Sorafenib reduces strongly, but reversible specific, CD8⁺ T-cell responses. C57BL/6 mice were pretreated for 2 weeks with the indicated dosage of tyrosine kinase inhibitors. Thereafter, animals were immunized twice in weekly interval with OVA_257-264 and adjuvants under continued treatment, as described in “Methods.” Negative controls were immunized with VSV NP_52-59 peptide. One week after the last immunization, mice were killed and spleen cells were analyzed ex vivo for peptide-specific CD8⁺ T cells by staining with anti-CD8-FITC, anti-CD3ϵ-PerCP, VSV NP_52-59/H2-K^b-APC, and OVA_257-264/H2-K^b-PE tetramers. (A) Staining for both tetramers gated on CD8⁺ CD3ϵ⁺ lymphocytes is shown for a representative negative (left) and positive (right) control sample. (B) Percentage of OVA_257-264/H2-Kb tetramer-positive cells among CD8⁺ T cells for individual mice and means (lines) are shown. Treatment with 80 mg/kg body weight for 4 weeks resulted in severe toxicity. Therefore only 2 samples were evaluable, and this group was excluded from statistical analysis. (C) In addition, percentage of CD25⁺ cells among blood CD4⁺ cells was analyzed. Means of triplicates are shown and error bars indicate the standard deviations of means. Significance was tested by unpaired, heteroscedastic Student t test (*P < .05). (D) Reduced levels of CD25⁺ FoxP3⁺ T cells among CD4⁺ splenocytes of nonimmunized mice can also be observed after 14 days of daily sunitinib treatment. (E,F) Reversibility of the observed effects was assessed in an additional experiment with discontinued treatment 48 hours before first immunization. Mice had been fed daily for 2 weeks with sorafenib, sunitinib, or vehicle only in the indicated concentrations. Analysis was performed as described for panels B and C.