Gamma Delta TCR and the WC1 Co-Receptor Interactions in Response to Leptospira Using Imaging Flow Cytometry and STORM

Morphological and Phenotypic Characteristics of the Bovine Nasopharyngeal Mucosa and Associated Lymphoid Tissue

The mammalian nasopharynx is an anatomically complex region of the upper respiratory tract that directly communicates with the nasal cavity, laryngopharynx, oesophagus and trachea. The nasopharyngeal mucosa contains moderate quantities of mucosa-associated lymphoid tissue (MALT) that is appropriately located for immunological sampling but also creates vulnerability to pathogens. In recent years, the nasopharynx has been inculpated in the pathogenesis of important diseases of cattle (foot-and-mouth disease) and humans (COVID-19), yet the tissue has never been described in detail in any species. In order to characterize the morphology and cellular composition of the bovine nasopharynx, samples of mucosa were collected from the nasopharynx of five 8-13-month-old steers and examined using light microscopy, immunohistochemistry and multichannel immunofluorescence. Morphologically, the nasopharyngeal epithelium was highly heterogeneous, with a continuum ranging from stratified squamous epithelium to highly attenuated, follicle-associated epithelium (FAE). Distribution of MALT was similarly regionally variable ranging from absent to clusters of multiple lymphoid follicles. Phenotypic characterization demonstrated dense distributions of dendritic cells and T lymphocytes surrounding lymphoid follicles, which comprised mostly B lymphocytes. The FAE overlaying the lymphoid follicles also contained higher numbers of dendritic cells and lymphocytes compared with the adjacent non-lymphoid epithelium, although cytotoxic T cells were notably scarce in the FAE. The bovine nasopharyngeal lymphoid tissue had comparable elements to other MALTs with specific differences that may help to elucidate the pathogenesis of infectious agents that have specific tropism for this tissue.

Next generation sequencing of transcribed genes in ruminant γδ T cell populations

Bovine γδ T cells are distinguished by expression of WC1, hybrid pattern recognition receptors and co-receptors to the T cell receptor (TCR), or their absence. WC1 molecules bind pathogens and the ability of γδ T cells to respond to pathogens largely correlates with their expression of particular WC1 genes. Following activation, the TCR and WC1 molecules co-localize and knocking down WC1 abrogates the ability of WC1-expressing γδ T cells to respond to antigen. It is known that these two major populations, WC1⁺ and WC1^-, differ in their TCR gene expression and previous studies showed other differences using semi-quantitative RT-PCR and serial analysis of gene expression. Differences in genes expressed would influence the functional outcome when WC1⁺ vs. WC1^- γδ T cells respond to pathogens. To identify unique aspects of their transcriptome, here we performed RNA-Seq of flow cytometrically sorted bovine WC1⁺ and WC1^- γδ T cells and compared them to all mononuclear cells in blood. The greatest differences in gene expression were found between γδ T cells and other mononuclear cells and included those involved in lymphocyte activation and effector processes. Only minor differences occurred between ex vivo WC1⁺ vs. WC1^- γδ T cells with those gene products being involved in cell adhesion and chemotaxis. After culturing cells from primed animals with Leptospira antigens major difference in the transcriptome was evident, with over 600 genes significantly differentially expressed including those focused on cytokine signaling. Unexpectedly, antigen-responding and non-responding populations of WC1⁺ γδ T cells had few differences in their transcriptomes outside of cytotoxic factors although they had more WC1-1, WC1-2 and WC1-13 transcripts. Through differential gene expression we were able to define properties of ex vivo and stimulated WC1⁺ cells which will be useful in understanding their functional biology.

γδ T cells in artiodactyls: Focus on swine

Vaccination is the most effective medical strategy for disease prevention but there is a need to improve livestock vaccine efficacy. Understanding the structure of the immune system of swine, which are considered a γδ T cell "high" species, and thus, particularly how to engage their γδ T cells for immune responses, may allow for development of vaccine optimization strategies. The propensity of γδ T cells to home to specific tissues, secrete pro-inflammatory and regulatory cytokines, exhibit memory or recall responses and even function as antigen-presenting cells for αβ T cells supports the concept that they have enormous potential for priming by next generation vaccine constructs to contribute to protective immunity. γδ T cells exhibit several innate-like antigen recognition properties including the ability to recognize antigen in the absence of presentation via major histocompatibility complex (MHC) molecules enabling γδ T cells to recognize an array of peptides but also non-peptide antigens in a T cell receptor-dependent manner. γδ T cell subpopulations in ruminants and swine can be distinguished based on differential expression of the hybrid co-receptor and pattern recognition receptors (PRR) known as workshop cluster 1 (WC1). Expression of various PRR and other innate-like immune receptors diversifies the antigen recognition potential of γδ T cells. Finally, γδ T cells in livestock are potent producers of critical master regulator cytokines such as interferon (IFN)-γ and interleukin (IL)-17, whose production orchestrates downstream cytokine and chemokine production by other cells, thereby shaping the immune response as a whole. Our knowledge of the biology, receptor expression and response to infectious diseases by swine γδ T cells is reviewed here.

Identification of leptospiral protein antigens recognized by WC1+ γδ T cell subsets as target for development of recombinant vaccines

Pathogenic Leptospira species cause leptospirosis, a neglected zoonotic disease recognized as a global public health problem. It is also the cause of the most common cattle infection that results in major economic losses due to reproductive problems. γδ T cells play a role in the protective immune response in livestock species against Leptospira while human γδ T cells also respond to Leptospira. Thus, activation of γδ T cells has emerged as a potential component in the optimization of vaccine strategies. Bovine γδ T cells proliferate and produce IFN-γ in response to vaccination with inactivated leptospires and this response is mediated by a specific subpopulation of the WC1-bearing γδ T cells. WC1 molecules are members of the group B scavenger receptor cysteine rich (SRCR) superfamily and are composed of multiple SRCR domains, of which particular extracellular domains act as ligands for Leptospira. Since WC1 molecules function as both pattern recognition receptors and γδ TCR coreceptors, the WC1 system has been proposed as a novel target to engage γδ T cells. Here, we demonstrate the involvement of leptospiral protein antigens in the activation of WC1⁺ γδ T cells and identified two leptospiral outer membrane proteins able to interact directly with them. Interestingly, we show that the protein-specific γδ T cell response is composed of WC1.1⁺ and WC1.2⁺ subsets, although a greater number of WC1.1⁺ ???? T-cell respond. Identification of protein antigens will enhance our understanding of the role γδ T cells play in the leptospiral immune response and in recombinant vaccine development.