Tumor cells modified to express one or more T cell costimulatory molecules are disclosed. Preferred costimulatory molecules are B7-2 and B7-3. The tumor cells of the invention can be modified by transfection with nucleic acid encoding B7-2 and/or B7-3, by using an agent which induces or increases expression of B7-2 and/or B7-3 on the tumor cell or by coupling B7-2 and/or B7-3 to the tumor cell. Tumor cells modified to express B7-2 and/or B7-3 can be further modified to express B7. Tumor cells further modified to express MHC class I and/or class II molecules or in which expression of an MHC associated protein, the invariant chain, is inhibited are also disclosed. The modified tumor cells of the invention can be used in methods for treating a patient with a tumor, preventing or inhibiting metastatic spread of a tumor or preventing or inhibiting recurrence of a tumor. A method for specifically inducing a CD4.sup.+ T cell response against a tumor and a method for treating a tumor by modification of tumor cells in vivo are disclosed.