To better understand the pathogenesis of JDM, we evaluated untreated children with very early onset JDM, characterizing the relative distribution of lymphocyte phenotypic subsets in peripheral blood (PBLy) and muscle (MLy) and determining if there were evidence of oligoclonality in the lymphocytes isolated from involved muscle identified by MRI. The six untreated DQA1*0501+ JDM (mean age 7.75 years) with mean disease duration of 1.7 months had studies of muscle (immunohistochemistry) and PBLy (two color flow cytometry) obtained concurrently. We had previously reported that these 6 children had 1) a peripheral blood lymphopenia, 2) increased% CD19+ B cells 3) increased proportions of CD8+ cells in muscle compared with PBLy and 4) lack of clonal expansion by sizing of the TCR Vβ gene products generated by PCR. Using two color flow cytometry (CD56, CD3) and immunohistochemistry (CD56, CD3) the relative distribution of NK cells was next evaluated. There were 4 times as many CD56+ lymphocytes in the muscle as in the PBLy (p=0.03). When TCR Vβ gene expression was investigated, Vβ8 and Vβ14 was identified in all 6 MBxLy. Sequencing through the CDR3 region of 24 clones from 3 JDM with similar ethnic background showed a unique motif, YPAP, in the NDN region at position 96 in 7 clones(28%). We conclude that 1) the increase in CD8+ cells in muscle from very early JDM may be due in part to elevated proportions of CD56+ NK cells and 2) the unique motif in the CDR3 NDN region of TCR Vβ8 in each of 3 children suggests an antigen driven process.