Early Immune Responses to Emerging and Enduring Respiratory Pathogens

Abstract

For most of recent history, Tuberculosis (TB) has been the leading cause of death by an infective agent worldwide. Tuberculosis is caused by Mycobacterium tuberculosis (Mtb), which infects human lungs following respiratory transmission. Epidemiological studies have estimated up to a tenfold higher risk of active TB infection among those with vitamin A deficiency (VAD), but the mechanism of this increased risk is currently undefined. All trans retinoic acid (ATRA) is the active metabolite of vitamin A and has previously been found to improve the ability of human macrophages to kill Mtb when given exogenously, by affecting macrophage cytokine production, autophagy and cholesterol pathways. However clinical trials of vitamin A, in the form of retinol, as an adjunct to TB therapy have failed to show any measurable clinical benefit. We show that Mtb infected human macrophages likely have a reduced capacity to oxidise retinol to ATRA, and a reduced capacity to deliver ATRA to its nuclear receptors due to the reduced expression of key enzymes and carrier proteins. This may partly explain why clinical trials of retinol have failed and suggests that ATRA itself is worthy of investigation in clinical trials. We also show that blocking the signalling of endogenously produced ATRA within human macrophages impairs early metabolic responses to Mtb and impairs the ability of macrophages to kill Mtb. This may partly explain why those with VAD are at increased risk of TB. Nontuberculous mycobacteria (NTM) also cause respiratory infections, typically in patients with underlying structural lung disease or immunodeficiencies. NTM infections have increasing incidence worldwide and have worse treatment outcomes than TB due in large part to frequent drug resistance and relapse. It is unknown if ATRA, which is an effective host-directed therapy (HDT) for Mtb in vitro and in vivo in animal models of TB, might also improve host responses to NTMs, such as the slow grower Mycobacterium chimaera and the fast grower Mycobacterium abscessus. We found that ATRA is an effective HDT for M. chimaera in vitro, but not for M. abscessus. ATRA altered macrophage metabolic responses to NTMs and altered macrophage cytokine responses to M. chimaera. ATRA treated macrophages secreted less anti-inflammatory IL-10 when infected with M. chimaera which may support phagolysosomal maturation. ATRA may hold promise as an adjunctive HDT for slow growing NTM infections. In more recent years, TB has been overtaken as the leading infectious killer by COVID-19. SARS-CoV-2 also infects human lungs via respiratory transmission. Older adults, in particular those over the age of 75, are at far higher risk of severe infections and mortality due to COVID-19. Little is known about human alveolar macrophage (AM) responses to SARS-CoV-2 antigens, such as the spike protein and envelope protein. We found that human AMs do not sense SARS-CoV-2 spike protein but do sense envelope protein via the pattern recognition receptor TLR4, secreting IL-1?, IFN?, IL-12p70, IL-6 and TNF? in response. AMs from donors over the age of 75 years produced significantly more cytokines than those from younger patients following stimulation with SARS-CoV-2 envelope protein. This may partly explain why older adults are at such high risk of severe lung inflammation in COVID-19. In this thesis we demonstrate how studying the early immune responses of human macrophages to a diverse set of emerging and enduring respiratory pathogens ? Mtb, NTMs and SARS-CoV-2 ? can reveal findings that are relevant to future laboratory and clinical research.