Neutrophils suppress mucosal-associated invariant T cells in humans.

Mucosal-associated invariant T (MAIT) cells are innate-like T lymphocytes that are abundant in mucosal tissues and the liver where they can respond rapidly to a broad range of riboflavin producing bacterial and fungal pathogens. Neutrophils, which are recruited early to sites of infection, play a nonredundant role in pathogen clearance and are crucial for controlling infection. The interaction of these two cell types is poorly studied. Here, we investigated both the effect of neutrophils on MAIT cell activation and the effect of activated MAIT cells on neutrophils. We show that neutrophils suppress the activation of MAIT cells by a cell-contact and hydrogen peroxide dependent mechanism. Moreover, highly activated MAIT cells were able to produce high levels of TNF-α that induced neutrophil death. We therefore provide evidence for a negative regulatory feedback mechanism in which neutrophils prevent overactivation of MAIT cells and, in turn, MAIT cells limit neutrophil survival.

Type I interferons are important co-stimulatory signals during T cell receptor mediated human MAIT cell activation.

Mucosal associated invariant T (MAIT) cells are abundant unconventional T cells that can be stimulated either via their TCR or by innate cytokines. The MAIT cell TCR recognises a pyrimidine ligand, derived from riboflavin synthesising bacteria, bound to MR1. In infection, bacteria not only provide the pyrimidine ligand but also co-stimulatory signals, such as TLR agonists, that can modulate TCR-mediated activation. Recently, type I interferons (T1-IFNs) have been identified as contributing to cytokine-mediated MAIT cell activation. However, it is unknown whether T1-IFNs also have a role during TCR-mediated MAIT cell activation. In this study, we investigated the co-stimulatory role of T1-IFNs during TCR-mediated activation of MAIT cells by the MR1 ligand 5-amino-6-d-ribitylaminouracil/methylglyoxal. We found that T1-IFNs were able to boost interferon-γ and granzyme B production in 5-amino-6-d-ribitylaminouracil/methylglyoxal-stimulated MAIT cells. Similarly, influenza virus-induced T1-IFNs enhanced TCR-mediated MAIT cell activation. An essential role of T1-IFNs in regulating MAIT cell activation by riboflavin synthesising bacteria was also demonstrated. The co-stimulatory role of T1-IFNs was also evident in liver-derived MAIT cells. T1-IFNs acted directly on MAIT cells to enhance their response to TCR stimulation. Overall, our findings establish an important immunomodulatory role of T1-IFNs during TCR-mediated MAIT cell activation.

Early Clearance of Mycobacterium tuberculosis Is Associated With Increased Innate Immune Responses.

BACKGROUND: A proportion of tuberculosis (TB) case contacts do not become infected, even when heavily exposed. We studied the innate immune responses of TB case contacts to understand their role in protection against infection with Mycobacterium tuberculosis, termed "early clearance." METHODS: Indonesian household contacts of TB cases were tested for interferon-γ release assay (IGRA) conversion between baseline and 14 weeks post recruitment. Blood cell populations and ex vivo innate whole blood cytokine responses were measured at baseline and, in a subgroup, flow cytometry was performed at weeks 2 and 14. Immunological characteristics were measured for early clearers, defined as a persistently negative IGRA at 3 months, and converters, whose IGRA converted from negative to positive. RESULTS: Among 1347 case contacts, 317 were early clearers and 116 were converters. Flow cytometry showed a resolving innate cellular response from 2 to 14 weeks in persistently IGRA-negative contacts but not converters. There were no differences in cytokine responses to mycobacterial stimuli, but compared to converters, persistently IGRA-negative contacts produced more proinflammatory cytokines following heterologous stimulation with Escherichia coli and Streptococcus pneumoniae. CONCLUSIONS: Early clearance of M. tuberculosis is associated with enhanced heterologous innate immune responses similar to those activated during induction of trained immunity.

ESBL- and Carbapenemase-Producing Enterobacteriaceae in Patients with Bacteremia, Yangon, Myanmar, 2014.

Among 42 gram-negative bloodstream isolates from inpatients in 3 hospitals in Yangon, Myanmar, admitted during July-December 2014, 16 (38%) were extended-spectrum ß-lactamase-producing Enterobacteriaceae and 6 (14%) produced carbapenemase. The high prevalence of multidrug-resistant gram-negative bacteria raises concerns about the empiric treatment of patients with sepsis in Yangon.

TLR signaling in human antigen-presenting cells regulates MR1-dependent activation of MAIT cells.

Mucosal-associated invariant T (MAIT) cells are an abundant innate-like T lymphocyte population that are enriched in liver and mucosal tissues. They are restricted by MR1, which presents antigens derived from a metabolic precursor of riboflavin synthesis, a pathway present in many microbial species, including commensals. Therefore, MR1-mediated MAIT cell activation must be tightly regulated to prevent inappropriate activation and immunopathology. Using an in vitro model of MR1-mediated activation of primary human MAIT cells, we investigated the mechanisms by which it is regulated. Uptake of intact bacteria by antigen presenting cells (APCs) into acidified endolysosomal compartments was required for efficient MR1-mediated MAIT cell activation, while stimulation with soluble ligand was inefficient. Consistent with this, little MR1 was seen at the surface of human monocytic (THP1) and B-cell lines. Activation with a TLR ligand increased the amount of MR1 at the surface of THP1 but not B-cell lines, suggesting differential regulation in different cell types. APC activation and NF-κB signaling were critical for MR1-mediated MAIT cell activation. In primary cells, however, prolonged TLR signaling led to downregulation of MR1-mediated MAIT cell activation. Overall, MR1-mediated MAIT cell activation is a tightly regulated process, dependent on integration of innate signals by APCs.

Genetic evaluation of ESBL-producing Escherichia coli urinary isolates in Otago, New Zealand.

OBJECTIVES: The incidence of infections with ESBL-producing Escherichia coli (ESBL-Ec) in New Zealand is increasing. ESBL-Ec most commonly cause urinary tract infections and are seen in both community and hospitalized patients. The reason for the increasing incidence of ESBL-Ec infections is unknown. METHODS: In this study, 65 urinary ESBL-Ec isolates from the Otago region in 2015 were fully genetically characterized to understand the mechanisms of transmission. The ESBL gene, E. coli STs, plasmid types and genetic context (e.g. insertion sequences) of ESBL genes were determined by a combination of whole genome and plasmid sequencing. The phylogenetic relationships of the isolates were compared with ESBL-Ec isolates sequenced as part of the 2016 nationwide survey. RESULTS: Significant diversity of E. coli strains, plasmids, and the genetic context of ESBL genes was seen. However, there was evidence of common mobile genetic elements in unrelated ESBL-Ec. CONCLUSIONS: Multiple introductions of ESBL resistance genes or resistant bacterial strains with limited horizontal transmission of mobile genetic elements accounts for the increased incidence of ESBL-Ec in this low prevalence area. Future studies should investigate modes of transmission of ESBL-Ec in the Otago region.

A prospective study of bloodstream infections among febrile adolescents and adults attending Yangon General Hospital, Yangon, Myanmar.

Data on causes of community-onset bloodstream infection in Myanmar are scarce. We aimed to identify etiological agents of bloodstream infections and patterns of antimicrobial resistance among febrile adolescents and adults attending Yangon General Hospital (YGH), Yangon, Myanmar. We recruited patients ≥12 years old with fever ≥38°C who attended YGH from 5 October 2015 through 4 October 2016. A standardized clinical history and physical examination was performed. Provisional diagnoses and vital status at discharge was recorded. Blood was collected for culture, bloodstream isolates were identified, and antimicrobial susceptibility testing was performed. Using whole-genome sequencing, we identified antimicrobial resistance mechanisms of Enterobacteriaceae and sequence types of Enterobacteriaceae and Streptococcus agalactiae. Among 947 participants, 90 (9.5%) had bloodstream infections (BSI) of which 82 (91.1%) were of community-onset. Of 91 pathogens isolated from 90 positive blood cultures, we identified 43 (47.3%) Salmonella enterica including 33 (76.7%) serovar Typhi and 10 (23.3%) serovar Paratyphi A; 20 (22.0%) Escherichia coli; 7 (7.7%) Klebsiella pneumoniae; 6 (6.6%), Staphylococcus aureus; 4 (4.4%) yeasts; and 1 (1.1%) each of Burkholderia pseudomallei and Streptococcus agalactiae. Of 70 Enterobacteriaceae, 62 (88.6%) were fluoroquinolone-resistant. Among 27 E. coli and K. pneumoniae, 18 (66.6%) were extended-spectrum beta-lactamase (ESBL)-producers, and 1 (3.7%) each were AmpC beta-lactamase- and carbapenemase-producers. Fluoroquinolone resistance was associated predominantly with mutations in the quinolone resistance-determining region. blaCTX-M-15 expression was common among ESBL-producers. Methicillin-resistant S. aureus was not detected. Fluoroquinolone-resistant, but not multiple drug-resistant, typhoidal S. enterica was the leading cause of community-onset BSI at a tertiary hospital in Yangon, Myanmar. Fluoroquinolone and extended-spectrum cephalosporin resistance was common among other Enterobactericeae. Our findings inform empiric management of severe febrile illness in Yangon and indicate that measures to prevent and control enteric fever are warranted. We suggest ongoing monitoring and efforts to mitigate antimicrobial resistance among community-onset pathogens.

Molecular mechanisms of antimicrobial resistance and phylogenetic relationships of Salmonella enterica isolates from febrile patients in Yangon, Myanmar.

BACKGROUND: Enteric fever is common in southeast Asia. However, there is little information on the circulating Salmonella enterica strains causing enteric fever in Myanmar. METHODS: We performed antimicrobial susceptibility testing and whole genome sequencing on S. enterica bloodstream isolates from febrile patients aged ≥12 y attending two hospitals in Yangon, Myanmar, from 5 October 2015 through 4 October 2016. We identified the serovar of S. enterica, determined antimicrobial susceptibility and the molecular mechanisms of resistance. We analysed phylogenetic relationships among Myanmar S. enterica isolates and those with isolates from neighbouring countries. RESULTS: Of 73 S. enterica isolated, 39 (53%) were serovar Typhi and 34 (47%) were Paratyphi A. All isolates were susceptible to ampicillin, chloramphenicol and trimethoprim-sulfamethoxazole but resistant to ciprofloxacin. We identified mutations in chromosomal genes gyrA, gyrB and parC as responsible for fluoroquinolone resistance. All S. enterica Typhi isolates were of 4.3.1 subclade (formerly known as H58) and formed two closely related genotypic clusters; both clusters were most closely related to isolates from India from 2012. All S. enterica Paratyphi A were lineage C, clade C4 and were closely related. CONCLUSION: Our study describes currently circulating S. enterica serovars in Myanmar, the genetic basis of their antimicrobial resistance and provides a genotypic framework for epidemiologic study.

TCR- or Cytokine-Activated CD8+ Mucosal-Associated Invariant T Cells Are Rapid Polyfunctional Effectors That Can Coordinate Immune Responses.

Mucosal-associated invariant T (MAIT) cells can be activated via either their T cell receptor (TCR), which recognizes MR1-bound pyrimidines derived from microbial riboflavin biosynthesis, or via cytokines. These two modes of activation may act in concert or independently, depending upon the stimulus. It is unknown, however, how MAIT cell responses differ with the mode of activation. Here, we define transcriptional and effector responses of human CD8+ MAIT cells to TCR and cytokine stimulation. We report that MAIT cells rapidly respond to TCR stimulation, producing multiple cytokines and chemokines, altering their cytotoxic granule content and transcription factor expression, and upregulating co-stimulatory proteins. In contrast, cytokine-mediated activation is slower and results in a more limited response. Therefore, we propose that, in infections by riboflavin-synthesizing bacteria, MAIT cells play a key early role in effecting and coordinating immune responses, while in the absence of TCR stimulation, their role is likely to differ.

Shared and Distinct Phenotypes and Functions of Human CD161++ Vα7.2+ T Cell Subsets.

Human mucosal-associated invariant T (MAIT) cells are an important T cell subset that are enriched in tissues and possess potent effector functions. Typically such cells are marked by their expression of Vα7.2-Jα33/Jα20/Jα12 T cell receptors, and functionally they are major histocompatibility complex class I-related protein 1 (MR1)-restricted, responding to bacterially derived riboflavin synthesis intermediates. MAIT cells are contained within the CD161++ Vα7.2+ T cell population, the majority of which express the CD8 receptor (CD8+), while a smaller fraction expresses neither CD8 or CD4 coreceptor (double negative; DN) and a further minority are CD4+. Whether these cells have distinct homing patterns, phenotype and functions have not been examined in detail. We used a combination of phenotypic staining and functional assays to address the similarities and differences between these CD161++ Vα7.2+ T cell subsets. We find that most features are shared between CD8+ and DN CD161++ Vα7.2+ T cells, with a small but detectable role evident for CD8 binding in tuning functional responsiveness. By contrast, the CD4+ CD161++ Vα7.2+ T cell population, although showing MR1-dependent responsiveness to bacterial stimuli, display reduced T helper 1 effector functions, including cytolytic machinery, while retaining the capacity to secrete interleukin-4 (IL-4) and IL-13. This was consistent with underlying changes in transcription factor (TF) expression. Although we found that only a proportion of CD4+ CD161++ Vα7.2+ T cells stained for the MR1-tetramer, explaining some of the heterogeneity of CD4+ CD161++ Vα7.2+ T cells, these differences in TF expression were shared with CD4+ CD161++ MR1-tetramer+ cells. These data reveal the functional diversity of human CD161++ Vα7.2+ T cells and indicate potentially distinct roles for the different subsets in vivo.

Molecular Analyses Define Vα7.2-Jα33+ MAIT Cell Depletion in HIV Infection: A Case-Control Study.

Mucosal-associated invariant T (MAIT) cells are an abundant antibacterial innate-like lymphocyte population. There are conflicting reports as to their fate in HIV infection. The objective of this study was to determine whether MAIT cells are truly depleted in HIV infection. In this case-control study of HIV-positive patients and healthy controls, quantitative real-time polymerase chain reaction was used to assess the abundance of messenger RNA (mRNA) and genomic DNA (gDNA) encoding the canonical MAIT cell T cell receptor (Vα7.2-Jα33). Comparison was made with flow cytometry. Significant depletion of both Vα7.2-Jα33 mRNA and gDNA was seen in HIV infection. Depletion of Vα7.2+CD161++ T cells was confirmed by flow cytometry. In HIV infection, the abundance of Vα7.2-Jα33 mRNA correlated most strongly with the frequency of Vα7.2+CD161++ cells. No increase was observed in the frequency of Vα7.2+CD161- cells among CD3+CD4- lymphocytes. MAIT cells are depleted from blood in HIV infection as confirmed by independent assays. Significant accumulation of a CD161- MAIT cell population is unlikely. Molecular approaches represent a suitable alternative to flow cytometry-based assays for tracking of MAIT cells in HIV and other settings.