*3.8. Antibody Response in Vaccinated Horses*

The humoral immune response in horses was assessed at 2, 4, 6, and 8 weeks after immunization (Figure 4B). Serum samples were collected to measure the SUDV GP-specific antibodies and neutralizing antibody. The level of SUDV GP-specific IgG antibody was detected from the second week (Figure 8A). After the fifth immunization, the titer of SUDV GP-specific IgG antibody of horse #392 was 1:40,960,

and for horse #18 was 1:8192. The results revealed that SUDV VLPs mixed with Freund's adjuvant could stimulate horses to produce humoral immune response. Regarding the research on SUDV therapeutic antibodies, we selected #392 horse serum for further study. The #392 serum was extracted and purified to obtain purified IgG; #392 horse serum and #392 purified IgG were simultaneously subjected to virus neutralization. These findings showed that the neutralizing antibody titer of #392 horse serum was 1:10,240 after the fifth immunization, correspondingly, the neutralizing antibody titer of purified IgG was 1:5120 (Figure 8B).

**Figure 8.** The detection of humoral immune responses in horses after immunized SUDV VLPs mixed with Freund's adjuvant. (**A**) Measurement of SUDV GP-specific IgG antibody by indirect ELISA and (**B**) detection of neutralizing activity by using pseudotyped viruses.
