CHAPTER 9 B-Cell Development

Two electron micrographs show the successive stages of B-cell development. — B cells at various stages of development seek contact with stromal cells expressing CXCL12 (pre-pro-B cells, left) or IL-7 (pro-B cells, *right*).

Learning Objectives

After reading this chapter, you should be able to:

Understand the experimental approaches used to identify and order the various cellular stages of B-cell development.
Explain the mechanisms that drive the progression of cells through the various stages of B-cell development in the bone marrow and spleen.
Describe how heavy- and light-chain gene rearrangement events are programmed into defined stages of B-cell development.
Compare and contrast the first and second checkpoints in B-cell development in terms of their stages, signaling, consequences, and importance.
Identify four processes ensuring self-tolerance that operate at the immature B and transitional B stages.
Examine the basic similarities and differences between B-2, B-1a, B-1b, and marginal zone B cells.

Millions of B lymphocytes are generated in the adult bone marrow every day and exported to the periphery. The rapid and unceasing generation of new B cells occurs in a carefully regulated sequence of events. Cell transfer experiments (similar to those described in Chapter 2) that identified hematopoietic stem cells (HSCs), in which genetically marked donor HSCs are injected into unmarked recipients, have indicated that B-cell development from HSC to mature B cell takes from 1 to 2 weeks.

B-cell development begins in the bone marrow with the asymmetric division of an HSC and continues through a series of progressively more differentiated progenitor stages to the production of common lymphoid progenitors (CLPs), which can give rise to B cells, T cells, or innate lymphoid cells. These early stages in hematopoiesis and lymphocyte development were described in Chapter 2 (see Figures 2-1 and 2-3). Progenitor cells destined to become T cells migrate to the thymus, where they complete their maturation (see Chapter 8). The majority of CLPs that remain in the bone marrow enter the B-cell development pathway (Overview Figure 9-1). As differentiation proceeds, developing B cells express a precisely controlled sequence of cell-surface receptors and adhesion molecules. Some of the signals received from these receptors induce the differentiation of the developing B cell; others trigger its proliferation at particular stages of development; and yet others direct its movements within the bone marrow environment. These signals collectively allow differentiation of the CLP through the early B-cell stages to form the immature B cell that leaves the marrow to complete its differentiation in the spleen. For the investigator, the expression of different cell-surface molecules at each stage of B-cell maturation provides an invaluable experimental tool with which to recognize and isolate B cells poised at discrete points in their development.

OVERVIEW FIGURE 9-1

B-Cell Development Begins in the Bone Marrow and Is Completed in the Periphery

B-cell development begins with a hematopoietic stem cell (HSC) and passes through progressively more committed lymphoid progenitor cell stages until it reaches the pro-B-cell stage. At this stage, the precursor cell is irreversibly committed to the B-cell lineage, and the recombination of the immunoglobulin genes begins. Once the completed IgM is expressed on the cell surface, the immature B cell leaves the bone marrow to complete its maturation through the transitional T1 and T2 stages in the spleen.

B-cell development begins with a hematopoietic stem cell in eight stages that lead to the generation of immunoglobulin antigens and commitment. Stage 1 shows the earlier development in nostrils, intestines, and hands. The formation of B-cells in blood vessels occurs in stages 2 and 3. B-cell development begins in the bone marrow in stage 4, which also includes TH and TC cells. Stages 5 and 6 show the proliferation of B-cells and T-cells in the liver and heart, respectively. In stages 7 and 8, the lymphatic vessels contain B-cells, T-cells, and antigens.

An enlarged image of the bone marrow shows the progressive stages of lymphoid progenitor cells. The formation of small rounded B-cells starts in the bone endosteum. These cells undergo various developmental stages in the bone marrow by ordered gene rearrangements. The differentiation starts from the binding of B-cells to the irregularly shaped receptors, the secretion of antibodies around B-cells, and the binding of autogenerated antigens of self antigens to the antibodies. The immunoglobulin receptors involved in the commitment lineage are Pre-BCR, IgM, and IgD.

Immature B-cells leave the bone marrow via the central sinus to the blood vessel. They pass through the transitional T1 and T2 stages in the spleen to form mature B cells. Mature B cells leave the bone marrow to the periphery. The stages of B-cell development are labeled as follows: Early Pro-B: DJ H recombination and start of V-DJ H recombination; late Pro-B: V-DJ H recombination; Pre-B: micro H chain expressed as pre-BCR, several rounds of cell division, and VJ L chain recombination; and Immature B: mIgM expression, negative selection, deletion, and receptor editing.

The primary function of mature B cells is to detect pathogens and other potentially harmful foreign antigens and to differentiate into plasma cells secreting antibodies that protect the host against the invaders. Therefore, the most critically important events occurring during B-cell development are the rearrangements of immunoglobulin receptor heavy- and light-chain gene segments to form the B-cell receptor for antigen, determining its specificity. Recall from Chapter 6 that immunoglobulin gene rearrangements begin with heavy-chain D-to-J_H gene segment rearrangement, followed by the stitching together of the V_H and DJ_H segments to allow synthesis of an intact μ heavy chain. This chain initially pairs with a surrogate light chain, allowing the cell-surface expression of the pre-B-cell receptor. This initial form of membrane immunoglobulin (Ig) activates several rounds of cell division, followed by rearrangement of the light-chain V and J gene segments, allowing the immature B cell to express membrane IgM (mIgM).

As is true for T cells (see Chapter 8), developing B cells must solve the problem of creating a diverse repertoire of receptors capable of recognizing an extensive array of antigens, while ensuring that self-reactive B cells are either eliminated or inactivated. Processes responsible for self-tolerance occur at several stages of B-cell development. Development of B cells is somewhat simpler than that of T cells, in that B-cell receptors recognize intact antigens, not antigen fragments presented by MHC proteins, and hence do not need to be selected for those that recognize self-MHC molecules. Also unlike T-cell development, B-cell development is almost complete by the time the B cell leaves the bone marrow; in mammals there is no thymus equivalent in which B-cell development is accomplished. Instead, immature B cells are released to the periphery, where they complete their developmental program in the spleen.

In this chapter, we will follow B-cell development from its earliest stages in the primary lymphoid organs to the generation of fully mature B cells in the secondary lymphoid tissues. Most of this chapter will focus on the predominant (or conventional) B-cell population, known as B-2 B cells (or follicular B cells), derived through HSC-initiated hematopoiesis. As for T cells, however, several B-cell subsets exist, and later in this chapter we will briefly address how the processes of differentiation of the minority subsets, that is, B-1 B cells and marginal zone (MZ) B cells, differ from the developmental program followed by the conventional B-2 B cells. We will conclude with a brief comparison of the maturational processes of T and B lymphocytes.