Therapeutic efficacy of adoptive immunotherapy is predicated on in vivo antigen-specific proliferation of donor T cells

Activated T cells with down-regulated L-selectin expression (L-sel^-) from tumor-draining lymph nodes represent a potent source of specific immune effectors in adoptive immunotherapy. Using congenic pairs of mice and carboxyfluorescein diacetate succinimidyl ester-labeled L-sel^- T cells, the current study analyzed in vivo proliferation of transferred cells. In the lung of MCA205 tumor-bearing mice, 6% or 0.3 × 10⁶ of the 5 × 10⁶ donor cells were identified 24 h after transfer. Vigorous proliferation of donor cells was evident on day 2, reaching a maximum on day 6. The proliferation was tumor-specific and CD4 T cells divided with greater magnitude than CD8 cells. Successful adoptive immunotherapy also required sublethal whole-body irradiation (WBI) of the recipient. WBI exerted its effects on facilitating specific T cell proliferation at the tumor site. Taken together, our results demonstrate that adoptively transferred T cells undergo extensive proliferation in response to the tumor and this response is associated with therapeutic efficacy.