Antigen-specific CD8 T cells accumulate and functionally reactivate in the bone marrow following brain infection

Bone marrow is a primary immune organ responsible for hematopoiesis, yet its role in shaping adaptive immune responses remains incompletely understood. In particular, whether antigen-specific CD8 T cell responses can arise and persist within the bone marrow following brain infection is unclear. We used intracranial infection with Theiler’s Murine Encephalomyelitis Virus (TMEV) in C57BL/6 mice, along with complementary antigen delivery (OVA + Poly I:C) and systemic infection (LCMV) models. Antigen-specific CD8 T cells were quantified in the brain, secondary lymphoid organs, and bone marrow using tetramer-based flow cytometry. Lymphocyte trafficking was modulated using FTY720, and functional memory responses were assessed via peptide reactivation. Hematopoietic stem and progenitor cell (HSPC) populations and myeloid compartments were analyzed by flow cytometry. Antigen-specific CD8 T cells accumulated in the bone marrow within 5–7 days following intracranial TMEV infection, with kinetics comparable to secondary lymphoid organs. Antigen was detectable in the bone marrow within 24 h of intracranial delivery and was primarily associated with migratory antigen-presenting cells. Antigen-specific CD8 T cells persisted long-term in the bone marrow and demonstrated robust expansion upon antigen re-encounter, even under conditions limiting lymphocyte egress. Reactivation of these cells was associated with expansion of Lineage⁻Sca-1⁺c-Kit⁺ (LSK) HSPCs and increased MHCII⁺ myeloid populations. This increase in the LSKs and MHCII + myeloid cells in the bone marrow niche was abrogated with CD8 T cell depletion. These findings establish the bone marrow as an active site of antigen-specific CD8 T cell accumulation, persistence, and functional reactivation following brain infection. Antigen-specific CD8 T cell activity within the bone marrow is associated with modulation of the hematopoietic niche, highlighting a previously underappreciated link between CNS-directed immune responses and systemic hematopoiesis.