Abstract
Although evidence suggests that peripheral B
cell death plays a role in maintenance of B cell
homeostasis including elimination of self-reactive
B cells, the site of B cell death in unimmunized
peripheral lymphoid organs has not yet been
studied. Here we demonstrate that cells exhibiting
reduced mitochondrial membrane potential and
those expressing active caspase 3, both of which
are characteristic for apoptosis, are specifically
accumulated in the marginal zone (MZ) B cell
compartment compared to the other B cell subsets
in normal unimmunized mouse spleen. These
apoptotic MZ B cells exhibit increased expression
of MHC class II and CD22, whose expression is
shown to be increased upon B cell activation,
but not other activation markers such as CD69
and CD86, suggesting that apoptotic MZ B cells
are activated with partial expression of B-cellactivation-
associated antigens. These results
indicate that MZ is the major site of peripheral B
cell apoptosis in unimmunized mice. Apoptosis of
MHC class IIhi CD22hi MZ B cells may contribute to
B cell homeostasis.
