CD14 collaborates with TRIF to mediate septic responses downstream of TNF receptors

Project Summary
CD14 is a coreceptor that facilitates the innate immune responses triggered by a broad range of pathogen-
derived molecules and host damage-derived danger signals. These properties, which contribute to the rapid
mobilization of the immune system, can also contribute to the exacerbation of inflammatory immune
responses, particularly when tissue damage is extensive. Because of this, CD14 is an attractive target for the
modulation of the 'cytokine storms' associated with severe bacterial sepsis, cancer immunotherapies, and the
acute respiratory distress syndrome (ARDS) that is seen in severe cases of COVID-19. Tumor necrosis factor
(TNFα) is one of the most prominent inflammatory cytokines associated with these storms, and TNFα-blocking
biologicals have proven useful in treating these inflammatory syndromes. However, we currently lack therapies
for the persistent immune paralysis associated with sepsis and ARDS. Here, we build on recent findings from
our collaborators, who have extended the in vitro coreceptor-like functions of CD14 to the death domain-
containing receptors FAS and TNFR1. This is clinically significant, because it suggests that CD14 participates
in the induction of inflammatory cytokines and in the effector responses set in motion by those very cytokines.
Further studies established that CD14 and TRIF are required for the induction of lethal sepsis by TNFα. In
conjunction with in vitro biochemical data, these findings indicate that CD14 modulates the signals initiated by
TNFR1 in precisely the same way that it augments LPS-induced signals: by engaging endosomal signaling
pathways that act via TRAM, TRIF, TBK1, and IRF3 to promote type-I interferon production and inflammatory
cell death. Thus, CD14 may contribute to septic immunoparalysis by inducing the death of immune cells
exposed to FasL or TNFα. For this reason, it is crucial to understand how CD14 interacts with these receptors.
However, these receptors do not share ligands, and CD14, which is a GPI-anchored protein, lacks the
transmembrane and cytoplasmic regions typically associated with signaling effectors. Via an exhaustive
analysis of the evolutionary histories of CD14, TNFα, and TNFR1, we identified features that appear to have
coevolved in placental mammals and that could plausibly link CD14 to TNFα/TNFR1 complexes. In order to
clarify how CD14 contributes to septic phenomena in vivo, this proposal will evaluate the hypothesized link
between CD14 and TNFα/TNFR1 complexes (Aim 1), and will determine whether CD14 promotes sepsis via
cell-intrinsic signaling events occurring in myeloid cells and/or stromal tissues (Aim 2). These efforts are
intended to rapidly narrow the scope of future enquiry by identifying viable mechanisms with the greatest
explanatory power. In so doing, this work will set the stage for examinations of these pathways in vivo.

Grant Number: 5R21AI176055-02
NIH Institute/Center: NIH
Principal Investigator: STEPHEN BUNNELL