Enhanced inhibition of murine tumor and human breast tumor xenografts using targeted delivery of an antibody-endostatin fusion protein.

Endostatin can inhibit angiogenesis and tumor growth in mice. A potential limitation of endostatin as an antitumor agent in humans is the short serum half-life of the protein that may decrease effective concentration at the site of tumor and necessitate frequent dosing. In an effort to improve antitumor activity, endostatin was fused to an antibody specific for the tumor-selective HER2 antigen to create an antibody-endostatin fusion protein (anti-HER2 IgG3-endostatin). Normal endostatin rapidly cleared from serum in mice (T(1/2)(2), = 0.6-3.8 hours), whereas anti-HER2 IgG3-endostatin had a prolonged half-life (90% intact; T(1/2)(2), 40.2-44.0 hours). Antigen-specific targeting of anti-HER2 IgG3-endostatin was evaluated in BALB/c mice implanted with CT26 tumors or CT26 tumors engineered to express the HER2 antigen (CT26-HER2). Radio-iodinated anti-HER2 IgG3-endostatin preferentially localized to CT26-HER2 tumors relative to CT26 tumors. Administration of anti-HER2 IgG3-endostatin to mice showed preferential inhibition of CT26-HER2 tumor growth compared with CT26. Anti-HER2 IgG3-endostatin also markedly inhibited the growth of human breast cancer SK-BR-3 xenografts in severe combined immunodeficient mice. Anti-HER2 IgG3-endostatin inhibited tumor growth significantly more effectively than endostatin, anti-HER2 IgG3 antibody, or the combination of antibody and endostatin. CT26-HER2 tumors treated with the endostatin fusion protein had decreased blood vessel density and branching compared with untreated CT26-HER2 or CT26 treated with the fusion protein. The enhanced effectiveness of anti-HER2 IgG3-endostatin may be due to a longer half-life, improved serum stability, and selective targeting of endostatin to tumors, resulting in decreased angiogenesis. Linking of an antiangiogenic protein, such as endostatin, to a targeting antibody represents a promising and versatile approach to antitumor therapy.

Increased rejection of primary tumors in mice lacking B cells: inhibition of anti-tumor CTL and TH1 cytokine responses by B cells.

We investigated the role of B cells in tumor immunity by studying immune responses of mice genetically lacking B cells to primary tumors. IgM(-/-) B cell-deficient mice (BCDM) exhibited enhanced resistance to 3 histologically diverse syngeneic tumors as compared to the wild-type (WT) mice. EL4 thymoma and MC38 colon carcinoma grew progressively in WT mice, but regressed spontaneously in BCDM whereas growth of B16 melanoma was slowed significantly in BCDM as compared to the WT mice. BCDM exhibited increased T cell infiltration of tumors, higher T(H)1 cytokine response and, in the case of MC38, a higher anti-tumor CTL response. The increased tumor resistance of BCDM did not seem to result from intrinsic changes in their non-B immunocytes because adoptive transfer of WT splenic B cells to BCDM abrogated tumor rejection and resulted in diminished anti-tumor T(H)1 cytokine and CTL responses. Studies involving BCR-transgenic mice indicated that B cells may inhibit anti-tumor T cell responses by antigen-nonspecific mechanisms since neither tumor-specific antibodies nor cognate T:B interactions were necessary for inhibition of tumor immunity by B cells. IFN-gamma secretion in splenocyte:tumor co-cultures of tumor-challenged BCDM was inhibited by WT but not CD40(-/-) B cells indicating that B cells may inhibit anti-tumor T(H)1 cytokine responses in a CD40-dependent manner. Adoptive transfer of CD40(-/-) B cells into BCDM resulted in restored growth of MC38 suggesting additional factors other than CD40 are involved in dampening anti-tumor responses. The effects of B cells on anti-tumor response warrant further study.