Sustained TLR7 triggering results in a weakened antigen-specific humoral immune response despite immune activation. (A) Attenuation of humoral immune response after treatment with UV-VSV. Mice were treated with PBS or 1 mg/kg per day R848 for 7 days and subsequently immunized with UV-VSV. Neutralizing IgM and IgG titers were monitored for the following 20 days. Means from mock- and R848-treated animals were compared using unpaired t test. N = 5. (B) Immunosuppression is not due to a general incapability of B cells to produce IgG. Total IgG levels were measured at day 0, after treatment with 1 mg/kg per day R848 for 7 and 20 days after UV-VSV inoculation. Means were compared using paired t test. N ≥ 5. (C,D) Splenic lymphoid subpopulations from R848-treated animals displayed an activated phenotype after 7 days of treatment. Receptor expression was analyzed by flow cytometry. (C) Dose-dependent increase of CD69 mean fluorescence intensity on B220⁺ B cells. Gray and black lines indicate cells stained with isotype or specific monoclonal antibody, respectively. One representative mouse from each group is shown. (D) TLR7-dependent increase in the percentage of lymphoid subsets expressing CD69. Error bars indicate standard error. N ≥ 5.