LEC/chTNT-3, a chemokine fusion protein generated previously in our laboratory, produces a 40-60% reduction in well-established solid tumors of the BALB/c mouse. In this study, CD25(+) T-cell depletion was used in combination with LEC/chTNT-3 treatment to enhance the therapeutic value of this approach. In two tumor models (Colon 26 and RENCA), this combination immunotherapy produced complete regression of established s.c. tumors after 5 consecutive days of i.v. treatment. To show that targeted LEC is critical to these results, similar combination studies were performed with chTNT-3/cytokine fusion proteins consisting of human interleukin 2, murine IFN-gamma, and murine granulocyte macrophage colony-stimulating factor using identical treatment regimens. These studies showed no significant improvement indicating that combination therapy with anti-CD25(+) antisera requires LEC localization to tumor to produce complete regression. To study the mechanism of this remarkable response, immunotherapeutic studies were repeated in knockout mice and showed that successful treatment with CD25(+) depletion was dependent on the presence of IFN-gamma but not perforin. Other studies using real-time PCR, ex vivo proliferation, and intracellular cytokine staining with lymphocytes from tumor draining lymph nodes, suggested that this combination treatment was associated with increased T-helper 1 cytokine expression, enhanced T-cell activation, and increased IFN-gamma production by T cells. Rechallenge experiments showed that combination LEC/chTNT-3 treatment and CD25(+) depletion produced long-acting memory cells capable of preventing re-engraftment of the same but not different tumor cell lines. These studies suggest that LEC/monoclonal antibody fusion proteins, when used in combination with CD25(+) T-cell depletion, is a viable method of immunotherapy for the treatment of solid tumors.