U5 snRNP200 complex antibodies delay tumor growth and can induce tumor necrosis in vivo. (A) Severe combined immunodeficient mice with THP-1 growing as a solid tumor under the skin were treated with cytotoxic AML-specific antibodies rAT1331 or rAT1337 or with an isotype control (AT1002). Treatment started when the tumors had a size of 100 mm³. Further tumor growth to a size of 350 mm³ was significantly delayed in mice treated with cytotoxic AML antibodies. (B) Mice were euthanized after 25 days, 8 days after the last antibody injection. Three representative hematoxylin and eosin stains of paraffin-embedded THP-1 tumors, 1 of each group, are shown. (C) Of each tumor, the area of necrosis was calculated as percentage of the total surface. Although total tumor sizes no longer differed between groups after 25 days, tumors from rAT1331-treated mice had significantly more necrotic tissue.