autoimmune encephalomyelitis model of multiple sclerosis following human neural stem cell transplant.

Multiple sclerosis (MS) is a chronic, inflammatory autoimmune disease that affects the central nervous system (CNS) for which there is no cure. In MS, encephalitogenic T cells infiltrate the CNS causing demyelination and neuroinflammation; however, little is known about the role of regulatory T cells (Tregs) in CNS tissue repair. Transplantation of neural stem and progenitor cells (NSCs and NPCs) is a promising therapeutic strategy to promote repair through cell replacement, although recent findings suggest transplanted NSCs also instruct endogenous repair mechanisms. We have recently described that dampened neuroinflammation and increased remyelination is correlated with emergence of Tregs following human NPC transplantation in a murine viral model of immune-mediated demyelination. In the current study we utilized the prototypic murine autoimmune model of demyelination experimental autoimmune encephalomyelitis (EAE) to test the efficacy of hNSC transplantation. Eight-week-old, male EAE mice receiving an intraspinal transplant of hNSCs during the chronic phase of disease displayed remyelination, dampened neuroinflammation, and an increase in CNS CD4+CD25+FoxP3+ regulatory T cells (Tregs). Importantly, ablation of Tregs abrogated histopathological improvement. Tregs are essential for maintenance of T cell homeostasis and prevention of autoimmunity, and an emerging role for Tregs in maintenance of tissue homeostasis through interactions with stem and progenitor cells has recently been suggested. The data presented here provide direct evidence for collaboration between CNS Tregs and hNSCs promoting remyelination.

Mzb1 protein regulates calcium homeostasis, antibody secretion, and integrin activation in innate-like B cells.

Marginal zone (MZ) B cells of the spleen and B1 cells, termed innate-like B cells, differ from follicular B cells by their attenuated Ca(2+) mobilization, fast antibody secretion, and increased cell adhesion. We identified and characterized Mzb1 as an endoplasmic reticulum-localized and B cell-specific protein that was most abundantly expressed in MZ B and B1 cells. Knockdown of Mzb1 in MZ B cells increased Ca(2+) mobilization and nuclear NFAT transcription factor localization, but reduced lipopolysaccharide-induced antibody secretion and integrin-mediated cell adhesion. Conversely, ectopic expression of an Lck-Mzb1 transgene in peripheral T cells resulted in attenuated Ca(2+) mobilization and augmented integrin-mediated cell adhesion. In addition to its interaction with the substrate-specific chaperone Grp94, Mzb1 augmented the function of the oxidoreductase ERp57 in favoring the expression of integrins in their activated conformation. Thus, Mzb1 helps to diversify peripheral B cell functions by regulating Ca(2+) stores, antibody secretion, and integrin activation.

Genetically targeted single-channel optical recording reveals multiple Orai1 gating states and oscillations in calcium influx.

Orai1 comprises the pore-forming subunit of the Ca(2+) release-activated Ca(2+) (CRAC) channel. When bound and activated by stromal interacting molecule 1 (STIM1), an endoplasmic reticulum (ER)-resident calcium sensor, Orai1 channels possess high selectivity for calcium but extremely small conductance that has precluded direct recording of single-channel currents. We have developed an approach to visualize Orai1 activity by fusing Orai1 to fluorescent, genetically encoded calcium indicators (GECIs). The GECI-Orai1 probes reveal local Ca(2+) influx at STIM1-Orai1 puncta. By whole cell recording, these fusions are fully functional as CRAC channels. When GECI-Orai1 and the CRAC-activating domain (CAD) of STIM1 were coexpressed at low levels and imaged using a total internal reflectance fluorescence microscope, cells exhibited sporadic fluorescence transients the size of diffraction-limited spots and the brightness of a few activated GECI proteins. Transients typically rose rapidly and fell into two classes according to duration: briefer "flickers" lasting only a few hundred milliseconds, and longer "pulses" lasting one to several seconds. The size, intensity, trace shape, frequency, distribution, physiological characteristics, and association with CAD binding together demonstrate that GECI-Orai1 fluorescence transients correspond to single-channel Orai1 responses. Single Orai1 channels gated by CAD, and small Orai1 puncta gated by STIM1, exhibit repetitive fluctuations in single-channel output. CAD binding supports a role in open state maintenance and reveals a second phase of CAD/STIM1 binding after channel opening. These first recordings of single-channel Orai1 currents reveal unexpected dynamics, and when paired with CAD association, support multiple single-channel states.

Nanoscale patterning of STIM1 and Orai1 during store-operated Ca2+ entry.

Stromal interacting molecule (STIM) and Orai proteins constitute the core machinery of store-operated calcium entry. We used transmission and freeze-fracture electron microscopy to visualize STIM1 and Orai1 at endoplasmic reticulum (ER)-plasma membrane (PM) junctions in HEK 293 cells. Compared with control cells, thin sections of STIM1-transfected cells possessed far more ER elements, which took the form of complex stackable cisternae and labyrinthine structures adjoining the PM at junctional couplings (JCs). JC formation required STIM1 expression but not store depletion, induced here by thapsigargin (TG). Extended molecules, indicative of STIM1, decorated the cytoplasmic surface of ER, bridged a 12-nm ER-PM gap, and showed clear rearrangement into small clusters following TG treatment. Freeze-fracture replicas of the PM of Orai1-transfected cells showed extensive domains packed with characteristic "particles"; TG treatment led to aggregation of these particles into sharply delimited "puncta" positioned upon raised membrane subdomains. The size and spacing of Orai1 channels were consistent with the Orai crystal structure, and stoichiometry was unchanged by store depletion, coexpression with STIM1, or an Orai1 mutation (L273D) affecting STIM1 association. Although the arrangement of Orai1 channels in puncta was substantially unstructured, a portion of channels were spaced at ∼15 nm. Monte Carlo analysis supported a nonrandom distribution for a portion of channels spaced at ∼15 nm. These images offer dramatic, direct views of STIM1 aggregation and Orai1 clustering in store-depleted cells and provide evidence for the interaction of a single Orai1 channel with small clusters of STIM1 molecules.

Imaging of effector memory T cells during a delayed-type hypersensitivity reaction and suppression by Kv1.3 channel block.

Effector memory T (Tem) cells are essential mediators of autoimmune disease and delayed-type hypersensitivity (DTH), a convenient model for two-photon imaging of Tem cell participation in an inflammatory response. Shortly (3 hr) after entry into antigen-primed ear tissue, Tem cells stably attached to antigen-bearing antigen-presenting cells (APCs). After 24 hr, enlarged Tem cells were highly motile along collagen fibers and continued to migrate rapidly for 18 hr. Tem cells rely on voltage-gated Kv1.3 potassium channels to regulate calcium signaling. ShK-186, a specific Kv1.3 blocker, inhibited DTH and suppressed Tem cell enlargement and motility in inflamed tissue but had no effect on homing to or motility in lymph nodes of naive and central memory T (Tcm) cells. ShK-186 effectively treated disease in a rat model of multiple sclerosis. These results demonstrate a requirement for Kv1.3 channels in Tem cells during an inflammatory immune response in peripheral tissues. Targeting Kv1.3 allows for effector memory responses to be suppressed while central memory responses remain intact.

Two-photon imaging of remyelination of spinal cord axons by engrafted neural precursor cells in a viral model of multiple sclerosis.

Neural precursor cells (NPCs) offer a promising approach for treating demyelinating diseases. However, the cellular dynamics that underlie transplanted NPC-mediated remyelination have not been described. Using two-photon imaging of a newly developed ventral spinal cord preparation and a viral model of demyelination, we describe the motility and intercellular interactions of transplanted mouse NPCs expressing green fluorescent protein (GFP) with damaged axons expressing yellow fluorescent protein (YFP). Our findings reveal focal axonal degeneration that occurs in the ventral side of the spinal cord within 1 wk following intracranial instillation with the neurotropic JHM strain of mouse hepatitis virus (JHMV). Axonal damage precedes extensive demyelination and is characterized by swelling along the length of the axon, loss of YFP signal, and transected appearance. NPCs engrafted into spinal cords of JHMV-infected mice exhibited diminished migration velocities and increased proliferation compared with transplanted cells in noninfected mice. NPCs preferentially accumulated within areas of axonal damage, initiated direct contact with axons, and subsequently expressed the myelin proteolipid protein gene, initiating remyelination. These findings indicate that NPCs transplanted into an inflammatory demyelinating microenvironment participate directly in therapeutic outcome through the wrapping of myelin around damaged neurons.

Molecular biophysics of Orai store-operated Ca2+ channels.

Upon endoplasmic reticulum Ca(2+) store depletion, Orai channels in the plasma membrane are activated directly by endoplasmic reticulum-resident STIM proteins to generate the Ca(2+)-selective, Ca(2+) release-activated Ca(2+) (CRAC) current. After the molecular identification of Orai, a plethora of functional and biochemical studies sought to compare Orai homologs, determine their stoichiometry, identify structural domains responsible for the biophysical fingerprint of the CRAC current, identify the physiological functions, and investigate Orai homologs as potential therapeutic targets. Subsequently, the solved crystal structure of Drosophila Orai (dOrai) substantiated many findings from structure-function studies, but also revealed an unexpected hexameric structure. In this review, we explore Orai channels as elucidated by functional and biochemical studies, analyze the dOrai crystal structure and its implications for Orai channel function, and present newly available information from molecular dynamics simulations that shed light on Orai channel gating and permeation.

Orai1 function is essential for T cell homing to lymph nodes.

In T lymphocytes, Ca(2+) release-activated Ca(2+) (CRAC) channels composed of Orai1 subunits trigger Ag-induced gene expression and cell proliferation through the NFAT pathway. We evaluated the requirement of CRAC channel function for lymphocyte homing using expression of a dominant-negative Orai1-E106A mutant to suppress Ca(2+) signaling. To investigate homing and motility of human lymphocytes in immunocompromised mouse hosts, we transferred human lymphocytes either acutely or after stable engraftment after a second transfer from the same blood donor. Human and mouse lymphocyte homing was assessed, and cells were tracked within lymph nodes (LNs) by two-photon microscopy. Our results demonstrate that human T and B lymphocytes home into and migrate within the LNs of immunocompromised NOD.SCID mice similar to murine lymphocytes. Human T and B cells colocalized in atrophied or reconstituted mouse LNs, where T cells migrated in a random walk at velocities of 9-13 µm/min and B cells at 6 µm/min. Expression of Orai1-E106A inhibited CRAC channel function in human and mouse T cells, and prevented homing from high endothelial venules into murine LNs. Ca(2+) signals induced by CCL21 were also inhibited in T cells expressing Orai1-E106A. With CRAC channels inhibited, the high-affinity form of LFA-1 failed to become active, and T cells failed to migrate across endothelial cells in a transwell model. These results establish a requirement for CRAC channel-mediated Ca(2+) influx for T cell homing to LNs mediated by high-affinity integrin activation and chemokine-induced transendothelial migration.

Toll-like receptor 4-activated B cells out-compete Toll-like receptor 9-activated B cells to establish peripheral immunological tolerance.

B-cell-induced peripheral T-cell tolerance is characterized by suppression of T-cell proliferation and T-cell-dependent antibody production. However, the cellular interactions that underlie tolerance induction have not been identified. Using two-photon microscopy of lymph nodes we show that tolerogenic LPS-activated membrane-bound ovalbumin (mOVA) B cells (LPS B cells) establish long-lived, highly motile conjugate pairs with responding antigen-specific OTII T cells but not with antigen-irrelevant T cells. Treatment with anti-CTLA-4 disrupts persistent B-cell-T-cell (B-T) contacts and suppresses antigen-specific tolerance. Nontolerogenic CpG-activated mOVA B cells (CpG B cells) also form prolonged, motile conjugates with responding OTII T cells when transferred separately. However, when both tolerogenic and nontolerogenic B-cell populations are present, LPS B cells suppress long-lived CpG B-OTII T-cell interactions and exhibit tolerogenic dominance. Contact of LPS B cells with previously established B-T pairs resulted in partner-swapping events in which LPS B cells preferentially migrate toward and disrupt nontolerogenic CpG mOVA B-cell-OTII T-cell pairs. Our results demonstrate that establishment of peripheral T-cell tolerance involves physical engagement of B cells with the responding T-cell population, acting in a directed and competitive manner to alter the functional outcome of B-T interactions.

Two-photon tissue imaging: seeing the immune system in a fresh light.

Many lymphocyte functions, such as antigen recognition, take place deep in densely populated lymphoid organs. Because direct in vivo observation was not possible, the dynamics of immune-cell interactions have been inferred or extrapolated from in vitro studies. Two-photon fluorescence excitation uses extremely brief (<1 picosecond) and intense pulses of light to 'see' directly into living tissues, to a greater depth and with less phototoxicity than conventional imaging methods. Two-photon microscopy, in combination with newly developed indicator molecules, promises to extend single-cell approaches to the in vivo setting and to reveal in detail the cellular collaborations that underlie the immune response.

The CRAC channel consists of a tetramer formed by Stim-induced dimerization of Orai dimers.

Ca(2+)-release-activated Ca(2+) (CRAC) channels underlie sustained Ca(2+) signalling in lymphocytes and numerous other cells after Ca(2+) liberation from the endoplasmic reticulum (ER). RNA interference screening approaches identified two proteins, Stim and Orai, that together form the molecular basis for CRAC channel activity. Stim senses depletion of the ER Ca(2+) store and physically relays this information by translocating from the ER to junctions adjacent to the plasma membrane, and Orai embodies the pore of the plasma membrane calcium channel. A close interaction between Stim and Orai, identified by co-immunoprecipitation and by Förster resonance energy transfer, is involved in the opening of the Ca(2+) channel formed by Orai subunits. Most ion channels are multimers of pore-forming subunits surrounding a central channel, which are preassembled in the ER and transported in their final stoichiometry to the plasma membrane. Here we show, by biochemical analysis after cross-linking in cell lysates and intact cells and by using non-denaturing gel electrophoresis without cross-linking, that Orai is predominantly a dimer in the plasma membrane under resting conditions. Moreover, single-molecule imaging of green fluorescent protein (GFP)-tagged Orai expressed in Xenopus oocytes showed predominantly two-step photobleaching, again consistent with a dimeric basal state. In contrast, co-expression of GFP-tagged Orai with the carboxy terminus of Stim as a cytosolic protein to activate the Orai channel without inducing Ca(2+) store depletion or clustering of Orai into punctae yielded mostly four-step photobleaching, consistent with a tetrameric stoichiometry of the active Orai channel. Interaction with the C terminus of Stim thus induces Orai dimers to dimerize, forming tetramers that constitute the Ca(2+)-selective pore. This represents a new mechanism in which assembly and activation of the functional ion channel are mediated by the same triggering molecule.

Mutations in Orai1 transmembrane segment 1 cause STIM1-independent activation of Orai1 channels at glycine 98 and channel closure at arginine 91.

Stim and Orai proteins comprise the molecular machinery of Ca(2+) release-activated Ca(2+) (CRAC) channels. As an approach toward understanding the gating of Orai1 channels, we investigated effects of selected mutations at two conserved sites in the first transmembrane segment (TM1): arginine 91 located near the cytosolic end of TM1 and glycine 98 near the middle of TM1. Orai1 R91C, when coexpressed with STIM1, was activated normally by Ca(2+)-store depletion. Treatment with diamide, a thiol-oxidizing agent, induced formation of disulfide bonds between R91C residues in adjacent Orai1 subunits and rapidly blocked STIM1-operated Ca(2+) current. Diamide-induced blocking was reversed by disulfide bond-reducing agents. These results indicate that R91 forms a very narrow part of the conducting pore at the cytosolic side. Alanine replacement at G98 prevented STIM1-induced channel activity. Interestingly, mutation to aspartate (G98D) or proline (G98P) caused constitutive channel activation in a STIM1-independent manner. Both Orai1 G98 mutants formed a nonselective Ca(2+)-permeable conductance that was relatively resistant to block by Gd(3+). The double mutant R91W/G98D was also constitutively active, overcoming the normal inhibition of channel activity by tryptophan at the 91 position found in some patients with severe combined immunodeficiency (SCID), and the double mutant R91C/G98D was resistant to diamide block. These data suggest that the channel pore is widened and ion selectivity is altered by mutations at the G98 site that may perturb α-helical structure. We propose distinct functional roles for G98 as a gating hinge and R91 as part of the physical gate at the narrow inner mouth of the channel.

Subunit stoichiometry of human Orai1 and Orai3 channels in closed and open states.

We applied single-molecule photobleaching to investigate the stoichiometry of human Orai1 and Orai3 channels tagged with eGFP and expressed in mammalian cells. Orai1 was detected predominantly as dimers under resting conditions and as tetramers when coexpressed with C-STIM1 to activate Ca(2+) influx. Orai1 was also found to be tetrameric when coexpressed with STIM1 and evaluated following fixation. We show that fixation rapidly causes release of Ca(2+), redistribution of STIM1 to the plasma membrane, and STIM1/Orai1 puncta formation, and may cause the channel to be in the activated state. Consistent with this possibility, Orai1 was found predominantly as a dimer when coexpressed with STIM1 in living cells under resting conditions. We further show that Orai3, like Orai1, is dimeric under resting conditions and is predominantly tetrameric when activated by C-STIM1. Interestingly, a dimeric Orai3 stoichiometry was found both before and during application of 2-aminoethyldiphenyl borate (2-APB) to activate a nonselective cation conductance in its STIM1-independent mode. We conclude that the human Orai1 and Orai3 channels undergo a dimer-to-tetramer transition to form a Ca(2+)-selective pore during store-operated activation and that Orai3 forms a dimeric nonselective cation pore upon activation by 2-APB.

CD95 triggers Orai1-mediated localized Ca2+ entry, regulates recruitment of protein kinase C (PKC) ß2, and prevents death-inducing signaling complex formation.

The death receptor CD95 plays a pivotal role in immune surveillance and immune tolerance. Binding of CD95L to CD95 leads to recruitment of the adaptor protein Fas-associated death domain protein (FADD), which in turn aggregates caspase-8 and caspase-10. Efficient formation of the CD95/FADD/caspase complex, known as the death-inducing signaling complex (DISC), culminates in the induction of apoptosis. We show that cells exposed to CD95L undergo a reorganization of the plasma membrane in which the Ca(2+) release-activated Ca(2+) channel Orai1 and the endoplasmic reticulum-resident activator stromal interaction molecule 1 colocalize with CD95 into a micrometer-sized cluster in which the channel elicits a polarized entry of calcium. Orai1 knockdown and expression of a dominant negative construct (Orai1E106A) reveal that on CD95 engagement, the Orai1-driven localized Ca(2+) influx is fundamental to recruiting the Ca(2+)-dependent protein kinase C (PKC) ß2 to the DISC. PKCß2 in turn transiently holds the complex in an inactive status, preventing caspase activation and transmission of the apoptotic signal. This study identifies a biological role of Ca(2+) and the Orai1 channel that drives a transient negative feedback loop, introducing a lag phase in the early steps of the CD95 signal. We suggest that these localized events provide a time of decision to prevent accidental cell death.

Protein kinase D orchestrates the activation of DRAK2 in response to TCR-induced Ca2+ influx and mitochondrial reactive oxygen generation.

DRAK2 is a serine/threonine kinase highly enriched in lymphocytes that raises the threshold for T cell activation and maintains T cell survival following productive activation. T cells lacking DRAK2 are prone to activation under suboptimal conditions and exhibit enhanced calcium responses to AgR stimulation. Despite this, mice lacking DRAK2 are resistant to organ-specific autoimmune diseases due to defective autoreactive T cell survival. DRAK2 kinase activity is induced by AgR signaling, and in this study we show that the induction of DRAK2 activity requires Ca(2+) influx through the Ca(2+) release-activated Ca(2+) channel formed from Orai1 subunits. Blockade of DRAK2 activity with the protein kinase D (PKD) inhibitor Gö6976 or expression of a kinase-dead PKD mutant prevented activation of DRAK2, whereas a constitutively active PKD mutant promoted DRAK2 function. Knockdown of PKD in T cells strongly blocked endogenous DRAK2 activation following TCR ligation, implicating PKD as an essential intermediate in the activation of DRAK2 by Ca(2+) influx. Furthermore, we identify DRAK2 as a novel substrate of PKD, and demonstrate that DRAK2 and PKD physically interact under conditions that activate PKD. Mitochondrial generation of reactive oxygen intermediates was necessary and sufficient for DRAK2 activation in response to Ca(2+) influx. Taken together, DRAK2 and PKD form a novel signaling module that controls calcium homeostasis following T cell activation.

Selective and site-specific mobilization of dermal dendritic cells and Langerhans cells by Th1- and Th2-polarizing adjuvants.

Dendritic cells (DCs) initiate and polarize adaptive immune responses toward varying functional outcomes. By means of intravital two-photon microscopy, we report that dermal dendritic cells (DDCs) and Langerhans cells (LCs) are differentially mobilized during contact sensitization and by adjuvants such as unmethylated CpG oligonucleotide (CpG) and LPS that induce T helper type 1 (Th1) responses, or papain that induces T helper type 2 (Th2) responses. In ear pinna, contact sensitization, CpG, LPS, and papain all mobilized DDCs in three distinct phases: increased motility and dendritic probing, directed migration, and entry into lymphatic vessels. During the same treatments, the adjacent LCs in ear pinna remained immotile over a 48-hr period of observation. In contrast, footpads lacked DDCs and Th1-polarizing adjuvants selectively induced a delayed mobilization of LCs after 48 hr. Th1 polarization of CD4(+) T cells was independent of the immunization site, whereas ear immunization favored Th2 polarization, correlating with site-specific DC distribution and dynamics. Our results provide an initial description of peripheral DC dynamics in response to adjuvants and imply that LC mobilization enhances a Th1 response and is not sufficient to trigger a Th2 response, whereas mobilization of DDCs alone is sufficient to trigger T-cell proliferation and to polarize initial T-cell activation toward a Th2 response.

The functional network of ion channels in T lymphocytes.

For more than 25 years, it has been widely appreciated that Ca2+ influx is essential to trigger T-lymphocyte activation. Patch clamp analysis, molecular identification, and functional studies using blockers and genetic manipulation have shown that a unique contingent of ion channels orchestrates the initiation, intensity, and duration of the Ca2+ signal. Five distinct types of ion channels--Kv1.3, KCa3.1, Orai1+ stromal interacting molecule 1 (STIM1) [Ca2+-release activating Ca2+ (CRAC) channel], TRPM7, and Cl(swell)--comprise a network that performs functions vital for ongoing cellular homeostasis and for T-cell activation, offering potential targets for immunomodulation. Most recently, the roles of STIM1 and Orai1 have been revealed in triggering and forming the CRAC channel following T-cell receptor engagement. Kv1.3, KCa3.1, STIM1, and Orai1 have been found to cluster at the immunological synapse following contact with an antigen-presenting cell; we discuss how channels at the synapse might function to modulate local signaling. Immuno-imaging approaches are beginning to shed light on ion channel function in vivo. Importantly, the expression pattern of Ca2+ and K+ channels and hence the functional network can adapt depending upon the state of differentiation and activation, and this allows for different stages of an immune response to be targeted specifically.