Evaluating the innate immune functions of porcine gamma-delta T cells
Date
2024-07-22
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
Type
Thesis
Degree Level
Doctoral
Abstract
Gamma-delta (γδ) T cells, a distinct subset of T cells, play a crucial role in immune responses and surveillance due to their unique characteristics. While γδ T cells are a rare cell population in human blood, they are enriched in many peripheral tissues. Unlike conventional T cells, γδ T cells have diverse mechanisms of antigen recognition. These mechanisms include the recognition of unconventional antigens, like stress molecules and non-peptide antigens, independent of the presentation via MHC molecules, enabling them to respond rapidly to invading pathogens or other stressors. Over the years many innate-like functions have been described for human and murine γδ T cells, for example, the direct recognition of pathogens through Pattern Recognition Receptors, the MHC-unrestricted activation and polyclonal proliferation. In terms of effector functions, γδ T cells mainly exhibit cytotoxicity against infected or other dysregulated cells and proinflammatory cytokine production (IL-17, IFNγ). In contrast to humans, pigs belong to the “γδ high species” characterized by a high frequency of circulating γδ T cells. Especially in young pigs about 50-60% of T cells are γδ T cells. Despite this abundance, the role and function of porcine γδ T cells within the immune system and developmental effects are largely unknown. Thus, this thesis tested the hypothesis that porcine γδ T cells can function as innate immune cells and can change phenotypically and functionally with age. First, we successfully characterized the phenotype of γδ T cells in circulation. We determined that the main phenotype of γδ T cells changes from CD2−CD8a−/dimCD27+ in young pigs to CD2+CD8ahighCD27− in 3-year-old pigs. Moreover, we identified TLR transcript in porcine γδ T cells with the highest level observed for TLR7 and TLR8. TLR7 and 8 were not only expressed on the transcript level but were also functionally relevant as we demonstrated a direct effect of TLR7/8 ligand (R848) on porcine γδ T cells. Cytokine production (IFNγ) by γδ T cells from young and adult pigs was potently stimulated by R848 in combination with IL-2 and IL-12 with stronger responses from adult pigs. On top of the differences in γδ T cell phenotype and cytokine production with age, γδ T cells from different age groups seem to differ in the downstream signaling induced by TLR7/8 activation as demonstrated by kinome analysis and flow cytometry. Our data revealed significant involvement of the kinases IRAK1/4, p38 and JNK in the TLR7/8 induced co-stimulation in adult γδ T cells, potentially indicating an involvement of the MyD88-dependent pathway. Gamma-delta T cells from young pigs, mainly consisting of CD2- γδ T cells, seem to utilize alternative signaling pathways that did not involve IRAK1/4 or JNK. As TLR7 and TLR8 play pivotal roles in recognizing viral RNA and IFNγ is a cytokine with direct antiviral properties, we may have uncovered an antiviral innate function of T cells: the direct recognition of RNA viruses leading to IFNγ production and subsequently activating downstream immune responses. Besides pro-inflammatory cytokine production, human γδ T cells are also known for their broad cytolytic activity towards infected/stressed cells. In this thesis, we discovered that only CD2+ γδ T cells express cytotoxic effector cell-associated markers and that γδ T cells can exhibit cytolytic functions in vitro. Unexpectedly, this cytotoxic activity was not exclusively directed at infected cells; rather, it appeared to be broader in scope, as uninfected cells were also targeted. It remains to be seen what target cell recognition mechanisms are employed by porcine γδ T cells, but it is plausible that the recognition of cellular stress plays a role. Overall, my thesis supports the understanding of γδ T cells as an immune cell subset engaged in innate immune responses and stress surveillance.
Description
Keywords
Gamma-delta T cells, pig, Immunology, Cell activation: Cytotoxicity
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Veterinary Microbiology
Program
Veterinary Microbiology