Factors regulating the development of B cell

In this article, I briefly explain the factors regulating the development of B cells.

Table of Contents

The development of immune cells

Immune cells develop from immature precursors in the primary lymphoid organs, i.e., in the bone marrow and thymus. The mature antigen-specific lymphocytes first come across antigens in the secondary lymphoid organs like the spleen, lymph nodes, and the specialized areas in the gut and other mucosal tissues.

The B and T lymphocytes arise from a single cell type, the hematopoietic stem cell. In the bone marrow, the B cell development starts with the asymmetric division of a hematopoietic stem cell. This division continues through a sequence of increasingly more differentiated progenitor stages. As a result, common lymphoid progenitors are produced. These common progenitors give rise to B cells, T cells, and innate lymphoid cells.

The majority of common lymphoid progenitors remaining in the bone marrow get an entry into the B-cell development pathway. The developing B cells express cell-surface receptors and adhesion molecules. The signals from these receptors induce the differentiation of the developing B cell into an immature B cell. The immature B cell leaves the bone marrow to complete its differentiation in the spleen.

During the development of B cells, the rearrangement of immunoglobulin receptor heavy and light chain gene segments occurs to form the antigen-specific B cell receptor, determining its specificity. The development of B cells is simpler than T cells. This happens because B cell development is almost complete by the time it leaves the bone marrow.

The events in the bone marrow during the development of B cell

The bone marrow contains microenvironments in which hematopoietic stem cells and various bone marrow stromal cells reside. The hematopoietic stem cells survive for a long period with the support of the proteins secreted by the stromal cells. The proteins also help in the division of the hematopoietic stem cells.

The precursors of B cells at various developmental stages interact with stromal cells to produce some proteins. These proteins help the developing cells to differentiate and progress within the bone marrow in an orderly manner. The receptor c-kit (CD117) is expressed by the hematopoietic stem cells and the early progenitor cells. It binds with ligand stem cell factor, thus promoting the differentiation of cells into progenitor cells while maintaining their niche.

When a developing B cell reaches the pre-pro-B-cell stage, it needs a signal from a chemokine CXCL12, secreted by certain stromal cells. This signal helps the B cell to enter into the pro-B-cell stage. Another group of stromal cells secrete the cytokine IL-17, which is recruited by pro-B cells to mature to the pre-B stage. The expression of specialized transcription factors is induced by many stromal cells for the development of B-cells.

Processes defining the stages of B-cell development

The cells possess different characteristic features during their developmental process. They have surface molecules, including adhesion molecules, and receptors for chemokines and cytokines. A group of active transcription factors determine the genes to be expressed at each step of the developmental process. While B cells develop, the developmental stages are also defined by the rearrangement of heavy and light-chain immunoglobulin genes.

To characterize B cell development, researchers applied various experimental approaches. The first one was to generate antibodies against the molecules present on the surface of bone marrow cells. Then, which of these molecules were present at the same time as other antigens, and the antigens defining the unique cell types were determined.

In the second approach, scientists were able to confirm the order of cell populations and their genetic rearrangements. They did it by first identifying cells with precise combinations of the cell-surface markers. They then evaluated those cells for the type of daughter cells, as well as for the occurrence of immunoglobulin gene rearrangements.

In the third approach, to arbitrate the effects of elimination of expression of a particular gene, scientists applied the power of knockout genetics. However, knockout genetics has a snag as it only defines the first stage in differentiation. In the first stage, there is a requirement of the transcription factor.

The studies shown by recent research

Recent researches have a different approach to conditional knockouts where a gene gets deleted in certain circumstances, like in a specific cell type or developmental stage. In another approach, known as knock-in experiments, a gene with a marker (green fluorescent protein) is inserted into the genome under the same regulatory control of a transcription factor. In the knock-in animal, the fluorescent marker can detect every cell in which the transcription factor is expressed.

Recent research on B cell development has discovered vital roles played by epigenetic changes. The changes affect a gene’s expression but do not affect the DNA sequence of the gene itself. Epigenetic changes involve DNA methylation, chromatin alterations like histone modification, and chromatin structure remodeling. The effects of microRNAs decrease the stability of mRNAs, thus inhibiting protein expression.

Chromatin immunoprecipitation is a technique used to study epigenetic changes. In this study, chromatin is fragmented into small pieces, and an antibody to a transcription factor is used to immuno-precipitate fragments of chromatin containing that protein. The genes to which that protein has bound can be known by analyzing the isolated fragments.

The role of transcription factors in developing a hematopoietic stem cell into a common lymphoid progenitor

Hematopoiesis is initiated by a unique set of surface proteins expressed by the hematopoietic stem cells. The receptor for the stem cell factor c-kit is one of the surface proteins, and their interaction spurs precise signals for the differentiation into multipotent progenitor cells. The stem cell-associated antigen-1 (Sca-1), another surface protein that along with the c-kit, is expressed in early progenitor cells. However, as cells proceed to the lymphoid cell lineage, their level of expression decreases.

The transcription factors like Ikaros, Purine box factor 1(PU.1), and E2A take part in the incipient stages of lymphocyte development. Ikaros brings in chromatin remodeling complexes to specific regions of the DNA. In this way, it makes sure the accessibility of the necessary genes for B-cell development.

The transcription factor PU.1, with its low and high levels, determines the lymphoid or myeloid differentiation. With its high levels, PU.1 favors myeloid differentiation, whereas with its low levels, it favors lymphoid differentiation.

The transcriptional repressor Gfi1 regulates the expression of PU.1. The transcription factor PU.1 is down regulated by Gfi1 to the levels needed for progressing down the lymphoid pathway. The transcription factors, Ikaros and PU.1, combine to cause the expression of the transcription factor E2A. E2A plays an important role in B-cell development.

The Fms-related tyrosine kinase 3 receptor (FLT-3) is expressed by progenitors, which binds to the membrane-bound FLT-3 ligand on bone marrow stromal cells. This binding signals the progenitor cell to start synthesizing the IL-7 receptor (IL-7R) α chain.

The cells entering the lymphoid lineage

When the cells progress to the lymphoid lineage, they start to express RAG1/2 and terminal deoxynucleotidyl transferase. Then, they begin the preparation for the rearrangement of their antigen receptor genes. The cell is defined as an early lymphoid progenitor cell by the expression of RAG1/2 and terminal deoxynucleotidyl transferase.

Some early lymphoid progenitor cells migrate out of the bone marrow and serve as T cell progenitors in the thymus. Others stay in the bone marrow as B-cell progenitors.

The increase in the expression of IL-7R decreases the expression of c-kit and Sca-1 proteins. Thus, this develops the early lymphoid progenitor (ELP) into a common lymphoid progenitor (CLP).

At the common lymphoid progenitor stage, the progenitor loses the myeloid potential. However, it retains the potential to mature into T cells, NK cells, and dendritic cells. The expression of the key transcription factor early B-cell factor 1 (EBF1) is activated by signals received from the IL-7R along with transcription factors E2A and Foxo1. The expression of early B-cell factor 1(EBF1) is necessary for B-cell differentiation.

The role of protein kinases JAK1,JAK3 and STAT5

The JAK/STAT pathway plays a role in signaling through the IL-7 receptor (IL-7R). The binding of IL-7 to IL-7R activates two protein kinases, JAK1 and JAK3. These two kinases phosphorylate and activate STAT5. This causes dimerization of STAT5, which gets an entry to the nucleus to act as a transcription factor.

The cell survival is supported by an anti-apoptotic factor Mcl-1, the expression of which is upregulated by STAT5. The cell proliferation at the pro-B-cell stage is activated by the protein products of the two genes, c-Myc and N-Myc, whose expression is increased by the transcription factor E2A.

Conclusion

Immune cells develop in the primary lymphoid organs, bone marrow, and the thymus. There are different factors regulating the development of a B cell. The B cell precursors at different stages of development interact with bone marrow stromal cells to produce some proteins that help in the differentiation of the developing cells and progress within the bone marrow in an orderly manner.

During the development of B cells, the rearrangement of immunoglobulin receptor heavy and light chain gene segments occurs to form the antigen-specific B cell receptor, determining its specificity.

At each step of B cell development, the genes to be expressed are determined by a group of active transcription factors. Various experimental approaches are applied by researchers to characterize the B cell development.

Various transcriptional factors play an important role in the development of a hematopoietic stem cell into a lymphoid progenitor cell. The transcription factors like Ikaros, Purine box factor 1(PU.1), and E2A take part in the incipient stages of lymphocyte development.