Modulation of T cell activation by localized K⁺ accumulation at the immunological synapse--a mathematical model

A role for TASK2 channels in the human immunological synapse

The immunological synapse is a transient junction that occurs when the plasma membrane of a T cell comes in close contact with an APC after recognizing a peptide from the antigen-MHC. The interaction starts when CRAC channels embedded in the T cell membrane open, flowing calcium ions into the cell. To counterbalance the ion influx and subsequent depolarization, K_v 1.3 and KCa3.1 channels are recruited to the immunological synapse, increasing the extracellular K⁺ concentration. These processes are crucial as they initiate gene expression that drives T cell activation and proliferation. The T cell-specific function of the K_2P channel family member TASK2 channels and their role in autoimmune processes remains unclear. Using mass spectrometry analysis together with epifluorescence and super-resolution single-molecule localization microscopy, we identified TASK2 channels as novel players recruited to the immunological synapse upon stimulation. TASK2 localizes at the immunological synapse, upon stimulation with CD3 antibodies, likely interacting with these molecules. Our findings suggest that, together with K_v 1.3 and KCa3.1 channels, TASK2 channels contribute to the proper functioning of the immunological synapse, and represent an interesting treatment target for T cell-mediated autoimmune disorders.

Periodic Membrane Potential and Ca2+ Oscillations in T Cells Forming an Immune Synapse

The immunological synapse (IS) is a specialized contact area formed between a T cell and an antigen presenting cell (APC). Besides molecules directly involved in antigen recognition such as the TCR/CD3 complex, ion channels important in the membrane potential and intracellular free Ca²⁺ concentration control of T cells are also recruited into the IS. These are the voltage-gated Kv1.3 and Ca²⁺-activated KCa3.1 K⁺ channels and the calcium release-activated Ca²⁺ channel (CRAC). However, the consequence of this recruitment on membrane potential and Ca²⁺ level control is not known. Here we demonstrate that the membrane potential (MP) of murine T cells conjugated with APCs in an IS shows characteristic oscillations. We found that depolarization of the membrane by current injection or by increased extracellular K⁺ concentration produced membrane potential oscillations (MPO) significantly more frequently in conjugated T cells than in lone T cells. Furthermore, oscillation of the free intracellular Ca²⁺ concentration could also be observed more frequently in cells forming an IS than in lone cells. We suggest that in the IS the special arrangement of channels and the constrained space between the interacting cells creates a favorable environment for these oscillations, which may enhance the signaling process leading to T cell activation.

Ion channels and anti-cancer immunity

The outcome of a malignant disease depends on the efficacy of the immune system to destroy cancer cells. Key steps in this process, for example the generation of a proper Ca(2+) signal induced by recognition of a specific antigen, are regulated by various ion channel including voltage-gated Kv1.3 and Ca(2+)-activated KCa3.1 K(+) channels, and the interplay between Orai and STIM to produce the Ca(2+)-release-activated Ca(2+) (CRAC) current required for T-cell proliferation and function. Understanding the immune cell subset-specific expression of ion channels along with their particular function in a given cell type, and the role of cancer tissue-dependent factors in the regulation of operation of these ion channels are emerging questions to be addressed in the fight against cancer disease. Answering these questions might lead to a better understanding of the immunosuppression phenomenon in cancer tissue and the development of drugs aimed at skewing the distribution of immune cell types towards killing of the tumour cells.

A mathematical model of T lymphocyte calcium dynamics derived from single transmembrane protein properties

Fate decision processes of T lymphocytes are crucial for health and disease. Whether a T lymphocyte is activated, divides, gets anergic, or initiates apoptosis depends on extracellular triggers and intracellular signaling. Free cytosolic calcium dynamics plays an important role in this context. The relative contributions of store-derived calcium entry and calcium entry from extracellular space to T lymphocyte activation are still a matter of debate. Here we develop a quantitative mathematical model of T lymphocyte calcium dynamics in order to establish a tool which allows to disentangle cause-effect relationships between ion fluxes and observed calcium time courses. The model is based on single transmembrane protein characteristics which have been determined in independent experiments. This reduces the number of unknown parameters in the model to a minimum and ensures the predictive power of the model. Simulation results are subsequently used for an analysis of whole cell calcium dynamics measured under various experimental conditions. The model accounts for a variety of these conditions, which supports the suitability of the modeling approach. The simulation results suggest a model in which calcium dynamics dominantly relies on the opening of channels in calcium stores while calcium entry through calcium-release activated channels (CRAC) is more associated with the maintenance of the T lymphocyte calcium levels and prevents the cell from calcium depletion. Our findings indicate that CRAC guarantees a long-term stable calcium level which is required for cell survival and sustained calcium enhancement.

4-Aminopyridine ameliorates mobility but not disease course in an animal model of multiple sclerosis

Neuropathological changes following demyelination in multiple sclerosis (MS) lead to a reorganization of axolemmal channels that causes conduction changes including conduction failure. Pharmacological modulation of voltage-sensitive potassium channels (K(V)) has been found to improve conduction in experimentally induced demyelination and produces symptomatic improvement in MS patients. Here we used an animal model of autoimmune inflammatory neurodegeneration, namely experimental autoimmune encephalomyelitis (EAE), to test the influence of the K(V)-inhibitor 4-aminopyridine (4-AP) on various disease and immune parameters as well as mobility in MOG₃₅₋₅₅ immunized C57Bl/6 mice. We challenged the hypothesis that 4-AP exerts relevant immunomodulatory or neuroprotective properties. Neither prophylactic nor therapeutic treatment with 4-AP altered disease incidence or disease course of EAE. Histopathological signs of demyelination and neuronal damage as well as MRI imaging of brain volume changes were unaltered. While application of 4-AP significantly reduced the standing outward current of stimulated CD4(+) T cells compared to controls, it failed to impact intracellular calcium concentrations in these cells. Compatibly, KV channel inhibition neither influenced CD4(+) T cell effector functions (proliferation, IL17 or IFNγ production). Importantly however, despite equal disease severity scores 4-AP treated animals showed improved mobility as assessed by 2 independent methods, 1) foot print and 2) rotarod analysis (0.332 ± 0.03, n=7 versus 0.399 ± 0.08, n=14, p<0.001, respectively). Our data suggest that 4-AP while having no apparent immunomodulatory or direct neuroprotective effects, significantly ameliorates conduction abnormalities thereby improving gait and coordination. Improvement of mobility in this experimental model supports trial data and clinical experience with 4-AP in the symptomatic treatment of MS.

The role of PSD-95 in the rearrangement of Kv1.3 channels to the immunological synapse

Establishment of the immunological synapse (IS) between T lymphocytes and antigen-presenting cells is a key step in the adaptive immune response. Several proteins accumulate in the IS, such as the Kv1.3 potassium channel; however, the mechanism of this translocation is unknown. PSD-95 and SAP97 are adaptor proteins that regulate the polarized cell surface expression and localization of Kv1 channels in neurons. We investigated whether these proteins affect the redistribution of Kv1.3 into the IS in non-excitable human T cells. We show here that PSD-95 and SAP97 are expressed in Jurkat and interact with the C terminus of Kv1.3. Disruption of the interaction between PSD-95 or SAP97 and Kv1.3 in Jurkat was realized by the expression of a C-terminal truncated Kv1.3, which lacks the binding domain for these proteins, or by the knockdown of the expression of PSD-95 or SAP97 using specific shRNA. Expression of the truncated Kv1.3 or knockdown of PSD-95, but not the knockdown of SAP97, inhibited the recruitment of Kv1.3 into the IS; the fraction of cells showing polarized Kv1.3 expression upon engagement in an IS was significantly lower than in control cells expressing the full-length Kv1.3, and the rearrangement of Kv1.3 did not show time dependence. In contrast, Jurkat cells expressing the full-length channel showed marked time dependence in the recruitment into the IS peaking at 1 min after the conjugation of the cells. These results demonstrate that PSD-95 participates in the targeting of Kv1.3 into the IS, implying its important role in human T-cell activation.