Role of MHC Class II on tumor cells. (A) shRNA-mediated down-regulation of MHC class II expression by MB49. Three RAG.KO mice per group were challenged (day 0) with each of the tumor cell lines, MB49mock, MB49-LacZ(i), and MB49-A^b(i). On day 5, they received 8 × 10⁵ Marilyn cells to induce the up-regulation of class II molecules and on day 13 the tumors were taken for analysis (gating on the CD45.2⁻ and 7-AAD⁻ population to exclude immigrating immune cells and dead cells). Histograms represent class II (A^b) staining of the tumor cells from one mouse from each group. Empty line is the isotype control. Bar graph represents the averages of the mean fluorescence of the 3 mice per group (gray area) compared with that of the isotype controls (white). (B) Marilyn cells reject tumors with knocked-down A^b: RAG.KO mice were challenged with either 10⁵ MB49 mock cells (no symbol), or MB49-∅ (circles), MB49-LacZ(i) (squares), or MB49-A^b(i) (diamonds) cells. One day later, half of the mice in each group received 10⁶ Marilyn cells (closed symbols). The average tumor size (10-20 mice total per group), from 3 individual experiments is shown. (C) Marilyn mice reject tumors that do not express A^b. Percentage of tumor free RAG.KO (broken line), MataHari (circles), or Marilyn (triangles) mice after challenge with 5 × 10⁶ βTC-tet (H-2^k) pancreatic tumor cells. Data were pooled from 2 experiments.