We next tested whether inhibiting secretory
autophagy would compromise intestinal defense
against oral S. Typhimurium infection. Inhibiting secretory autophagy by TUDCA treatment of
S. Typhimurium–infected mice led to increased
numbers of S. Typhimurium in the intestine, mesenteric lymph nodes (MLNs), liver, and spleen
(Fig. 3I). Lysozyme gavage of TUDCA-treated infected mice rescued the increased bacterial burden
(Fig. 3I), suggesting that the increased bacterial
numbers were not due to other effects of TUDCA.
Thus, secretory autophagy is essential for host
defense against invasive bacteria.
Activation of antibacterial autophagy in intestinal enterocytes requires epithelial cell expression
of the Toll-like receptor (TLR) signaling adaptor MyD88 (7). Secretory autophagy was inhibited
in Paneth cells of Myd88-deficient mice (Fig. 4, A
and B), producing a diffuse distribution of lysozyme similar to that seen in mice hypomorphic
for Atg16L1 (3) (Fig. 4, A and C). Secretory autophagy was still evident in infected mice with an
epithelial cell–specific deletion of Myd88 (Myd88DIEC)
(Fig. 4, A and B), which indicates that epithelial
cell Myd88 is dispensable. In contrast, infected
mice harboring a dendritic cell (DC)–specific Myd88
deletion (Myd88DDC) failed to show secretory
autophagy and exhibited a diffuse distribution
of lysozyme (Fig. 4, A to C). Thus, Paneth cell
secretory autophagy requires DC MyD88.
The requirement for DC Myd88 suggested the
involvement of a known cellular relay in which
DC TLRs capture bacterial signals and relay them
to epithelial cells via type 3 innate lymphoid cells
(ILC3) and their secretion of interleukin-22 (IL-
22) (19). Although secretory autophagy occurred
upon infection of mice lacking T cells (Rag1−/−),
it was inhibited in infected Rorc−/− mice, which
lack both T helper 17 cells and ILC3 (Fig. 4, D
and E). This suggests that ILC3 are essential for
secretory autophagy of lysozyme. Supporting the
requirement for ILC3, treatment of infected
Myd88−/− mice with recombinant IL-22 rescued
the diffused distribution of lysozyme and restored
secretory autophagy of lysozyme in Paneth cells
without affecting lysozyme transcript levels (Fig.
4, F to H, and fig. S12). These results argue that
Paneth cell secretory autophagy requires activa-
tion of the DC-ILC3 circuit, which may provide a
cell-extrinsic licensing signal that allows secretory
autophagy to be rapidly activated upon detection
of Paneth cell–intrinsic ER stress.
Our results illuminate how the intestine preserves antimicrobial function in the face of a
pathogenic bacterial infection (fig. S13) and suggest how simultaneous disruption of both ER
stress and autophagy pathways leads to severe
inflammation in mice (17). Our findings also provide potential clues about how inflammation can
arise in human inflammatory bowel disease.
Genes that govern both the ER stress response
and autophagy are frequently mutated in people
with Crohn’s disease (2, 4). Because disruption of
either pathway precludes secretory autophagy,
our results suggest how infection of intestinal
epithelial cells could trigger chronic inflammation in people with these genetic abnormalities.
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We thank B. Levine for discussions and C. L. Behrendt-Boyd for
assistance with mouse experiments. This work was supported by
the NIH (grant DK070855 to L.V.H.; grants AI118807 and AI128151
to S.E. W.), the Burroughs Wellcome Foundation (Investigators in
the Pathogenesis of Infectious Diseases Award to L.V.H.), the
Welch Foundation (grant I-1874 to L.V.H.; grant I-1858 to S.E. W.),
and the Howard Hughes Medical Institute (L.V.H.). S.B. was
supported by a Gruss-Lipper postdoctoral fellowship.
Materials and Methods
Figs. S1 to S14
25 November 2016; resubmitted 12 April 2017
Accepted 14 July 2017
Published online 27 July 2017
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