By
From Schering-Plough, Laboratory for Immunological Research, 69571 Dardilly, France
Isolation of large numbers of surface IgD+CD38 naive and surface IgD
CD38
memory B
cells allowed us to study the intrinsic differences between these two populations. Upon in vitro
culture with IL-2 and IL-10, human CD40-activated memory B cells undergo terminal differentiation into plasma cells more readily than do naive B cells, as they give rise to five- to eightfold more plasma cells and three- to fourfold more secreted immunoglobulins. By contrast, naive B cells give rise to a larger number of nondifferentiated B blasts. Saturating concentrations
of CD40 ligand, which fully inhibit naive B cell differentiation, only partially affect that of
memory B cells. The propensity of memory B cells to undergo terminal plasma cell differentiation may explain the extensive extra follicular plasma cell reaction and the limited germinal center reaction observed in vivo after secondary immunizations, which contrast with primary
responses in carrier-primed animals. This unique feature of memory B cells may confer two
important capacities to the immune system: (a) the rapid generation of a large number of effector cells to efficiently eliminate the pathogens; and (b) the prevention of the overexpansion and
chronic accumulation of one particular memory B cell clone that would freeze the available peripheral repertoire.
Memory B lymphocytes are mainly generated in the
germinal centers (GCs)1 of secondary lymphoid organs (1). Within these structures, proliferating B blasts
can increase the affinity of their surface Igs through somatic
mutation of their Ig variable region genes and positive selection of high affinity mutants (9). Isotype switch of
the Igs can also occur during GC reaction (17). After leaving the GCs, memory B cells either join the recirculating pool of lymphocytes, or home to antigen draining sites
such as the marginal zone of the spleen (20). Memory B
cells display several intrinsic differences with naive B cells:
(a) lower threshold for activation, (b) ability to directly
present antigen to helper T cells, and (c) longer life span
(21). Although these features of memory B cells are essential for the immune system to make a robust secondary
antibody response, they may lead to an overexpansion of a
particular memory B cell clone, which would overload the immune system during chronic infections or antigenic
stimulations (26). Several hypotheses can be put forward to
explain how the immune system controls the size of memory cell clones: (a) decreasing potential hypothesis: memory
(T and B) cells generated after each round of stimulation
acquire a decreased potential to generate new memory cells
and an increased potential to undergo terminal differentiation into effector cells (26, 27), (b) growth factor/costimulatory signal starvation hypothesis: at the late stage of immune responses, the clonally expanded memory blasts (T
and B) undergo apoptosis in the absence of growth factors/
costimulating molecules (26, 27), and (c) hypothesis of Fas
ligand-mediated apoptosis: T cells can undergo autocrine
Fas ligand-mediated apoptosis (28), and B cells can be
sensitized to Fas ligand-mediated apoptosis by CD40 triggering (31).
Recently, large amounts of human memory B cells have
been purified from human tonsils and blood, based on their
IgD Antibodies and Reagents.
The mouse mAbs used for the phenotypic studies were FITC-conjugated anti-CD20 (IOB20; Immunotech, Marseille, France) and PE-conjugated anti-CD38
(Leu17; Becton Dickinson Monoclonal Center, Mountain View,
CA). Antibodies used for cell purification and cell culture were anti-CD4 (Q4120) and biotinylated anti-IgD (HJ9) purchased from
Sigma Chemical Co. (St. Louis, MO), anti-CD38 (T16), anti-IgCD38
or IgM+IgD
or IgA1+ phenotypes (32).
These cells contained somatically mutated IgV genes, an
indication of their germinal center origin. The isolation of
these memory B cells allowed us to directly test the decreasing potential hypothesis by culturing memory and naive B cells in vitro. Herein, we describe a novel important
feature of memory B cells: their bias towards terminal
plasma cell differentiation.
(6E1), and anti-Ig
(C4) purchased from Immunotech, and anti-CD2, -CD3, -CD8 ascites produced in our own laboratory using
the OKT hybridomas obtained from American Type Culture
Collection (Rockville, MD). Antibodies used for immunoenzymatic stainings are described in the corresponding section. Anti-CD40 ligand (LL48) -blocking mAb and CD40 ligand (CD40L)
-transfected murine fibroblasts were produced in our laboratory (31).
Purification of B Cell Populations.
Naive and memory B cells
were purified from human tonsils obtained from children undergoing routine tonsillectomy, as previously described (33). In brief,
tonsils were finely minced in RPMI 1640 (GIBCO BRL, Paisley,
UK). Cell suspension was washed twice and T cells were depleted
by sheep RBC rosetting and centrifugation at room temperature
on ditrizoate-ficoll (density = 1,077; Eurobio, Les Ulis, France).
B cells were then incubated with biotinylated anti-human IgD
antibodies. For naive cell purification, two rounds of positive selection were performed with a magnetic activated cell sorter (MACS®;
Miltenyi Biotec, Bergisch Gladbach, Germany). For memory cell preparation, two rounds of negative magnetic beads depletion
(Streptavidin-coated Dynabeads; Dynal, Oslo, Norway) were performed. Both resulting IgD+ and IgD populations were further
depleted of T cells and CD38+ (i.e., GC) B cells by incubation
with anti-CD2, -CD3, -CD4, -CD8, and -CD38 antibodies followed by two rounds of depletion with anti-mouse IgG-coated
magnetic beads (Dynal). This procedure lead to 98-99.5% pure naive and 95-99.5% pure memory B cell populations.
Proliferation Assays. For DNA synthesis, 2.5 × 104 B cells were cultured together with 5 × 103, 75 Gy-irradiated, CD40L- transfected fibroblasts in 200 µl Iscove medium (GIBCO) complemented with 5% FCS (GIBCO) for 12 d in the presence of IL-2 and IL-10. DNA synthesis was assessed by incubation with 1 µCi of tritiated thymidine (Amersham, Les Ulis, France) during the last 8 h of culture. For cellular expansion, 1.5 × 105 B cells were cultured with 5 × 104 CD40L-transfected fibroblasts for 12 d in the presence of IL-2 and IL-10. Cells were harvested and counted in tripan blue (GIBCO) to exclude dead cells.
Two-step Cell Cultures.
1.5-2 × 107 purified naive or memory B cells were cultured for 3 d in 20 ml Iscove medium complemented with 5% FCS in the presence of IL-2, IL-10, and
CD40L-transfected fibroblasts (5:1, B cells/fibroblast). Cells were
then harvested, washed, and recultured with or without CD40L.
In another set of experiments, anti-Ig and Ig
antibodies were
used to trigger B cell receptor (BCR) at 2 µg/ml final concentration. Secondary cultures consisted of 1.5 × 105 B cells in 1 ml Iscove medium containing IL-2 and IL-10, together with 5 × 104
irradiated murine fibroblasts. Murine fibroblasts were either CD40L-transfected cells or nontransfected cells together with anti-CD40L-blocking antibody at 2 µg/ml to block the signals
given by CD40L-transfected cells that could have been harvested
from the primary cultures. All secondary cultures were done in
triplicate. After 4 d, cultures were harvested, supernatants frozen
for antibody titer assays, and cells kept for analysis.
Cell Cultures with Progressive Triggering of CD40. To assess the effect of progressive triggering of CD40 antigen on naive and memory B cells, a second two-step culture was established. Cells were grown in primary cultures as in the previous two-step culture system. After 3 d, cells were recultured under seven different conditions. As the number of CD40L molecules on transfected fibroblasts varies from one experiment to another, a fixed cell ratio, rather than a fixed number of molecules, was chosen to avoid differences in the fibroblast feeder effects. Therefore, 1.5 × 105 B cells were cultured for 4 d in 1 ml Iscove medium containing IL-2 and IL-10, together with 5 × 104 irradiated fibroblasts. One culture condition was established with CD40L-transfected cells whose CD40L molecules number has been determined. These cells are then diluted with nontransfected irradiated fibroblasts for other culture conditions at the ratios of 1/2, 1/4, 1/8, and 1/16. Two other cultures were also set using parental cells, with or without anti-CD40L antibody at 2 µg/ml. All secondary cultures were set in triplicates and designed as the number of CD40L molecules present in the culture per B cell.
Ig Secretion Assays. IgA, IgG, and IgM concentrations in culture supernatants were measured using ELISA. Total Ig levels are given as the summation of these values.
Cell Sorting.
Naive and memory cells were cultured for 3 d in
the presence of IL-2, IL-10, and CD40L-transfected fibroblasts.
They were then harvested and recultured for an additional 4 d
with IL-2, IL-10, and parental fibroblasts. After harvesting, debris
and dead cells were depleted from the cultures by centrifugation
on ditrizoate-ficoll (Eurobio). Cells were then stained with
FITC-conjugated anti-CD20 and PE-conjugated anti-CD38 antibodies. Both CD20/lowCD38high and CD20+CD38
populations were sorted using a FACStar® (Becton Dickinson Immunocytometry Systems, San Jose, CA).
Giemsa and Immunoenzymatic Stainings.
7 × 104 sorted cells
were cytocentrifuged on microscope slides. Some slides were
stained with Giemsa-Gurr solution, whereas others were kept for
immunoenzymatic staining. Human Igs were revealed by anti-
human and
light chain antibodies (A8B5 and N10/2 clones,
respectively, IgG1 isotypes; Dako), whereas IgM isotype Igs were
revealed by anti-human IgM mAb (145-8, IgG1 isotype; Becton
Dickinson Monoclonal Center). Enzymatic activity was developed with Fast Red substrate (Dako). All immunoenzymatically
colored slides were lightly counterstained with Mayer's hematoxylin solution.
Using a two-step culture system, we
previously demonstrated that continuous triggering of CD40
antigen on GC cells inhibits their terminal differentiation
into plasma cells (PC; 37). To determine the influence of
CD40L on the capacity of memory and naive B cells to generate PCs, similar culture conditions were used. Both populations were cultured for 3 d over CD40L-transfected fibroblasts in the presence of IL-2 and IL-10. Activated B
cell blasts were then recultured for 4 d with nontransfected
fibroblasts, IL-2, IL-10, and an anti-CD40L-blocking antibody to block the CD40L-transfected fibroblasts carried
over from the primary culture. Although naive B cells yielded
16.4 ± 6.6% CD20/lowCD38high plasma cells (mean ± SD, n = 7; Fig. 1 B; Table 1), memory B cells yielded
62.4 ± 11.9% plasma cells (mean ± SD, n = 4; Fig. 1 D;
Table 1). Accordingly, naive B cells yielded three times more nondifferentiated CD20+CD38low B blasts than did
memory cells. Addition of CD40L during the secondary
culture (Fig. 1, A and C) considerably inhibited the plasma
cell differentiation of B cell blasts, generated from both naive and memory cells (Table 1).
FACS®-sorted CD20/lowCD38high cells generated from
both naive and memory B blasts display the morphology of
terminally differentiated PCs (Fig. 2 A), as well as an intense Ig
and Ig
light chain staining (Fig. 2 C). In contrast, CD20+CD38low populations display the morphology
of blasts with a weak surface Ig expression (Fig. 2, B and
D). Although 50% of plasma cells generated from naive B
cells contain intracytoplasmic IgM (Fig. 2 E), only 20% of
plasma cells generated from memory B cells expressed IgM
(Fig. 2 F).
High Concentrations of CD40L Do Not Completely Block the Terminal Differentiation of Memory B Cells.
To further understand the propensity of memory B cells to undergo plasma
cell differentiation, secondary cultures of naive and memory blasts were set up in the presence of increasing density of CD40L. For that purpose, absolute numbers of CD40L
molecules per fibroblast were estimated using quantitative
flow cytometry and CD40L-transfected fibroblasts were
gradually diluted with their parental cells (see Material and
Methods). As shown in Fig. 3 A, increased CD40 ligation
of memory cells results in a decreased production of plasma
cells and a concommitant increase of B blasts (Fig. 3 B). In
fact, there is a linear correlation between the log (1/
CD40L available per memory blast) and the percentage of generated plasma cells (r2 = 0.945, 0.966, and 0.983 from
three experiments). Note that CD40L-transfected fibroblasts were indeed carried over from the primary cultures,
as the addition of anti-CD40L antibody to the cultures
with nontransfected fibroblasts further enhanced the plasma
cell generation. As shown in Fig. 3 C, in the absence of CD40L in the secondary culture, memory cells can generate up to eight times more PCs than do naive cells. Note
that very high amounts of CD40L molecules in the secondary cultures do not completely inhibit the generation of
PCs from memory cells, since up to 2 × 105 CD40L molecules/B blast led to the generation of 3.6 × 105 PCs from
an initial input of 1.5 × 105 blasts (Fig. 3 C).
Increasing the number of CD40L molecules available in
the cultures not only inhibited the plasma cell generation,
but also the secretion of Igs (Fig. 4 A). Furthermore, in all
culture conditions, memory B cells produced more total Igs
than naive B cells (Fig. 4 A). With regard to secreted isotype, although naive and memory B cells produced comparable levels of IgM (Fig. 4 D), memory cells, as expected,
produced considerably more IgG and IgA (Fig. 4, B and C).
Naive and Memory B Cells Proliferate Equally Well. We then
questioned whether the poor differentiation capacity of naive B cells, as compared to that of memory B cells, may indeed reflect a reduced activation and proliferation capacity.
Thus, purified naive and memory B cells were cultured
over CD40L-transfected fibroblasts with IL-2 and IL-10,
and proliferation was assessed by measuring thymidine incorporation, as well as viable cell numbers. As shown in
Fig. 5, naive B cells proliferate at least as much as memory
B cells do.
Memory, but Not Naive, B Cells Undergo Rapid PC Differentiation in Cultures even after Anti-BCR Triggering.
Anti-Igs were shown to prevent B cell differentiation (38). Since
naive B cells, but not memory B cells, were isolated by positive selection using anti-IgD, we questioned whether
the difference in the differentiation capacity between naive
and memory B cells could be due to the BCR triggering.
Accordingly, in the first 3 d of primary cultures, 2 µg/ml of
anti-Ig and 2 µg/ml of anti-Ig
antibodies were added
into the cultures in the presence of CD40L, IL-2, and IL-10.
At the end of the culture, cells were washed and seeded in
a 4 d secondary culture with IL-2, IL-10, and different concentrations of CD40L. Fig. 6 shows that in the presence
of three different CD40L concentrations (9.6 × 104/cell,
4.8 × 104/cell, no CD40L), 3, 6, and 13% of CD38+
CD20
plasma cells were generated from the naive B cells.
In the same culture conditions, 21, 29, and 43% of CD38+
CD20
plasma cells were generated from the memory B
cells. This experiment indicates that memory B cells, but
not naive B cells, preferentially undergo plasma cell differentiation even after BCR triggering.
This paper describes the striking differentiation ability of
memory versus that of naive B cells. This correlates with
previous histophysiological observations in vivo showing
that secondary antigenic challenge in carrier-primed rats
leads to a massive extrafollicular PC reaction and a poor
follicular GC reaction in the spleen. In contrast, only small
extrafollicular PC reactions, but large GC reactions develop
upon primary immunization (Fig. 7; 39, 40). Likewise, in
mice infected with reoviruses, adoptively transferred memory B cells give rise to a large extrafollicular PC reaction,
but a small GC reaction; in contrast, transferred naive cells
generate a large GC reaction (41). Thus, the differences in
the capacity of memory versus naive B cells to differentiate is an intrinsic property of the B cells, rather than of the microenvironments. The propensity of memory B cells to undergo rapid differentiation into effector cells may confer
two important properties to the immune system. First, it allows the rapid generation of large numbers of effector cells,
whose products (antibodies) efficiently eliminate pathogens. This novel feature of memory B cells, together with
their low threshold for activation, and their ability to home
to the antigen draining sites and to directly present antigen to T cells, may all contribute to the velocity of secondary
antibody responses. Second, it prevents the overexpansion
and accumulation of a particular memory B cell clone that
would otherwise overload the immune system and freeze
the available Ig repertoire (26). Since PCs have a relatively
short lifespan and do not proliferate in response to further
stimulations (42), the majority of memory B cells will
undergo clonal exhaustion by differentiating into effector
cells during secondary immune responses. Interestingly, T
memory cells show a similar tendency not to expand and
overload the whole immune system, as towards the end of
a primary immune response specific T blasts are rapidly
eliminated (45).
The finding that CD40L inhibits the differentiation of both activated memory and naive B cells, complements the previous observations made with GC B cells (37) or total B cells isolated from blood and tonsils (49, 50). Thus, CD40L represents a differentiation suppressor during not only the GC, but also the extrafollicular reactions (51). Indeed, CD40L-expressing T cells have been reported by immunohistology, both within the GCs and the extrafollicular T zones (52).
Although CD40L inhibits the PC differentiation of naive, GC, and memory B cells, a fraction of the memory cell subset seems to be resistant to this effect. Differential effects of CD40L on mature B cell subsets have already been noticed. For instance, CD40 triggering is an important survival but a minor proliferative signal for GC cells (55), whereas it provides a strong and long-term proliferative signal to resting naive and memory B cells (58). The molecular mechanisms underlying the propensity of memory B cells to undergo terminal differentiation are still unknown. CD40 triggering on human GCs and resting mature B cells results in the activation of different protein kinases (62, 63). Further comparative studies of CD40 signaling pathways in naive, GC, and memory B cells should now be carried on to explain how mature B cells change their responses to CD40 triggering at different stages of their immunopoiesis.
Address correspondence to Dr. Yong-Jun Liu, Schering-Plough, 27 chemin des Peupliers, BP 11, 69571 Dardilly Cedex, France. Phone: 33-4-72-17-27-00; FAX: 33-4-78-35-47-50; E-mail: lir{at}schering-plough.fr
Received for publication 30 December 1996 and in revised form 3 July 1997.
C. Arpin is the recipient of a grant from Fondation Mérieux.The authors wish to thank S. Bonnet-Arnaud and M. Vatan for superb editorial assistance; I. Durand for flow cytometry; Dr. J. Chiller for supporting this work; and Drs. F. Brière, I. Fugier-Vivier, and C. Mueller for critical reading of the manuscript.
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