Download B cells take their time: sequential IgG class switching over

Immunology and Cell Biology (2014) 92, 645–646
& 2014 Australasian Society for Immunology Inc. All rights reserved 0818-9641/14
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NEWS AND COMMENTARY
Sequential IgG class switching
B cells take their time: sequential IgG class switching
over the course of an immune response?
Menno C van Zelm
Immunology and Cell Biology (2014) 92, 645–646; doi:10.1038/icb.2014.48; published online 17 June 2014
B
cells are unique in their capability to
modify their immunoglobulin (Ig) molecules during an immune response. Affinity
maturation is established by somatic hypermutations (SHMs) and selection for antigen
binding. Furthermore, through class switch
recombination (CSR), the Ig constant regions
can be changed from IgM into IgD, IgE, one
of the two IgA, or one of the four IgG
isotypes.
In this issue of Immunology and Cell
Biology, Jackson et al.1 show a relationship between the extent of affinity
maturation and Ig subclass usage through
large-scale sequence analysis of IgA and IgG
transcripts. SHM levels in variable regions
were increased in transcripts switched to
more downstream-located IgG constant
regions, that is, IgG3oIgG1oIgG2oIgG4
(Figure 1a). A similar pattern was seen for
the selection for replacement mutations,
except for IgG4. These results confirmed
earlier findings and the authors’ temporal
model, which states that there is a programmed order of IgG subclass usage during
an immune response. Specifically, the laterformed IgG2 and IgG4 might function to
restrict inflammation due to their inability to
fix complement (Figure 1b).2
The authors’ model for sequential IgG
switching is further supported by the presence of IgG3 and IgG1 switch region
remnants in IgG1 þ and IgG2 þ B cells,
respectively.3 Interestingly, the authors did
not find differences in mutation loads
between IgA1 and IgA2.1 A possible
explanation lies in the fact that IgA can be
produced in both T-cell-dependent and
T-cell-independent manners. Especially local
MC van Zelm is at the Department of Immunology,
Erasmus MC, University Medical Center, Rotterdam,
The Netherlands
E-mail: [email protected]
intestinal responses can generate IgA2
without an intermediate switch via IgG1 or
IgA1, excluding the possibility of a temporal
mechanism.3,4 Potentially, the same is true
for IgE þ B cells, which can be derived in
T-cell-dependent responses via IgG1 or
directly from IgM in T-cell-independent
responses.5 In contrast, IgG responses are
completely T-cell dependent, because
individuals with inherited mutations in
CD40L that lack germinal centers show a
complete absence of IgG.6 This makes IgG a
better model to study the relation between
SHM levels and sequential CSR.
SHM levels are directly related to cell
divisions in a germinal center,7 and can
therefore be used as a measure for the
extent
of
an
antibody
response.
Unexpectedly, urban dwellers did not carry
fewer SHMs in their IgA and IgG transcripts
than villagers from Papua New Guinea, who
live in areas with a high burden of infectious
disease.1 Apparently, the repeated antigen
exposure did not lead to higher mutation
loads per IgA or IgG subclass. Recent studies
with Sanger sequencing of transcripts
generated with IgA and IgG consensus
primers showed similar SHM levels between
children (5–14 years) and adults (23–35
years) per Ig subclass.6,8,9 However, adults
showed more dominant IgG2 and IgA2
subclass usage (Figure 1d). Thus, repeated
antigen exposure might not lead to higher
SHM levels per Ig subclass, but rather to
more sequential CSR. This would suggest
that sequential switching takes place not only
in the course of one antigen response, but
also in consecutive responses through the
recruitment of IgG3 þ and IgG1 þ memory B
cells (Figure 1c).3 It would therefore be
interesting to study whether the Papua New
Guinean individuals carry relatively more
IgG2/4 than urban dwellers.
Are SHM levels then independent of the
extent of antigen exposure? The observations
in urban dwellers vs villagers and in children
vs adults suggest that this is the case for
healthy individuals.1 B-cell responses can also
be driven by chronic inflammation. Indeed,
patients with Sarcoidosis, a multi-system
disorder involving abnormal collections of
inflammatory cells (granulomas), show B-cell
involvement with increased IgG2 usages and
higher SHM levels per Ig subclass.8 It is
therefore entirely possible that SHM levels
are tightly regulated, also independent of
sequential IgG, and that this mechanism is
only disrupted in disease states.
Advances in multi-color flow cytometry
have enabled the phenotyping of separate
memory B cells with distinct extents of
antibody maturation. CD27-IgG þ B cells
have lower degrees of proliferation, SHM
and IgG2 usage than CD27 þ IgG þ B cells.3
These CD27 IgG þ B cells could therefore
represent earlier products from an immune
response than CD27 þ IgG þ B cells, fitting
with the temporal model. However, the
higher SHM levels in CD27 þ IgG þ B cells
were not related to the increased IgG2 usage,
as CD27 B cells carry fewer SHMs in each
of the IgG subclasses than CD27 þ B cells.10
Thus, also in distinct antigen-experienced
B-cell subsets SHM levels and sequential
class switching seem uncoupled.
In conclusion, the study by Jackson et al.1
fits with the temporal model of human IgG
function. The temporal model is the first to
describe how IgG subclasses might act
together to provide a strong and ratelimiting immune response. Although SHM
levels and sequential IgG class switching are
not directly coupled in all types of antibody
responses, the authors provide clear
indications for a temporal effect of SHM
and sequential Ig class switching. Still, in vivo
News and Commentary
646
a
V DJ
Cμ Cδ
Cγ3
Sμ
Sγ3
Cγ1
Ψε
Sγ1
Ψγ
Cα1
Cγ2
Sα1
b
Sγ2
Cγ4
Sγ4
Cε
Sε
Cα2
Sα2
c
germinal center
germinal center
T cell
IgG3/1+
naive
B cell
T cell
naive
B cell
IgG2/4+
IgG3+
IgG1+
IgG2+
IgG4+
IgG3 IgG4
10% 1%
IgG3 IgG4
5% 5%
d
IgA2
22%
102
IgA1
78%
IgA2
46% 183
IgA1
54%
IgG2
22%
107
IgG1
68%
p < 0.01
p < 0.0001
child
IgG2
IgG1
33% 214 56%
adult
child
adult
Figure 1 Ig class switching and SHM levels in children and adults. (a) Schematic representation of the human IGH constant gene regions. (b) The
temporal model of IgG class switching, where over the course of a germinal center response sequential switching towards IgG34IgG14IgG24IgG4
occurs with increasing levels of SHMs. (c) Model for sequential IgG switching in secondary germinal center responses, where primary IgG memory B cells
re-enter a germinal center in a secondary response, undergo additional SHMs and switch to a more downstream IgG subclass.3 (d) IgA and IgG subclass
distributions in transcripts derived with consensus primers in previous studies.5,6,8 Ig subclass distributions were compared between children and adults
using the w2 test.
these effects will be blurred by secondary
antigen encounters (Figure 1c), the diversity
of antigens, anatomical locations of immune
responses and the formation of diverse antigen-experienced B-cell subsets in humans.
Furthermore, the diversity of antigens, anatomical locations of immune responses and
the diversity of antigen-experienced human
B-cell subsets can blur the effects of temporal
CSR. To overcome these limitations, it would
be important to design an analysis strategy
for antigen-specific B cells. These could be
isolated at several time points following
primary and secondary vaccinations of
healthy volunteers with a T-cell-dependent
antigen (for example, rabies vaccine). Such
analyses would provide new insights into the
temporal and sequential nature of antibody
responses and could advance our understanding of how humoral immunity is regulated by memory B cells, affinity maturation
and Ig effector functions.
Immunology and Cell Biology
ACKNOWLEDGEMENTS
I would like to thank M van der Burg, ED de Geus,
C Grosserichter-Wagener, JJ Heeringa and BG de
Jong for critical reading of the manuscript. This
work was supported by the Sophia Children’s
Hospital Fund (grant S698), and fellowships from
the Erasmus University Rotterdam (EUR) and the
Erasmus MC.
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