Unconventional T Cells: Gamma Delta, iNKT and MAIT
Some T cells never wait to be told. γδ T cells, iNKT cells and MAIT cells carry a T-cell receptor but use it like a pattern-recognition receptor — and they respond in hours, without clonal expansion. They are not restricted by peptide-MHC at all. γδ T cells read a phosphorylated metabolite through butyrophilin, iNKT cells read lipid on CD1d, and MAIT cells read a riboflavin breakdown product on MR1 — a molecule vertebrates cannot make, which is why MAIT cells are a bacterial sensor rather than an antigen-specific one. All three can also be switched on by cytokine alone, with the TCR uninvolved. That combination — innate timing, conserved ligands, tissue residency and unusual abundance in liver, gut and blood — is what makes them worth a map of their own. Click any protein for the matching Assay Genie ELISA kit or In Vivo antibody.
Three lineages, three non-peptide ligands. γδ T cells — specifically the Vγ9Vδ2 subset that dominates human blood — are triggered when isoprenoid pyrophosphates accumulate inside a cell, either because a microbe is making them or because a tumour's mevalonate pathway is dysregulated. They do not see the metabolite directly: it binds the intracellular domain of BTN3A1 (CD277), which changes the butyrophilin's outside face. That is an inside-out sensing mechanism with no parallel in conventional T-cell biology. iNKT cells use a near-invariant Vα24-Jα18 TCR to read lipid and glycolipid presented on CD1d. MAIT cells use MR1 to read 5-OP-RU, an unstable adduct of a riboflavin synthesis intermediate. Vertebrates do not synthesise riboflavin, so the ligand is unambiguous evidence of a microbe — MR1 is arguably the most elegant pattern-recognition receptor in the immune system, and it happens to be an MHC fold.
Innate timing on a T-cell chassis. Because the ligands are conserved rather than clonal, these cells sit at high frequency in the naive repertoire — MAIT cells can be several per cent of blood T cells and far more in liver — and they arrive in tissue pre-differentiated. They do not need days of proliferation to become useful, which is why they are the T cells you can detect responding at four hours. They are also tissue-resident: CXCR6 holds MAIT cells in liver sinusoids and CCR6 marks the IL-17-committed pool. And crucially, all three lineages can be activated by cytokine alone. IL-12 plus IL-18 will drive IFN-γ from MAIT cells with no TCR engagement at all, which means a TCR-blocking experiment can leave the response entirely intact — a result that is routinely misread as the reagent having failed.
Two output programmes, set by transcription factor. PLZF (ZBTB16) is the iNKT identity factor and is what makes these cells effector-like from the moment they leave the thymus. T-bet marks the type 1 arm — IFN-γ, TNF, granzyme B and perforin. RORγt marks the type 17 arm — IL-17A, IL-22 and GM-CSF, driven by IL-23 and IL-1β. The split matters clinically: the RORγt arm is the one implicated in psoriatic and gut inflammation, while the T-bet arm is the one being engineered into allogeneic cell therapies. PD-1 restrains both, and IL-10 restrains the type 1 output specifically.
Why the In Vivo layer here is unusually deep. This is the rare map where the functional-grade reagents sit at the centre rather than the edges. There is a pan-γδ TCR antibody, a Vγ2 TCR antibody, an iNKT-specific Vα24-Jα18 antibody, and depleting or blocking antibodies for CD3, NK1.1, NKG2A/C/E, CD4, CD8α, CD25, CD127, CD2, CD28, IL-2, TNF, GM-CSF and PD-1 — fifteen functional-grade nodes out of thirty-six. That means most of the interesting experiments on this map are ones you can actually run: deplete the lineage, block the receptor, remove the cytokine, and separate TCR-dependent from cytokine-driven activation. One honest gap: the MAIT Vα7.2 TCR itself has no catalogue reagent, so CD161 is the practical surface marker, and BTN3A1 links to a recombinant protein rather than an antibody. Both are stated on the map rather than glossed. For research use only; not for use in diagnostic or therapeutic procedures.
Every protein node links to a product — ELISA kit, In Vivo antibody or research antibody.