Btk-deficient mice show abrogated production of antibodies and splenocytes proliferation compared with the wild-type control. C57BL/6 and Btk-deficient mice were primed intravenously with 50 μg per mouse with OVA-Exo and boosted with the OVA protein 4 weeks later. Sera were collected 7 days after boosting, and the presence of OVA-specific (A) IgG and (B) IgG1, and (C) IgG2a antibodies was detected by ELISA using serial dilution of the sera. Results are expressed as optical density at 1:900 serum dilutions. Individual data and mean are presented (n = 7 per group). (D) Splenocytes from the aforementioned boosted mice were restimulated in vitro with OVA or OVA-Exo for 48 hours, and proliferation was detected by thymidine incorporation assay. (E) Spleen sections from mice immunized with OVA-Exo stained with anti–FDC-M2 (follicular dendritic cells; red) and anti-B220 (B cells; pseudo-colored blue). Bar represents 150 μm. (F) A proposed model describing the mechanism of how indirectly loaded exosomes facilitate interaction between B and T cells for efficient activation of the immune response. APC indicates antigen-presenting cell; Exo, exosome.