Tryptophan and derived metabolites availability are key modulators ofthe host-pathogen crosstalk during Haemophilus influenzaelung infection

Tryptophan modulates lung homeostasis and host immunity through indoleamine 2,3-dioxygenase (IDO) and aryl hydrocarbon receptor (AhR) signaling. IDO-mediated limitation of tryptophan availability within the lungs can be overcome by pathogens able to synthesize their own tryptophan, and a decline in IDO and AhR function is observed in patients suffering chronic obstructive pulmonary disease (COPD). These observations led us to investigate the effect of tryptophan availability on the host-pathogen interplay during lung infection by Haemophilus influenzae, a pathobiont frequently associated to COPD exacerbations. In this work, the use of chemically defined bacterial culture conditions, metabolite determination and genome-wide gene expression profiling, showed that exogenous tryptophan lowers this amino acid biosynthetic machinery gene expression but increases its intracellular pool, maybe related to over-expression of the tnaAB genes, which also leads to indole excretion. In turn, the biochemical connection between serine into tryptophan by TrpB conversion of indole and serine to tryptophan, is expanded at the regulatory level as the sdaCA 3'UTR-overlapping mRNA exerts an antisense negative regulation on the convergent mtr gene expression. Although tryptophan uptake and biosynthesis relate to each other at the gene expression level, tryptophan biosynthesis is essential for H. influenzae in vitro growth, as shown by inactivating genes encoding enzymes at different stages of the biosynthetic pathway, and supported by the growth inhibitory effect of the TrpE allosteric inhibitor indole propionic acid, a tryptophan derivative produced by the gut microbiota. Indeed, tryptophan biosynthesis is needed for H. influenzae in vivo survival, in such a way that the pathogen overcomes tryptophan limitation within the host, and also modulates IDO and AhR signalling towards pro-inflammatory responses likely contributing to COPD vicious circle progression.