Target Discovery and Validation Reviews and Protocols

Overview of Immune System 287
Once a functional heavy chain is made, it associates with other proteins into
a signaling complex known as the pre-BCR (Fig. 4). In addition to a homodimer
of Igµ, the pre-BCR contains the products of the surrogate light chain
(SLC) genes λ5 and VpreB and a heterodimeric signaling module made up of
the transmembrane proteins Igα and Igβ (CD79α and CD79β) (17,24). A
developing pro-B-cell that makes a functional Ig heavy chain is rescued from
apoptosis and enters the pre-B-cell compartment, whereas nonproductive VDJH
rearrangement on both alleles leads to programmed cell death (25). Cell surface
expression of a functional pre-BCR induces two to five cell divisions, leading to
a large pre-B-cell population that subsequently becomes small resting pre-B-cells
(24,25). Notably, this process generates B-cell clones expressing a common µ
heavy chain but with the potential to express a diverse array of light chains,
thereby extending B-cell repertoire. Similarly to T-cells, the pre-BCR functions
as a biological sensor that informs pro-B-cells that they have succeeded in
rearranging a functional Ig heavy chain (17).
Once a functional/productive µ heavy chain is formed, the cell proceeds to
the pre-B-stage, expresses the recombinase, and renders the light chain loci
(λ and κ) accessible to the recombination machinery. The κ-chain locus is
generally rearranged before the λ-chain locus; the latter chain only initiates
rearrangement if the κ-locus rearrangement has failed to generate a productive
light chain. In contrast to T-cells, the mechanism of allelic exclusion also occurs
at the light chain loci.
As soon as a functional light chain has paired with the preexisting µ-heavy
chain, a complete IgM molecule is displayed on the cell surface in association
with Igα and Igβ, forming the BCR of immature B-cell (Fig. 4). If the newly
expressed receptor encounters a strong interaction with self-antigens, B-cell
development is inhibited, and the cell is deleted by apoptosis. This process is
the first negative selection process that B-cells undergo in the bone marrow.
Negative selection at this stage would therefore promote elimination of
unwanted autoreactive specificities while preserving immature B-cells reactive
with foreign antigens. However, a large proportion of autoreactive B-cells
escape clonal deletion by altering their BCRs specificity, so that they are no
longer autoreactive, a process called “receptor editing” (26,27) (Fig. 4). Thus,
immature B-cells maintain the rearrangement machinery upregulated to allow
for secondary rearrangements at the IgL chain gene loci. In addition to clonal
deletion, autoreactive B-cells are inactivated via a second mechanism called
anergy (25). Anergic B-cells express a unique surface phenotype in that they
express much reduced sIgM (because of receptor modulation after antigenic
exposure), normal IgD levels, and are desensitized to further receptor-induced
signal transduction. The “choice” of an immature B-cells to undergo either
clonal deletion or clonal anergy is mainly determined by the concentration