Differential regulation of early and late stages of B lymphocyte development by the {micro} and {delta} membrane heavy chains of Ig

Regulation of B lymphocyte development by the µ-membrane(µ_m) and -membrane (_m) heavy chains of Ig was examinedin an Ig transgenic mouse model. Mice were bred on a commonC57BL/6 (B6) background, and expressed rearranged and hypermutatedheavy and light chain transgenes encoding high-affinity receptorsfor the foreign antigen hen egg rysozyme (HEL). At no stagewere they exposed to HEL. variation of the Ig heavy chain constructyielded four different types of Ig transgenlc mice in whichdeveloping B lineage cells either expressed µ_m and _m inthe normal physiological sequence (µ_m then µ_m+_m),or produced µ_m alone, µ_m alone or µ_m +_m fromthe onset of heavy chain expression in the bone marrow. ImmatureB220^low, HSA^high and mature B220^high, HSA^low B cells were producedin all mice regardless of their developmental pattern of µ_mand _m expression. However, production of immature B cells wasmost efficient when µ_m heavy chain was expressed aloneduring early B cell development Thus expression of _m duringthis period either in the presence or absence of µ_m resultedin a 2- to 3-fold reduction in the numbers of immature B cellsin the spleen as well as altered levels of surface B220 andHSA on these cells in spleen and bone marrow respectively. Bycontrast, normal maturationally regulated expression of _m ledto the presence of increased numbers of mature B cells in thespleen and lengthened the average lifespan of these cells asdetermined by in vivo incorporation of 5-bromo-2'-deoxyurtdlne.These results pointed to selective effects of µ_m and _mheavy chains on regulation of the early and late stages of Bcell development respectively, and provided a rational basisfor co-expression of µ_m and _m as well as the delayed expressionof _m during normal B cell development     

The skewed heavy-chain repertoire in peritoneal B-1 cells is predetermined by the selection via pre-B cell receptor during B cell ontogeny in the fetal liver

As many as 5–15% of B-1 cells in the peritoneal cavityof adult mice produce antibodies reactive to phosphatidylcholine(PtC) and the vast majority of them express B cell receptors(BCRs) composed of V_H11-µH chains utilizing the J_H1 segmentand V9-L chains. This extremely skewed repertoire of PtC-reactiveB-1 cells is traditionally attributed to the expansion of particularclones in response to self or exogenous antigens. Here, we showthat the strong bias toward the J_H1 usage among V_H11-µHchains is already established prior to the BCR assembly, namelyat the transition from the large to the small pre-B cell stageduring B cell ontogeny in the fetal liver. Among V_H11-µHclones isolated from large pre-B cells where the J_H1 skewingwas not established yet, the J_H1 users showed the highest abilityto form pre-B cell receptor (pre-BCR) and to induce cellularproliferation and differentiation when expressed in fetal liverpro-B cells. Thus, the J_H1 users were positively selected andamplified at the pre-BCR checkpoint. When co-expressed withV9-L chains to form BCR, the J_H1 users almost exclusively conferredthe PtC reactivity on BCR even though other J_H users could alsoform BCR on the cell surface. Therefore, the pre-BCR-mediatedpositive selection of the J_H1 users among V_H11-µH chainsappears to be beneficial to the efficient generation of ‘innate-type’PtC-reactive B cells during the fetal B cell development, evenbefore the self-renewal or the antigen-driven clonal expansionof B-1 cells takes place in the peritoneal cavity.     

Peripheral CD4+ T‐cell tolerance is induced in vivo by rare antigen‐bearing B cells in follicular,marginal zone,and B‐1 subsets

B cells are efficient APCs when they internalize antigen via BCR‐mediated uptake. Adoptively transferred antigen‐presenting B cells can induce T‐cell tolerance to foreign and self antigens; however, it is unknown whether endogenous B cells presenting self‐peptides interact with naïve T cells and contribute to peripheral T‐cell self‐tolerance. Moreover, the relative abilities of mature B‐cell subsets to induce T‐cell tolerance have not been examined. To address these questions, we created a new mouse model wherein a very small fraction of B cells expresses an antigen transgene that cannot be transferred to other APCs. We limited antigen expression to follicular, marginal zone, or B‐1 B‐cell subsets and found that small numbers of each subset interacted with naïve antigen‐specific T cells. Although antigen expressed by B‐1 B cells induced the most T‐cell division, divided T cells subsequently disappeared from secondary lymphoid tissues. Independent of which B‐cell subset presented antigen, the remaining T cells were rendered hypo‐responsive, and this effect was not associated with Foxp3 expression. Our data show that physiologically relevant proportions of B cells can mediate peripheral T‐cell tolerance, and suggest that the mechanisms of tolerance induction might differ among follicular, marginal zone, and B‐1 B‐cell subsets.