The Journal of Immunology, 2000, 164: 2924-2930.
Copyright © 2000 by The American Association of Immunologists
Identification of a Precursor to Phosphatidyl Choline-Specific B-1 Cells Suggesting That B-1 Cells Differentiate from Splenic Conventional B Cells In Vivo: Cyclosporin A Blocks Differentiation to B-11
Larry W. Arnold,
Suzanne K. McCray,
Calin Tatu2 and
Stephen H. Clarke3
Department of Microbiology and Immunology, University of North Carolina, Chapel Hill, NC 27599
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Abstract
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The origin of B-1 cells is controversial. The initial paradigm
posited that B-1 and B-2 cells derive from separate lineages. More
recently it has been argued that B-1 cells derive from conventional B
cells as a result of T-independent Ag activation. To understand B-1
cell differentiation, we have generated Ig transgenic (Tg) mice using
the H and L chain genes (VH12 and V
4) of
anti-phosphatidyl choline (anti-PtC) B cells. In normal mice
anti-PtC B cells segregate to B-1. Segregation is intact in
VH12 (6-1) and VH12/V
4 (double)
Tg mice that develop large numbers of PtC-specific B cells. However, if
B-1 cell differentiation is blocked, anti-PtC B cells in these Tg
mice are B-2-like in phenotype, suggesting the existence of an
Ag-driven differentiative pathway from B-2 to B-1. In this study, we
show that double Tg mice have a population of anti-PtC B cells that
have the phenotypic characteristics of both B-2 and B-1 cells and that
have the potential to differentiate to B-1 (B-1a and B-1b). Cyclosporin
A blocks this differentiation and induces a more B-2-like phenotype in
these cells. These findings indicate that these cells are intermediate
between B-2 and B-1, further evidence of a B-2 to B-1 differentiative
pathway.
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Introduction
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B-1
cells constitute a small but nevertheless significant subset in mice
(1). They differ from the more abundant conventional or
B-2 cells in a number of respects that presumably reflect their
different roles in the immune system. In addition to their differences
in cell-surface phenotype and anatomical distribution
(1, 2, 3), they have characteristics suggesting that they are
Ag-selected. For example, V gene analysis suggests that some have
undergone Ag-driven clonal expansion (4, 5), and they
contribute the majority of circulating IgM (1, 6, 7).
Moreover, they are resistant to anti-IgM-induced tolerance
(8), and those in the peritoneum express constitutively
phosphorylated STAT-3 (9). Both of these characteristics
are shared with Ag-activated B cells.
A further argument for Ag selection is that B cells of certain
specificities segregate to B-1. These specificities include
polyreactive and autoreactive B cells. Among the latter are B cells
specific for single-stranded DNA, rheumatoid factor, Thy-1, red blood
cells, and the common membrane phospholipid, phosphatidyl choline
(PtC)4 (7, 10, 11, 12, 13, 14). Among anti-Thy-1 B cells, Thy-1 itself is
responsible for the accumulation of these cells in the B-1 subset,
indicating that self-Ags play a role in segregation (13).
PtC-specific B cells are particularly notable because they can comprise
as much as 10% of the B-1 repertoire in unmanipulated mice
(12) due to clonal selection and expansion after birth
(15), but they are undetectable among conventional B cells
(12, 15).
We have been interested in how B cells of certain specificities
segregate to B-1. The mechanism of segregation is intrinsically linked
to the relationship between B-1 and B-2 cells. There are two principal
hypotheses to explain this relationship, each of which predicts a
different mechanism of segregation. One is that B-1 and B-2 cells
derive from separate lineage-committed precursors (the lineage
hypothesis) (16, 17, 18). Other researchers (19, 20) and Haughton et al. (21) have proposed that B-1
cells derive from B-2 cells and are induced to differentiate to B-1 by
stimulation with thymus-independent type 2 (TI-2) Ags (the induced
differentiation hypothesis). By this hypothesis, B-2 cells are referred
to as B-0 cells to reflect their potential to differentiate to B-1
(21). The evidence of Ag selection described above does
not discriminate between these hypotheses. Discrimination will require
identification of the precursors of B-1 cells.
To understand the origin of B-1 cells, we have examined the
differentiation of PtC-specific B cells through the use of anti-PtC
transgenic (Tg) mice. Mice carrying the anti-PtC H and L chain
genes VH12 and V
4 were
generated (15). Our analysis indicates that segregation to
B-1 occurs after Ig gene rearrangement and that segregation appears to
be a function of the ability to bind PtC; cells that do not bind PtC or
that bind only weakly are B-0, whereas those that bind well are B-1
(22). The role of Ag binding is further suggested by Lam
and Rajewsky (23) in their report showing direct
correlation between VH12 receptor surface density
and B-1 cell development in mice carrying a VH12
transgene that has been inserted into an endogenous
JH locus. A similar conclusion about surface
receptor density was reached in an analysis of anti-red blood cell
B-1 cells (24). The segregation of PtC-binding B cells to
B-1 could be explained by either negative selection of anti-PtC B-0
cells (the lineage hypothesis) or differentiation of anti-PtC B-0
cells to B-1 cells (the induced differentiation hypothesis). To
discriminate between these possibilities, we combined the anti-PtC
Tgs with the X-linked immunodeficiency (xid) mutation
(25), a null mutation of Brutons tyrosine kinase
(26, 27, 28, 29). This kinase is a component of the signaling
pathway from the IgM receptor complex (30). The loss of
Brutons tyrosine kinase results in impaired TI-2 responses and poor
B-1 cell development (1, 31). Our analysis indicated that
the majority of splenic anti-PtC B cells have the conventional B
cell phenotype of xid mice (25), suggesting
that B-0 cells are precursors to B-1 cells and that differentiation is
dependent on signaling from surface IgM. The lineage hypothesis does
not predict that anti-PtC B cells with the xid mutation
would have a B-0 phenotype.
To identify precursors of B-1 cells in non-xid mice in vivo,
we focus in this report on a population of PtC-specific B cells in
double (Dbl) Tg mice that have an ambiguous phenotype
(25). We demonstrate by adoptive transfer and treatment
with cyclosporin A (CsA) that these cells are intermediate in
differentiation between B-0 and B-1, indicating a line of
differentiation between these two subsets consistent with a single B
cell lineage.
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Materials and Methods
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Mice
The Dbl Tg mice used in this study were bred and maintained in
our animal colony as described (15). C.B17 recipients were
purchased from Taconic (Germantown, NY).
Flow cytometry
The Abs used for immunofluorescence were against
IgMa (DS-1), IgMb (AF6-78),
B220 (RA3-6B2), CD5 (53-7.3), CD43, and CD23, were obtained from
PharMingen (San Diego CA), and were fluoresceinated, biotinylated, or
conjugated to PE. In three-color experiments, directly fluoresceinated,
PE-conjugated, and biotinylated Abs were combined. The biotinylated Ab
binding was revealed by addition of streptavidin-RED670 (Life
Technologies, Gaithersburg, MD). To detect PtC-binding B cells,
liposomes encapsulating carboxyfluorescein were used as previously
described (15). Contours are 5% probability.
Cell sorting and adoptive transfer
Splenic B cells from Dbl Tg or BALB/c mice were stained with
anti-B220-FITC and anti-CD23-PE and the
B220+, CD23int, and
B220+ CD23- cells were
sorted separately on a MoFlo (Cytomation, Ft. Collins, CO) high-speed
sorter. A total of 24 x 106 sorted Dbl Tg
CD23int cells or BALB/c
CD23+ cells were injected i.v into C.B17 mice
that had been irradiated with 500 rad 2 days before cell transfer. The
sorted Dbl Tg CD23int cells were contaminated
with 35% CD23- B-1 cells, and therefore, B-1
cell control transfers were with 2 x 105
CD23- B-1 cells into C.B17 mice that had been
irradiated as described above. Mice were analyzed by flow cytometry
35 days posttransfer. All experiments were performed three or more
times.
CsA treatment
C.B17 mice received i.p. injections of 50 mg/kg CsA (Novartis
Pharmaceuticals, Basle, Switzerland) in 30 µl beginning the day
before transfer and then daily for 4 days. After 4 days, spleen and
peritoneal cells were taken for flow cytometry analysis. Dbl Tg mice
were treated for 2 wk with daily doses of CsA as above and were
analyzed by flow cytometry. For the transfers of CsA-treated cells from
Dbl Tg mice, mice were treated with daily injections of CsA for 2 wk,
and the spleen cells were stained for B220 and CD23. The
B220+ CD23+ cells were
sorted and
4 x 106 cells were injected
i.v. into irradiated C.B17 mice as described. After 6 days, spleen
cells were taken and stained for analysis by flow cytometry.
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Results
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The existence of a B-1 intermediate cell in VH12 Tg
mice
B-0 and B-1 cells are distinguishable using an array of
cell-surface markers. B-0 cells are IgMlow,
B220high, CD23+,
CD43-, and CD5-, whereas
B-1 cells are IgMhigh,
B220low, CD23-, and often
CD43+ and CD5+
(25). In addition, B-1 cells are larger and more granular
than B-0 cells. Although the majority of anti-PtC B cells in Dbl Tg
mice have the typical B-1 phenotype (15), 520% have an
unusual phenotype. These cells express CD23 typical of B-0 cells,
albeit at lower levels, and express the B-1 cell markers CD5 and CD43
at levels equivalent to those of B-1 cells (Ref. 25 and
Fig. 1
). They are also intermediate to
B-0 and B-1 in B220 expression (B220int) and cell
size, and they express less IgM than B-1 cells do. Thus, their
phenotype suggests that they are cells in transition from B-0 to B-1
(B-1int).

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FIGURE 1. Dbl Tg mice have a population of PtC-specific B cells with a phenotype
intermediate to B-0 and B-1 (B-1int). The phenotype of
IgMa+ CD23- B-1 cells (Dbl Tg B-1) and
IgMa+ CD23+ B-1int cells (Dbl Tg
B-1int) shown in the boxes on the two-parameter histogram
(left) are compared with each other and with those of
B-0 cells from a non-Tg littermate (Non-Tg B-0). Expression levels of
CD5, CD23, CD43, B220, IgMa, and cell size are shown using
one-parameter histograms. The IgM level on non-Tg B-0 cells is not
shown because they are IgMb.
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To test the possibility that B-1int cells are
precursors to B-1, the B220+,
CD23int cells from Dbl Tg spleens were sorted
(Fig. 2
A) and adoptively
transferred to sublethally irradiated C.B17 mice. Three to five days
later, spleen cells from recipient mice were analyzed by flow
cytometry. The recovered donor cells were PtC-specific at all time
points. At 3 days posttransfer, most transferred
IgMa cells were similar in phenotype to the
starting population, i.e., CD23int,
CD5+, and CD43+, although
some had decreased expression of CD23 (Fig. 2
B). However, by
day 5 most transferred cells had lost CD23 expression. In addition,
they had increased IgM expression and cell size, and some had decreased
B220 (Fig. 2
B). Interestingly, a substantial fraction of the
recovered B cells had lost expression of CD5 but retained expression of
CD43. Thus, it appears that B-1int cells can give
rise to both B-1a (CD5+) and B-1b
(CD5-) cells. B-1 cells that contaminate the
sorted B-1int population are not responsible for
these results because transfer of B-1 cells at a number equal to that
determined by flow cytometry analysis to contaminate (35%) the
sorted B-1int cell populations yielded few or no
recoverable cells 5 days posttransfer (data not shown). Thus, between 3
and 5 days posttransfer, B-1int cells had
differentiated to B-1, supporting the claim that they are precursors to
B-1. Sorted BALB/c splenic B-0 cells transferred to C.B17 mice did not
change within this time period, indicating that the manipulations
(staining with Abs, cell sorting, etc.) have not induced
differentiation to B-1 (Fig. 2
B).

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FIGURE 2. Adoptively transferred Dbl Tg B-1int cells differentiate to
B-1 cells. A, The CD23+ B-1int
cell population used in the cell transfer experiments is indicated.
B, Sorted B-1int cells were injected i.v.
into sublethally irradiated C.B17 mice. After 3 or 5 days the
transferred cells in the spleens of recipient mice were analyzed by
flow cytometry. Shown is the phenotypic comparison of cells 3 and 5
days after transfer of Dbl Tg B-1int cells with BALB/c
CD23+ splenic B-0 cells taken 5 days posttransfer.
C, B-1int cells from Dbl Tg mice were
transferred to sublethally irradiated mice that were
(B-1int CsA) or were not (B-1int No CsA)
treated daily with CsA beginning the day before transfer. The
phenotypes of the cells of these populations are compared with that of
B-1 cells from Dbl Tg mice (B-1 No CsA).
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Cells at various stages of differentiation to B-1 appear to be evident
in VH12-only Tg (6-1) mice. In 61 mice, the
transgene-encoded H chain can associate with multiple L chains and
generate both PtC-binding and -nonbinding B cells (15).
Cells that stain brightly with liposomes
(PtCbright) are B-1, whereas those cells that
stain less brightly (PtCint) or not at all
(PtC-) are B-0 (15, 22). The
difference in liposome binding ability is due to L chain use. The
PtCbright cells predominantly use the
V
4/5H gene (15). This is the gene
used by anti-PtC B-1 cell lymphomas and hybridomas, and it is the
V
transgene in Dbl Tg mice (9, 15). Based on CD23 expression, there are three subpopulations of
PtCbright cells: CD23high,
CD23int, and CD23- (Fig. 3
). The CD23-
cells are the majority cell type in these mice and are B-1 (i.e.,
CD5+, CD43+,
B220low, and IgMhigh),
whereas the CD23int cells have a phenotype
identical with the B-1int cells of Dbl Tg mice
(i.e., CD5+, CD43+, and
B220int). The CD23high
cells express CD23 at levels nearly equivalent to those of B-0 cells,
but many are CD43+, CD5+,
and B220int (Fig. 3
). However, a smaller
proportion of CD23high cells has this phenotype
relative to the B-1int population, suggesting
that PtCbright CD23high
cells are at an earlier stage of B-1 cell differentiation than are
B-1int cells.

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FIGURE 3. 6-1 mice show a progression of differentiation of PtCbright
cells from B-0 to B-1. 6-1 mice have multiple populations of B cells
when stained with liposomes and CD23 as indicated by the boxes in the
two-parameter histogram (left). These populations differ
in phenotype as indicated by the one-parameter histograms to the
right. The populations are coded as shown with the boxes
in the two-parameter histogram.
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CsA blocks differentiation to B-1
TI-2 Ags are proposed to be responsible for driving
differentiation to B-1 (19, 20, 21). Because B cell responses
to CsA in vivo and in vitro (32, 33) can block TI-2
stimulation, we examined the effect of CsA treatment on B-1 cell
differentiation. Dbl Tg B-1int cells were
transferred to sublethally irradiated C.B17 mice that received daily
CsA treatment beginning the day before transfer. As shown in Fig. 2
C, the transferred cells did not differentiate to B-1 after
4 days, even though B-1int cells transferred into
untreated mice differentiated to B-1. Relative to
B-1int cells transferred into untreated mice,
CsA-treated cells showed essentially no change in CD23 levels, a
decrease in CD5, CD43, and IgM expression, a decrease in cell size and
granularity, and an increase in B220 expression. Thus, CsA blocks
differentiation to B-1 and appears to cause
B-1int to acquire a more B-0-like phenotype,
suggesting that continual Ag stimulation is required for
differentiation to B-1.
Daily treatment of Dbl Tg mice with CsA for 2 wk results in the
accumulation of PtC-specific cells with a more B-0-like phenotype (Fig. 4
). The B-1int
cells from CsA-treated mice are CD5- and
IgMlow, and all have reduced their CD43 levels
with many showing little or no CD43 expression. Despite a more B-0-like
phenotype, CsA-treated cells nonetheless retain the ability to
differentiate to B-1 (Fig. 5
).
Transferred CD23+ cells from CsA-treated Dbl Tg
mice decreased CD23 expression and increased CD5 and CD43 expression
(Fig. 5
).

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FIGURE 4. Long-term treatment of Dbl Tg mice with CsA leads to an accumulation of
PtC-specific cells that have a B-0 phenotype. Dbl Tg mice treated daily
with CsA or left untreated were compared by flow cytometry for
phenotypic differences. Shown is the phenotype of the IgM+,
CD23+ population. For comparison, the CD23+
cells of a non-Tg littermate are shown (Normal).
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FIGURE 5. PtC-specific B-0 cells from CsA-treated Dbl Tg mice differentiate to
B-1 after adoptive transfer to sublethally irradiated mice. Shown are
the phenotypic comparison of the CD23+ cells from
CsA-treated Dbl Tg mice (Sorted B-1int CsA) and the cells
recovered from recipient mice 6 days later (Recovered Cells). For
reference, the phenotype of B-1 cells (IgM+
CD23-) from a Dbl Tg mouse that have not been treated with
CsA (B-1 No CsA) is included.
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Discussion
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In this paper, we describe anti-PtC B cells that have the
phenotypic characteristics of both B-0 and B-1 cells. Our analysis of
these cells suggests that they are intermediate
(B-1int) in a differentiative pathway from B-0 to
B-1; they differentiate to B-1 upon adoptive transfer, and they acquire
a more B-0-like phenotype upon exposure to CsA. However, CsA does not
induce a complete loss of CD5 and CD43 expression in the transfer
experiments (Fig. 2
C), and CD43 expression is not completely
lost by long-term CsA treatment (Fig. 4
). We attribute this to the
influence of constant Ag stimulation driving differentiation toward
B-1, and to the short time frame of the transfer experiments. However,
a more complete block in differentiation, mediated by the
xid mutation, results in PtC-specific B cells that have the
phenotype of conventional (B-0) cells of xid mice and that
do not express either CD5 or CD43 (25). Taken together
then, we conclude that anti-PtC B cells differentiate from B-0 to
B-1 as a result of signaling through IgM. We further conclude, based on
the effect of CsA on B-1int cells, that
B-1int differentiation is reversible. Whether
differentiation of cells that have reached the B-1 cell stage is
reversible is presently unknown. This pathway, which is illustrated in
Fig. 6
, was initially proposed by Wortis
and coworkers (19, 20) as a result of their observation
that splenic B-0 cells can be induced to differentiate to B-1 in vitro
by treatment with anti-IgM and IL-6.

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FIGURE 6. Schematic of B-1 cell differentiation based on the analysis of
PtC-specific B cell differentiation. In this schematic, it is presumed
that PtC-specific B cells pass through a transitional B cell stage
before becoming B-0 cells that express CD23, although the existence of
PtC-specific transitional cells has not been demonstrated. We propose
that B-0 cells will differentiate to B-1 if stimulated by an
appropriate Ag, presumably a TI-2 Ag. The mice in whom PtC-specific
cells of each type have been observed are given across the top. Normal
mice have a significant population of PtC-specific B-1 cells
(12 ), and it is presumed that they have PtC-specific
B-1int cells but that their number is too low to detect.
xid blocks differentiation from B-0 to
B-1int (25 ). Other mutations affect this
pathway as discussed in the text. CsA blocks differentiation of
B-1int to B-1 and probably blocks differentiation of B-0 to
B-1int. It may also cause B-1int cells to
revert to B-0. The effect of CsA on cells that have already
differentiated to B-1 is not known, as indicated by the dotted reverse
arrow.
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Our findings have bearing on the relationship between B-1a and B-1b
cells. Transferred B-1int cells give rise to both
B-1a and B-1b cells. Thus, cells of these B-1 subsets must diverge late
in differentiation from B-0. Moreover, because
B-1int cells are CD5+,
these data indicate that B-1b cells initially express CD5 and then lose
CD5 expression during differentiation. The significance of CD5 loss to
the function of these cells is unknown. One possibility is that CD5 may
regulate the differentiation to Ab-secreting cells because CD5 is a
negative regulator of IgM receptor signaling (34).
Regardless of its role, because the B-1int cells
in this study are all PtC-specific, the divergence to the B-1a and B-1b
subsets must not be on the basis of specificity.
A B-0 to B-1 differentiation pathway provides a B cell activation
framework for the interpretation of the numerous B cell receptor
signaling and coreceptor knockout models that differentially affect B-1
and B-0 cells. For example, CD19 knockout and CD21 knockout mice lack
B-1 cells (35, 36, 37, 38), as do mice lacking the cytoplasmic
kinases Vav and protein kinase C-ßI/II (39, 40, 41).
Conversely, mice that overexpress CD19 and mice deficient in Src
homology domain 2-containing phosphotyrosine phosphatase-1 have
excessive numbers of B-1 cells (36, 37, 42). In light of
our evidence of a B-0 to B-1 differentiative pathway, the effects of
these signaling molecules on B-1 cells should be viewed in the context
of Ag-driven B-0 to B-1 cell differentiation rather than as having
differential effects on B-0 and B-1 lineages independent of Ag
stimulation. This interpretation is consistent with the known roles
these molecules play in B cell Ag activation.
B-1int cells appear to be present in non-Tg mice.
B cells expressing both CD23 and CD5 are present in the spleens of 7-
to 10-day-old non-Tg mice (20). These cells may be
equivalent to the B-1int cells described here,
which would mean that they are in transition to B-1 as suggested
(20). They may be visible in very young non-Tg mice
because the B-1 population is developing rapidly; B-1 cells reach their
highest frequency (4045%) at day 9 (43).
B-1int cells may be visible in adult Dbl Tg mice
because, like in young non-Tg mice, a large proportion of the B cells
are differentiating to B-1.
Ag appears to be required for anti-PtC B-1 cell differentiation, as
suggested by the effects of the xid mutation
(25) and CsA. In addition, we have shown a correlation
between the ability to bind PtC and segregation to B-1
(22), and Lam and Rajewsky (23) have shown
that Ag receptor density is important in anti-PtC B-1
differentiation, indicating that signaling through surface IgM is
critical. The Ag responsible for anti-PtC differentiation to B-1 is
unknown. PtC is an abundant phospholipid, so the Ag could be self. If
so, the irradiation of recipient mice for B-1int
cell transfers may have increased the levels of this Ag and contributed
to driving B-1 differentiation. Transfer to unirradiated mice yielded
too few recovered donor B cells to test this possibility. However,
involvement of a non-self-Ag cannot be ruled out. In fact, anti-PtC
Abs appear to have a protective effect against certain bacterial
infections (44), suggesting this possibility.
Whatever the Ag driving PtC-specific B-1 differentiation, it and the
other Ags that drive B-1 differentiation are most likely TI-2 Ags; TI-2
stimulation in vitro can induce a B-1 phenotype (19, 20),
and B-1 cells are present in nude mice (1). Moreover,
xid mice, which respond poorly to TI-2 Ags
(45), have few B-1 cells (1). The
specificities known to be associated with B-1 are typically Ags that
cannot elicit T cell responses (carbohydrates, lipids, and DNA
(7, 10, 11, 12, 13, 14)), further supporting the idea that TI-2 Ags
drive B-1 cell differentiation. Both self- and non-self-Ags can be
involved in B-1 cell development; Hayakawa et al. (13)
have shown that development of B-1 cells specific for a carbohydrate
epitope on the self-Ag Thy-1 is dependent on the presence of Thy-1
itself, and Whitmore et al. (46) have shown that
immunization with the non-self-Ag polyvinyl pyrrolidone (PVP) produces
a population of anti-PVP B-1 cells.
A variety of factors that affect B cell specificity have been suggested
to be key to the burst in B-1 cell differentiation in the neonate
(20). These include the absence in neonatal lymphocytes of
terminal deoxynucleotidyl transferase (47, 48, 49, 50, 51), which
results in little or no N region addition among neonatal B cells (in
contrast to adult bone marrow-derived cells) and a bias in the neonate
relative to the adult in VH gene rearrangements
(52, 53, 54). But other factors including the availability of
Ags and other nonantigenic factors (e.g., cytokines or other cells)
that drive B-1 cell differentiation or differences in the specificities
produced as mice age could be involved. A recent report suggests that B
cell differentiation in the neonate is less dependent on the efficient
formation of a pre-B cell receptor than it is in the adult
(55). Specifically, B cells expressing the
VH11 gene were favored in the neonate over the
adult. Because VH11 is also used to encode
anti-PtC Abs (4, 5), this mechanism may favor the
expression of cells that have B-1 specificities. Nevertheless, the
neonatal repertoire appears to be biased by a variety of mechanisms
that may be evolutionarily selected to rapidly fill the B-1 repertoire,
leading to a rapid enlargement of the B-1 compartment relative to the
B-0 compartment in neonates. In this regard, we have demonstrated that
anti-PtC B cells are selected at multiple differentiative
checkpoints to ensure that a high frequency of
VH12-expressing B cells have an anti-PtC
VHCDR3 (56) and express
V
4/5H (22), and Booker and
Haughton (57) have shown that the
VH11 and VH12 genes are
evolutionarily conserved. This is evidence of strong evolutionary
selection for the development of an anti-PtC response, presumably
to promote the survival of every individual.
B-1 cells may be receptive to T cell help (58). If this is
a pathway generally open to these cells and they can be drawn into
germinal centers, they may undergo affinity maturation through somatic
hypermutation and selection as well as heavy chain class switch.
Because there is a bias toward self-specificities among B-1 cells, they
could be an important reservoir of anti-self B cells in autoimmune
diseases. It is possible that B-1 cells are activated with T cell help
and are driven to produce pathogenic Abs through somatic mutation and
selection in a germinal center. In light of the evidence that B cells
can move between subsets, an assessment of the potential of these B
cells to be activated by T-dependent Ags in autoimmunity is
warranted.
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Acknowledgments
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We thank William Nostrum and the staff of the Flow Cytometry
Facility at the University of North Carolina for their assistance.
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Footnotes
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1 This work was supported by National Institutes of Health Grants AI29576 and AI43587, Grant 79017 from the American Cancer Society, and a grant from the Arthritis Foundation. 
2 Current address: Department of Physiology, Western University "Vasile Goldis" Arad, Vasile Goldis 1-3 Ap. 10, RO-2900 Arad, Romania. 
3 Address correspondence and reprint requests to Dr. Stephen H. Clarke, Department of Microbiology and Immunology CB#7290, 804 MEJB, University of North Carolina, Chapel Hill, NC 27599. E-mail address: 
4 Abbreviations used in this paper: PtC, phosphatidyl choline; Tg, transgenic; TI-2, thymus-independent type 2; xid, X-linked immunodeficiency; Dbl, double; CsA, cyclosporin A; int, intermediate. 
Received for publication October 1, 1999.
Accepted for publication January 4, 2000.
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