The role of host lymphoid populations in the response of mouse EMT6 tumor to photodynamic therapy.

Photodynamic therapy (PDT) treatment of murine EMT6 mammary sarcoma using Photofrin (10 mg/kg) and light (110 J/cm2) cured all these lesions growing in syngeneic BALB/c mice. In contrast, the same treatment produced initial ablation but no long-term cures of EMT6 tumors growing in either scid or nude mice, the immunodeficient strains sharing the same genetic background with BALB/c mice. No difference was detected in either the level of Photofrin accumulated per g of tumor tissue or the extent of tumor cell killing during the first 24 h after PDT of EMT6 tumors growing in BALB/c or scid mice. The assumption that the difference in tumor cures could be ascribed to the absence of functional lymphoid cells in scid and nude mice was supported by the results of experiments involving the adoptive T-cell transfer into scid mice or bone marrow transfer between BALB/c and scid mice. The adoptive transfer of splenic virgin T lymphocytes from BALB/c mice into scid mice performed 9 days before PDT of EMT6 tumors growing in the recipients was successful in delaying the recurrence of treated tumors. Adoptive transfer done immediately after PDT or 7 days after PDT had no obvious benefit. Even better improvement and a high cure rate of PDT-treated tumors was obtained with scid mice reconstituted with BALB/c bone marrow. In contrast, a marked drop in tumor cure rate was observed with BALB/c mice reconstituted with scid bone marrow. These results suggest that the activity of host lymphoid populations was essential for preventing the recurrence of EMT6 tumors following the PDT treatment used in this study. The contribution of PDT-induced immune reaction may, therefore, be of critical importance for the cure with at least some tumors.