(A) Serum anti-BP180 antibody titers in control and BP180-immunized mice at week 4, as measured by ELISA (n = 8). (B) Representative direct immunofluorescence images showing IgG and C3 deposition (green) in back skin sections. Scale bar = 100 μm. (C) Frequencies of CD4⁺CD20⁺ T cells in the spleen, lymph nodes, and peripheral blood of control and BP180-immunized mice (n = 6). (D) Frequencies of CXCR5⁺PD-1⁺ cells among CD4⁺CD20⁺ and CD4⁺CD20^– T cells in the spleen, lymph nodes, and peripheral blood of control and BP180-immunized mice (n = 5). (E) MFI of IL-21 in CD4⁺CD20⁺ or CD4⁺CD20^– T cells in the spleen of control and BP180-immunized mice (n = 5). (F) Anti-BP180 antibody titers in cell culture supernatants from each group were measured by ELISA (n = 5). (G) Serum anti-BP180 antibody titers in each group of Rag1^–/– mice that received adoptive transfer of B cells along with T cell subsets (or B cells alone as control), measured by ELISA (n = 5). (H) Frequencies of GL7⁺Fas⁺ germinal center (GC) B cells among B220⁺ B cells and IgD^–CD138⁺ plasma cells in Rag1^–/– recipient mice (n = 5). (I) Frequencies of CXCR5⁺PD-1⁺ T cells within CD4⁺ T cells from negative control–knockout (NC-KO) and MS4A1-knockout (MS4A1-KO) T cells (n = 4). (J) MFI of IL-21 in CD4⁺ T cells from control and MS4A1-KO T cells (n = 4). All data are presented as the mean ± SD. An unpaired 2-tailed Student’s t test was used for A, C, I, and J; 1-way ANOVA with Tukey’s post hoc test was used for F–H; and 2-way ANOVA with Tukey’s post hoc test was used for D and E. *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001.