FcγRIIB1 expression is associated with inhibition of development of human melanoma tumors in nude mice. (a and d) In vitro proliferation of FcγRIIB1-negative (721.2, A375, or HT144) and FcγRIIB1-positive (721.1, A375IIB1, or HT144IIB1 transfectants) melanoma lines, determined by an MTT assay on days 1–4. (b and e) Tumor uptake in nude mice, determined by subcutaneous inoculation of 2 × 10⁶ 721.1 and 721.2 cells (b) or 1 × 10⁶ A375, HT144, A375IIB1, or HT144IIB1 cells (e). Results are expressed as percentage of mice (n = 30) that remained tumor-free after challenge in three experiments. (c and f) Tumor growth, measured by volume after injection of 4 × 10⁶ 721.1 and 721.2 cells (c) or 2 × 10⁶ A375, HT144, A375IIB1, or HT144IIB1 cells (f). Ten mice were used per experiment in each group, and mean tumor volume values, with SD, of mice bearing tumor are shown. Data are representative of three independent experiments. *P < 0.03, **P < 0.002 between FcγRIIB1-positive and FcγRIIB1-negative tumors.