Dynamic imaging reveals promiscuous crosspresentation of blood-borne antigens to naïve CD8⁺ T cells in the bone marrow

The bone marrow (BM) hosts memory lymphocytes and supports secondary immune responses against blood-borne antigens, but it is unsettled whether primary responses occur there and which cells present the antigen. We used 2-photon microscopy in the BM of live mice to study these questions. Naïve CD8+ T cells crawled rapidly at steady state but arrested immediately upon sensing antigenic peptides. Following infusion of soluble protein, various cell types were imaged ingesting the antigen, while antigen-specific T cells decelerated, clustered, upregulated CD69, and were observed dividing in situ to yield effector cells. Unlike in the spleen, T-cell responses persisted when BM-resident dendritic cells (DCs) were ablated but failed when all phagocytic cells were depleted. Potential antigen-presenting cells included monocytes and macrophages but not B cells. Collectively, our results suggest that the BM supports crosspresentation of blood-borne antigens similar to the spleen; uniquely, alongside DCs, other myeloid cells participate in crosspresentation.