Bone marrow as a secondary lymphoid organ: mature T-cell subsets in bone marrow from rheumatoid arthritis and osteoarthritis patients

Recent data indicate that bone marrow plays an important role not only as a primary lymphoid organ responsible for haemopoiesis, but also as a secondary lymphoid organ with capability of antigen presentation exceeding that of lymph nodes. Although in chronic inflammatory/immune disease, like rheumatoid arthritis (RA), bone marrow participates in the initiation and/or perpetuation of the disease, there is little information about the real number of lymphocyte subpopulations in the bone marrow of these patients and how they can be modulated by T-cell growth factors. IL-15 acting through IL-15 receptors (including high-affinity IL-15R alpha chain) is a key cytokine influencing the development of natural killer cells in bone marrow, and proliferation and maintenance of the memory CD3+ cell pool. However, there is no information about the levels of IL-15 in bone marrow.