Rafael Di Marco Barros, Natalie A. Roberts, Robin J. Dart, Pierre Vantourout, Anett Jandke,
Oliver Nussbaumer, Livija Deban, Sara Cipolat, Rosie Hart, Maria Luisa Iannitto, Adam Laing,
Bradley Spencer-Dene, Philip East, Deena Gibbons, Peter M. Irving, Pablo Pereira, Ulrich Steinhoff,
and Adrian Hayday

Hayday Team Photo

A small group of the authors photographed in the newly-opened Francis Crick Institute, London, December 2016. From left to right: Dr Natalie Roberts, Rafael diMarco Barros, Dr Anett Jandke, Professor Adrian Hayday, Dr Deena Gibbons.

A conserved biology emerges linking mucosal and skin intraepithelial lymphocyte (IEL) biology and mucosal IEL biology in rodents and humans

Context

It is by now well established that the gut harbours a very large T cell compartment. Less well appreciated is the anatomical segmentation of that compartment into intraepithelial lymphocytes (IEL) and those located beneath the basement membrane, more obviously in flux with the systemic circulation. The distinction between these two sub-compartments is exemplified by the disproportionately high representation among IEL of T cells bearing the gamma delta (γδ) T cell receptor (TCR). Unlike αβ T cells, γδ T cells are not restricted by MHC. Their enrichment among IEL is particularly true in rodents, but seems increasingly evident in humans too. Moreover, this may reflect an ancient biology, since the gut epithelium of jawless vertebrates is enriched in immunocytes expressing putative antigen receptors of the VLR-C family, as opposed to VLR-A+ immunocytes found in the lamina propria.

The physiology of IEL has yet to be properly characterised. Nonetheless, unlike most αβ T cells, TCRγδ+ IEL can employ an activating receptor, NKG2D, to respond very rapidly to ligands upregulated on neighbouring epithelial cells by myriad forms of cell and tissue perturbation. This so-called “Lymphoid Stress Surveillance Response” (LSSR) places IEL in the afferent arm of the mucosal immune response, with the aggregate outcome being to preserve tissue integrity and organ function.

Nonetheless, the LSSR does not offer a role for the TCR, a deficiency emphasised by the striking tissue-specific associations of IEL with discrete TCRs. Thus, murine skin IEL are dominated by Vγ5+ cells; uterine IEL by Vγ6+ IEL, and small intestinal IEL by Vγ7+ cells; while human colonic IEL were reported to be enriched in Vγ4+ cells. These relationships of discrete T cell subsets with particular organs cannot obviously be replicated by NK cells or innate-lymphoid cells (ILC), which lack diversity in their antigen receptors.

Findings

The paper by diMarco Barros et al. unexpectedly identifies the Butyrophilin-like 1 (Btnl1) gene, expressed by post-mitotic enterocytes, as the basis for Vγ7+ cells dominating the murine intestinal IEL compartment. Within a discrete developmental time-window, between 15 and 25 days post partum, Btnl1 drives the local and selective differentiation and expansion of hitherto rare Vγ7+ IEL. Because its effects are TCR-specific, and require neither microbes nor food antigens, Btnl1 evokes the MHC-mediated selection of TCRγδ+ thymocytes. However, the actions of Btnl1 are wholly extra-thymic. Moreover, locally-acting Btnl1 may underpin the maintenance of the Vγ7+ IEL compartment in adults.

These findings offer new insight into Btnl genes, and the Btn genes (to which Btnl genes are by definition related), all of which are at best poorly understood. Thus, diMarco Barros et al show that Btnl6, likewise expressed primarily by enterocytes, is also involved specifically in Vγ7+ IEL regulation. Furthermore, Btnl1 and Btnl6 are closely related to Skint1, a murine gene uniquely expressed by keratinocytes and thymic epithelial cells, which is critical for the selection of epidermal Vγ5+ IEL, thus establishing a conserved biology across mucosal and skin IEL biology that had heretofore been doubted. Likewise, diMarco Barros et al establish a conserved biology across rodent and human IEL biology by showing that BTNL3 and BTNL8, expressed by human colonic epithelial cells, specifically regulate human Vγ4+ IEL. Interestingly, human BTN3A1 is a critical and specific regulator of human peripheral blood Vγ9Vδ2+ T cells. Thus, six of eighteen known human Btnl/Btn genes are now assigned major, TCR-specific roles in γδ T cell regulation, with others likely to follow suit. Although Btnl/Btn genes are mammalian, they share close similarities with avian Bg genes that have been implicated in chicken γδ T cell biology.

What’s next?

The mechanism-of-action by which Btnl/Btn genes regulate γδ cells is unknown, particularly whether or not they engage the γδ TCR. In that regard, are BTNL/BTN proteins modified post-translationally, or is their expression sensitive to epithelial cell perturbation, thereby permitting them to communicate particular pathophysiologic states to the local T cell compartment? And what benefit accrues from epithelial cells in different sites employing different Btnl/Btn genes to communicate with site-specific T cells, when the utilisation of NKG2D ligands such as MICA (human) and Rae1 (mouse) is generic across organs? Answers to this last question can dissolve our ignorance over the fundamental physiologic roles of mucosal IEL, and should be obtained by reprogramming the expression of different Btnl/Btn genes both spatially and temporally. Because the system is conserved, mouse experiments can be paralleled by tracking human genetic data for associations of BTNL/BTN genes with inflammatory diseases and/or cancer, thereby implicating the BTNL/BTN-γδ T cell axis in maintaining human barrier integrity. Inevitably, the next step may be for the axis to become a target for clinical intervention.

Read the article here