In situ T cell responses against melanoma comprise high numbers of locallyexpanded T cell clonotypes

It is well established that melanoma cells express Ags that are recognized by autologous T cells in vitro. Tumor-infiltrating lymphocytes in situ comp rise clonotypic T cells, suggesting that their expansion is driven by Ag st imulation. Still, little is known about the detailed characteristics of the in situ T cell response. In the present study, we scrutinized this respons e by analyzing multiple metastatic lesions for the presence of clonotypic T cells. This approach was chosen to distinguish whether the clonal T cell e xpansion occurs as a systemic or localized phenomenon. TCR clonotype mappin g of six s.c. metastases from two patients revealed the presence of multipl e (from 40 to >60) clonotypic T cells in all lesions. Clonotypic T cells we re present in TCR P-variable regions expressed both at high and low levels. Comparison of the T cell clonotypes present in different lesions from indi vidual patients demonstrated that, in general, clonotypes were exclusively detected in a single lesion, Hence, anti-melanoma T cell responses are much more heterogeneous than previously anticipated and accommodate a predomina nce of strictly localized T cell clonotypes.