Mucosal Homing and Tissue-Resident Memory
A T cell does not choose which tissue to defend — the tissue chose it, during priming. Dendritic cells in the gut-draining lymph nodes carry RALDH2 and make retinoic acid, and retinoic acid plus TGF-β writes a gut address onto the T cells they prime: α4β7 and CCR9 up, CCR7 and CD62L down. Those receptors read MAdCAM-1 and CCL25 on intestinal endothelium and nowhere else, which is why gut-primed cells go to the gut. Entry is only half of it. To stay, a cell must switch off egress: CD69 sequesters S1PR1, and the cell can no longer follow the sphingosine-1-phosphate gradient back into circulation. What remains is a tissue-resident memory cell — CD103-anchored to E-cadherin, held by Runx3 and Blimp-1, sustained by IL-15 — and it is invisible to a blood draw. Click any protein for the matching Assay Genie In Vivo antibody or ELISA kit.
Tissue tropism is imprinted, not chosen. A naive T cell has no tissue preference. It acquires one in the lymph node where it is primed, from the dendritic cell that primes it. Intestinal CD11c+ dendritic cells that have migrated from the lamina propria express RALDH2 (ALDH1A2) and convert dietary vitamin A into retinoic acid; retinoic acid signalling through RARα, together with TGF-β1, induces α4β7 (LPAM-1) and CCR9 on the responding CD4 and CD8 cells. The same signal reciprocally downregulates CCR7 and CD62L, the receptors for peripheral lymph-node entry. This is the part that is easy to miss: gut imprinting is not an addition, it is a swap. A cell cannot be optimised for both the gut and the peripheral nodes at once, and the same dendritic cell that sends a cell to the intestine has taken away its access elsewhere.
Two addresses, read together. α4β7 is the heterodimer of CD49d (α4/ITGA4) and integrin β7, and its ligand MAdCAM-1 is expressed on intestinal post-capillary venules and almost nowhere else — which is precisely what makes the pair druggable without global immunosuppression. CCR9 reads CCL25 (TECK), constitutively produced by small-intestinal epithelium. The pairing is why anti-α4β7 is gut-selective while anti-α4 is not: CD49d also partners β1 to make VLA-4, which reads VCAM-1 everywhere including the CNS. LFA-1 and ICAM-1 supply the shared arrest step, and CCR6–CCL20 and CXCR3–CXCL9/CXCL10 add the inflammatory recruitment that operates on top of the constitutive route.
Residency is the absence of exit, not the presence of glue. The step that converts a recently arrived effector into a resident is negative. KLF2 normally drives S1PR1, and S1PR1 lets a cell follow the sphingosine-1-phosphate gradient out of tissue and back into lymph. CD69, induced on activation in situ, binds S1PR1 and drives its internalisation; KLF2 falls; the exit receptor disappears and the cell is trapped. Everything else that defines a TRM cell is built on that trapping. CD103 (αEβ7), induced by TGF-β, anchors it to epithelial E-cadherin; CD49a binds collagen IV; CD101 and PD-1 mark the restrained phenotype that keeps a permanently armed cell from destroying the barrier it defends; Runx3 and Blimp-1 hold the transcriptional programme; IL-15 keeps it alive. The practical consequence is uncomfortable: TRM cells do not recirculate, so blood immunophenotyping systematically misses the population that actually does the work in mucosal tissue.
What the resident population does, and how to test it. TRM cells are the fastest responders in the tissue, producing granzyme B, IFN-γ, IL-17A and IL-22 on re-encounter; IL-22 in particular drives epithelial repair and MUC2 production rather than killing, which is why the same population is protective in infection and pathogenic in colitis. CD25+ regulatory cells share the niche and set its threshold. The In Vivo tie-in: this map has an unusually deep functional-grade layer, and the controls are the point. Anti-LPAM-1 (α4β7) blocks gut entry selectively; anti-β7 (clone FIB21) and anti-CD49d (clone PS/2) let you separate the β7 and α4 contributions, which is the in-model equivalent of the vedolizumab-versus-natalizumab distinction; anti-CD11a removes the shared arrest step; anti-CD62L tests whether a response is node-dependent at all; and anti-CD103 (clone M290) probes retention once the cells are already there. Blocking entry and blocking retention give different answers, and only running both tells you which one your phenotype depends on. For research use only; not for use in diagnostic or therapeutic procedures.
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