The Role of Tryptophan Metabolism in Pulmonary Fibrosis

Project Summary / Abstract
Idiopathic Pulmonary Fibrosis (IPF) is a progressive fibrotic disease of the lungs that is more common in aging
and men, with 1 in 200 adults over age of 70 diagnosed. Patients have an estimated 2-5 years mean survival
time following diagnosis, and only lung transplant is considered curative. Understanding the causes and
immunological environment of the fibrotic lung could provide new targets for therapeutic or prophylactic
treatment. Tryptophan metabolism is associated with increased inflammation in the lungs of IPF patients, as well
as in our bleomycin-induced murine lung fibrosis model, producing several metabolites, such as kynurenine (kyn)
that are recognized by the Aryl-hydrocarbon receptor (AHR). AHR has been well-characterized as affecting the
immune system through several mechanisms both pro- and anti-inflammatory. Preliminary data from our lab
show that inhibiting one of the early enzymes in the kynurenine pathway (KP) of tryptophan metabolism, TDO,
results in significant protection from fibrosis in our murine bleomycin (blm) model. Data from human IPF patients
support that TDO is uniquely expressed in alveolar fibroblasts of IPF patients, but not healthy controls.
Furthermore, AHR is mostly highly expressed in dendritic cells (DCs) which play a critical role in fibrosis. We
hypothesize that KP metabolites from fibroblasts are activating AHR in DCs to stimulate pro-inflammatory
signals which in turn drive pulmonary fibrosis. The long term goal of this proposal is to understand how
tryptophan metabolism impacts lung fibrosis in order to identify novel therapies for IPF. The objectives of this
project are to determine how TDO inhibition affects downstream metabolites, and whether those metabolites
are signaling to relevant immune cells which are important for fibrosis, specifically DCs, in a pro-inflammatory
way. The specific aims of my proposal are: 1) to determine the in vivo effects of TDO inhibition on the kyn
pathway and dendritic cells, and 2) to evaluate the mechanism of fibroblast-dendritic cell cross talk through ex
vivo co-culture and conditioned media experiments. Under the first aim, I will perform metabolomics on healthy
mice and on mice with blm-induced fibrosis with or without treatment with a TDO inhibitor in order to understand
how fibrosis alters the metabolome of the lung and to establish the effect of TDO inhibition on downstream
immunomodulatory metabolites. I will also perform RNAseq on primary DCs from the same conditions to
determine how their maturation and pro-inflammatory pathways are altered, as well as evaluate their functional
capacity to polarize naïve T-cells. Under the second aim, I will test primary fibroblasts, DCs, and T-cells of both
our murine blm model and human IPF patients to determine the specific mechanism of anti-fibrotic activity by
both directly treating cells with TDO-inhibitors and Tryptophan metabolites of interest, as well as using
conditioned media experiments to evaluate signaling.

Grant Number: 1F30HL172413-01A1
NIH Institute/Center: NIH
Principal Investigator: Hannah Carter