Postnatal and adult immunoglobulin repertoires of innate-like CD19(+)CD45R(lo) B Cells

The diversity in antibody repertoire relies on different B cell populations working efficiently to fulfil distinct specific functions. We recently described an innate-like CD19(+)CD45R(-/lo) (19(+)45R(lo)) cell population in postnatal unstimulated adult mice, a heterogeneous population containing cells expressing immunoglobulin M (IgM) and others behaving as differentiated mature B lymphocytes (intracytoplasmic IgG1, AID(+), Blimp-1(+)RAG2(-)). In the present study, we characterized the Ig repertoire expressed by splenic 19(+)45R(lo) cells, assuming that they would bear a restricted repertoire biased for germline rearrangements and low mutation rates similar to other innate-like cells. Sequences from 19(+)45R(lo) cells displayed a variety of V, D and J regions, and the analysis of the CDR-H3 region revealed an intermediate overall CDR-H3 length and moderate hydrophobicity. Both IgM and switched sequences of PD15 19(+)45R(lo) cells had shorter CDR-H3 region and fewer non-template N nucleotides than adult sequences, as expected for profiles that correspond to an immature phenotype. Regarding the mutation rate in the VH regions, IgG1 sequences already carried a high rate of replacement mutations at PD15, which increased further in the sequences obtained from adult mice. Moreover, statistical models suggest that a proportion of the switched sequences in adult 19(+)45R(lo) cells had experienced antigen selection, unlike other innate-like B cell compartments.We thank B. Palacios for assisting with the cloning and sequencing, A.G. de la Campa, A. Trento, I. Cuesta and L. Garcia-Albert for their help with the bioinformatics software analyses, and M.A.R. Marcos for his contribution to the original planning of the experiments. This work was supported by grants from the Fondo de Investigaciones Sanitarias (FIS, MPY 1450/11) and from the Ministerio de Ciencia e Innovación (MICINN, SAF2009-12596) and Ministerio de Economia y competitividad (MINECO, SAF2012-33916). I. Cortegano received a postdoctoral fellowship from the MICINN