T cell migration is essential for T cell responses, allowing for detection of cognate antigen at the surface of an Antigen-Presenting Cell (APC) and for interactions with other cells involved in the immune response. Although appearing random, growing evidence supports that T cell motility patterns are strategic and governed by mechanisms that are optimized for both activation-stage and environment-specific attributes. In this Opinion Article, we will discuss how to understand the combined effects of T cell- intrinsic and -extrinsic forces upon these motility patterns when viewed in highly complex tissues filled with other cells involved in parallel motility. In particular, we will examine how insights from ‘search theory’ describe T cell movement across exploitation-exploration gradients, in the context of activation versus effector function and in the context of lymph nodes versus peripheral tissues.

Lung metastasis is the lethal determinant in many cancers1,2 and a number of lines of evidence point to monocytes and macrophages having key roles in its development3–5. Yet little is known about the immediate fate of incoming tumour cells as they colonize this tissue and even less known about how they make first contact with the immune system. Primary tumours liberate circulating tumour cells (CTCs) into the blood and we have developed a stable intravital two-photon lung imaging model in mice6 for direct observation of the arrival of CTCs and subsequent host interaction. Here we show dynamic generation of tumour microparticles in shear flow in the capillaries within minutes of CTC entry. Rather than dispersing under flow, many of these microparticles remain attached to the lung vasculature or independently migrate along the inner walls of vessels. Using fluorescent lineage reporters and flow cytometry, we observed ‘waves’ of distinct myeloid cell subsets that load differentially and sequentially with this CTC-derived material. Many of these tumour-ingesting myeloid cells collectively accumulated in the lung interstitium along with the successful metastatic cells and, as previously understood, promote the development of successful metastases from surviving tumour cells3. Although the numbers of these cells rise globally in the lung with metastatic exposure and ingesting myeloid cells undergo phenotypic changes associated with microparticle ingestion, a consistently sparse population of resident conventional dendritic cells, among the last cells to interact with CTCs, confer antimetastatic protection. This work reveals that CTC fragmentation generates immune-interacting intermediates, and defines a competitive relationship between phagocyte populations for tumour loading during metastatic cell seeding.

During immune surveillance, T cells survey the surface of antigen-presenting cells. In searching for peptide-loaded major histocompatibility complexes (pMHCs), they must solve a classic trade-off between speed and sensitivity. It has long been supposed that microvilli on T cells act as sensory organs to enable search, but their strategy has been unknown. We used lattice light-sheet and quantum dot-enabled synaptic contact mapping microscopy to show that anomalous diffusion and fractal organization of microvilli survey the majority of opposing surfaces within 1 minute. Individual dwell times were long enough to discriminate pMHC half-lives and T cell receptor (TCR) accumulation selectively stabilized microvilli. Stabilization was independent of tyrosine kinase signaling and the actin cytoskeleton, suggesting selection for avid TCR microclusters. This work defines the efficient cellular search process against which ligand detection takes place.

Introduction:In order for T cells to mount an adaptive immune response and enact cell-mediated immunity, they must first successfully detect rare cognate antigen. This detection is achieved by surfacebound T cell receptors (TCRs), binding to peptide-major histocompatibility complexes (pMHC). With some temporal latency, this binding event induces TCR signaling and T cell effector function. In order for TCR recognition to take place, T cells must efficiently survey surfaces of antigen presenting cells (APCs), which may display mainly non-stimulatory pMHC and only rare cognate antigen, in a process involving close (nanometer-scale) membrane apposition. Additionally, those rare pMHC ligands are distributed nonuniformly on subsets of APCs and only within specific lymph nodes. Thus, T cells must solve a classic search tradeoff between speed and sensitivity: faster movements provide larger overall coverage with costs at the level of sensitivity. Successful search, which results in ligand detection, is ultimately required for effector function and T cell-mediated adaptive immune response. Although surface deformations are indicated in this recognition process, the full understanding of search strategy requires real-time full 3-dimensional analysis that has not been possible using fixed or low-resolution approaches.Rationale: It has long been supposed that small microvilli on T cell surfaces are used as sensory organs to enable the search for pMHC, but their strategy has not been amenable to study. We used time-resolved lattice light sheet (LLS) microscopy and Qdot-enabled synaptic contact mapping (SCM) microscopy to show how microvilli on the surface of T cells search opposing cells and surfaces prior to and during antigen-recognition. Results:In characterizing microvilli movement on T cell surfaces, we uncovered fractal organization of the microvilli, suggesting consistent coverage across scales. We found that their movements surveyed the majority of opposing space within one minute, which is equivalent to the roughly one minute half-life of T cell-APC contacts in vivo. Individual microvilli local dwell times were sufficiently long to permit discrimination of pMHC half-lives. Protrusion density was similar in non-synapse and synapse regions and did not change appreciably during synapse development, suggesting that T cells did not "intensify" search upon recognition. TCR recognition resulted in selective stabilization of receptor-occupied protrusions as seen by longer microvilli dwell times in synapse regions with pMHC and increased persistence of TCR-occupied contacts. Microvillar scanning in synapse regions lacking pMHC showed dynamics similar to non-synaptic regions, supporting the dependence of TCR stabilization on ligand recognition. Subsequent TCR movements took place upon the stabilized protrusions, even while transient ones tested new regions. In the absence of tyrosine kinase signaling, microvillar search and TCR-occupied protrusion stabilization continued. Intrinsic stabilization was also independent of the ac...

Immunization results in the differentiation of CD8+ T cells, such that they acquire effector capabilities and convert into a memory pool capable of rapid response upon re-exposure. The initial priming of T cells takes place via an immunological synapse (IS) formed with an antigen-presenting cell (APC). By disrupting synaptic stability at different times, we show that CD8+ T cell differentiation requires cell interactions beyond those made with APC. We identify a `Critical Differentiation Period' (CDP) characterized by and requiring the interaction between primed T cells. We show that T-T synaptic interactions play a major role in the generation of protective CD8+ T cell memory. T-T synapses and allow T cells to polarize critical interferon-γ secretion towards one another. “Collective” activation and homotypic clustering therefore drive private cytokine sharing and act as regulatory stimuli for T cell differentiation.