Persistent Polyclonal B Cell Lymphocytosis B Cells Can Be Activated through CD40-CD154 Interaction

Chronic Lymphocytic Leukemia B-Cell Normal Cellular Counterpart: Clues From a Functional Perspective

Chronic lymphocytic leukemia (CLL) is characterized by the clonal expansion of small mature-looking CD19+ CD23+ CD5+ B-cells that accumulate in the blood, bone marrow, and lymphoid organs. To date, no consensus has been reached concerning the normal cellular counterpart of CLL B-cells and several B-cell types have been proposed. CLL B-cells have remarkable phenotypic and gene expression profile homogeneity. In recent years, the molecular and cellular biology of CLL has been enriched by seminal insights that are leading to a better understanding of the natural history of the disease. Immunophenotypic and molecular approaches (including immunoglobulin heavy-chain variable gene mutational status, transcriptional and epigenetic profiling) comparing the normal B-cell subset and CLL B-cells provide some new insights into the normal cellular counterpart. Functional characteristics (including activation requirements and propensity for plasma cell differentiation) of CLL B-cells have now been investigated for 50 years. B-cell subsets differ substantially in terms of their functional features. Analysis of shared functional characteristics may reveal similarities between normal B-cell subsets and CLL B-cells, allowing speculative assignment of a normal cellular counterpart for CLL B-cells. In this review, we summarize current data regarding peripheral B-cell differentiation and human B-cell subsets and suggest possibilities for a normal cellular counterpart based on the functional characteristics of CLL B-cells. However, a definitive normal cellular counterpart cannot be attributed on the basis of the available data. We discuss the functional characteristics required for a cell to be logically considered to be the normal counterpart of CLL B-cells.

B-cell signaling in persistent polyclonal B lymphocytosis (PPBL)

Persistent polyclonal B lymphocytosis (PPBL) is a benign hematological disorder characterized by a selective expansion of circulating polyclonal marginal zone (MZ)-like B cells. Previous reports demonstrated that cases of PPBL showed poor activation, proliferation and survival of B cells in vitro, yet the underlying defect remains unknown. Here we report for the first time an attenuated activation of the canonical NF-κB (nuclear factor of kappa light polypeptide gene enhancer in B cells) and mitogen-activated protein kinase/extracellular signal-regulated kinase pathway after CD40 stimulation. This defect was selective, as alternative NF-κB signaling after CD40 stimulation and both B-cell receptor- and Toll-like receptor 9-mediated activation remained unaffected. Reduced canonical NF-κB activation resulted in decreased IκBα and CD40 expression in resting cells. In PPBL patients, expression of Bcl-xL in MZ-like B cells did not increase upon activation, consistent with the high apoptosis rates of PPBL-derived B cells that were observed in vitro. The B-cell phenotype of mice with selective knockouts of early components of the CD40 signaling pathway resembles PPBL, but sequencing corresponding genes in sorted MZ-like B cells of PPBL patients did not reveal relevant genetic alterations. Nevertheless, the frequently observed mutations in early signaling components of the NF-κB pathway in MZ lymphomas underline the relevance of our findings for the pathogenesis of PPBL.

Estrogen receptor signal in regulation of B cell activation during diverse immune responses

The role of signalling through oestrogen receptors (ERs) in the regulation of B cell activation is an area of growing importance not only in terms protective immunity but also in the determination of the mechanisms of the onset of autoimmune disorders and cancers. The mode of signalling action of this single chain nuclear receptor protein molecule depends on its ability to bind to the promoters of Pax5, HOXC4 and apolipoprotein B RNA-editing enzyme activation-induced cytidine deaminase (AID) genes. ER-mediated transcriptional regulation induces class switch recombination of the immunoglobulin heavy chain variable (VH) to DH-JH genes and somatic hypermutation in developing B cells. The mode of action of ER is associated with BCR-signal pathways that involve the regulator proteins BAFF and APRIL. Additionally, the plasma membrane-bound G protein-coupled oestrogen receptor-1 (GEPR1) directs diverse cell signalling events in B cells that involve the MAPK pathways. These signals are immensely important during progenitor and precursor B cell activation. We have focused our goals on the medicinal aspects of ER-signalling mechanisms and their effects on polyclonal B cell activation.