Science - USA (2019-02-15)

result was further confirmed by a transcriptome-
wide analysis, where the Spearman correlations
between B2→B1 and B1 control samples were as
high as those between biological replicates and
significantly higher than those between B2→B1
and B2 control samples, closely clustering B1 and
B2→B1 samples together (Fig. 4B). Thus, the
B2→B1 conversion entails profound genetic alter-
ations, leading to a B1-typical transcriptome profile
in B2→B1 cells.
Taken together, mature B2 cells of the various
subsets exhibit plasticity toward B1 cell differen-
tiation. The cells undergo B1 differentiation upon
substitution of their BCR by an autoreactive BCR
that is known to drive B1 cell development in
early postnatal life. In both instances, differenti-
ation is initially linked toextensive cell prolifer-
ation, likely reflecting a special mode of B cell
activation by autoantigen ( 4 , 20 ). The present
experimental system offers a unique opportunity
to study this process, which is key to the un-
derstanding of BCR repertoire selection in the B1
compartment. In contrast, only low numbers of
B1 cells switching to the expression of the B2-
typical BCR were recovered from the animals,
with a surface phenotype intermediate between
B1 and B2. This is likely a consequence of the fact
that B1 cells represent B cells that have under-
gone antigen-driven activation, proliferation, and
differentiation. The profound genetic and mor-
phological changes that the cells undergo during
these processes may well be irreversible.
Jerne’s notion that autoreactive receptors ini-
tially drive lymphocyte development, with the
subsequent selection of escape somatic mutants,
was accommodated in the classical model of
B cell development through ordered V(D)J gene
rearrangements, in first the IgH and then the IgL

loci of progenitor B cells ( 1 , 31 , 32 ). In this model,

IgH chain–containing pre-BCRs constitutively

drive clonal expansion sustained by the surrogate

light chain, followed by sequential IgL chain gene

rearrangements and IgL chain editing. Although

these ordered gene rearrangements clearly dom-

inate postnatal B cell development, recent evi-

dence indicates that during fetal development,

very much in agreement with Jerne, IgH and IgL

chains are often initially coexpressed ( 33 ). In this

situation, autoreactive BCRs instead of pre-BCRs

drive the expansion and selection of B cells,

which at that developmental stage predominantly

develop into the B1 variety ( 11 , 34 ).

Although the control of these different path-

ways of differentiation remains to be fully eluci-

dated, the present results establish that mature

B2 cells are not irreversibly committed to a B2

“lineage”but maintain the potential to develop

into B1 cells. That these cells do so upon engraft-

ment of an autoreactive, B1 cell–typical BCR

demonstrates the instructive role of BCR speci-

ficity and mode of signaling in driving B1 cell

differentiation, in the absence of B1-lineage

precommitment.

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ACKNOWLEDGMENTS

We thank S. Clarke for reagents and the Vk4mice;J.diSanto

for providing the Rag2−/−Cg−/−mice; J. Wang, K. Jenssen,

V. Blinder, E. Derudder, T. Chakraborty, J. Xia, D. Ghitza,

C.Aristoff,A.Monti,A.Tetreault,J.Grundy,andA.Pellerin

for technical help in Boston; K. Petsch, C. Grosse, J. Pempe,

S. Mackowiak, and the MDC animal caretakers for their help;

and H. P. Rahn for excellent FACS-related support. We apologize

Grafet al.,Science 363 , 748–753 (2019) 15 February 2019 5of6

A

Neurl3

Dennd3Sox4

Csrp2

Emid1

Lrrk2

1300014i06rik

Gdf11

IcoslFcer2a

Fam101b

Zfp608

Ccr6

Cpm

Pstpip2

Myo1dPlscr1

Cd300lf

Gpr55

Csf2rb

Folr4

AhrTbc1d9

Bhlhe41

Zbtb32

Cyp11a1

Fam160a1

Lysmd2

−2 −1 0 1 2

Row Z−Score

02040

Color Key

and Histogram

Count

0.8 0.9 1

Correlation

024

6

8

Color Key

and Histogram

Count

B

B1-up signature

B2-up signature

B1 Ctrl B2→B1 B2 Ctrl B2 Ctrl B2→B1 B1 Ctrl

B1 Ctrl

B2

→

B1

B2 Ctrl

Fig. 4. B2→B1 cells acquire the transcriptome profile of B1 cells.
(A) Heat map with sample clustering of gene expression levels of 14 genes
specifically expressed in B1 cells (red) and 14 genes specifically expressed
in B2 cells (blue) previously defined as B1 signature ( 30 ). RNA was isolated
from splenic control B2 cells from B2 mice (B2 Ctrl), peritoneal 5C5+cells

from B1 mice (B1 Ctrl), and B2→B1 cells isolated from the peritoneal

cavity of recipients 30 days after transfer (B2→B1). (B) Heat map

of Spearman correlations between each sample, based on the expression

levels of 7629 expressed genes. Each B2→B1 sample is based on one

donor animal and was pooled from three or four recipients.

RESEARCH | REPORT

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