Immunobiology of the biliary tract system

Abstract

The biliary tract is a complex tubular organ system spanning from the liver to the duodenum. It is the site of numerous acute and chronic disorders, many of unknown origin, that are often associated with cancer development and for which there are limited treatment options. Cholangiocytes with proinflammatory capacities line the lumen and specialised types of immune cells reside in close proximity. Recent technological breakthroughs now permit spatiotemporal assessments of immune cells within distinct niches and have increased our understanding of immune cell tissue residency. In this review, a comprehensive overview of emerging knowledge on the immunobiology of the biliary tract system is provided, with a particular emphasis on the role of distinct immune cells in biliary disorders.

Keywords: cholangiocytes; innate lymphocytes; macrophages; structural immunology; unconventional T cells.

Fig. 1. Overview of NK cell and ILC subsets including master transcription factors regulating these cells and the key effector cytokines they produce.

Unlike T and B cells, ILCs do not express highly variable antigen receptors. Below, the major 1LC subsets are introduced (previously reviewed in detail here:^,). NK cells are cytotoxic and proinflammatory (release IFNγ, TNF, and chemokines such as CCL3, CCL4, and CCL5) ILCs that are prevalent in the circulation and enriched in certain peripheral tissues such as the liver. They are defined as CD56⁺CD3^- lymphocytes and express the master transcription factors Eomes and T-bet. ILC1s are cytokine-producing cells (IFNγ) defined by expressing the master transcription factor T-bet while lacking Eomes. A mouse-human species difference for ILC1s is that they are prevalent in mouse liver tissue (NK1.1⁺CD49a⁺CD49b⁻ cells) whilst the human functional counterpart would be CD56^brightCD16⁻ liver-resident NK cells. 1LC2s express the master transcription factor GATA3 and exhibit Th2 cytokine responses. 1LC3s are identified by the master transcription factor RORγt and have the capacity to produce both IL-17 and IL-22. AREG, amphiregulin; CCL, C–C motif chemokine ligand; Eomes, eomesodermin; GATA3, GATA binding protein 3; 1FN, interferon; 1L, interleukin; 1LCs, innate lymphoid cells; NK, natural killer; RORγt, retinoid orphan receptor-γt.

Fig. 2. Overview of major unconventional T-cell populations, their cytokine responsiveness, TCR restriction, ligands, and major effector functions.

Below, the central unconventional T-cell populations are introduced (previously reviewed in detail here: MAIT cells are defined by the expression of a 5-OP-RU tetramer or co-expression of TCR-Va7.2 and CD161 and recognise vitamin B2 (riboflavin) metabolites presented on the non-polymorphic MR1. MAIT cells are highly enriched in the human liver but scarce in mouse liver. They exhibit Th1 (IFNγ) and Th17 (IL-17) immunity in response to bacterial infections or proin-flammatory cytokines. γδ T cells represent a distinct T-cell linage expressing a TCR that can recognise a wide array of exogenous and endogenous molecules such as bacterial toxins, microbial lipids (via CD1d), viral proteins, and phosphoantigens (via butryophilins). γδ Tcells exhibit proinflammatory Th1 and Th17 functions that can either be protective or pathogenic during immune responses. γδ T cells are enriched in the human liver. CD1d-restricted NKT cells display an invariant TCR (typically Va24 paired with Vb11) that recognises glycolipids presented on CD1d. These cells have been called a “Swiss-army knife” of the immune system having the capacity to produce a broad range of Th1, Th2, and Th17 cell-associated cytokines. CD1d-restricted NKT cells are highly prevalent in mouse liver but scarce in human liver. IFN, interferon; IL, interleukin; MAIT, mucosal-associated invariant T; MR1, MHC class I related-1 molecule; NKT, natural killer T; PLZF, promyelocytic leukaemia zinc finger (or ZBTB16); TCR, T-cell receptor; Th, T helper; TNF, tumour necrosis factor.

Fig. 3. Overview of liver macrophages including major identifying surface markers.

Historically, all macrophages residing in the liver were considered Kupffer cells. However, recent lineage tracing studies in mice and scRNAseq experiments in humans and mice, both in steady state and disease settings, have revealed considerable heterogeneity within the liver macrophage compartment (previously reviewed here:). At steady state, human liver macrophages can roughly be divided into CD68⁺MARCO⁺ and CD68⁺MARCO^- subsets.^,, Different subpopulations within these two main subsets have also been identified, and the composition changes in disease settings, including the appearance of scar-associated TREM2⁺CD9⁺ macrophages originating from monocytes. The CD68⁺MARCO⁺ subset corresponds to murine liver-resident long-lived Kupffer cells and is immunoregulatory while CD68⁺MARCO^- macrophages are recently recruited from blood and are more proinflammatory.^,, Beyond this, in mice, capsule macrophages are also present55 but have not been reported in humans. Finally, the murine lipid-associated macrophages (Spp1⁺Gpnmb⁺Trem2⁺CD9⁺), recruited during metabolic inflammation, might represent a murine counterpart of scar-associated macrophages.^, CX3CR1, C-X3-C motif chemokine receptor 1; Gpnmb, glycoprotein nmb; MARCO, macrophage receptor with collagenous structure; scRNAseq, single-cell RNA-sequencing; Spp1, secreted phosphoprotein 1; Trem2, triggering receptor expressed on myeloid cells 2.

Fig. 4. Structural immune cell gradients in the liver and bile ducts.

(A) Schematic overview of a liver module and a bile duct including the spatial localisation of indicated myeloid and lymphoid cell types. (B) Gradients of immune cell presence at steady state in the liver lobule and in close proximity to intra- and extrahepatic bile ducts.