The Roles of Type 2 Cytotoxic T Cells in Inflammation, Tissue Remodeling, and Prostaglandin (PG) D2 Production Are Attenuated by PGD2 Receptor 2 Antagonism.

Human type 2 cytotoxic T (Tc2) cells are enriched in severe eosinophilic asthma and can contribute to airway eosinophilia. PGD2 and its receptor PGD2 receptor 2 (DP2) play important roles in Tc2 cell activation, including migration, cytokine production, and survival. In this study, we revealed novel, to our knowledge, functions of the PGD2/DP2 axis in Tc2 cells to induce tissue-remodeling effects and IgE-independent PGD2 autocrine production. PGD2 upregulated the expression of tissue-remodeling genes in Tc2 cells that enhanced the fibroblast proliferation and protein production required for tissue repair and myofibroblast differentiation. PGD2 stimulated Tc2 cells to produce PGD2 using the routine PGD2 synthesis pathway, which also contributed to TCR-dependent PGD2 production in Tc2 cells. Using fevipiprant, a specific DP2 antagonist, we demonstrated that competitive inhibition of DP2 not only completely blocked the cell migration, adhesion, proinflammatory cytokine production, and survival of Tc2 cells triggered by PGD2 but also attenuated the tissue-remodeling effects and autocrine/paracrine PGD2 production in Tc2 induced by PGD2 and other stimulators. These findings further confirmed the anti-inflammatory effect of fevipiprant and provided a better understanding of the role of Tc2 cells in the pathogenesis of asthma.

Diverse Streptococcus pneumoniae Strains Drive a Mucosal-Associated Invariant T-Cell Response Through Major Histocompatibility Complex class I-Related Molecule-Dependent and Cytokine-Driven Pathways.

Mucosal-associated invariant T (MAIT) cells represent an innate T-cell population that can recognize ligands generated by the microbial riboflavin synthesis pathway, presented via the major histocompatibility complex class I-related molecule (MR1). Streptococcus pneumoniae is a major human pathogen that is also associated with commensal carriage; thus, host control at the mucosal interface is critical. The recognition of pneumococci by MAIT cells has not been defined nor have the genomics and transcriptomics of the riboflavin operon. We observed robust recognition of pneumococci by MAIT cells, using both MR1-dependent and MR1-independent pathways. The pathway used was dependent on the antigen-presenting cell. The riboflavin operon was highly conserved across a range of 571 pneumococci from 39 countries, dating back to 1916, and different versions of the riboflavin operon were also identified in related Streptococcus species. These data indicate an important functional relationship between MAIT cells and pneumococci.

Neuromedin U promotes human type 2 immune responses.

Type 2 immunity mediates the immune responses against parasites and allergic stimuli. Evidence from studies of cell lines and animals implies that neuromedin U (NmU) acts as a pro-inflammatory mediator of type 2 inflammation. However, the role of NmU in human type 2 immunity remains unclear. Here we investigated the expression of NmU in human blood and airways, and the expression of NmU receptors by human immune cells in blood and lung tissue. We detected human NmU (hNmU-25) in blood and airways with higher concentrations in the latter. NmU receptor 1 (NmUR1) was expressed by most human immune cells with higher levels in type 2 cells including type 2 T helpers, type 2 cytotoxic T cells, group-2 innate lymphoid cells and eosinophils, and was upregulated in lung-resident and activated type 2 cells. We also assessed the effects of NmU in these cells. hNmU-25 elicited type 2 cytokine production by type 2 lymphocytes and induced cell migration, including eosinophils. hNmU-25 also enhanced the type 2 immune response to other stimuli, particularly prostaglandin D2. These results indicate that NmU could contribute to the pathogenic processes of type 2 immunity-mediated diseases in humans via its pro-inflammatory effects on type 2 lymphocytes and eosinophils.

CD8+ Tc2 cells: underappreciated contributors to severe asthma.

The complexity of asthma is underscored by the number of cell types and mediators implicated in the pathogenesis of this heterogeneous syndrome. Type 2 CD4+ T-cells (Th2) and more recently, type 2 innate lymphoid cells dominate current descriptions of asthma pathogenesis. However, another important source of these type 2 cytokines, especially interleukin (IL)-5 and IL-13, are CD8+ T-cells, which are increasingly proposed to play an important role in asthma pathogenesis, because they are abundant and are comparatively insensitive to corticosteroids. Many common triggers of asthma exacerbations are mediated via corticosteroid-resistant pathways involving neutrophils and CD8+ T-cells. Extensive murine data reveal the plasticity of CD8+ T-cells and their capacity to enhance airway inflammation and airway dysfunction. In humans, Tc2 cells are predominant in fatal asthma, while in stable state, severe eosinophilic asthma is associated with greater numbers of Tc2 than Th2 cells in blood, bronchoalveolar lavage fluid and bronchial biopsies. Tc2 cells strongly express CRTH2, the receptor for prostaglandin D2, the cysteinyl leukotriene receptor 1 and the leukotriene B4 receptor. When activated, these elicit Tc2 cell chemotaxis and production of chemokines and type 2 and other cytokines, resulting directly or indirectly in eosinophil recruitment and survival. These factors position CD8+ Tc2 cells as important and underappreciated effector cells contributing to asthma pathogenesis. Here, we review recent advances and new insights in understanding the pro-asthmatic functions of CD8+ T-cells in eosinophilic asthma, especially corticosteroid-resistant asthma, and the molecular mechanisms underlying their pathologic effector function.