A landscape for the dynamics of an immune response

Affinity maturation of B cells involves not only a few but a whole spectrum of relevant mutations

Affinity maturation of B lymphocytes within germinal centers involves both diversification of their B-cell receptors (BCRs) by somatic hypermutation (SHM) and a crucial receptor-mediated selection step. However, in contrast to recent advances in revealing the molecular mechanism of SHM, the fundamentals of the selection process are still poorly understood, i.e. it is often not clear how and how many mutations contribute to improving a BCR during the response against a given antigen. A general drawback in assessing the mutations relevant to the selection process is the difficult task of rating the relative contributions of selection and intrinsic biases to the experimentally observed mutation patterns of BCRs. The approach proposed here is premised on statistical comparison of the frequency distributions of nucleotide substitutions as observed in datasets of hypermutated BCRs against their frequency distribution expected under the null hypothesis of no selection. Thereby, we show that the spectrum of mutations relevant to maturation of canonical anti-(4-hydroxy-3-nitrophenyl)acetyl BCRs is much broader than previously acknowledged, going beyond the scope of single key mutations. Moreover, our results suggest that maturation not only involves selection by means of affinity but likewise expression and stabilization of BCRs.

Initial repertoire of anti-(4-hydroxy-3-nitrophenylacetyl) antibodies as potential donors for effective affinity maturation

We previously found that there are two distinct antibody (Ab) maturation pathways for the immune response of C57BL/6 mice to 4-hydroxy-3-nitrophenylacetyl (NP), one involving Abs with high evolvability (group-H) and the other involving Abs with low evolvability (group-L). Commitment to whichever pathway is followed pre-determined in B cells at an early developmental stage. Candidates for the group-L or -H pathway are thus expected to pre-exist in the initial repertoire of the immune response. In the present study, we examined the initial Ab repertoire from the viewpoint of the latent potential of these Abs for effective affinity maturation. At first, we prepared anti-NP B cell hybridomas at 1 week postimmunization. Although the diversity of the obtained repertoire was maintained mainly by the third complementarity determining region of the heavy chain (CDR-H3), their changes in the near UV circular dichroism resulting from NP-binding allowed for classification into three groups according to the same rules applied in the pathway classification of the maturated Abs. This suggested that the innate structural properties of CDR-H3 were conserved throughout maturation. In other words, in exploring the structure of CDR-H3, it is possible to distinguish the latent potentials of Abs in effective affinity maturation even those making up the initial Ab repertoire. We then examined an artificially designed group-H Ab prototype and found its NP-binding ability sufficient for engagement in the initial repertoire. The question arose here as to why the majority of the actual initial repertoire consisted of the group-L ancestors regardless of their middling NP-binding affinity, which called for further discussion from the viewpoint of the dynamics possibly shaping the repertoire.