The present invention relates to methods of using the expression of ILTL3 ligand or ILT3 on certain types of cancer cells as a diagnostic tool. Methods are provided for treating ILT3-ligand expressing cancers, such as T-cell acute lymphoblastic leukemia (T-cell acute lymphoblastic leukemia), for example by administering ILT3, the extracellular domain of ILT3 or ILT3Fc conjugated to a cytotoxic agent to kill the targeted cancer cell. Other methods are provided for treating cancers that express ILT3 on their surface, such as monocytic forms of AML, for example by administering anti-ILT3 antibodies conjugated to a cytotoxic agent.
