Blk haploinsufficiency impairs the development, but enhances the functional responses, of MZ B cells

Abstract

Blk was identified two decades ago as a B cell-specific member of the Src family of tyrosine kinases. Recent studies, however, have discovered that Blk is expressed in many cell types outside of the B lineage, including early thymic precursors, IL-17-producing γδ T cells and pancreatic β-cells. In light of these recent discoveries, we performed a more comprehensive analysis of Blk expression patterns in hematopoietic cells and found that Blk is differentially expressed in mature B cell subsets, with marginal zone (MZ) B cells expressing high levels, B1 B cells expressing intermediate to high levels, and follicular (FO) B cells expressing low levels of Blk. To determine whether these differences in Blk expression levels reflected differential requirements for Blk in MZ, B1 and FO B cell development, we analyzed the effects of reducing and eliminating Blk expression on B cell development. We report that both Blk-haploinsufficiency and Blk-deficiency impaired the generation of MZ B cells. Moreover, although there were fewer MZ B cells in Blk+/− and Blk−/− mice compared to Blk+/+ mice, Blk mutant MZ B cells were hyper-responsive to B cell receptor stimulation, both in vitro and in vivo. Thus, this study has revealed a previously unappreciated role for Blk in the development and activation of MZ B cells.

Concordance of Increased B1 Cell Subset and Lupus Phenotypes in Mice and Humans Is Dependent on BLK Expression Levels

“…To compare the phenotypes among different Blk genotypes or between young and aged groups of mice correspondingly, we only present data from females in all of the results below. We confirmed an increase of peritoneal B1a (PerC B1a) cells in young Blk−/− mice (Figure 2A & 2B) using the same gating strategy as previously (24). Interestingly, aged mice had at least, 10 times more PerC B1a cells than young mice (Figure 2B).…”

“…This clearly indicated that the contribution to autoimmunity is derived from reduction or loss of Blk . Samuelson et al reported that reduced expression of Blk enhanced nephrosis in B6.lpr/lpr mice, but failed to observe any obvious autoimmune symptoms in the B6.Blk+/ - mice (17, 24). The aged group of female mice in our report is 1-yr-old, but they used 5- to 6-month-old mice.…”

“…Revisiting immune phenotypes in the Blk knockout mouse in the C57BL/6 background revealed a role for Blk in the production of higher levels of anti-nuclear antibodies (ANAs), increased B1a cell numbers in the peritoneal cavity, and the presence of hyper-responsive marginal zone B (MZ B) cells (24). …”

“…We therefore utilized Blk+/+ , Blk+/− and Blk−/− mice, representing differential expression levels of Blk mRNA and BLK protein (24), and performed a comprehensive analysis of their phenotypes to investigate if these animals develop any kidney disease. In parallel, we investigated several peripheral blood cell populations of healthy human donors genotyped for the human SNP rs2736340 in the promoter region of the BLK gene (3).…”

Agammaglobulinemia: causative mutations and their implications for novel therapies

“…For the relatively mild agammaglobulinemias, such approaches should only be used if the associated risks can be minimized. Keeping that in mind, a curative treatment would be preferred, as even the current, AKT AKT1/2-deficiency results in impaired generation of marginal zone and B1 B-cells and survival of follicular B-cells [87] BCL10 BCL10 deficiency results in defective development and function of follicular, marginal zone and B1 B-cells [88] BLK BLK deficiency causes impaired generation and activation of marginal zone B-cells [89] CARD11 CARMA1 deficiency results in impaired B-cell proliferation in response to BCR activation, defective maturation of MZ B-cell population and disrupted CD5 + cell compartment in the peritoneal cavity (similar to the xid mouse) [90] CD19 CD19 deficiency does not seriously affect the early B-cell generation in the bone marrow, the mice have reduced numbers of B-cells in peripheral lymphoid tissues and low serum Ig levels [31] CNB1 Calcineurin deficiency results in reduced numbers of B1 B-cells in the spleen and peritoneal cavity, and there are higher total serum IgM levels [91] ERK1/ERK2 ERK1 and ERK2 deficiency causes impaired pre-BCR-mediated cell expansion and pro-B to pre-B-cell partial developmental block [92] FOXO1/FOXO3 Conditional inactivation of FOXO1 at the early pro-B-cell stage results in a complete arrest in B-cell development, and its inactivation in late pro-B-cells causes arrest at the pre-B-cell stage. FOXO3 k. o. shows significant reduction in pre-B-cell numbers in the bone marrow, and also increased basal levels of some Ig subclasses [93] [94]…”