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Department of Pediatrics, University of Utah School of Medicine, Salt Lake City, UT 84132;
Mallinckrodt Department of Pediatrics and Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, MO 63110; and
Department of Microbiology and Immunology, University of Oklahoma School of Medicine, Oklahoma City, OK 73190
| Abstract |
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| Introduction |
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The adult immune repertoire to Haemophilus influenzae type b capsular polysaccharide (Hib PS) is dominated by a limited number of heavy chain (IgVH) and light chain (IgVL) variable region genes (7, 8, 9, 10). Studies of serum anti-Hib PS Ab suggest that this repertoire varies with age and Ag formulation (11, 12, 13). Identification of the molecular basis of this diversity may shed light on the age-dependent development of the immune response to TI Ags. To better understand age-related changes in the human anti-Hib PS repertoire, we determined variable region gene sequences encoding anti-Hib PS mAbs of young children immunized with a TD Hib PS vaccine. Our data confirm that quantitative and qualitative differences exist between the anti-Hib PS repertoire of young children and the typical repertoire of adults following immunization.
| Materials and Methods |
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A total of 19 young children were studied. Seventeen children were immunized at 2, 4, 6, and 15 to 18 mo of age with Hib oligosaccharide-CRM197 mutant Corynebacterium diphtheriae protein conjugate vaccine (HbOC; Lederle Laboratories, Wayne, NJ). Blood was obtained 5 to 10 days following immunization at 15 to 18 mo for Ab determination and to obtain immune PBL. Similar studies were performed on sera and lymphocytes of two infants (aged 12 and 18 mo) who had been immunized 7 days earlier with varicella vaccine (Varivax, Merck, West Point, PA) but who had not received Hib vaccine within 3 mo. Adult subjects received a single immunization with HbOC (six subjects), Hib-diphtheria toxoid conjugate vaccine (Connaught Laboratories, Swiftwater, PA; four subjects), or unconjugated Hib PS vaccine (Praxis, Rochester, NY; one subject). By 6 yr of age the vast majority of individuals have acquired natural immunity to Hib, and all adult subjects from whom preimmune sera were available (10 of 11) were found to have detectable anti-Hib PS serum Ab (7, 14). Cell lines obtained from both adults and infants after immunization, therefore, represent secondary immune responses.
Serum anti-Hib PS Abs and mAbs
Serum total anti-Hib PS Ab was measured by RIA with
125I-labeled Ag (15). IgG and IgM anti-Hib PS Ab
concentrations were measured by ELISA using multiwell plates (model
3590, Costar, Cambridge, MA) coated with Hib PS coupled to
poly-L-lysine. Ab was detected by incubation with
biotinylated goat anti-human IgG or IgM Abs (BioSource, Camarillo,
CA) followed by detection with avidan-coupled alkaline phosphatase and
p-nitrophenylphosphate enzyme substrate as previously
described (16). The level of IgG-anti-Hib PS Ab was measured
quantitatively by comparison with a reference serum from the Center for
Biologics Evaluation and Research (Bethesda, MD) that contained
66 µg/ml of IgG anti-Hib PS Ab. The results of the IgM assays
were expressed as units relative to a serum pool (A144) that we
prepared from adults immunized with Hib PS. The amount of Hib PS
required for 50% inhibition of serum Ab or mAb binding to Hib PS
(ID50) was measured by inhibition ELISA as previously
described (16, 17). Low affinity anti-Hib PS mAbs were defined as
those that were not inhibitable by 10 µg/ml of soluble Hib PS in this
ELISA assay. Inhibition of binding by Escherichia coli K100
PS was measured using formalin-fixed pelleted bacteria containing K100
PS (18). The expression of HibId-1 and HibId-2 Ids (associated with the
use of the V
II A2 and V
VII DPL19 light chain variable segments,
respectively) was measured by ELISA as previously described (12).
Binding to type 3 Streptococcus pneumoniae PS and to tetanus
toxoid was measured by ELISA as described for Hib PS. Streptococcal
membranes were prepared from M type 5 Streptococcus pyogenes
as described by Cunningham (19). Type 3 S. pneumoniae PS was
provided by Dr. Philip Vella, Merck. The type 3 PS was coupled to
poly-L-lysine as previously described (15). Tetanus toxoid
was obtained from Wyeth (Marletta, PA). Binding to streptococcal
membranes, ssDNA, dsDNA (Calbiochem-Behring, La Jolla, CA), myosin,
cardiolipin, and smRNP (Sigma, St. Louis, MO) was measured by
ELISA. For these assays, the Ags were diluted 10 µg/ml in
carbonate-bicarbonate buffer, pH 9.6, and were coated directly onto
microtiter plates (Immulon 4, Dynatech, Chantilly, VA). Bound Ab
was detected with alkaline phosphatase-conjugated goat anti-human
Ig Ab (Sigma) as previously described (8). Binding of mAbs to other
bacterial and self Ags was defined as present, or positive, when the
absorbance was greater than twice the background.
Anti-Hib PS mAbs
Hybridoma cell lines secreting human anti-Hib PS mAb were
produced, and variable region genes encoding heavy (IgVH)
and light (IgVL) chains amplified from mRNA were
cloned and sequenced as described previously (7, 16). Briefly,
107 to 108 immune PBLs were fused to 5 x
107 murine myeloma SP2/O-Ag14 cells, and heterohybridoma
cells were selected by resistance to 1 x 10-4 M
hypoxanthine, 4 x 10-5 M thymidine, and 4 x
10-7 M aminopterin as described by Carroll (20). Hybridoma
cell lines secreting anti-Hib PS binding Ab were initially detected
by ELISA, using multiwell plates (model 3590, Costar) coated with Hib
PS coupled to poly-L-lysine (16). Hib PS-binding Ab was
detected by incubation with biotinylated goat anti-human
or
Abs (BioSource, Camarillo, CA) followed by detection with
avidan-coupled alkaline phosphatase and
p-nitrophenylphosphate enzyme substrate as previously
described. All hybridomas secreting anti-Hib PS binding mAb in a
sufficient quantity of absorbance of >1.0 units by ELISA on culture
supernatant were cloned by limiting dilution.
| Results |
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The geometric mean serum Ab concentration of serum IgG anti-Hib PS Ab from infants postimmunization was 31 µg/ml (range, 3588.0), and that of adult subjects was 24 µg/ml (range, 760). The geometric mean concentration of IgM anti-Hib PS Abs in infants after immunization was 27 ELISA units (range, 2262), and that in adult subjects was 124 (range, 24611; p = 0.03). The mean ID50 of serum anti-Hib PS IgG in infants was 95 ng/ml, compared with 156 ng/ml in adults (p = 0.03). The mean ID50 of serum anti-Hib PS IgM in infants was 38 ng/ml, compared with 54 ng/ml in adults.
Anti-Hib PS monoclonal cell lines
A total of 2094 mAb cell lines were generated from 16 infants immunized with HbOC vaccine. Of these, 78 (3.7%) reacted with Hib PS. Individually, however, there was great variation in the frequency of Hib PS-reactive cell lines (011%). To estimate the frequency of naturally occurring Hib PS-reactive cells, 391 hybridomas were produced from two infants of comparable age who had not been recently immunized with Hib vaccine. Four of 307 cell lines (1.4%) from one child and none of 84 cell lines from the other bound Hib PS. A similar analysis was performed on four adult subjects, all immunized with HbOC vaccine. Of 237 heterohybridoma cell lines obtained 7 to 9 days after immunization, 72 (30.4%) were reactive with Hib PS.
Most anti-Hib PS binding mAbs were not completely evaluable because these cell lines often secrete Ab only transiently. Of a total of 41 stable heterohybridomas from infants, 14 (34%) were not inhibitable by Hib PS, suggesting a low affinity interaction. All of the cell lines secreting anti-Hib PS mAbs that were inhibitable by Hib PS were recovered from three children, whereas noninhibitable mAbs were obtained from five children. Stable cell lines were recovered from three of four adult subjects immunized with HbOC vaccine, and all were inhibitable by Hib PS (n = 116). In this and previous studies, a total of 337 (78%) of 432 cell lines obtained from immunized adult subjects were inhibitable by Hib PS, including 89 of 102 (87%) from subjects immunized with Hib-diphtheria toxoid conjugate and 122 of 204 (60%) of those immunized with plain Hib PS.
Twenty-seven percent of Hib PS binding mAbs from infants were of the IgG isotype, 35% were IgA, and 38% were IgM. Of cell lines from infants that were inhibitable by Hib PS, 46% were IgG, 46% were IgA, and 8% were IgM. Cell lines from adults immunized with HbOC, which were all inhibitable by Hib PS, were 71% IgG, 26% IgA, and 4% IgM.
Of the 27 Hib PS-inhibitable mAbs from infants that could be further
evaluated, 8 of 10
mAbs expressed the HibId-1 Id (associated with
the V
II A2 germline V
segment), and 16 of 17
mAbs expressed
the HibId-2 Id (associated with the V
VII DPL19 variable segment;
Table I
) (7, 12). Each of these Ids is
frequently expressed in the adult anti-Hib PS repertoire. No
mAb that was not inhibitable by Hib PS expressed the HibId-1 or HibId-2
Ids. All evaluable anti-Hib PS mAbs from adults immunized with HbOC
vaccine used
light chains; 24 of 44 (54.5%) were HibId-1
positive.
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A subset of anti-Hib PS Abs cross-reacts with the structurally
similar capsular PS of the K100 serotype of E. coli (7, 21).
All six Hib PS-inhibitable mAbs from infants cross-reacted with K100
PS. Three of these mAbs were tested for cross-reactivity with
streptococcal membrane Ags, pneumococcal type 3 PS, and tetanus toxoid,
and no reactivity was seen. Three of nine mAbs that were not
inhibitable by Hib PS cross-reacted with K100 PS, and six of eight
tested cross-reacted with streptococcal membranes (Table I
).
Reactivity with self Ags
We previously demonstrated that a small proportion of anti-Hib
PS mAb obtained from adults cross-react with ss- and dsDNA (8). Like
mAbs obtained from adults, high affinity mAbs from children had little
or no cross-reactivity with self-Ags (Table I
). In contrast, Hib
PS-reactive mAbs from infants that were not inhibitable by soluble Hib
PS usually cross-reacted with one or more self Ags, including cardiac
myosin, smRNP, DNA, and cardiolipin.
Variable region gene use
To identify the molecular basis of the anti-Hib PS repertoire
of infants we cloned and sequenced IgVH and
IgVL encoding the most stable of the mAb cell
lines (Tables 2 and
3). Two mAbs that were inhibitable by Hib
PS were studied. The mAbs MM1.5 and MM4.1 were obtained from the same
subject. These cell lines have identical IgVH genes,
encoded by heavy chain variable (VH) segments 100%
homologous to the V323/DP-47 germline gene, DN1 segments, and
germline JH4 genes. IgVL encoding these
hybridomas use the V
II A2 germline gene and germline J
3 segments
and express the Hibid1 Id. Like mAbs from adults, both IgV
have an
arginine residue at the VJ junction. Five nucleotide differences from
the A2 germline sequence are shared between MM1.5 and MM4.1, and the
former has an additional mutation resulting in a coding change in FR3.
The shared mutations suggest that these two hybridomas are clonally
related, although we were unable to identify heavy chain or light chain
rearrangements by Southern blotting to confirm this (data not shown).
Low affinity, non-Hib PS-inhibitable mAbs of infants are encoded by at
least five members of the VH3a gene family: V37, V321,
V323, V330, and DP-51, and a VH4 family member, V434.
D segments encoding these IgVH are, in general, long and
can be readily compared with known germline genes (DXP1, DIR1/2, and
DN1; Table II
). In one instance (MAX4) the D is generated by a D-D
fusion consisting of a DN1 segment and inverted DIR1 gene (22, 23). In
each of these IgVH, nongermline-encoded nucleotides are
present at V-D and D-J joints, which may represent N or P
addition (24, 25).
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or V
segments. Two mAbs from unrelated subjects
are encoded by V
I L1/J
3 combinations, and two are encoded by
V
VII DPL2/J
2/3 combinations. Although one mAb uses the V
II A2
gene that dominates the response of adults, this is paired with a
V321-encoded heavy chain, rather than the V323/V
II A2
combination observed in adults. Interestingly, this light chain also
lacks a VJ arginine residue and does not express the HibId-1 Id,
whereas, to our knowledge, all other V
IIA2-encoded light chains with
this residue are HibId-1 positive. The invariant VJ arginine residue
observed in anti-Hib PS Ab of adults is, in fact, absent in all
non-Hib PS inhibitable mAbs from infants (7). The noninhibitable mAb
MAX5 that is encoded by the adult VH segment, V323, is
paired with a V
III L6-encoded IgVL.
The homology of VH segments encoding infants mAbs to
germline genes ranges from 95.9 to 100.0% (Table II
). Half are
unmutated. The homology of V
and V
segments to candidate germline
segments is also high, ranging from 98.0 to 100.0% (Table III
). In
both heavy and light chains, however, there is little evidence of
Ag-driven somatic mutation (Table IV
).
Only the DUN1, DUN2, and RL1 IgVH have R:S ratios that
exceed the expected ratio of 2.9:1 (26). No IgVL
gene has an elevated R:S ratio.
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| Discussion |
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Attempts to generate hybridomas from infants aged 2 to 4 mo immunized with one or two doses of HbOC vaccine were unsuccessful. Although fusions generated large numbers of immortalized cell lines, few yielded mAbs specific for Hib PS, and none was sufficiently stable for additional characterization (data not shown). Larger numbers of Hib-PS-specific cell lines were obtained from older children aged 15 to 18 mo after four doses of HbOC vaccine, but this was a significantly lower proportion than was obtained from adults. Differences in the frequency of anti-Hib PS-specific mAbs may be an artifact of the fusion process; however, an identical protocol was followed for all age groups, suggesting an intrinsic difference in the numbers of Hib PS-specific circulating lymphocytes. Using a plaque assay to quantitate specific Ab-secreting lymphocytes, Munoz and Insel found between 2.0 and 16.8% of Ig-secreting cells in adults 8 days after immunization with plain Hib PS vaccine were specific for this Ag (27). Barington et al. described two adults immunized with Hib-TT in whom 52.5 and 82.9% of affinity-purified PBL were Hib PS specific (28). In this study 15 to 61% of cell lines from four adults immunized with HbOC bound Hib PS. Although each of these studies used different methodologies, collectively these data suggest that the frequency of Hib PS-specific lymphocytes is higher in adults than in children and higher in response to TD vaccines than to TI formulations.
The variable region gene repertoire of adults immunized with different
Hib vaccine formulations has been extensively characterized (7, 8, 29, 30). Most anti-Hib PS mAbs and most serum Ab of adults are encoded
by V315/DP-38 and V323/DP-47 VH segments (8, 10, 31).
These IgVH are further characterized by short (mean, 23
bases; range, 948 bases) and highly conserved CDR-3 regions (8).
Light chain variable region gene use in the adult anti-Hib PS
response is more diverse than IgVH use; however, on the
average, >50% of serum Ab of adults expresses the HibId-1 Id,
suggesting that the V
II A2 variable segment encodes a large
proportion of expressed anti-Hib PS Ab (32). V
II A2-encoded
IgVL are almost exclusively paired with
V323/DP-47 encoded IgVH. Almost all anti-Hib PS mAbs
obtained from adults have an arginine residue at the VJ joint (7, 8, 33, 34).
Whereas anti-Hib PS mAbs obtained from adults immunized with HbOC were all inhibitable by Ag, almost 30% of mAbs obtained from young children were not, suggesting that Ab secreted by these cell lines has low affinity for Ag. The greater frequency of high affinity Abs noted in adults may be attributable to the rapid proliferation of a pool of memory cells that have undergone affinity maturation as a result of repeated Ag exposure. Variable region genes encoding secondary anti-Hib PS immune responses of both adults and children, however, exhibit little evidence of affinity maturation. A more likely explanation may be that different vaccine formulations elicit distinct, but overlapping, Ab repertoires. Consistent with this is the observation that in adults, the proportion of high affinity mAbs is dependent on vaccine formulation, with low affinity mAbs most prevalent in response to TI plain PS vaccine (our unpublished data). The avidity of serum Ab of adults elicited by different Hib PS conjugate vaccines also varies (13, 35). Low affinity anti-Hib PS mAbs from adults are more likely to be encoded by infrequently expressed variable region genes, suggesting a greater fraction of serum Ab elicited by TI vaccine formulations may be encoded by an unconventional repertoire (our unpublished observation). In young children, the serum expression of certain light chain variable region families and Ids also varies with vaccine formulation, implying that variability in gene usage may be responsible for the functional differences observed in response to alternative TD Ag formulations (12). Our current studies confirm that the Ab repertoire of adults and infants elicited by one of these vaccines, HbOC, is fundamentally different when examined at the level of B cell responses.
The overall anti-Hib PS repertoire of infants is more diverse than
that of older subjects. Variable region genes encoding low affinity,
noninhibitable anti-Hib PS Ab of infants are encoded by at least
six different VH segments. Two of these genes, V323/DP-47
and V321/DP-77, have been described in the adult anti-Hib PS
repertoire (8, 29). In anti-Hib PS mAbs of adults, V323/DP-47 is
almost invariably paired with light chains encoded by the predominant
V
II A2 germline gene, whereas in the noninhibitable mAb MAX5, it is
paired with a V
III L6-encoded light chain. Likewise, the only V
II
A2-encoded noninhibitable anti-Hib PS Ab is not paired with
V323/DP-47, but with V321/DP-77. Whereas the IgVH CDR-3
of adult anti-Hib PS mAbs is typically short and highly conserved,
the IgVH CDR-3 region of noninhibitable anti-Hib PS
mAbs from infants is longer (mean, 64 nucleotides; range, 3684 bases)
and diverse. Notably, the invariant arginine residue at the light chain
VJ joint of adult anti-Hib PS IgVL is not
observed in any noninhibitable anti-Hib PS mAbs obtained from an
infant. Although not formally proven, these data suggest that three
molecular features are important for high affinity interaction with Hib
PS: heavy and light chain pairing, short IgVH CDR-3
regions, and the presence of an arginine residue at
IgVL VJ joints.
The higher affinity anti-Hib PS mAbs of young children immunized
with HbOC may also be encoded by a subtly different repertoire from
that of adults. Two of these mAbs are encoded by the canonical
V323/DP-47/V
II A2 combination. The IgVH D segment,
however, uses a DN1 gene in germline configuration and is relatively
long (39 nucleotides). The limited number of cell lines studied here
does not allow us to determine whether the minor differences between
these variable region genes and those encoding higher affinity adult
anti-Hib PS Abs are a usual feature.
Immunization with HbOC may selectively stimulate a population of B cells that is distinct from those elicited by other Ag formulations and that will remain stable, or over time and repeated Ag exposure it is possible that very small increases in affinity may shape the anti-Hib PS repertoire of these children to more closely resemble that of adults. Consistent with the former hypothesis is the observation of phenotypically similar, but functionally different, memory B cell subpopulations that may differ in responsiveness to secondary challenge with TI or TD Ag formulations in some murine immune responses (36). To date, the anti-Hib PS repertoire of adults has been studied in individuals who were primarily immunized with Hib PS as a result of natural exposure to Hib itself or to cross-reactive bacterial Ags, whereas both primary and subsequent exposures of infants were to a TD formulation (9, 10, 12, 13, 16, 31, 37). Further studies will be required to ascertain whether the anti-Hib PS repertoire of infants primarily immunized with TD Ag formulations evolves to resemble that of subjects with natural immunity or if the early repertoire persists.
Our data suggest that there may be considerable individual differences in response to Hib PS immunization. Other investigators have noted differences in serum Ab concentration and light chain expression elicited by different lots of the same vaccine (13, 37). These differences may result from variability in Ag preparations, diversity in the immunologic maturity of subjects, differing environmental Ag exposure, or intrinsic genetic differences in the Ig repertoire (38).
Compared with that in adult mice, a high frequency of unselected mAbs obtained from neonates expresses poly- and autoreactive Ab (39). The noninhibitable anti-Hib PS Ab of infants may be similar primordial polyreactive Abs (40). The low affinity (noninhibitable) mAb secreted by cell lines from infants are not reflected in serum Ab responses. We previously noted that serum Ab responses may not reflect contributions from all cell lines recovered by fusion of peripheral lymphocytes following immunization. This may occur because cells that contribute the majority of serum Ab may reside in large numbers in lymphoid tissues, whereas the cell lines secreting low affinity, cross-reacting Ab may secrete relatively small amounts of Ab in vivo. Alternatively, or additionally, the binding of low affinity Abs may be inhibited by high affinity Abs in the ELISA.
The relationship between polyreactive low affinity anti-Hib PS Abs and high affinity protective Ig is not clear. It is unlikely that such Igs contribute significantly to protective immunity. It is possible, however, that somatic mutation may increase the affinity for Ag and result in the selection of unconventional anti-Hib PS Abs from low affinity precursors. Although all the adult subjects studied by Silverman and Lucas expressed anti-Hib PS IgVH encoded by VH323/DP-47 and/or VH315/DP-77, a minority also had small amounts of serum Ab encoded by VH4 and/or VH1 family members; these may be examples of this phenomenon (10).
In summary, we demonstrate that the anti-Hib PS Ab repertoire of infants immunized with HbOC vaccine differs from that of adults. A significant proportion of B cells secrete Ab that binds to Hib PS with low avidity. These Abs often are polyreactive and are encoded by a variable region gene repertoire distinct from that of adults. The variable region gene repertoire of higher affinity anti-Hib mAbs of infants is more similar to the conventional repertoire of adults, but also has subtle differences that may be responsible for the distinct immunologic characteristics of each TD conjugate vaccine. Further studies will be required to determine whether primary exposure to Hib PS in a TD Ag formulation results in the modification of the mature anti-Hib PS repertoire.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. E. Adderson, Room 2R036, University of Utah Medical Center, 50 N. Medical Dr., Salt Lake City, UT 84132. E-mail address: ![]()
3 Abbreviations used in this paper: PS, polysaccharide; TI, T independent; TD, T dependent; Hib PS, Haemophilus influenzae type b polysaccharide; HbOC, Haemophilus influenzae type b oligosaccharide conjugate. ![]()
Received for publication March 16, 1998. Accepted for publication June 18, 1998.
| References |
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and V
segments and VJ combinations. J. Clin. Invest. 89:729.
II gene and one of several JK genes are joined by an invariant arginine to form the most common L chain V region. J. Immunol. 143:4110.[Abstract]
II gene and one of several J
genes are joined by an invariant arginine to form the most common L chain V region. J. Immunol. 143:4110.
light chain variable region that predominates the vaccine-induced human antibody response to the Haemophilus influenzae b polysaccharide. J. Clin. Invest. 88:1811.
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