Antibody protection against MAYV depends on the IgG subclass and Fc effector functions. (A) 4-wk-old C57BL/6 male and female FcγR^−/− mice were administered 100 µg of the indicated anti-MAYV mAbs and 100 µg of anti-Ifnar1 mAb 1 d before subcutaneous inoculation with 10³ FFU of MAYV-BeH407. Data are from two experiments. (B) Recombinant, isotype-switched mAbs (MAY-115 and MAY-134) were tested by ELISA for binding to soluble mouse and human FcγRs. Binding is presented relative to the mouse IgG2a isotype control of MAY-115 (left) or MAY-134 (right). Data are the mean and SD of two experiments performed in triplicate. (C and D) Neutralizing activity of isotype-switched MAY-115 (C) or MAY-134 (D) was measured by FRNT against MAYV-BeH407. Data are representative of three experiments performed in triplicate. (E) 4-wk-old FcγR^−/− mice were administered 100 µg of the indicated MAY-115 IgG and 100 µg of anti-Ifnar1 mAb 1 d before subcutaneous inoculation with 10³ FFU of MAYV-BeH407. Data are from two experiments. (F–I) Protection by isotype-switched mouse IgG2a mAbs. MAY-115 (F and G) and MAY-134 (H and I) isotype-switched mAbs were tested as prophylaxis against lethal challenge (F and H) or musculoskeletal disease (G and I) as described in Figs. 5 and 6. Survival data are from two experiments. Swelling data are the mean and SD from two experiments (n = 10 mice, two-way ANOVA with Sidak’s post-test). (J–L) Protection by isotype-switched mouse IgG1 mAbs. MAY-117 (J) and MAY-130 (K) isotype-switched mAbs were tested as prophylaxis against lethal challenge (L). Survival data are from two experiments (A, E, F, H, and L; n = 10 mice, log-rank test with Bonferroni correction compared with the isotype control or indicated treatment groups). The experiments in F and H or G and I were performed concurrently, and thus a single isotype control mAbs was used and included in each graph. *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001.