1. Perivascular clusters of dendritic cells provide critical survival signals to B cells in bone marrow niches Anita Sapoznikov et al. Nature immunology 2008; March 2

2. Background The bone marrow is generally considered a primary lymphoid organ that provides a unique microenvironment with defined niches for stem cells, lymphogenesis and myelogenesis. Early B cell development is confined to the bone marrow, immature B cells then leave the bone marrow and mature in the spleen. Notably, up to a quarter of the mature B cell pool positive for IgD and IgM can be found recirculating in the steady state through the bone marrow. Moreover, bone marrow–resident mature B cells can participate in situ in T cell–independent humoral immune responses to blood-borne microbes.

3. Background Bone marrow constitutes a storage site for terminally differentiated B cells. Mature CD4+ and CD8+ T cells continuously home to the bone marrow. Bone marrow represents a principal reservoir for both CD4+ and CD8+ memory T cells. Bone marrow–resident naive and memory T cells can both be activated in situ, which suggests the presence of ‘professional’ antigen- presenting cells in the bone marrow.

4. Question What is the phenotypic characterization and function of bone marrow–resident DCs (bmDCs)?

5. Characterization of bmDCs

6. Organization of bmDCs into perivascular clusters

7. Two-photon microscopy of cranial bone marrow cavities of x3cr1gfp/+ mice (bmDCs, green; blood vessels, red)

8. Two-photon microscopy of the colocalization of adoptively transferred B,T cells, host bmDCs and blood vessels in the bone marrow cavity of a Cx3cr1gfp/+ mouse

9. Conditional ablation of bmDCs

10. Ablation of bmDCs causes loss of recirculating B cells

12. The bmDCs provide a survival signal to mature B cells

14. The survival of mature B cells in bone marrow is independent of BAFF-producing bmDCs

15. B cells require MIF-producing bmDCs for survival Flow cytometry of mature B cells in the bone marrow for surface expression of CD44 and CD74, components of the MIF receptor complex

16. B220+CD93–CD23+ mature cells from the spleens and bone marrow of Mif –/– mice and wild- type control mice

17. splenic DCs in control wild-type mice reconstituted with bone marrow from Mif –/– mice and CD11c-DTRtg mice (Mif –/–;DTRtg-WT)

18. bone marrow and spleen B cell compartments of the following bone marrow chimeras: wild-type mice reconstituted with bone marrow from CD11c-DTRtg mice (DTRtg-WT), from Mif –/– mice and CD11c-DTRtg mice (Mif –/–DTRtg-WT) or from Baff –/– mice and CD11c- DTRtg mice (Baff –/–DTRtg-WT)

19. distribution of Mif –/– B cells (CD45.1–) and Mif +/+ B cells (CD45.1+) in the bone marrow B cell compartment.