B cells and aging: balancing the homeostatic equation.

The interplay of selective and homeostatic processes dominates the behavior of B lineage subsets following B cell antigen receptor (BCR) expression, and extends to determinants of immune response quality and the persistence of immunologic memory. A key concept emerging from these considerations is that primary events acting upstream of mature B lymphocyte pools can profoundly impact downstream populations as the system attempts homeostatic adjustments. Since, advancing age is accompanied by profound changes in B cell generation and homeostasis, establishing the relative contributions of primary lesions versus compensatory homeostatic processes is critical to understanding these perturbations. Exploration of this problem requires an understanding of: (1) the identity, dynamics, and progenitor/successor relationships of marrow and peripheral B cell subsets; (2) the nature and interactions of selective and homeostatic processes acting in these subsets; (3) how these change with age. Our data show that BLyS and its receptors mediate peripheral B cell homeostasis, and that the size, dynamics and behavior of all B cell subsets influenced by B Lymphocyte Stimulator change with age. These findings suggest that homeostatic processes mediated through B Lymphocyte Stimulator are altered with age, and that these perturbations may primarily reflect compensatory homeostatic adjustments to upstream reductions in B cell generation.