IFN-gamma-induced trained immunity as a strategy to enhance anti-microbial responses in human monocytes and macrophages.

Abstract

Infectious diseases remain a significant global health threat, exacerbated by the rise in multi-drug- resistant infections and the potential for epidemics. Mycobacterium tuberculosis (M.tb) and Staphylococcus aureus are major contributors to infection-related mortality. Host-directed therapies (HDT) aim to enhance patient’s immune system to fight infection adjunct to antibiotics. Trained immunity is a functional reprogramming of innate immune responses whereby myeloid cells are metabolically and epigenetically rewired, resulting in a heightened ability of innate immune cells to respond to infection. As trained immunity can enhance myeloid antimicrobial responses, it may be an effective HDT for infectious disease. Interferon-γ (IFN-γ) has been identified as a key mediator of trained immunity. Adenoviral vectors have been shown to induce trained immunity in the murine lung through an IFN- γ-dependent mechanism. The ability of adenoviral vectors to induce trained immunity in peripheral blood in humans in vivo was unknown at the outset of this project. It was hypothesised that the adenoviral vector ChAdOx1 nCoV-19, developed against COVID-19, would induce trained immunity in humans in vivo. Vaccination with ChAdOx1 nCoV-19 led to enhanced myelopoiesis, metabolic reprogramming of monocytes, and increased cytokine and chemokine production in response to bacterial ligands or M.tb, compared to pre-vaccine controls. Additionally, resting monocytes exhibited significant IFN-γ production post-vaccination, suggesting that this may be a potential mechanism for the induction of trained immunity. Thus, it was hypothesised that IFN-γ alone would induce trained immunity in human monocytes in vitro, leading to metabolically reprogrammed monocyte-derived macrophages (MDM) with heightened immune responses to M.tb or S. aureus. IFN-γ training enhanced antigen presentation marker expression and increased glycolytic activity in MDM upon stimulation with M.tb or lipopolysaccharide (LPS), compared to untrained controls. IFN-γ trained MDM also produced more cytokines and chemokines upon challenge with LPS, M.tb, or S. aureus, and exhibited reduced bacillary burdens when infected with M.tb or S. aureus. Additionally, IFN-γ training monocytes enhanced cytokine responses to M.tb in individuals who had a mutation in their TIRAP gene. This cohort of individuals are known to be vulnerable to infectious diseases including sepsis, tuberculosis, and S. aureus bacteraemia. Therefore, IFN-γ may be effective as a HDT against bacterial infections, especially in subpopulations who are vulnerable to infections. Finally, as M.tb primarily affects the lungs, the ability of IFN-γ to induce trained immunity in human alveolar macrophages (AM) was assessed. As AM are continuously exposed to lipopolysaccharide (LPS), which can induce tolerance in monocytes, whether LPS could induce tolerance in human AM was also explored. It was hypothesised that IFN-γ would induce trained immunity in AM in response to LPS or M.tb, and that LPS training would induce tolerance in AM in response to subsequent LPS challenge. Both IFN-γ trained and LPS trained AM altered their cell surface marker expression in AM. IFN-γ trained AM produced more IL-6, IL-8, and G-CSF in response to M.tb, whereas LPS tolerised AM had reduced cytokine production upon LPS rechallenge. Collectively, these data demonstrate that trained immunity can enhance antibacterial responses in human myeloid cells, both in vivo and in vitro. Trained immunity, induced by IFN-γ, may be effective as a HDT against infections in humans by enhancing glycolytic metabolism and antimicrobial function in monocytes and AM.