The role of bone marrow-derived stromal cells in the maintenance of plasma cell longevity

Protective circulating Abs originate primarily from long-lived plasma cells in the bone marrow. However, the molecular and cellular basis of plasma cell longevity is unknown. We investigated the capacity of primary bone marrow-derived stromal cells to maintain plasma cell viability in vitro. Plasma cells purified from the bone marrow or lymph nodes died rapidly when plated in media, but a subpopulation of plasma cells survived and secreted high levels of Ab for up to 4 wk when cocultured with stromal cells. Ab secretion was inhibited by the addition of anti-very late Ag-4 to plasma cell/stromal cell cocultures indicating that direct interactions occur and are necessary between stromal cells and plasma cells. The addition of rIL-6 to plasma cells cultured in media alone partially relieved the sharp decline in Ab secretion observed in the absence of stromal cells. Moreover, when stromal cells from IL-6^-/- mice were used in plasma cell/stromal cell cocultures, Ab levels decreased 80% after 7 days as compared with wild-type stromal cells. Further, IL-6 mRNA message was induced in stromal cells by coculture with plasma cells. These data indicate that bone marrow plasma cells are not intrinsically long-lived, but rather that plasma cells contact and modify bone marrow stromal cells to provide survival factors.