Id-specific CD4⁺ T cells accumulate and become activated in Id+ B-lymphoma tumors. SCID mice were injected with 1.25 × 10⁵ F9 or F55 cells and 13 days later received 10⁶ Id-specific CD4⁺ cells purified from TCR-transgenic mice. At this time point, none of the mice had tumors. Five days later, single-cell suspensions from 2 F55 tumors (∼2 mm) and 2 F9 tumors (8-10 mm) were analyzed by flow cytometry. (A) Number of Id-specific (GB113⁺) CD4+ T cells per 106 F9 and F55 tumor cells. (B) Number of activated (CD69⁺ GB113+) CD4+ cells per 106 F9 and F55 tumor cells. (C) Gated GB113⁺ CD4+ T cells in F9 and F55 tumor cell suspensions were analyzed for CD69 expression and size (forward scatter [FSC]).