B-1a Cell Diversity: Nontemplated Addition in B-1a Cell Ig Is Determined by Progenitor Population and Developmental Location

Abstract

Natural antibodies produced by B-1a cells are required for immediate protection against infection. The protective capacity of natural antibodies is attributed to germ-line like structure, which includes the relative absence of N-region addition. Previous studies have shown B-1a cell immunoglobulin from aged mice contains abundant N-additions. B-1a cells have been shown to derive from a specific Lineage negative (Lin-)CD45Rlo/-CD19+ progenitor found both in fetal liver and adult bone marrow. Here, we report identification of a fetal liver population characterized phenotypically as Lin-CD45R-CD19-, which gives rise to IgM+IgDloCD45RloCD5+Mac-1+CD19hiCD43+CD23lo B-1a cells upon adoptive transfer to SCID recipients. These B-1a cells derived from the Lin-CD45R-CD19- fetal liver population produce natural antibody that binds pneumococcal antigens, but this immunoglobulin contains substantial N-addition despite initial absence of TdT. Furthermore, we show extensive N-addition is also present in B-1a cells derived from the Lin-CD45Rlo/-CD19+ B-1 progenitor found in the bone marrow. Together these results demonstrate B-1a cell N-addition depends on the type of progenitor and the location of the progenitor during its development. These findings have implications for how regulation of different progenitors from fetal liver and bone marrow may play a role in the age-related increase in N-region addition by B-1a cells in normal animals.

Age-Related Decline in Natural IgM Function: Diversification and Selection of the B-1a Cell Pool with Age

“…On the other hand, IgM from BMD B-1a cells contained a large number of N-additions at both 1 month and 3 months post transfer with 9% and 7% of sequences lacking N-additions at both junctions, respectively. These results are consistent with our previously published data demonstrating BMD B-1a cells are derived from progenitors with high levels of TdT and produce IgM with abundant N-additions (40, 44). Interestingly, at 10 months post transfer IgM from BMD B-1a cells displayed a decrease in the number of N-additions at both junctions.…”

“…An alternative explanation for these results might be if bone marrow (BM) from aged mice produce B-1a cells expressing IgM with low levels of N-addition, which if true, would contrast with previous work demonstrating BMD B-1a cells from young mice produce IgM with abundant N-additions (40). Thus, we considered the possibility that changes observed in N-addition from BMD B-1a cell IgM 10 months after adoptive transfer of bone marrow (Figure 6) could be due to an age-related change in the function of BM progenitors.…”

“…It has been previously shown that bone marrow derived (BMD) B-1a cells produce Ig with significantly more N-region addition than fetal liver derived B-1a cells (38, 40, 44). Therefore, we hypothesized the age-related change in natural antibody sequence and decreased protective capacity of natural IgM could be due to an increase in diversity produced by emigration of BMD B-1a cells into the adult B-1a cell pool.…”

“…Peritoneal CD5+ B-1 cells were sorted onto a 48-well AmpliGrid (Advalytix). Single cell PCR was then carried out as previously described (40). Sequencing data was deposited into NCBI’s Genbank (http://www.ncbi.nlm.nih.gov/genbank/) and the accession numbers for the sequences are listed in Supplemental Figure 7.…”

“…As the animal moves into adult life TdT expression allows for some diversity in the B-1a cell pool. In particular, the adult B-1a progenitors express high levels of TdT, which give rise to B-1a cells producing Ig with many N-additions (40). In the work presented here we demonstrate bone marrow B-1a cell progenitors add to the adult B-1a cell pool and both the fetal and bone marrow derived B-1a cells are exposed to selection pressures with increasing age.…”