RESEARCH ARTICLE SUMMARY
◥IMMUNOLOGY
Expression of Foxp3 by T follicular helper cells
in end-stage germinal centers
Johanne T. Jacobsen†, Wei Hu†, Tiago B. R. Castro, Sigrid Solem, Alice Galante, Zeran Lin,
Samuel J. Allon, Luka Mesin, Angelina M. Bilate, Ariën Schiepers, Alex K. Shalek,
Alexander Y. Rudensky, Gabriel D. Victora
INTRODUCTION:To produce antibodies that
protect effectively against pathogens, B cells
must undergo a Darwinian process of somatic
hypermutation of their immunoglobulin genes
followed by selective proliferation of variants
with improved affinity for antigen. This process,
known as affinity maturation, can dramatically
increase the affinity and potency of antibodies
elicited by infection and vaccination. Affinity
maturation takes place in germinal centers
(GCs), structures that form within secondary
lymphoid organs in response to infection or
immunization. Because somatic mutations ac-
cumulate progressively with time, the duration
of the GC reaction is an important determi-
nant of the extent to which antibodies can
mutate and mature. Despite the importance
of this parameter, our understanding of the
factors that determine the duration of a GC and
the timing of its contraction remains limited.
RATIONALE:In addition to B cells, a minor but
critical component of the GC reaction are T
follicular helper (TFH) cells characterized by
expression of chemokine receptor CXCR5, in-
hibitory receptor PD-1, and transcription fac-
tor Bcl6. TFHcells provide mitogenic signals
essential for the proliferative expansion of
high-affinity B cells and thus play a key role
in antibody affinity maturation. The size of the
TFHcell population closely correlates with,
and likely determines, the magnitude of the
GC reaction, and interruption of the signals
provided by TFHcells to B cells leads to GC
dissolution, making TFHcells likely candidates
for regulators of GC duration. The positive
effects of TFHcells are counterbalanced by
a population of GC-resident T cells that ex-
press Foxp3, the master transcription factor
of the regulatory T cell (Treg) lineage. The best
characterized of these Foxp3 populations are
T follicular regulatory (TFR) cells, which arise
through acquisition of a TFH-like phenotype
by thymic-derived Tregsand control aspects
of the B cell response ranging from B cell
foreign and self-antigen specificity to isotype
switching. Given the well-established role
of CD4 T cells in sustaining GCs and theemerging role of Foxp3 in suppressing this
reaction, we sought to determine whether
expression of Foxp3 by GC-resident T cells
could also play a role in GC longevity.RESULTS:Imaging of the GC reactions in mice
over time showed that the density and num-
ber of Foxp3 T cells within these structures
surged acutely in the days preceding the on-
set of GC contraction. In contrast to their
early- or peak-GC counterparts, late-GC Foxp3
T cells expressed high levels of TFHcell surface
markers CXCR5 and PD-1 and engaged in
long-lived dynamic interactions with GC B cells
that resembled those observed for TFHcells,
suggesting late-GC Foxp3 T cells may differ
in ontogeny from the canonical TFRpopula-
tion abundant at earlier time points. T cell
receptor sequencing suggested that, unlike
TFRcells, late-GC Foxp3 T cells likely arise
through up-regulation of Foxp3 by TFHcells in
the days immediately preceding GC regression.
This conversion was confirmed by adoptive
transfer experiments, in which transferred
Foxp3 naïve T cells acquired Foxp3 expression
in late but not in early GCs. Transcriptionally,
late-GC Foxp3 T cells closely resembled TFH
cells while showing shifts in expression of Treg-
related signatures that included loss of key
T cell help-related genes. A gain-of-function
experiment showed that ectopic Foxp3 ex-
pression by TFHcells, although incapable of
completely shifting these cells toward a TFR
phenotype, was sufficient to change expres-
sion of Treg-related gene signatures and trigger
GC contraction.CONCLUSION:Our data indicate that, in ad-
dition to canonical TFRcells, there exists a
second population of Foxp3 GC T cells that
arises immediately before GC contraction,
through the up-regulation of Foxp3 and limited
acquisition of Treg-likefeaturesbyTFHcells.
Functional experiments support a model in
which the contraction, and eventual shutdown,
of late-stage GCs is promoted by acquisition
of Foxp3 by this TFHcell population. These
findings raise the possibility that GC shut-
down is an active process rather than simply
a result of the progressive consumption of
antigen by GC B cells. Manipulating this
process may provide an avenue toward
extending GC lifetime, potentially contribut-
ing to the induction of highly mutated anti-
bodies by vaccination.
▪RESEARCHSCIENCEsciencemag.org 16 JULY 2021¥VOL 373 ISSUE 6552 297
The list of author affiliations is available in the full article online.
*Corresponding author. Email: [email protected]
(J.T.J.); [email protected] (G.D.V.)
These authors contributed equally to this work.
Cite this article as J. T. Jacobsenet al.,Science 373 ,
eabe5146 (2021). DOI: 10.1126/science.abe5146READ THE FULL ARTICLE AT
https://doi.org/10.1126/science.abe5146Germinal center B
TFH
TFR
Foxp3+ TFHEnd-stage
germinal
centerContracted
germinal centerFoxp3+ TFHEarly
germinal
centerA surge in Foxp3+T cells precedes GC contraction.GCs are usually temporary structures that emerge
upon infection or immunization, contracting and eventually disappearing upon cessation of antigenic
stimulation. At their peak, GCs contain two major populations of T cells, TFHcells and TFRcells. Of these,
only the latter population expresses the Treg-associated transcription factor Foxp3. We find that the days
immediately preceding GC contraction are characterized by a surge in the numbers of Foxp3-expressing
T cells, arising at least in part from up-regulation of Foxp3 by the TFHpopulation. Gain-of-function
experiments suggest that Foxp3 expression by TFHcells may aid in the process of GC shutdown.