Activated Th1 T cells migrate through confined environments using small, dynamic focal adhesions containing integrins, talin, and vinculin, contradicting the classical view that T cell migration is integrin-independent amoeboid movement (Caillier et al., Journal of Cell Biology, 2024). The source work leaves open how T cells generate traction when no adhesion ligands are available, what these focal adhesions do beyond anchoring - including which cues cells deposit or modify in the extracellular matrix so that follower cells move faster - and whether focal adhesion-mediated confined migration extends beyond activated Th1 cells to other immune cell types (the authors conjecture that "other immune cells behave similarly") and to other T cell subsets and activation states. These questions remain open: later work models hybrid amoeboid-mesenchymal mechanisms, documents intermittent integrin-mediated contractility across several immune cell types in 3D matrices, and proposes candidate adhesion-free mechanisms (glycocalyx-mediated unspecific adhesion under compression, hydraulic nuclear-piston propulsion), but no accepted molecular account of truly integrin-independent T cell migration exists, the identity and causal role of the deposited follower-guidance cue remain undetermined, and no systematic survey across immune cell types or T cell subsets and activation states has been performed.

Background

T cells continually patrol the densely packed interstitial spaces of tissues during immune surveillance, and their ability to migrate through such confining, mechanically heterogeneous environments is a prerequisite for immune responses, including the infiltration of cytotoxic T cells into solid tumors. Classically, cell migration has been divided into two modes. Mesenchymal migration, typical of fibroblasts, is slow and depends on large, long-lived focal adhesions - multiprotein complexes built around integrins (transmembrane heterodimeric receptors that bind extracellular matrix (ECM) proteins such as fibronectin and recruit adaptors like talin and vinculin to link the ECM to the actin cytoskeleton and transmit traction forces). Amoeboid migration, named after amoeboid crawling, is fast, rounded, and was long assumed to be independent of integrins and adhesions; immune cells, including T cells, were its textbook examples. This dichotomy was cemented by the finding that dendritic cells lacking all integrins still migrate rapidly in three-dimensional collagen gels by flowing and squeezing (Lämmermann et al., 2008), by measurements showing that intranodal T cell migration is tuned by substrate friction rather than adhesion strength (Hons et al., 2018), and by demonstrations of adhesion-independent force generation through cortical actin friction during bleb-based movement (blebs being pressure-driven membrane protrusions that detach from the actin cortex; Bergert et al., 2015), membrane-flow-driven migration (O'Neill et al., 2018), and locomotion guided by environmental topography (Reversat et al., 2020); the amoeboid, adhesion-independent view of immune-cell migration was consolidated in a major review (Paluch et al., 2016).

Caillier et al. (2024) challenged this picture for activated primary Th1 cells (helper T cells polarized toward the type-1 effector lineage). Using microfabricated confinement (a roughly 5 micrometer gap above substrates coated with defined matrix proteins), traction-force microscopy (which infers cell-generated forces from the deformations of an elastic, marker-patterned substrate), and imaging of fluorescently tagged adhesion proteins, they showed that: (i) Th1 cells require both confinement and ECM protein to migrate efficiently - on fibronectin without confinement the cells show only Brownian motion, and in serum-free medium on passivated, integrin-ligand-free surfaces migration collapses even under confinement, while small amounts of ECM rescue it; (ii) confined Th1 cells form small, short-lived focal adhesions containing the canonical mesenchymal adhesion proteins (integrins, talin, vinculin), localized to sites of contractile traction stress, enabling the cells to pull themselves through confined spaces; and (iii) Th1 cells preferentially follow tracks of previous T cells and move faster along them, an effect abolished by a pan-inhibitor of matrix metalloproteinases (MMPs, secreted proteases that degrade ECM), suggesting that the cells biochemically or physically modify the ECM - for example by leaving a chemokine behind, deforming the ECM, or leaving membranous tubular networks - to guide follower cells, analogous to the chemokine trails neutrophils deposit for CD8+ T cells in airways (Lim et al., 2015). The authors conclude that the boundary between amoeboid and mesenchymal migration is 'blurry at best', explicitly note that the mechanisms behind true integrin-independent migration "remain to be explored" and that these focal adhesions likely play important additional functional roles - such as leaving behind chemokine trails or guiding other cells - that "future work will be needed to decipher"; on generality beyond activated Th1 cells, they conjecture that "other immune cells behave similarly".

Problem Statement

The problem, as posed by the source work, is to complete the mechanistic account of how T cells generate traction in confined tissue environments. It comprises three linked questions, all of which must be addressed; the first two are raised explicitly in the source's closing discussion, and the third elaborates the authors' conjecture that other immune cells likely behave similarly:

(1) Mechanisms of traction generation in both regimes. What are the biophysical and molecular mechanisms of truly integrin-independent migration - how does a T cell propel itself through a confined space that offers no ligands for adhesion - and how do these adhesion-free mechanisms interdigitate with integrin-mediated focal adhesions during confined migration? Equivalently: what is the actual molecular content of the 'blurry' boundary between amoeboid and mesenchymal migration modes for immune cells?

(2) Functional roles of T cell focal adhesions beyond anchoring. What do migrating T cells deposit or modify in their environment at focal adhesions or in their wake - chemokine trails, MMP-dependent ECM degradation (MMPs, matrix metalloproteinases, being the secreted proteases that degrade extracellular matrix), mechanical ECM deformation, or membranous tubular networks - and do these modifications causally guide follower cells, as suggested by the observation that Th1 cells preferentially follow and move faster along tracks of other T cells (an effect abolished by a pan-MMP inhibitor)?

(3) Generality. Does focal adhesion-mediated confined migration extend to other immune cell types (B cells, NK cells, neutrophils, monocytes, dendritic cells) and to other T cell subsets and activation states (naive T cells, CD8 effector and memory cells, regulatory T cells, Th17 cells, tissue-resident memory cells, exhausted cells), or is it specific to activated Th1 cells (the helper T cells of the type-1 effector lineage studied in the source work)?

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