The bone marrow constitutes a functional secondary lymphoid organ. (A) M315 Id levels in mice challenged IV with 1 × 106 MOPC315.BM cells, as determined by ELISA. Adoptive T-cell therapy (ACT)–treated mice were injected with 2 × 106 Id-spec. TCR-Tg CD4+ T cells on day 18 after the tumor challenge, whereas controls did not receive T cells. Results are shown as means ± standard deviation (SD) (n = 8 mice per treatment group). The dotted line reflects the lower limit of detection (LLD) in ELISA. (B) Survival of mice treated as in panel A, with the addition of a TCR-Tg control group (n = 8 mice per treatment group). (C) Bioluminescence imaging of wild-type BALB/c and TCR-Tg BALB/c mice challenged subcutaneously with 2 × 105 and IV with 1 × 106 MOPC315.BM-Luc cells on the indicated days after tumor cell injection. All mice received a daily IP injection of fingolimod (FTY720) for the duration of the experiment. Red arrows indicate the subcutaneous tumor injection site. (D) Flow cytometry quantitation of Id-specific (GB113+) CD4+ T cells within the femoral bone marrow of nonchallenged mice or IV MOPC315.BM tumor challenged mice receiving daily injections of FTY720 or NaCl. Results are shown as means ± SD (n = 5-8 per treatment group). (E) M315 Id levels of wild-type BALB/c and TCR-Tg BALB/c mice challenged IV with 1 × 106 MOPC315.BM and receiving daily IP injections of FTY720 or NaCl. Results are shown as means ± SD. The dotted line reflects the LLD in the ELISA. (F) Dorsal whole-body bioluminescence quantitation of tumor burden in tumor-challenged mice, as described in panel E. Results are shown as means ± SD.