Abstract 4279: A new class of immunomodulators derivatized from the Tat protein of HIV-1 in a murine breast cancer model (2015)

Abstract Introduction: Immunotherapies such as immune checkpoint inhibitors have had limited success in treating breast cancers. We observed that women with HIV infection and even AIDS have an unusually low incidence of breast cancer. The Tat protein of HIV-1 modulates innate immunity through activation of antigen presenting cells (APCs), and thus could be the reason for this finding. A Tat protein derivative designed to eliminate elements contributing to HIV-1 mediated immunosuppression was expressed as a recombinant protein in E. coli and found to stimulate monocytes to differentiate into antigen presenting cells. This protein, designated PIN-2, was tested in the syngeneic 4T1 murine breast cancer model, a poorly immunogenic and invasive triple negative breast cancer (TNBC) cell line. Methods: In vitro activity of PIN-2 was confirmed using a human monocyte activation assay that determined cell surface expression of CD80 and CD86 by flow cytometry after 72h of incubation with PIN-2, compared with controls. BALB/c mice were implanted orthotopically in the mammary fat pad with 10000 4T1 cells (ATCC No. CRL-2593). Treatment was initiated 7 days after implantation. Mice were administered 100ng PIN-2 intravenously 3x/week for 3 weeks. Tumors were measured 2x/week starting 12 days after implantation using a standard caliper method. Mice were sacrificed 29 days after implantation and examined macroscopically for spontaneous pulmonary metastases. Results: PIN-2 significantly increased human monocyte cell surface expression of costimulatory molecules CD80 and CD86 in vitro at concentrations as low as 30ng/mL culture compared to the negative control (P&amp;lt;0.01). The activation was dose dependent. In vivo, PIN-2 treatment reduced tumor volume to less than 60% of the tumor volume of untreated mice 29 days after tumor implantation (686 mm3 tumor size in placebo vs. 407mm3 in treated mice, p&amp;lt;0.01). Moreover, PIN-2 significantly inhibited the development of spontaneous 4T1 lung metastases (p&amp;lt; 0.05 when compared to placebo). Preliminary examination indicates that 90% of the mice in the control group had visible nodules on the surface of the lungs. Macroscopically visible metastases were reduced to only 50% of the PIN-2 treated mice. There was no evidence of weight loss or apparent toxicity by PIN-2. Conclusion: PIN-2 stimulates cell surface expression of costimulatory molecules that serve as markers for activated antigen presenting cells and are essential for cytotoxic T-cell activation. It reduces primary and metastatic tumor burden in an orthotopic murine model of TNBC. These data suggest that PIN-2 functions as an innate immune agonist to facilitate adaptive anti-tumor immune responses in a poorly immunogenic model of breast cancer. Moreover, PIN-2 appears to be well tolerated, supporting further evaluation of PINS in human breast cancer clinical trials. Citation Format: Christoph M. Hotz-Behofsits, Sophie J. Hanscom, Joshua B. Goldberg, Colin B. Bier. A new class of immunomodulators derivatized from the Tat protein of HIV-1 in a murine breast cancer model. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4279. doi:10.1158/1538-7445.AM2015-4279