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Department of Genetics and Pathology, Rudbeck Laboratory, Uppsala University, Uppsala, Sweden; and
Division of Rheumatology, Department of Medicine, Washington University School of Medicine, St. Louis, MO 63110
| Abstract |
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| Introduction |
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The need for C in initiation of Ab production is usually most
pronounced when suboptimal Ag concentrations are used and can be
partially overcome with optimal immunizations. For example, mice
depleted of C by treatment with CVF produced near normal responses to
Ag administered in CFA (11). Guinea pigs genetically
deficient in C2 or C4 (12) as well as gene-targeted mice
lacking C3 or C4 (13) produced significant Ab responses to
high, but not to low, doses of bacteriophage
X174. It is not clear
whether high doses of Ag can also overcome the need for CR1/2. In
initial studies in which CR1/2 was blocked by mAbs, a strong Ab
response was seen with optimal doses of erythrocytes, whereas
suboptimal doses were unable to induce responses (5).
However, using CR1/2-/- mice only marginal
responses were detected even after repeated immunizations with high
doses of Ag (7, 8, 9).
Whereas deficiencies in the classical pathway components C2 and C4 lead to impaired humoral responses, mice lacking factor B of the alternative pathway appear to have normal Ab responses (14). This implies that classical pathway C activation, known to be initiated by Ab/Ag complexes, is of major importance for the induction of a normal Ab response. Abs in complex with their specific Ag are known to have strong negative or positive feedback regulatory effects on humoral responses (15). Three isotypes can enhance Ab responses: IgG (16, 17, 18, 19), IgE (20, 21), and IgM (22, 23, 24). Enhancement by IgG can take place in the absence of C (18). IgE does not activate C, and IgE-mediated enhancement is known to be completely dependent on the low affinity receptor for IgE, CD23 (20, 21). However, a link between the C system and feedback regulation by Abs was seen in an experimental system in which IgM anti-SRBC administered together with suboptimal doses of SRBC enhanced the Ab response; IgM unable to activate C does not enhance, and normal IgM cannot enhance in C-depleted animals (24). In the present report we have investigated whether Ags in complex with IgM, IgG2a, or IgE as well as Ags administered with adjuvants are able to induce Ab responses in animals lacking CR1/2.
| Materials and Methods |
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Male CR1/2-/- mice (H-2Ab) (7) were mated with female CBA/J mice (H-2Ak; Bommice, Bomholtgaard, Ry, Denmark). This was done to bring the CR1/2-/- mice to the H-2Ak haplotype, as H-2Ab mice are low responders to IgG and IgE immune complexes (25, 26). The F1 generation was intercrossed, and mice homozygous for the H-2Ak allele and mutant CR1/2 (CR1/2-/-) or wild-type CR1/2 (CR1/2+/+) were selected and used for breeding of CR1/2-/- and CR1/2+/+ mice. The offspring from these mice, 25 mo of age, matched for age and sex within each experiment, were used. Although the optimal strains would be fully congenic mice, the most important gene locus (I-A) for the studied Ab responses was similar in CR1/2-/- and CR1/2+/+ animals. Animals were bred in the animal facilities at the Department of Animal Development and Genetics, Uppsala University (Uppsala, Sweden). Female BALB/c mice (Bommice) were used in the experiments in which CR1/2 was blocked with anti-CR1/2 mAbs. These mice were kept at the animal facilities at the Biomedical Center, Uppsala University.
PCR and primers
The H-2A haplotype (H-2Ak or
H-2Ab) was analyzed by two independent PCR
reactions. The Ak allele was detected using
primers
K2 (5'-TTC CAA GTT GTG TTT TCC TG-3') and
K1:2 (5'-TAT
CAG TCT CCT GGA GAG ATT G-3'). The Ab allele was
detected using primers
K1:2 (described above) and
B2 (5'-ACT CCC
AAG TTG TGT TTT ACT A-3'). Gene amplification was performed in a
50-µl volume of 10 mM Tris-HCl (pH 8.3), 50 mM KCl, 5 mM
MgCl2, 0.2 mM dNTPs, 0.22 µM of primers
K2
and
B2, 0.28 µM of primer
K1, and 2.5 U AmpliTaq DNA polymerase
(Perkin-Elmer/Cetus, Norwalk, CT) for 25 cycles (95°C for 30 s;
55°C for 40 s; 72°C for 30 s). Both
K1:2/
K2 and
K1:2/
B2 give fragments 180 bp in size.
The CR1/2 genotype was analyzed by two independent PCR. The CR1/2-/- genotype was detected using the primer pair P3 (5'-CGC TGT TCT CCT CTT CCT CAT C-3') and P4 (5'-GAT GGA TAC TTT CTC GGC AGG AGC-3'). The CR1/2+/+ genotype was detected using the primer pair P5 (5'-TGT CAG GCT CCT CCT AAA ATT ATC-3') and P6 (5'-CTT TAC AAA GAC GGA TTT CTA TA-3'). Gene amplification was performed in a 50-µl volume of 10 mM Tris-HCl (pH 8.3), 50 mM KCl, 3 mM MgCl2, 0.2 mM dNTPs, 0.22 µM each of primers (P3/P4 or P5/P6), and 0.5 U of AmpliTaq DNA polymerase (Perkin-Elmer/Cetus) for 35 cycles (94°C for 30 s; 65°C for 30 s; 72°C for 2 min (CR1/2-/- allele primer pair) or 94°C for 30 s; 55°C for 30 s; 72°C for 2 min (CR2+/+ allele primer pair)). The P3/P4 PCR gives a 400-bp DNA fragment, while the P5/P6 PCR gives a DNA fragment of 690 bp.
Antibodies
Rat IgG2b anti-mouse CR1/2 mAbs were derived from the
hybridoma cell line 7G6 (27) and purified as previously
described (5). The hybridoma cell lines IGELb4 (mouse IgE
anti-2,4,6-trinitrophenyl (anti-TNP)) (28) and C4007B4
(7B4, mouse IgG2a anti-TNP) (17) were cultured in DMEM
with 5% FCS. IgG2a was purified on a protein A-Sepharose column
(Amersham Pharmacia Biotech, Uppsala, Sweden) according to the
manufacturers recommendations. IgE was purified by affinity
chromatography on a Sepharose column conjugated with a rat
anti-mouse
mAb, 187.1.10 (29). Bound IgE was
eluted with 0.1 M glycine-HCl, pH 2.8. Abs were dialyzed against PBS,
sterile-filtered, and stored at -20°C. Protein concentrations were
determined by absorbance at 280 nm, assuming that an absorbance of 1.5
equals 1 mg/ml of Ab. Polyclonal IgM anti-SRBC was prepared from
sera obtained from mice 5 days after i.p. immunization with 10% SRBC
in PBS. Sera were diluted 1:2 in 0.05 M sodium phosphate/0.15 M NaCl,
pH 7.4, and isolated by fluid phase liquid chromatography gel
filtration using Superdex 200 prep grade, High Load 26/60 (Amersham
Pharmacia Biotech). The IgM-containing column fractions with reactivity
to SRBC, as determined by ELISA (30), were pooled and
concentrated using a P10 Amicon concentration filter (Amicon, Beverly,
MA) according to the manufacturers instructions. The direct
hemagglutination titer was determined as previously described
(23).
Antigens
OVA (grade V, A-5503), BSA (fraction V, A-3059), and TNP (picrylsulfonic acid/hydrate) were obtained from Sigma (St. Louis, MO). TNP was conjugated to BSA in 0.28 M cacodylate buffer, pH 6.9. After 70 min of incubation at room temperature the reaction was stopped by an excess of glycyl-glycin (1 mg/ml; Merck, Darmstadt, Germany). Proteins were dialyzed against PBS, sterile-filtered, and stored at 4°C. The number of TNP residues/BSA was determined as described previously (31), and a conjugate with 12 TNP/BSA was used. SRBC and horse erythrocytes (HRBC) were obtained from the National Veterinary Institute (Uppsala, Sweden) and stored in sterile Alsevers solution at 4°C. Erythrocytes were prepared by washing three times in sterile PBS for use as Ags or in HBSS for use in plaque-forming cell (PFC) assays.
Immunizations
Mice were immunized in their tail veins with 0.1 ml of IgG2a anti-TNP/BSA-TNP or IgE anti-TNP/BSA-TNP complexes formed by incubating Ag and Abs together at 37°C for 1 h in PBS along with 20 µg of OVA (as a specificity control). In experiments involving SRBC and IgM, Ab was administered i.v. in 0.1 ml of PBS, followed after 1 h by SRBC and HRBC (as a specificity control) in 0.1 ml of PBS i.v. Equal volumes of CFA (Difco, Detroit, MI) and BSA were emulsified, and two 50-µl emulsions (20 µg of BSA/mouse) were given s.c. in each flank. Alum (Pierce, Rockford, IL) was emulsified with BSA as described by the manufacturer, and two 50-µl emulsions (20 µg of BSA/mouse) were given s.c. in each flank.
ELISA and PFC assays
Mice were bled from the tail veins, and individual sera were analyzed by ELISA for BSA- or SRBC-specific IgG as previously described (30, 32). A BSA-specific standard serum, affinity-purified on BSA-Sepharose, was used to determine the concentrations of BSA-specific IgG. Standard curves and calculations were performed using a Softmax program (Molecular Devices, Menlo Park, CA). To determine the total numbers of B cells producing SRBC-specific IgM, a modified version of the Jerne hemolytic PFC assay was used (33).
Statistical analysis
Statistical differences between the control and the experimental groups were determined by Students t test.
| Results |
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IgM is known to enhance Ab responses to particulate Ags, such as erythrocytes and malaria parasites, whereas enhancement of responses to soluble protein Ags has been shown only occasionally, using KLH as Ag (17, 34). It may be that IgM needs to bind to a large Ag to take on the configurational change necessary for its ability to activate C, which is known to be required for the enhancing effect (24). For these reasons IgM-mediated enhancement was here studied in the classical SRBC system. This system also allows detection of single Ab-producing B cells using the sensitive hemolytic PFC assay (35).
CR1/2-/- and CR1/2+/+
mice were injected with 2 x 105 SRBC with
or without IgM anti-SRBC. All groups were also given HRBC as a
specificity control. IgM increased the IgG anti-SRBC response in
CR1/2+/+ mice (Fig. 1
a), but not in
CR1/2-/- mice (Fig. 1
b). There was
no enhancement of the response to HRBC (data not shown). To determine
whether the nonresponsiveness is also present at the single-cell level,
CR1/2-/- and CR1/2+/+
mice were challenged as described above, and the numbers of IgM
anti-SRBC-producing spleen cells were analyzed. No enhancement was
detected in CR1/2-/- mice, whereas IgM induced
a 4-fold enhancement of the direct PFC response in
CR1/2+/+ mice (Table I
, Expt. 1). Using an alternative
approach, normal BALB/c mice were treated with 200 µg of
anti-CR1/2 mAbs (7G6) 24 h before challenge with SRBC and IgM,
a protocol shown previously to inhibit Ab responses to erythrocytes
(5). The results show that IgM was unable to enhance Ab
responses after in vivo blocking of CR1/2 (Table I
, Expt. 2). Again, no
enhancement of the HRBC-specific response was seen.
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IgE/Ag and IgG/Ag complexes are able to enhance Ab production in CR1/2-/- mice
IgG and IgE have dual immunoregulatory capacity. Whereas
administration together with soluble Ags results in enhancement,
administration with particulate Ags usually causes specific suppression
of Ab responses (33, 34, 36). Therefore, to study IgE- and
IgG-mediated enhancement, soluble protein Ags were used.
CR1/2-/- and CR1/2+/+
mice were challenged with 20 µg of BSA-TNP alone or in complex with
50 µg of IgE or IgG2a anti-TNP mAbs. All animals received 20 µg
of OVA as a specificity control. Sera were analyzed by ELISA 728 days
after priming. Both strains responded to challenge with IgG2a/BSA-TNP
(Fig. 2
, a and b)
or IgE/BSA-TNP (Fig. 2
, c and d). The magnitude
of the response to IgG2a/Ag was slightly lower in
CR1/2-/- mice, but the difference compared with
CR1/2+/+ mice was not significant. No enhancement
of the IgG anti-OVA responses was detected (data not shown).
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To determine whether CR1/2-/- mice are
able to respond to Ag administered with adjuvant,
CR1/2-/- and CR1/2+/+
mice were challenged s.c. with various amounts of BSA in CFA or alum.
Fig. 3
shows that both
CR1/2-/- and CR1/2+/+
mice were able to produce high titers of BSA-specific IgG. The response
to 2 µg was lower in CR1/2-/- than in
CR1/2+/+ mice, but the difference was not
significant.
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| Discussion |
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-chain (19). The FcR
-chain is associated with
Fc
RI and Fc
RIII, but has not been shown to affect the C system.
Therefore, the results suggested that Fc
Rs are of primary importance
in IgG-mediated enhancement. The ability of IgG to up-regulate Ab
responses without CR1/2, demonstrated in the present report,
strengthens this conclusion. The exact molecular mechanisms by which
IgE and IgG up-regulate Ab responses are not understood. Efficient
uptake and presentation of Ag in the form of IgE or IgG complexes to T
cells is known to take place in vitro (39, 40, 41, 42, 43, 44) and may
also be the general mechanism in vivo. If this is indeed the case, it
suggests that when efficient T cell help is available, signaling
through the BCR, without costimulation via the CR2/CD19/TAPA-1 complex,
is sufficient to activate B cells. Because IgG and IgE are more
abundant in secondary than in primary Ab responses, it may be that
secondary Ab responses are less C dependent than primary Ab responses,
which is in line with previous experimental data (7, 8). Adjuvants have classically been used to augment immune responses, but their mechanism of action is unclear. We show here that mice lacking CR1/2 are able to respond to T-dependent Ags administered in adjuvants. These results agree with previous data showing that CVF-treated animals respond to Ag administered in adjuvant (11, 45) and are another example of CR1/2-independent induction of humoral responses.
We also confirm previous observations that
CR1/2-/- mice have severely impaired serum Ab
responses to SRBC alone (7) and extend them to show that
this is paralleled by fewer Ag-specific B cells in the spleens of
CR1/2-/- mice after Ag challenge. This finding
is compatible with the observations of smaller and fewer GCs in
CR1/2-/- mice following
X174 challenge
(8).
Previous findings that IgM, Ag, and C work together to enhance Ab responses (24) and that normal Ab responses require CR1/2 (5, 7, 8, 9) implied that IgM-mediated enhancement operates via CR1/2. We here provide the first experimental evidence that this is indeed the case. We have used preformed Ag-specific IgM, but in a physiological, primary response Ag would initially form complexes with natural IgM. The importance of natural IgM in the induction of immunity is supported by recent observations. Gene-targeted mice, lacking secretory IgM but able to express surface IgM and to secrete IgG and IgA, had diminished Ab responses after immunization with KLH-4-hydroxy-3-nitrophenyl (46, 47). Responses were reconstituted when IgM from normal mouse serum was administered to targeted mice before challenge (46). Therefore, it appears likely that Ab responses to low doses of Ag involves recognition of the Ag by natural IgM followed by C activation and ligation of CR1/2, which would lead to production of early specific IgM and, in turn, further enhance the positive feedback loop.
The precise mechanism behind the role of CR1/2 in Ab responses is not known. Positive signaling to Ag-specific B cells via IgM/Ag/C coligated to the CR2/CD19/TAPA-1 complex may be the signal necessary to break threshold responses to low dose Ags (48, 49). CR1/2-facilitated Ag presentation to T cells by opsonization of immune complexes has been seen using in vitro systems (50, 51, 52), although T cell priming in vivo can occur without functional CR1/2 (53) or without the presence of C3 (13). CR1/2 also affect B cell retention/survival within lymphoid follicles and GCs (54). Regardless of which molecular mechanism(s) is involved, the data presented here reveal two important aspects of the in vivo role of CR1/2. First, CR1/2 is a crucial receptor in triggering responses to IgM/Ag complexes, which is probably the first step in Ab production. Second, the need for CR1/2 can be circumvented when Ags are administered in complex with IgG2a or IgE, isotypes typical of secondary responses, or together with adjuvants, a system mimicking natural inflammatory responses.
| Acknowledgments |
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| Footnotes |
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2 Current address: Microbiology and Tumorbiology Center, Karolinska Institute, Stockholm, Sweden SE 17177. ![]()
3 S.E.A. and J.D. contributed equally to this paper. ![]()
4 Address correspondence and reprint requests to Dr. Birgitta Heyman, Department of Genetics and Pathology, Rudbeck Laboratory, SE-751 85 Uppsala, Sweden. ![]()
5 Abbreviations used in this paper: CFV, cobra venom factor; CR, complement receptor; BCR, B cell receptor; TNP, 2,4,6-trinitrophenyl; HRBC, horse erythrocytes; PFC, hemolytic plaque-forming cells; GC, germinal center; KLH, keyhole limpet hemocyanin; Fc
R, Fc receptor for IgG. ![]()
Received for publication March 28, 2000. Accepted for publication June 14, 2000.
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