TMEM16F mediates bystander TCR-CD3 membrane dissociation at the immunological synapse and potentiates T cell activation

The tether function of the anoctamins

The core functions of the anoctamins are Cl- channel activity and phosphatidylserine (and perhaps other lipids) scrambling. These functions have been extensively studied in various tissues and cells. However, another function of the anoctamins that is less recognized and minimally explored is as tethers at membrane contact sites. This short review aims to examine evidence supporting the localization of the anoctamins at membrane contact sites, their tether properties, and their functions as tethers.

Transient hydroxycholesterol treatment restrains TCR signaling to promote long-term immunity

T cell receptor (TCR) plays a fundamental role in adaptive immunity, and TCR-T cell therapy holds great promise for treating solid tumors and other diseases. However, there is a noticeable absence of chemical tools tuning TCR activity. In our study, we screened natural sterols for their regulatory effects on T cell function and identified 7-alpha-hydroxycholesterol (7a-HC) as a potent inhibitor of TCR signaling. Mechanistically, 7a-HC promoted membrane binding of CD3ε cytoplasmic domain, a crucial signaling component of the TCR-CD3 complex, through alterations in membrane physicochemical properties. Enhanced CD3ε membrane binding impeded the condensation between CD3ε and the key kinase Lck, thereby inhibiting Lck-mediated TCR phosphorylation. Transient treatments of TCR-T cells with 7a-HC resulted in reduced signaling strength, increased memory cell populations, and superior long-term antitumor functions. This study unveils a chemical regulation of TCR signaling, which can be exploited to enhance the long-term efficacy of TCR-T cell therapy.

Self-programmed dynamics of T cell receptor condensation

A common event upon receptor-ligand engagement is the formation of receptor clusters on the cell surface, in which signaling molecules are specifically recruited or excluded to form signaling hubs to regulate cellular events. These clusters are often transient and can be disassembled to terminate signaling. Despite the general relevance of dynamic receptor clustering in cell signaling, the regulatory mechanism underlying the dynamics is still poorly understood. As a major antigen receptor in the immune system, T cell receptors (TCR) form spatiotemporally dynamic clusters to mediate robust yet temporal signaling to induce adaptive immune responses. Here we identify a phase separation mechanism controlling dynamic TCR clustering and signaling. The TCR signaling component CD3ε chain can condensate with Lck kinase through phase separation to form TCR signalosomes for active antigen signaling. Lck-mediated CD3ε phosphorylation, however, switched its binding preference to Csk, a functional suppressor of Lck, to cause the dissolvement of TCR signalosomes. Modulating TCR/Lck condensation by targeting CD3ε interactions with Lck or Csk directly affects T cell activation and function, highlighting the importance of the phase separation mechanism. The self-programmed condensation and dissolvement is thus a built-in mechanism of TCR signaling and might be relevant to other receptors.

SLAPSHOT reveals rapid dynamics of extracellularly exposed proteome in response to calcium-activated plasma membrane phospholipid scrambling

To facilitate our understanding of the often rapid and nuanced dynamics of extracellularly exposed proteomes during signaling events, it is important to devise robust workflows affording fast time resolution without biases and confounding factors. Here, we present Surface-exposed protein Labeling using PeroxidaSe, H2O2, and Tyramide-derivative (SLAPSHOT), to label extracellularly exposed proteins in a rapid, sensitive, and specific manner, while preserving cellular integrity. This experimentally simple and flexible method utilizes recombinant soluble APEX2 peroxidase that is applied to cells, thus circumventing biological perturbations, tedious engineering of tools and cells, and labeling biases. APEX2 neither requires metal cations for activity nor contains disulfide bonds, conferring versatility for a wide spectrum of experimental setups. We applied SLAPSHOT followed by quantitative mass spectrometry-based proteomics analysis to examine the immediate and extensive cell surface expansion and ensuing restorative membrane shedding upon the activation of Scott syndrome-linked TMEM16F, a ubiquitously expressed calcium-dependent phospholipid scramblase and ion channel. Time-course data ranging from one to thirty minutes of calcium stimulation using wild-type and TMEM16F deficient cells revealed intricate co-regulation of known protein families, including those in the integrin and ICAM families. Crucially, we identified proteins that are known to reside in intracellular organelles, including ER, as occupants of the freshly deposited membrane, and mitovesicles as an abundant component and contributor to the extracellularly exposed proteome. Our study not only provides the first accounts of the immediate consequences of calcium signaling on the extracellularly exposed proteome, but also presents a blueprint for the application of SLAPSHOT as a general approach for monitoring extracellularly exposed protein dynamics.

Simulation of receptor triggering by kinetic segregation shows role of oligomers and close-contacts

The activation of T cells, key players of the immune system, involves local evacuation of phosphatase CD45 from a region of the T cell's surface, segregating it from the T cell receptor. What drives this evacuation? In the presence of antigen, what ensures evacuation happens in the subsecond timescales necessary to initiate signaling? In the absence of antigen, what mechanisms ensure that evacuation does not happen spontaneously, which could cause signaling errors? Phenomena known to influence spatial organization of CD45 or similar surface molecules include diffusive motion in the lipid bilayer, oligomerization reactions, and mechanical compression against a nearby surface, such as that of the cell presenting the antigen. Computer simulations can investigate hypothesized spatiotemporal mechanisms of T cell signaling. The challenge to computational studies of evacuation is that the base process, spontaneous evacuation by simple diffusion, is in the extreme rare event limit, meaning direct stochastic simulation is unfeasible. Here, we combine particle-based spatial stochastic simulation with the weighted ensemble method for rare events to compute the mean first passage time for cell surface availability by surface reorganization of CD45. We confirm mathematical estimates that, at physiological concentrations, spontaneous evacuation is extremely rare, roughly 300 years. We find that dimerization decreases the time required for evacuation. A weak bimolecular interaction (dissociation constant estimate 460 μM) is sufficient for an order of magnitude reduction of spontaneous evacuation times, and oligomerization to hexamers reduces times to below 1 s. This introduces a mechanism whereby externally induced CD45 oligomerization could significantly modify T cell function. For large regions of close contact, such as those induced by large microvilli, molecular size and compressibility imply a nonzero reentry probability of 60%, decreasing evacuation times. Simulations show that these reduced evacuation times are still unrealistically long (even with a fourfold variation centered around previous estimates of parameters), suggesting that a yet-to-be-described mechanism, besides compressional exclusion at a close contact, drives evacuation.

Non-Stimulatory pMHC Enhance CD8 T Cell Effector Functions by Recruiting Coreceptor-Bound Lck

Under physiological conditions, CD8⁺ T cells need to recognize low numbers of antigenic pMHC class I complexes in the presence of a surplus of non-stimulatory, self pMHC class I on the surface of the APC. Non-stimulatory pMHC have been shown to enhance CD8⁺ T cell responses to low amounts of antigenic pMHC, in a phenomenon called co-agonism, but the physiological significance and molecular mechanism of this phenomenon are still poorly understood. Our data show that co-agonist pMHC class I complexes recruit CD8-bound Lck to the immune synapse to modulate CD8⁺ T cell signaling pathways, resulting in enhanced CD8⁺ T cell effector functions and proliferation, both in vitro and in vivo. Moreover, co-agonism can boost T cell proliferation through an extrinsic mechanism, with co-agonism primed CD8⁺ T cells enhancing Akt pathway activation and proliferation in neighboring CD8⁺ T cells primed with low amounts of antigen.