# Absent the pre-B cell receptor checkpoint, the B-1a immunoglobulin CDR-H3 repertoire normalizes by convergent selection

## TL;DR

The study shows that without a specific immune checkpoint, B-1a cells still develop a similar antibody repertoire through natural selection.

## Contribution

The study reveals convergent selection in B-1a cells compensates for the absence of a preBCR checkpoint.

## Key findings

- Absence of preBCR selection leads to differences in gene segment usage and nucleotide sequences in B-1a, B-1b, and B-2 CDR-H3s.
- At the amino acid level, B-1a CDR-H3 repertoires partially converge with wild-type sequences, especially with tyrosine selection.
- This convergence does not restore original VDJ architecture and is less evident in B-1b and B-2 cells.

## Abstract

Peritoneal cavity (PerC) B cells arise via a specific set of ontogenetic and antigen-driven selection pressures. A large portion of the PerC B-1 population, including B-1a and B-1b cells, is fetal-derived and thus, due to a lack of N nucleotide addition, produces an immunoglobulin repertoire that is characteristically germline-encoded. While conventional B-2 cells depend on efficient pairing of surrogate light chain (SLC) and μ Heavy Chain (HC) to pass through the preBCR checkpoint, the extent to which the passage of the B-1 population through the preBCR checkpoint affects the composition of its repertoire has remained an open question. In this work, we used mice deficient in λ5, a key component of the SLC, to test whether the absence of preBCR selection would yield natural, germline encoded, PerC B-1 CDR-H3 repertoires similar to WT. And, if not, whether a more WT-like repertoire at the protein level would be recreated by convergent cellular evolution. We found that the absence of preBCR selection led to categorical differences in DH-specific gene segment usage, RF preference, and N addition in PerC B-1a, B-1b, and B-2 CDR-H3s when compared to WT. Despite this marked divergence in nucleotide sequence, at the translated amino acid level, the B-1a CDR-H3 repertoire generated in the absence of preBCR selection exhibited partial convergence to the WT sequence, most notably with selection for tyrosine; this phenomenon was less apparent in B-1b and B-2 cells. This convergence was not evident at the nucleotide level and did not restore the underlying VDJ architecture. These results suggest that while the initial CDR-H3 repertoire is altered genetically in the absence of preBCR selection, natural self-antigen selection and self-renewal in the B-1a compartment promote the creation of a CDR-H3 repertoire that has considerable overlap in amino acid sequence with its wild-type counterpart.