Combination immunotherapy with anti-hCD137 (urelumab) and anti-hPD-1 (nivolumab) monoclonal antibodies (mAb) in a humanized mouse model enhances the human T-cell infiltrate in xenografted tumors. Using multiplexed quantitative immunofluorescence, we profiled T- and B-cells in the tumor microenvironment. In immunodeficient Rag2^−/−IL2Rγ null mice subcutaneously bearing human gastric carcinoma and transferred with peripheral blood mononuclear cells from the same patient, urelumab and nivolumab increased the T-cell infiltrates that were penetrating into the tumor. The presence of T lymphocytes was associated with slow tumor progression. In contrast, tumor-infiltrating lymphocytes (TIL) were restricted to the tumor periphery when treatment consisted of control hIgG4 mAb or either urelumab or nivolumab as single agents. The combination of urelumab and nivolumab seems to help overcome a peripheral barrier so TILs can enter the tumor core. For details, see article by Sanmamed and colleagues on page 3466.