The responses of conventional T cells and mucosal-associated invariant T cells to nontypeable Haemophilus Influenzae infection

Abstract

Nontypeable Haemophilus influenzae (NTHi) is a component of the normal lung microbiome, but is also highly associated with respiratory tract infections and exacerbations of major chronic respiratory diseases such as chronic obstructive pulmonary disease (COPD). It is not known how commensal bacteria can become pathogenic and cause inflammation in the lung, but is likely due to a breakdown in local immunity. T cell immunity may be key to controlling NTHi infection, although the responses of T cells to NTHi are not well understood. Mucosal-associated invariant T (MAIT) cells are a newly-discovered subset of innate-like T cells, which may play a role in controlling NTHi, as they recognise non-peptide antigens derived from the highly conserved vitamin B2 pathway. The role of MAIT cells in lung immunity to NTHi is also unclear.

The aim of this thesis was to study the cytokine and cytotoxic responses of MAIT cells to NTHi infection, comparing these responses to those of conventional T cells. The antigen presentation and co-stimulatory mechanisms that regulate MAIT cell activation have also been explored.

Conventional T cell and MAIT cell responses to NTHi were investigated using a human ex vivo lung tissue explant model and an autologous monocyte-derived macrophage (MDM)-T cell co-culture model. Cytokine and cytotoxic responses were measured by a combination of flow cytometry, ELISA and ELISpot. Blocking antibodies were used to determine the role of antigen presentation and cytokine signalling in conventional T cell and MAIT cell activation.

Lung MAIT cells significantly upregulated the cytokines; IFNγ, IL-17a and TNFα, and the cytotoxic markers; granzyme B and CD107a, following NTHi infection. A greater proportion of MAIT cells were active compared to conventional T cells. In the blood-derived MDM-T cell co-culture, IFNγ expression by MAIT cells was dependent on MR1 antigen presentation and IL-12 signalling in a time-dependent manner. Cytotoxic responses were regulated by MR1 but also by a novel mechanism where IL-12 and IL-7 signalling synergised to induce granzyme B expression by upregulation of the IL-12 receptor. In contrast, conventional T cell responses predominantly relied on antigen presentation for activation. MAIT cell responses were also impaired by treatment with corticosteroids, which are commonly used to manage inflammation in chronic lung diseases, but are associated with a higher risk of developing pneumonia in COPD patients.

Overall, the data in this thesis provide evidence for a role for MAIT cells in controlling NTHi infection in the lung and also highlight key differences in the regulation of innate and adaptive T cells. A further understanding of the mechanism underpinning T cell responses to NTHi infection may yield new therapeutic opportunities and improve the outcome for patients with respiratory diseases.

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Identifiers

ORCID for Karl Staples: orcid.org/0000-0003-3844-6457

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