Inflammatory immune cells may impair the preBCR checkpoint, reduce new B cell production, and alter the antibody repertoire in old age

Abstract

Aging impairs development of new B cells and diminishes the expression of protective antibodies. Reduced numbers of B cell precursors generally occur in old (~2 yrs.) mice. At the pro-B to pre-B cell transition, the pre-B cell receptor (preBCR) checkpoint directs pre-B cell expansion and selection of the pre-B cell immunoglobulin (Ig) μ heavy chain variable region repertoire. The preBCR is comprised of Ig μ heavy chain + surrogate light chains (SLC; λ5/VpreB). In old B cell precursors, SLC is decreased and fewer pre-B cells form the preBCR. In pro-B cells, SLC is complexed with cadherin 17 to form a "pro-B cell receptor" whose signaling is postulated to increase apoptotic sensitivity. We propose that inflammation in old mice, in part mediated by the age-associated B cells (ABC), promotes apoptosis among pro-B cells, particularly those relatively high in SLC. The remaining pro-B cells, with lower SLC, now generate pre-B cells with limited capacity to form the preBCR. Ig μ heavy chains vary in their capacity to associate with SLC and form the preBCR. We speculate that limited SLC restricts formation of the preBCR to a subset of Ig μ heavy chains. This likely impacts the composition of the antibody repertoire among B cells.

Figure 1. Aging reduces pro-B and pre-B cells, decreases surrogate light chains, and impairs pro-B cell receptor and pre-B cell receptor expression

Bone marrow pro-B cells and pre-B cells are reduced in old (~2 yrs) vs. young adult (~3 mo.) mice. Lower surrogate light chain proteins, λ5 and VpreB, occur in old pro-B cells. SLC protein losses likely affect both the “pro-B cell receptor” (cadherin 17 [Cadh 17] + SLC) and the pre-B cell receptor (preBCR) (Ig µ heavy chain + SLC). Three early pre-B cells are shown which have different Ig µ heavy chain V region sequences. These µ heavy chains differ in their capacity to associate with SLC to form the preBCR (µ1>µ2 and µ3). In young adult mice, overall SLC is relatively high and all of these early pre-B cells express the preBCR, proliferate, and produce late stage pre-B cells. We propose that in old age pro-B cells with higher SLC levels preferentially are depleted, leaving pro-B cells lower in SLC. Under these conditions, only early pre-B cells with µ1 (higher binding to SLC) are capable of forming the preBCR, proliferating, and undergoing further B cell maturation.

Figure 2. In old pro-B cells with low surrogate light chain, increased ERK MAPK activity reduces E47/E2A and Bim proteins

Old pro-B cells have lower SLC and active E47/E2A and Bim proteins. We hypothesize that the “pro-B cell receptor” (cadherin 17 [Cadh 17] + SLC) in young adult pro-B cells inhibits ERK MAPK activation, resulting in lower levels of phosphorylated (active) ERK MAPK. This minimizes both phosphorylation and subsequent ubiquitination and degradation of E47/E2A and Bim proteins. Therefore, in young pro-B cells, E47/E2A and Bim levels are relatively high. In old SLC^Lo pro-B cells, cadherin 17/SLC complexes are reduced, signaling via the “pro-B cell receptor” is diminished, and phosphorylated (active) ERK MAPK is increased. Active ERK MAPK phosphorylates both E47/E2A and Bim proteins, targeting them for further ubiquitination and proteosomal degradation. This results in reduced levels of E47/E2A and Bim proteins in old SLC^Lo pro-B cells.

Figure 3. A hypothesis: Inflammatory ABC promote loss of B cell precursors in old mice, diminish new B cell production, and alter antibody repertoires

In this model, ABC and likely other cell types (NK cells?) increase inflammation in old bone marrow. This promotes apoptosis in B cell precursors and reduces B lymphopoiesis. B cell precursors relatively high in surrogate light chain (SLC) are preferentially targeted for apoptosis. Remaining B cell development continues on an “SLC^Lo” pathway and the lower levels of SLC available may constrain preBCR assembly. As proposed in Figure 1, those µ heavy chains with greater capacity to form the preBCR under low SLC conditions will be favored and this may alter the frequency of particular B cell specificities in old age. While overall B cell production is reduced in old age (unfilled down arrow), the generation of the antibody repertoire is altered with increased representation of particular clonotypes, e.g., T15^neg anti-PC (unfilled up arrow).