Immunotherapy for Infectious Diseases

Humoral Immunity 17
In contrast to T-cell recognition, B-cells recognize native antigen that is not necessarily
associated with cells. B-cell development also begins in an isolated environment
in the bone marrow, where high avidity self-reactive B-cells are deleted (26,27).
Although it was generally thought that most B-cell-negative selection occurred in the
bone marrow (28), several lines of evidence point to a key distinction from T-cell development.
First, the bone marrow appears to export a larger proportion of the B-cells that
it produces than the thymus (29,30). These newly exported B-cells are relatively immature
cells that migrate from the bone marrow to the outer T-cell zones of the white pulp
of the spleen (31). These newly emigrated splenic B-cells express high levels of the
heat-stable antigen (HSA), a maturation marker common to developing B- and
T-cells (32). By contrast, HSA hi T-cells are found only in the thymus, as maturing
T-cells lose HSA expression before migrating to the periphery (33–36). Second, when
the recirculating B-cell repertoire has attained an adult size and steady state, only a
small fraction of these recent bone marrow emigrants persists after reaching the splenic
T-cell zone (31,37,38). The cells that do persist have a skewed V-region repertoire
(39,40). This splenic restriction point in B-cell production eliminates unwanted B-cells
by the same order of magnitude as occurs for T-cells exclusively in the thymus. A key
question is whether immature B-cells are selected against within the splenic T-cell zone
because they fail a positive selection step for particular specificities or because they
trigger a negative selection step against particular specificities.
The first evidence that immature B-cells are negatively selected in the spleen came
from Cyster et al. (41), who showed that self-reactive B-cells recognizing circulating
lysozyme antigen accumulate in the T-cell zone of the spleen and are excluded from
migration into the B-cell follicles, with an efficiency that is directly proportional to the
level of self-ligand present, the affinity of the receptor, its signaling properties, and the
presence of competing B-cells (41–44). Self-reactive cells that are excluded from
the follicular recirculating repertoire are short lived (1–3 days), whereas, cells that
enter the B-cell follicles are long lived and recirculate for 1–4 weeks (42).
The significance of this follicular exclusion checkpoint in negative selection of selfreactive
B-cells has recently been extended by studies tracking the development of
B-cells specific for double-stranded DNA (anti-dsDNA), a clinically important specificity,
in the context of a polyclonal B-cell repertoire. Mandik-Nayak et al. (45) have
shown that prototypic anti-dsDNA B cells are not deleted in the bone marrow but are
exported to the spleen as relatively immature cells with a short half-life relative to
the bulk of the repertoire. They also show that these autoreactive cells localize to the
interface between the B-cell and T-cell zones of the spleen. Together with the lysozyme
model antigen data, and the evidence that many immature cells are competitively
selected against at this site, it seems likely that B-cells bearing many different autoreactive
specificities will join the peripheral B-cell population and be subject to selection
at this stage and site within the spleen.
The exclusion of newly produced autoreactive B-cells from the B-cell follicles
places these potentially pathogenic cells in a site known to be important for the initiation
of antibody responses to foreign antigens—the outer T-cell zone (46, 47). Indeed,
autoantibody-producing cells in autoimmune mice appear and accumulate in the outer
T-cell zone (48), and it has been proposed that the pathogenic autoantibody production
results from a failure of B-cell tolerance in this site (49). Self-reactive cells that are