Bone marrow contains virus-specific cytotoxic T lymphocytes

Immunizing bone marrow donors prior to bone marrow transplant (BMT) has the potential for adoptively transferring specific immunity against opportunistic pathogens. Studies have shown that long-term antibody production occurs in the bone marrow and that specific humoral immunity may be transferred from donor to recipient following BMT. However, the magnitude and duration of T-cell memory in the bone marrow compartment has not been adequately investigated. In this study, virus-specific CD8⁺ T-cell responses in the bone marrow were compared with those observed in the spleen of mice acutely infected with lymphocytic choriomeningitis virus (LCMV). During the acute stages of infection, most CD8⁺ T cells in the spleen and bone marrow showed upregulated surface expression of the activation/memory marker, LFA-1 (LFA-1(hi)). After clearing LCMV infection, the antiviral immune response subsided to homeostatic levels and the ratio of CD8⁺/LFA-1(hi) to CD8⁺/LFA- 1(lo) T cells in the spleen and bone marrow of LCMV immune mice returned to the value observed in naive mice. Virus-specific ex vivo effector cytotoxic T-lymphocyte (CTL) responses could be identified in both spleen and bone marrow compartments at 8 days postinfection. LCMV-specific CTL precursor (CTLp) frequencies peaked in the bone marrow at 8 days postinfection and averaged one in 200 to one in 650 CD8⁺ T cells, a frequency similar to that observed in the spleen. After clearing the acute infection, potent LCMV- specific CTL memory responses could be demonstrated in the bone marrow for at least 325 days postinfection, indicating long-term persistence of antiviral T cells st this site. Adoptive transfer of LCMV-immune bone marrow into severe combined immunodefeciency (SCID) mice provided protection against viral challenge, whereas SCID mice that received naive bone marrow became chronically infected upon challenge with LCMV. These results indicate that after acute viral infection, virus-specific memory T cells can be found in the bone marrow compartment and are maintained for an extended period, and when adoptively transferred into an immunodeficient host, they are capable of conferring protection against chronic viral infection.