Antibody and B-cell responses following ISU-1 and 135 infections. Spike-specific IgG (A) and (B) IgA endpoint titers were determined in serum, BAL, and nasal swabs using ELISA. (C) Homologous and cross-neutralizing antibody titers were determined using virus neutralization assay in serum and BAL. Each point represents one animal with error bars defining SEM. Multiple Mann–Whitney test was performed to compare the changes between 2 groups. Asterisks represent significant differences (*P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001). (D) Spearman rank correlation between respiratory pathology and antibody responses from both ISU-1- and 135-infected samples at 11 DPI. Asterisks represent significant changes (*P < 0.05, **P < 0.01). (E) FACS staining was performed on BAL cells to identify total B and plasma cells. (F) Gating strategy for identifying BAL-derived putative T follicular helper–like cells and their proportions at 1, 5, 11, and 20 DPI are shown. Each point represents a single animal, and midline represents the mean. Two-way ANOVA and Bonferroni multiple tests were performed to compare temporal dynamics within each group and changes in frequency between 2 groups on single time point. Asterisks represent significant changes (*P < 0.05, **P < 0.01).