This study shows that bone marrow (BM) stroma expresses constitutively multiple adhesion molecules (ICAM-1, VCAM-1, MadCAM-1, P-selectin) relevant for the homing and infiltration of BM by blood derived T lymphocytes, and also the co-stimulatory molecule CD80, relevant for T cell activation. T cells were capable of homing to BM but not to thymus. Homing to BM involved the integrins LFA-1α and α4 which interact with the above constitutively expressed cell adhesion molecules (CAMs). CD3 T cells were detected together with BM resident CD11c dendritic cells (DCs), often enriched in follicle-like structures in BM parenchyma. Cognate interactions between transferred antigen specific transgenic CD4 T cells and antigen laden BM-DCs led to formation of multicellular clusters in situ in BM, to generation of lymphoblasts and to clonal T cell expansion within such clusters. The great majority of BM-CD4 T cells had a memory phenotype suggesting that the BM microenvironment facilitates maintenance of CD4 memory. These results extend and corroborate our previous findings on BM-CD8 T cell mediated immune responses. Together these findings suggest that DC-T cell interactions in BM play an important role in immune responses to blood-borne antigen and in the establishment of systemic immunity and long-term memory.

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