Characterization of Periostin, a Novel Factor for B Lymphopoiesis

In bone marrow, B lymphopoiesis-supporting cells, like stromal cells, form a microenvironment called a 'niche' and governs the survival and differentiation of hematopoietic stem cells (HSCs) into mature B cells. Research suggests that supportive cells in the bone marrow express unidentified molecules that regulate lymphopoiesis. In rabbits, B lymphopoiesis halts by four months of age, and the only B lineage cells present are mature B cells. By the use of subtractive hybridization, we found several molecules that are differentially expressed in young and adult rabbits. One of these molecules, periostin, is an extracellular matrix protein thought to function as an adhesion molecule during bone formation and can support osteoblastic cell line attachment and spreading. This led to the hypothesis that the bone marrow microenvironment may change as a rabbit ages, becoming unable to support B cell development. By the use of periostin knock out mice, I found that periostin is not required for B lymphopoiesis in vivo. However, if periostin expression is down-regulated by the use of siRNA in vitro, key cytokines required for B lymphopoiesis, IL-7 and SCF, are consequently down-regulated. Taken together these data suggest that periostin is important for B lymphopoiesis and may be a factor in the arrest of B lymphopoiesis.