The role of cytotoxic CD161-expressing CD4+ T-cells in host defense during chronic inflammatory diseases of the gut.
Immunology
Final Report Abstract
The aim of my DFG research fellowship was to characterise a population of intestinal CD161-expressing CD4 T cells that were capable of producing the cytotoxic effector molecule GzmB upon stimulation. As high levels of CD161 expression are also found on innate-like T cells including MAITs and iNKTs, I used a variety of different methods, including imaging, flow cytometry and sequencing, to determine if colonic CD161 expressing CD4 T cells share characteristics with these cells. While these experiments revealed the use of a diverse TCR repertoire, similar to the one of conventional T cells, they also showed a notable enrichment of genes upregulated in MAITs and iNKTs, suggesting a shared transcriptional program. This finding has important consequences for the functionality of colonic CD161 expressing CD4 T cells, as it suggests that they have the tools necessary to be activated independently of their TCRs via cytokines like IL12, IL23 and IL18, meaning that they potentially could play roles in any kind of infection and inflammatory condition that leads to the production of the appropriate cytokines. Expression of GzmB in these cells, was mainly depended of TCR signalling though. Interestingly, while I was able to confirm GzB expression, my data showed that colonic CD161 expressing CD4 T cells largely lack expression of perforin. As perforin is essential for the delivery of GzmB into the cytosol of target cells, I concluded from these results that colonic CD161 expressing CD4 T cells cannot function as classic cytotoxic effector cells. Due to these findings I shifted the focus of my work away from the originally planed cytotoxicity assays and their follow-up in cells from IBD-patients and tried to establish an alternative hypothesis instead. GzmB is capable of cleaving molecules of the extracellular matrix independently of perforin and hence its production by the colonic CD161 expressing CD4 T cells could impact of on the tissue of the colon as a whole. In vitro proof-of-principle experiments confirmed GzmBs potential to do so, however future studies in more sophisticated models (organoids, mucosoids) are required to fully answer this question. In experiments on MAIT cells, I was able to show that genetic signatures associated with tissue homeostasis and wound repair are upregulated upon TCR-dependent activation. Interestingly, in transcriptional analyses of the colonic CD161 expressing CD4 T cells I had previously found higher expression of the same factors already in the steady state. Importantly, in functional in vitro assays my colleagues and me where capable of showing a positive impact of activated MAIT cells on repairassociated processes, demonstrating that the expression of these signature can translate into actual repair function. While additional experiments are needed to fully understand the role of colonic CD161 expressing CD4 T cells, taken together, the results of this project hint towards the model that T cells with an innate-like transcriptomic program like them are important players in regulating tissue homeostasis via a diverse array of effector molecules.
Publications
- TCR and Inflammatory Signals Tune Human MAIT Cells to Exert Specific Tissue Repair and Effector Functions, Cell Reports, 2019 Sep 17; 28(12):3077-3091.e5
Tianqi Leng, Hossain Delowar Akther, Carl-Philipp Hackstein et al.
(See online at https://doi.org/10.1016/j.celrep.2019.08.050)
