RESEARCH ARTICLE SUMMARY
◥IMMUNOLOGY
CD97 promotes spleen dendritic cell homeostasis
through the mechanosensing of red blood cells
Dan Liu, Lihui Duan, Lauren B. Rodda, Erick Lu, Ying Xu, Jinping An, Longhui Qiu, Fengchun Liu,
Mark R. Looney, Zhiyong Yang, Christopher D. C. Allen, Zhongmei Li, Alexander Marson, Jason G. Cyster*
INTRODUCTION:Antigen capture and presen-
tation by conventional dendritic cells (cDCs) is
crucial for the initiation of adaptive immune
responses. These responses often take place
inside secondary lymphoid organs. The spleen
is the largest secondary lymphoid organ, has
an open blood circulation, and fosters T cell
and antibody responses against blood-borne
pathogens. A major population of cDCs within
the spleen, called cDC2s, is situated in a blood-
exposed location at the interface of the white
pulpandtheredpulp,knownasthebridging
channel. These cDC2s are very efficient at cap-
turing antigens from the blood and presenting
them to T cells, but the factors that enable
cDC2s to sense their location with respect to
blood flow are not well understood.RATIONALE:G protein–coupled receptors (GPCRs)
that signal through Ga 13 -containing hetero-
trimeric G proteins can cause chemorepulsion
and thereby help to confine cells in tissue
niches. Whether these G proteins and associated
GPCRs are involved in cDC positioning and
function has been unclear.RESULTS:Ga 13 and the downstream effector
ArhGEF1 were required for cDC2 positioning in
blood-exposed regions of the mouse spleen.
Using in vivo CRISPR-based screening, weidentified adhesion GPCR family member E5
(Adgre5, or CD97) as a Ga 13 -coupled receptor
needed for splenic cDC2 positioning. In the
absence of Ga 13 , ArhGEF1, or CD97, there was
a deficiency of splenic cDC2s but not cDC1s.
Intravital two-photon microscopy and spleen
transplant experiments showed that this defi-
ciency reflected a loss of splenic cDC2s into
the blood circulation. As a member of the ad-
hesion GPCR family, CD97 has a large extra-
cellular domain and is expressed as a noncovalent
heterodimer of the extracellular N-terminal
fragment (NTF) and the GPCR domain. The
CD97 NTF binds CD55 on other cells. CD97
engagement by CD55 on red blood cells (RBCs)
under conditions of shear stress led to the
removal of the CD97 NTF and the activation
of the CD97 GPCR domain. A mutant form of
CD97 that could not undergo autoproteolytic
cleavage or NTF extraction was not able to
restore the cDC2 compartment of CD97 null
mice. Deficiency in the CD55-CD97-Ga 13 pathway
was associated with a reduced ability to mount
T follicular helper cell and antibody responses
to modified RBCs and to blood-borne bacteria.
CD97-deficient splenic cDC2s had increased
F-actin content and markedly altered gene ex-
pression. Gene set enrichment analysis sug-
gested that part of the altered gene expression
profile was due to increased activity of the
G-actin–repressed Mrtf transcription factors.
The transcription factor IRF4 is important for
cDC2 homeostasis, and CD97 expression in
cDC2s was promoted by IRF4. Overexpression
of CD97 could partially restore splenic cDC2s
in IRF4-deficient mice.CONCLUSION:Homeostasis of cDC2s within
the spleen depends on CD97 mechanosensing
of circulating CD55+RBCs. This pathway en-
ables cDC2s to sense exposure to blood flow
and thereby avoid loss into the blood circula-
tion. We speculate that by leading to activation
of Rho, CD97 signaling causes the cell to retract
membrane processes from flow-exposed loca-
tions. Rho signaling may also enhance integrin-
mediated adhesion of cDC2s within the spleen.
CD97 signaling allows cDC2s to position at the
tissue-blood interface, a location that optimizes
antigen capture and enables very rapid immune
responses to systemic pathogens. CD97 is widely
expressed in the immune system and other tis-
sues, including some tumors and inflamed tis-
sues, and may allow additional cell types to sense
their location on the basis of interactions with
CD55+cells under conditions of fluid flow.▪RESEARCHSCIENCEscience.org 11 FEBRUARY 2022•VOL 375 ISSUE 6581 629
The list of author affiliations is available in the full article online.
*Corresponding author. Email: [email protected]
Cite this article as D. Liuet al.,Science 375 , eabi5965
(2022). DOI: 10.1126/science.abi5965READ THE FULL ARTICLE AT
https://doi.org/10.1126/science.abi5965SinusRed pulpWhite pulp LymphocytecDC2RBCFlow FlowCD97CD55Gα 13
ArhGEF1
cDC2 Retention/HomeostasisRhoNTF97
GPCRGPCRStachelGPSSpleenArteryVeinSpleen cross sectionBCRPFF
FFTcDC2
B cells
i.v. CD11cDiagram of bridging channel (BC)
RBCCD97 mechanosensing of CD55+RBCs enables splenic cDC2s to position near blood flow.The spleen
is a highly vascularized organ. In a cross-sectional view, B cell follicles (F) and T cell zones (T) highlight white-
pulp cords that are separated from the red pulp (RP) by cDC2-containing bridging channels (BC; dashed
box). Intravenously administered antibody to CD11c labels blood-exposed cDC (labeled cDC2 appear white).
Splenic cDC2s in bridging channels are near open-ended terminal arterioles and are exposed to RBCs as
they pass into the red pulp. cDC2s express CD97, and the interaction with CD55 on RBCs exerts a pulling
force on CD97, causing extraction of the NTF and activation of the GPCR domain. This pathway promotes the
retention of blood-exposed cDC2s in the spleen. i.v., intravenous; GPS, GPCR proteolysis site.