LAUSR

Significance The bone marrow (BM) harbors critical components of the adaptive immune system able to provide long-lasting protection against pathogens. Among those, CD4 memory T cells are potent helpers of immune reactions in secondary lymphoid organs. Here we analyze their reactivation in the BM in secondary immune reactions. The CD4 memory T cells form clusters with antigen-presenting cells and proliferate vigorously. Although these clusters contain many B lymphocytes, their formation is not dependent on them and no germinal centers develop. Rather, antigen-specific CD4 memory T cells are significantly amplified and, after termination of the immune reaction, they remain in the BM as resting cells. The BM thus provides a dynamic reservoir of CD4 memory T cells, adapting quantitatively to antigenic challenges. The bone marrow maintains memory CD4 T cells, which provide memory to systemic antigens. Here we demonstrate that memory CD4 T cells are reactivated by antigen in the bone marrow. In a secondary immune response, antigen-specific T cells of the bone marrow mobilize and aggregate in immune clusters together with MHC class II-expressing cells, mostly B lymphocytes. They proliferate vigorously and express effector cytokines, but they do not develop into follicular T-helper cells. Neither do the B lymphocytes develop into germinal center B cells in the bone marrow. Within 10 days, the immune clusters disappear again. Within 30 days, the expanded antigen-specific memory CD4 T cells return to memory niches and are maintained again individually as resting cells. Thus, in secondary immune responses in the bone marrow T-cell memory is amplified, while in germinal center reactions of secondary lymphoid organs humoral memory is adapted by affinity maturation.