Adrian Hayday : Immuno Surveillance

It is increasingly apparent that the immune system is tasked with both protecting from infection, and with responding to tissue that has been dysregulated by non-microbial challenges, such as irradiation. This has profound relevance for tumour immunology, since it considers the immune potential to recognise and respond to cells en route to malignant transformation. In 2011, we improved the cellular and molecular definition of such ‘lymphoid stress-surveillance’ (LSS) responses and, in so doing have established a direct linkage to atopy (IgE production) more commonly viewed in the context of allergies. Therefore, we have formulated a framework in which atopy and tumour surveillance jointly protect the body from environmental toxins and their effects. This provokes us to explore the developmental origins and regulation of this immunoprotective capability.

Immune responses to infectious agents share key features with responses to dysregulated tissues. For example, MICA (in human), Rae-1 (in mouse), and other MHC class I-like ligands for the activating NKG2D receptor, are displayed on epithelial cells infected by viruses and some bacteria, as well as on cells subject to ‘sterile stresses’, such as genotoxicity or oxidative stress. Thus, LSS by NKG2D-expressing cells – NK cells, γδT cells, and some cytolytic CD8⁽⁺⁾ αβT cells – may complement other immune mechanisms to effect recognition and eradication of infected and transformed cells. Highlighting their critical immunoprotective role, NKG2D ligands are frequently targeted by immune-evasion mechanisms of viruses and tumors.

Our studies have demonstrated that LSS is an extremely fast-acting response, that relies on the developmental acquisition of key functional potentials by particular lymphoid cells (Hayday, 2009; Immunity. 34: 184-96). With the potential to protect against non-microbial damage, regulate tissue inflammation, and suppress carcinogenesis, LSS has understandable translational interest.Hence, we have maintained clinic-based investigations in parallel with our basic programme.

Tumour surveillance and atopy

We analyse LSS in the murine skin, modelling the response of NKG2D+ intraepidermal T cells to tissue perturbation, by using simple transgenic molecular switches to activate Rae-1 specifically on normal keratinocytes. In 2011, we completed construction of several new strains of tg mice with inducible NKG2D ligands, and in each found intraepidermal T cells to rapidly and substantially upregulate interleukin 13 (IL-13), a cytokine causally linked to atopy. This contrasted with the prevailing view of intraepidermal T lymphocytes, as cytolytic cells, directly promoting eradication of dysregulated keratinocytes. Following this up, we found that standard protocols for inducing skin allergies in non-tg mice commonly upregulate Rae-1, IL-13, and the keratinocyte cytokines, IL-25 and IL-33, that are linked to allergy. Use of mutant mice revealed that upregulation of each cytokine depended on intraepidermal T cells and LSS, unexpectedly revealing that immune cells condition the epithelium to respond appropriately to environmental conditions (Strid, et al., 2011; Science. 334:6060: 1293-7).

What relates LSS to atopy?

We hypothesise that tissue dysregulation is most often induced by toxins and that, during LSS, allergy-associated cytokines induce IgE which promotes the physical expulsion of such toxins while simultaneously limiting their dissemination by lowering blood pressure. However, the so called Th2 responses underpinning atopy are generally regarded as deleterious for tumour immunoprotection. Thus, we propose that Th2 responses embrace at least two processes: one initiated by conventional microbial-sensing systems that invokes antigen-specific IgG1 and IgE, and that may be ineffective at tumour protection; and another initiated by LSS that rapidly invokes IgE, and that may promote tumour immunoprotection. Indeed, we recently found that LSS mutants impair IgE production, whereas microbial-sensing mutants primarily compromise IgG1. Such segregation is novel, provides an additional dimension to LSS, and is supported by emerging molecular data on the regulation of immunoglobulin synthesis. Furthermore, a meta-analysis we contributed to, found weak but significant negative correlations of human IgE levels with carcinogenesis. We, therefore, seek to define the pathway from LSS to IgE, to identify allergens that promote it, to image the pathway in real time in murine skin and to elucidate its positive effects on tumour immunoprotection.

Human stress-surveillance

Facilitated by the King’s College Comprehensive Biomedical Research Centre, we found that all human donors tested mount NKG2D-mediated LSS, but with very high and unanticipated interindividual variation. Our studies attribute this to ‘personalised tuning’ (Shafi, et al., 2011: Sci Transl Med. 3(113): 113RA124) reflecting genotype,  environment, and eQTL. We now aim to understand this tuning, its implications for immuno-evasion by viruses and tumours, and its potential contributions to tumour surveillance, graft rejection, and inflammation.

Figure 1 A schematic of tuning. A. Following dysregulation, MICA levels (red curves, shown on an arbitrary scale) on cells increase to different extents in  onors 1 and 2, dictated by their MICA haplotype, eQTL, and other factors. B. The MICA responsive gd T (yellow) and NK cells (green) in each donor are best activated by the levels of MICA most commonly induced by cellular dysregulation. C. Defined levels of MICA induction are phenocopied by CHO-FRT cells transfected with specific MICA alleles. Hence targeting of those transfectants depends on the alignment with an individual’s MICA-responsive gd T and NK cells.

Building stress-surveillance

LSS represents a novel, rapid-acting, lymphocyte biology dependent on distinct developmental pathways for the lymphocytes that mediate it. We have identified in developing gd thymocytes a signaling pathway induced by an epithelial determinant, Skint-1, that imprints on intraepidermal T cells the capacity to rapidly make IFNg (Turchinovich and Hayday, 2011; Immunity. 35: 59-68). There remain myriad questions, including: the acquisition of IL-13- producing and cytolytic potentials; the mechanism of-action of Skint-1 (does it encode a ligand for the intraepidermal T cell receptor?) and the analogous developmental cues for IL-17- producing γδT cells, another lymphocyte subset making key contributions to rapid immune responsiveness.

Stress-surveillance in the gut

Murine epidermis has proved a tractable experimental system providing basic insights and a foundation for translational studies. However, we also wish to know whether LSS is likewise mounted by intestinal intraepithelial lymphocytes (IEL), a major T cell compartment.We have developed a novel IEL culture system, identifying direct functional interactions with epithelial cells. Intriguingly, such interactions are regulated by Butyrophilin-like (Btnl) 1, a gene product highly similar to Skint-1 (Bas et al., 2011; Proc Natl Acad Sci. 108: 4376-81). Hence, parallel studies of gut and skin should reveal common and tissue-specific components of LSS germane to immunoprotection and immunopathology.