Intraperitoneal (IP) adoptive transfer of FoxP3/synaptotagmin VII (Syt VII) double-mutant mouse lymph node cells induces robust myositis in recombination-activating gene 1 (RAG-1)–null recipients. FoxP3-deficient (FoxP3−/Y) lymph node preparations from mice with either a heterozygous or mutant Syt VII genotype were made at the time the mice were killed, and transferred by intraperitoneal (IP) injection into RAG-1–null mice. A, Tissue inflammation in muscle sections obtained 4 weeks after adoptive transfer of FoxP3−/Y/Syt VII+/− mouse cells or FoxP3−/Y/Syt VII−/− mouse cells. Hematoxylin and eosin (H&E) stained; original magnification × 80 (left) and × 200 (right). B, Histopathologic scores of the H&E-stained sections, as determined by blinded analysis. Each symbol represents an individual mouse. Bars show the mean ± SD. C, Immunohistochemical staining for CD4, CD8, B220, and F4/80 in muscle sections obtained from FoxP3−/Y/Syt VII−/− mice 4 weeks after adoptive transfer. All images are representative of trends observed in 6 mice (n = 2 individual experiments). Original magnification × 200. D, Results of digital image analysis showing the pixel intensity values for CD4 T cells, CD8 T cells, F4/80 macrophages, and B220 B cells. Values are the mean ± SD. ∗ = P ≤ 0.001 versus Syt VII+/− (B) and versus all other cell subtype markers (D).