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The Journal of Immunology, 1999, 162: 2129-2136.
Copyright © 1999 by The American Association of Immunologists

Functional Characterization of the Somatic Hypermutation Process Leading to Antibody D1.3, a High Affinity Antibody Directed Against Lysozyme

Patrick England, Roland Nageotte, Martial Renard, Anne-Laure Page and Hugues Bedouelle1

Protein Engineering Group (Centre National de la Recherche Scientifique-URA 1129), Unité de Biochimie Cellulaire, Institut Pasteur, Paris, France


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
The impact of somatic hypermutation on the affinity of Abs directed against protein Ags remains poorly understood. We chose as a model the secondary response Ab D1.3 directed against hen egg lysozyme. During the maturation process leading to this Ab, five replacement somatic mutations occurred. After reconstituting the germline Ab from which D1.3 originated, we assessed the energetic and kinetic importance of each of the somatic mutations, individually or combined, using the BIAcore apparatus. We found that the mutations induced an overall 60-fold improvement of affinity, principally due to a decrease in the kinetic rate of dissociation. We showed that their effects were additive and context independent; therefore, in the case of D1.3, the order in which somatic mutations were introduced and selected is unimportant. Interestingly, most of the affinity improvement was due to a single somatic mutation (Asn50->Tyr in VL), involving a residue that belongs to the functional interface between Ab D1.3 and lysozyme. This replacement could either establish new Van der Waals contacts between the Ab and the Ag or help stabilize the conformation of a closely situated crucial residue of the Ab paratope. The four other mutations played only a marginal part in affinity maturation; potential reasons for which these mutations were nevertheless selected are discussed.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Affinity maturation is an outstanding feature of the development of T cell-dependent Ab responses against antigenic challenge 1 . In mammals, three different, but closely interrelated, types of phenomena lead to this maturation. First, a great diversity of Ag binding sites is generated, through V(D)J germline gene segment recombination with joining errors, followed by somatic hypermutation of the assembled variable genes. Then, the higher affinity variants are selected at the surface of B cells 2 .

The molecular basis of somatic hypermutation remains largely unknown, although it has recently been shown that DNA repair following error-prone replication is involved 3 . Point mutations are introduced in a stepwise manner, at an estimated rate of 10-4–10-3/base/generation (106 times more than the spontaneous rate of mutation), with a strong bias for transitions over transversions 4 . The existence of intrinsic hot spots of mutation has been revealed 5 .

Somatic hypermutation occurs mainly among rapidly dividing B lymphocytes in the dark zone of the lymphoid germinal centers. Clonally related B cells, each displaying only one type of membrane Ig, then migrate into the light zone of the germinal center where they compete for Ag binding (for review, see 6 . To avoid apoptosis, B cells must indeed take up Ag, which is presented in its native form at the surface of long term Ag-retaining follicular dendritic cells (FDC)2, 7 . Ag-selected B cells can then give rise to bone marrow Ab-forming cells or undergo new cycles of amplification/hypermutation and selection before becoming memory lymphocytes 8 .

The Ag-selected pattern of somatic mutations has been well characterized in the case of small haptenic Ags, but only for a few Abs has the functional role of these somatic mutations been assessed by site-directed mutagenesis 9, 10, 11, 12, 13 . In these cases, it was found that only some of the somatic mutations actually improved affinity (3- to 14-fold each), while most of them had no significant effect on the affinity of the Ab for its cognate Ag.

In the case of protein Ags, much less work has been performed. For cytochrome c, a protein hosting a strongly immunodominant epitope, the pattern of somatic mutations could be characterized in clonally related Abs, but the functional significance of these mutations was not assessed by site-directed mutagenesis 14 . In the case of an anti-idiotypic Ab, the affinity maturation process could be reconstituted and shown to be stepwise, but the different individual affinity improvements were not quantified 15 .

To our knowledge, no comprehensive study of the effects of somatic mutations on the affinity of an Ab toward a protein Ag has been performed to date. Such studies are nevertheless crucial, as results obtained from anti-hapten Abs cannot be expected to extrapolate to Abs directed against protein Ags. The nature and the shape of the two types of interfaces are altogether different; in particular, Ab-protein interfaces bury a surface at least 3 times larger than Ab-hapten interfaces and statistically involve 2–3 times more contacts 16 . This observation opens up the possibility that more somatically mutated residues are effectively involved in the improvement of the affinity for protein Ags than for haptens.

Furthermore, controversy has recently arisen about the real importance of affinity maturation in the shaping of high affinity repertoires of Abs directed against protein Ags 17 . First, Zinkernagel and collaborators 18 isolated early Abs directed against the vesicular stomatitis virus that were devoid of somatic mutations and nevertheless showed nanomolar avidities, without further significant improvement through somatic hypermutation. Second, no significant differences were observed between the average avidities of Abs from anti-lysozyme primary, secondary, or later responses 19 .

We precisely chose as a model Ag the well-characterized monomeric protein hen egg white lysozyme (HEL). More than 100 mAbs have been raised against HEL 19, 20, 21, 22, 23, 24 . Cross-reaction studies using panels of evolution-related avian lysozymes have allowed identification of the general region of the Ag and some specific amino acid residues bound by each mAb. Interestingly, unlike many other protein Ags, it appears that the whole surface of HEL is potentially antigenic 19 . This correlates well with the sequence data, which are available for only about 10 mAbs; none has been proven to be clonally related, although two have been shown to use a close combination of germline gene segments 25 .

In this study we assessed the impact on the affinity for HEL of each replacement somatic mutation that occurred during the maturation process leading to a particular mAb. The mouse mAb D1.3 is derived from a secondary immune response 24 . The genes coding for its variable domains, VH and VL, have been cloned and sequenced 26 . The interface between the Fv fragment of D1.3 and HEL has been well characterized at both the structural 27 and the functional 28, 29, 30 level.

On the one hand, we reverted each of the somatic mutations that D1.3 contains, and on the other hand, we introduced them into the reconstituted germline antibody from which D1.3 originated.3 Some mutations were introduced simultaneously to study their additivity. The kinetic parameters of the interaction between the mutant Fv fragments of D1.3 and immobilized HEL were measured using the BIAcore apparatus.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Identification of somatic mutations

We used both the FastA search and alignment software included in the version 9.1 of the Wisconsin Genetics Computer Group package (Madison, WI) and the DNAPLOT alignment software (developed by W. Müller and H.-H. Althaus, University of Cologne, Cologne, Germany; website: http://www.genetik.uni-koeln.de/dnaplot/) to compare the DNA sequences of the VH and VL domains of D1.3 with the sequences of the murine Ig germline gene segments that are available in the GenBank and EMBL databases.

Mutagenesis and production of somatic mutants

Phagemid pVD91 has been previously described 31 ; it allows the expression of the hybrid protein VH::VL-MalE, in which the VL variable domain of Ab D1.3 is covalently linked to the N-terminus of MalE and noncovalently associated with VH to form a heterodimeric Fv fragment. We introduced mutations into the VH::VL-MalE hybrid by oligonucleotide site-directed mutagenesis of pVD91. The codon changes in positions H-56 (AAC->AGC), H-86 (CAC->CAG), L-50 (TAT->AAC), L-51 (ACA->GCA), and L-52 (ACA->AAA) were introduced individually or simultaneously. The mutageneses were performed as described, using the ssDNA of pVD91 or its mutant derivatives as templates 32 . The sequences of the mutated genes (either VH or VL) were checked by the dideoxy chain termination method, using the T7 Sequencing Kit (Pharmacia Biotech, Uppsala, Sweden). We produced and purified the native and mutant VH::VL-MalE hybrids and checked their full reactivity toward HEL, as previously described 30 .

Binding assays and evaluation of the kinetic data

HEL was covalently immobilized on the carboxymethylated dextran matrix of a CM5 sensor chip to a level of 500–600 resonance units, using the Amine Coupling Kit (BIAcore AB, Uppsala, Sweden). The molecular interactions between the VH::VL-MalE hybrids and immobilized HEL were measured with the BIAcore 2000 apparatus as previously described 30 . The association and dissociation profiles were analyzed with a nonlinear least squares algorithm implemented in the BIAevaluation 2.1 software package (BIAcore AB), using double-exponential functions of time as previously described 30 .

No differences in the calculated association and dissociation rates were observed in control experiments, in which reduced surface densities of immobilized HEL (down to 50 resonance units) and/or higher flow rates (up to 30 µl/min) were used, showing that the interactions were not limited by mass transfer. Furthermore, we observed no significant difference in the kinetics of dissociation whether free HEL was present or not (at a saturating concentration of 1.5 µM or more) as a competitive ligand for VH::VL-MalE, showing that no rebinding of VH::VL-MalE to immobilized HEL occurred during the dissociation phase with the conditions used.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Identification of the somatic mutations in D1.3

We performed a thorough database search to identify the murine Ig germline gene segments for which the alignment with the genes coding for the variable domains of D1.3 was best. The identification was unambiguous for the mature gene coding for the VL domain of D1.3. Indeed, the identity between the 286 5'-terminal nucleotides coding for VL-D1.3 and the germline subgroup VK-V, isotype 12/13, variable (V) segment K2 33 was 97.9%, compared with 74% for VK isotype 10. The identity between the 38 3'-terminal nucleotides coding for VL-D1.3 and the germline joining (J) segment JK1 was 100%, compared with 84.2% for JK2.

For the VH domain, there was no ambiguity concerning the V and J segments used. The identity between the 295 5'-terminal nucleotides coding for VH-D1.3 and germline subgroup VH-IB V-segment PJ14 34 was 98.6%, compared with 90–91% for the next best two V segments. The identity between the 41 3'-terminal nucleotides coding for VH-D1.3 and germline J segment JH2 was 100% compared with 83% for JH4. As for the diversity (D) region, the germline segment DSP2.10 (or DSP2.11) 35 was used in reading frame 2 (Fig. 1Go).



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  		FIGURE 1. Comparison of the nucleotide sequences of the variable domains of D1.3 with those of the closest murine germline gene segments. A dash signifies that the nucleotide is the same in the two sequences. Somatic mutations are marked in standard characters when they are silent and in shaded characters when they lead to an amino acid replacement. Mutations due to junction errors during the V(D)J recombination are indicated in italics. a, Heavy chain variable domain of D1.3 (D1.3-VH) aligned with the PJ14, DSP2.10, and JH2 gene segments. b, Light chain variable domain of D1.3 (D1.3-VL) aligned with the K2 and JK1 gene segments. {dagger}, The codon numbering used is continuous to facilitate the comparison with the crystal structure data file. The codon numbering is the same using the Kabat convention, except for codons 83, 84, and 85 of VH, which become, respectively, 82a, 82b, and 82c, and codons 86–116, which become 83–113.

 
The alignment shows that three junctional errors may have occurred during the recombination of gene segments leading to the VH mature gene, two at the V-D junction and another at the D-J junction. Subsequent somatic hypermutation appears to have resulted in 10 somatic mutations in the V segment-encoded parts of VH and VL, four transitions and six transversions. Five of these mutations were silent, while five others led to amino acid replacements (Table IGo and Fig. 2Go). Among these, only the residue at position 50 of VL was in contact with the Ag in the three-dimensional structure of the Fv fragment of D1.3 liganded with HEL 27 .


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  		Table I. Replacements of amino acid residues during the somatic hypermutation process leading to Ab D1.3

 


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  		FIGURE 2. Structural locations of the residues of Ab D1.3 replaced during somatic hypermutation. The {alpha}-carbon traces of the variable fragment of Ab D1.3 (VH, solid lines; VL, dashed lines) and of HEL (ribbon representation) are shown. The side chains of the mutated residues are identified by a label. The atomic coordinates were obtained from the Protein Data Bank (reference no. 1VFB).

 
Rationale for mutagenesis

In a previous study we cloned the genes coding for the Fv fragment of Ab D1.3 into a phagemid that allows its expression as a fusion with protein MalE from Escherichia coli 31 . We have shown that the presence of MalE does not interfere with the interaction between HEL and the Fv fragment of D1.3 30 . Therefore, in this study we used Fv-MalE hybrids, in which the VL domain is covalently linked to the N-terminus of MalE and noncovalently associated with VH, without cleavage between the fusion partners.

We first reconstituted the germline VH and VL domains from which D1.3 originated, i.e., we simultaneously reverted the five nonsilent above-mentioned somatic mutations (Table IGo) by oligonucleotide site-directed mutagenesis. The protein that is encoded by this quintuple reversion gene is hereafter called germline Ab. We then reintroduced individually each of the five nonsilent somatic mutations (H-S56N, H-Q86H, L-N50Y, L-A51T, and L-K52T) in this germline antibody (upper part of Fig. 3Go).



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  		FIGURE 3. Schematic representation of the somatic hypermutation process and the mutagenesis strategy. Details are given in the text (see Results).

 
To determine whether the effects of the somatic mutations were dependent on their structural context, we also reverted individually each of the five nonsilent somatic mutations in Ab D1.3 (H-N56S, H-H86Q, L-Y50N, L-T51A, and L-T52K). The resulting single reversion mutants of D1.3 were equivalent to germline Abs in which four somatic mutations (all except the reverted mutation) had occurred (lower part of Fig. 3Go).

Finally, to determine whether the effects of somatic mutations were additive, we constructed two additional derivatives of the germline Ab, one double mutant (H-S56N/Q86H) and one triple mutant (L-N50Y/A51T/K52T) (middle part of Fig. 3Go).

Effects of the somatic mutations on the kinetic parameters of the interaction with HEL

For each mutation, we expressed and purified the corresponding Fv-MalE hybrid, with a yield of 0.5–2 mg of pure and fully reactive protein/l of culture medium at an OD600 of 1.5. No correlation between the yield and the nature of the mutation was observed (data not shown).

The kon, koff, and K'd measured at 20°C for the germline Ab, Ab D1.3, and the 12 different mutants are given in Tables II and III. Both the kon and koff values varied by factors of <2, at the limit of significance, except for mutation L-N50Y, its reversion counterpart L-Y50N, and the multiple mutants that included these mutations. The koff and K'd values of mutant L-N50Y were indeed 30-fold lower than those of the germline Ab (Table IIGo), while the koff and K'd values of mutant L-Y50N were 50-fold higher than those of Ab D1.3 (Table IIIGo).


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  		Table II. Effects of the somatic mutations on the kinetics of interaction between the germline Ab and HEL1

 

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  		Table III. Effects of the reversion mutations on the kinetics of interaction between Ab D1.3 and HEL1

 
We calculated the variation in the free energy of interaction ({Delta}{Delta}G) resulting from each of the five somatic mutations when introduced into the germline Ab or Abs at intermediate stages of the maturation process (Table IVGo). None of the mutations had markedly different effects according to the context in which they were introduced.


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  		Table IV. Effects of the somatic mutations on the free energy of dissociation from HEL and their dependence on the structural context

 
We addressed the question of the synergy between the somatic mutations by constructing some of the possible double- or triple-mutant intermediates that might have occurred during the affinity maturation process. In these cases, no significant difference between the {Delta}{Delta}G for the multiple mutant and the sum of the {Delta}{Delta}G values for the single mutants could be observed if one took into account the SEs. For instance, for the triple mutation L-N50Y/A51T/K52T, {Delta}{Delta}G(L-N50Y/A51T/K52T) = 2.08 ± 0.13 kcal/mol, while {Delta}{Delta}G(L-N50Y) + {Delta}{Delta}G(L-A51T) + {Delta}{Delta}G(L-K52T) = 1.87 ± 0.15 kcal/mol.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Nature of the mutations selected during somatic maturation

By comparing the nucleotide sequences of the variable genes coding for the VH and VL domains of Ab D1.3 with the sequences of the germline gene segments coding for mouse Igs, which are available in various databases, we determined that 10 somatic mutations had occurred during the maturation process leading to D1.3. Six of the mutations were transversions, and four were transitions, even though somatic hypermutation is intrinsically biased toward transitions over transversions 4 . Thus, the mutations may have been Ag selected. Furthermore, we observed that all the silent mutations were situated outside the CDRs (Fig. 1Go), whereas four replacement mutations of five were situated in the CDRs (Table IGo). The fact that a somatic replacement occurs inside a CDR has often been considered to suggest Ag selection. However, such an assertion is grossly misleading, as many CDR residues do not contribute to the energy of interaction with the Ag, while some non-CDR may play an important part. This is notably the case for Ab D1.3 28, 29, 30, 36 .

Only one of the five residues that are replaced by the hypermutation process, L-Tyr50, is actually in contact with the Ag in the crystallographic structure of the complex between the Fv fragment of Ab D1.3 and HEL 27 . Structural studies of other complexes between Abs and Ags have shown that the replacement of noncontact residues by somatic hypermutation is a widespread phenomenon 37, 38, 39, 40 . However, only seldom has the functional role of distal somatic replacements been assessed and shown to be important 12, 37 . No such functional characterization has yet been conducted in the case of protein Ags.

Effects of the somatic mutations and structural interpretation

We determined the kinetic parameters of the interaction between HEL and each somatic mutant using the BIAcore apparatus. First, we showed that the global affinity improvement induced by the five somatic replacements, i.e., the difference in affinity for HEL between the germline Ab and Ab D1.3, was approximately 60-fold (Table IIGo). This result establishes clearly that for Abs directed against HEL, a significant maturation of affinity through somatic hypermutation can occur, at least in one example. The reason why the average avidity of anti-HEL Abs does not vary between the primary and the secondary response in the study by Newman et al. 19 might be that most of the affinity maturation occurs very fast, before the early primary response Abs were isolated by day 7, or that the end-point titration assay used can only efficiently detect differences in affinity of at least an order of magnitude (as suggested by the authors themselves) and/or that the scattering of experimental points is high enough to conceal the maturation at the level of individual clones. The 60-fold improvement reported here is indeed in the upper range of affinity enhancements observed when comparing germline and hypermutated Abs directed against protein antigens 14, 15, 18, 41 , whereas in the case of haptenic Ags, much higher affinity improvements through somatic hypermutation have been reported, up to 30,000-fold 40 . It has been suggested that the physiological role of somatic hypermutation and selection is to allow Abs to attain a 109-1010 M-1 avidity ceiling regardless of the starting point 17 . We have shown here that the parental germline Ab of D1.3 has a micromolar affinity for the monovalent Ag HEL (Table IIGo), and therefore the impact of affinity maturation can be important in this case. However, the affinities of other anti-HEL, and more generally anti-protein, germline Abs may be either higher or lower. In the case of highly multivalent Ags such as the vesicular stomatitis virus, some germline Abs may already attain nanomolar avidities, making further somatic hypermutation superfluous 18 .

Second, we found that the affinity maturation leading to Ab D1.3 appeared to be exclusively due to a decrease in the rate of dissociation (koff) of the complex between the Ab and the Ag, which amply made up for a small decrease in the rate of association (kon). The preponderant part played by koff in affinity maturation by somatic hypermutation could be a general phenomenon, as it has been observed for all the Abs that have been characterized at a kinetic level whether they were directed against protein or haptenic Ags 14, 40, 41, 42 . From a theoretical standpoint, this observation could be explained by the fact that point mutations in an Ab paratope do not significantly change its overall hydrophobicity and electrostatic charge, which are preponderant for the long range attraction forces that rule kon, whereas they can have a strong effect on short range interactions, such as Van der Waals or hydrogen bonds, which rule koff 30 . This interpretation is backed by the fact that, unlike point mutations, the global CDR replacements that occur upon repertoire shift lead principally to improvements in kon 42 .

The main and most unexpected feature of the affinity maturation process leading to Ab D1.3 was that it appeared to be driven almost exclusively by one somatic mutation, L-N50Y. The 30-fold decreases in koff and K'd that it induced, are the highest reported to date for a single somatic mutation in any Ab. Indeed, the 9000-fold increase in affinity that has been reported in one case does not correspond to a replacement due to somatic hypermutation, but to a joining error during the V(D)J recombination of germline gene segments 43 . Abs that are directed against small haptens and for which only one or few somatic mutations play a preponderant part in affinity maturation have often been observed 9, 10, 11, 12 . However, it had been hypothesized that affinity maturation of Abs directed against protein Ags would involve a large number of small affinity improvements 15 . Our results clearly refute this theory and show that the hapten paradigm can also be applied to Abs directed against protein Ags, at least in the case of Ab D1.3.

The importance of somatic mutation L-N50Y can be better understood by analyzing the functional effects of other mutations at this position and the contacts that residue L-Tyr50 establishes in the crystal structure of the complex between HEL and the Fv fragment of Ab D1.3 27, 28, 30 . Mutation L-Y50F had previously been constructed in Ab D1.3 and shown to induce only a 2- to 4-fold increase in koff and K'd 28, 30 . These data suggest that the main effect of L-N50Y is not the establishment of contacts between the O{eta}H group of L-Tyr50 and HEL, either directly or mediated by water molecules 27 . The appearance of an aromatic cycle at position 50 could be the key feature of L-N50Y: tyrosine would have been selected because one of its corresponding codons (TAT) could be reached through a single mutation from the original asparagine codon (AAT) used in the germline V-K2 gene segment, unlike those for phenylalanine or tryptophan. The observation of the crystal structure can help explain the importance of an aromatic cycle in position L-50. On the one hand, the aromatic cycle of L-Tyr50 and the side chains of two residues of HEL (Asp18 and Asn19) form Van der Waals contacts, which could be energetically important. On the other hand, the aromatic cycles of L-Tyr50 and L-Tyr32 are parallel and closely in contact in the crystal structure; this geometry is favorable for aromatic stacking, and L-Tyr50 could stabilize the side chain of L-Tyr32 in an optimal configuration for the establishment of hydrogen bonds with HEL. The correct positioning of the O{eta} atom of L-Tyr32 is indeed crucial, as shown by mutation L-Y32F, which induces a 30- to 40-fold increase in koff and K'd 28, 30 .

The contrast between the importance of mutation L-N50Y and the apparent needlessness of the other four somatic replacements in terms of affinity maturation was striking; indeed, mutations L-A51T and L-K52T had no detectable effect, while H-S56N and H-Q86H induced hardly significant decreases in koff and K'd (Table IIGo). We contemplated the possibility that these mutations, although unimportant individually, played a synergistic part in affinity maturation. In this case, the {Delta}{Delta}G induced by a multiple somatic replacement ({Delta}{Delta}Gmultiple) would be higher than the sum of the {Delta}{Delta}G values induced by its constituent single mutations ({Sigma}({Delta}{Delta}Gsingle)). However, we observed neither long range synergy between mutations H-S56N and H-Q86H ({Delta}{Delta}Gmultiple = 0.15 ± 0.11 kcal/mol; {Sigma}({Delta}{Delta}Gsingle) = 0.27 ± 0.12 kcal/mol) nor close range synergy between mutations L-Y50N, L-A51T, and L-K52T ({Delta}{Delta}Gmultiple = 2.08 ± 0.13 kcal/mol; {Sigma}({Delta}{Delta}Gsingle) = 1.87 ± 0.15 kcal/mol). This suggested that the individual effects of the somatic mutations were independent and additive, and in particular that mutations L-A51T and L-K52T had no close range indirect effects on the residue in position L-50, whose importance has been discussed above.

To further investigate potential synergies between the somatic mutations, we reverted each of them, alone or in combination, in Ab D1.3. In this configuration, mutations H-H86Q and L-T52K had no detectable effect, while H-N56S and L-T51A induced hardly significant increases in koff and K'd, and multiple reversion mutations did not display any synergistic effect (Table IIIGo). Our results also showed that the somatic mutations induced the same {Delta}{Delta}G, taking into account the SEs, when introduced into the germline antibody or reverted in Ab D1.3, alone or in combination (Table IVGo). The effect of the somatic mutations on affinity was therefore context independent, suggesting that during the in vivo process of somatic maturation that led to Ab D1.3, the order in which mutations occurred and were selected was unimportant. This conclusion cannot be extrapolated to other Abs, especially if their affinity maturation relies on several key mutations, instead of one as for Ab D1.3. Indeed, in each of the two other cases where the affinity maturation by hypermutation has been characterized to the same extent of thoroughness as here (i.e., for two Abs directed against haptens p-azophenylarsonate and phosphocholine), three key mutations drive the affinity maturation process and the effects of the somatic mutations are context dependent and nonadditive 12, 13, 38 .

Hypotheses about the potential role of neutral somatic mutations

As discussed above, the reason why somatic mutations L-A51T, L-K52T, H-S56N, and H-Q86H were selected remains unclear. One possibility is that the small improvements in affinity induced by the mutations L-A51T, H-S56N, and H-Q86H are significant at some stage of the selection process. It is unlikely that these marginal improvements grant any selective advantage to an Ab during the competition for Ag capture from the FDC, but small differences in affinity may lead to profound changes in further steps, such as the processing and presentation of Ag to the T lymphocytes 44, 45 .

Another possibility is that somatic mutations L-A51T, L-K52T, H-S56N, and H-Q86H are truly neutral in terms of affinity maturation. In this case, three different types of explanations could be given for the presence of these mutations.

1) Some of the neutral mutations might have been coselected together with mutation L-N50Y or may reflect a background of mutational noise. At present, the frequency at which the Ab-bearing B lymphocytes are confronted to the Ag-bearing FDC is unknown. Theoretical calculations have suggested that hypermutation could generate as many as three somatic mutations before the Abs face selection in the presence of Ag 46 . Although these predictions await experimental confirmation, they could explain the existence of at least some of the four neutral somatic mutations in Ab D1.3. It has also been shown that high doses of injected immunogens, such as those given to the mice that produced Ab D1.3, favor a strong background of neutral somatic mutational noise 47 .

2) Some mutations may reflect intrinsic mutational hot spots of the somatic hypermutation machinery. H-S56N is the only mutation, in the case of Ab D1.3, that could be due to the presence of a characterized hot spot of hypermutation; indeed, the original germline codon for H-Ser56 (AGC) is in the context of the well-established (A/G)G(C/T)(A/T) hot spot motif (AGCA in this case; Fig. 1Go) 5 .

3) The neutral mutations may grant a selective advantage according to criteria other than affinity. For example, it has been shown that mutations in V genes may affect the folding of Abs 48 , their secretion 49, 50 , their glycosylation 51 , or their physical stability 52, 53 . The analysis of the crystallographic structure of the Fv fragment of Ab D1.3 shows that the side chain of residue L-Thr51 is totally buried inside the molecule, while those of residues L-Thr52, H-His86, and H-Asn56 are largely accessible to the solvent. Somatic mutation L-A51T could establish new contacts between the side chain of L-Thr51 and the backbone and/or the side chains of residues L-Asn31, L-Ser65, L-Gly66, and L-Tyr71, and therefore stabilize the ß-pleated structure of VL. Other important factors might be the level of intracellular expression, the solubility, the interaction with folding chaperones, and the susceptibility toward proteolytic degradation. The maturation process may also affect the relative stabilities of the different conformational isomers of the same Ab, leading to a more favorable isomeric equilibrium 54 . Moreover, Ag-driven selection operates on membrane-bound Igs, and somatic mutations may affect their interaction with other membrane constituents.

Most likely, a combination of the three points discussed above is involved in the selection of mutations L-A51T, L-K52T, H-S56N, and H-Q86H. To test whether these neutral mutations are significant and whether the importance of mutation L-N50Y leads to its recurrent selection, and to identify whether other patterns of somatic mutations could result in efficient affinity maturation, it would be interesting to isolate, in HEL-immunized mice, other mAbs that would use exactly the same V(D)J germline gene segment combination as Ab D1.3. Until now, no such mAb has been identified, although partial N-terminal peptide sequencing suggests that mAb HyHEL7 may use the same VH and V{kappa} gene segments as D1.3 25 . However, the D and J segments used may not be the same, as HyHEL7 does not appear to recognize the same HEL epitope as D1.3 19 . Another possibility would be, taking as a starting point the germline Ab from which D1.3 originated, to perform artificial affinity maturation experiments using in vitro or in vivo random mutagenesis protocols combined with selection by phage display 28, 55 . We are currently conducting such experiments.


   Footnotes
 
1 Address correspondence and reprint requests to Dr. Hugues Bedouelle, Unité de Biochimie Cellulaire, Institut Pasteur, 28 rue du Docteur Roux, 75724 Paris Cedex 15, France. E-mail address: Back

2 Abbreviations used in this paper: FDC, follicular dendritic cell; HEL, hen egg lysozyme; MalE, maltose-binding protein of Escherichia coli; Fv, variable fragment; kon, association rate constant; koff, dissociation rate constant; K'd, equilibrium dissociation constant; {Delta}{Delta}G, variation of free energy; CDR, complementarity-determining region. Back

3 Mutants are named with the prefix denoting the heavy chain (H) or the light chain (L) variable domain, followed by the single-letter code for the residue found in the wild-type Ab (germline or D1.3), then the sequence position, and finally the residue in the mutant. Back

Received for publication July 20, 1998. Accepted for publication November 4, 1998.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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