Abstract 72: Radiation induces LAP, latency-associated peptide of TGF-beta, on the surface of lymphoid cells in the tumor microenvironment
Authors: Kopsiaftis Stavros , Song Xiufeng , Rao Patricia E. , Fox Barbara S. 
Published: August 2018

Abstract
LAP, latency-associated peptide of TGFβ, is a protein that is co-synthesized with TGFβ and serves to effectively cage the cytokine, holding it in a latent state in the extracellular matrix and on the surface of immunosuppressive cell types. In the tumor microenvironment, activation of the LAP-TGFβ complex by integrins or proteases releases mature TGFβ, producing an immunosuppressive environment and fostering EMT. Anti-LAP antibodies have been shown to be effective at slowing tumor growth in mouse models (Gabriely et al., 2017) and offer a promising new approach to treating cancer. Anti-LAP antibodies are thought to mediate anti-tumor activity both by inhibiting the release of active TGFβ and through reducing the number of LAP+ inhibitory cells.
Radiation is used as a first line therapy for many forms of cancer and is known to be a potent inducer of TGFβ production. This study was designed to determine the effects of radiation on the expression of LAP in the tumor microenvironment. 30 Balb/c mice were inoculated sc in the right flank with 106 CT26 colorectal cancer cells. When the mean tumor volume was approximately 300 mm3, half of the mice received a single dose of 20 Gy of targeted ionizing radiation. Groups of 3 mice from each cohort were sacrificed 1, 2, 3, 7 and 14 days after radiation and spleen and TILs collected for analysis by flow cytometry. As expected, radiation reduced tumor growth, with mean tumor volumes of 250 mm3 at day 14 vs 3600 mm3 in the non-irradiated control group. This was accompanied by a marked increase in CD45+ cells in the tumor which started on day 2 post-radiation and peaked on day 7, when 84% of live cells recovered from tumors were CD45+ (vs 25% in the non-irradiated controls). Radiation induced a significant (p&lt;0.001) increase in CD8+ T cell numbers in the tumor, and prevented the increase in M2 macrophage numbers that occurred in the non-irradiated tumors. Radiation increased LAP expression on multiple cell types. Most notably, expression of LAP on DC and M2 macrophages increased steadily in response to radiation, with 33% of DC and 64% of M2 macrophages expressing LAP, significantly more than in control animals (10%, p=0.007 and 25%, p=0.01, respectively). In addition, 30% of M-MDSC from irradiated tumors expressed LAP, compared to 6% of non-irradiated tumors (p=0.03); expression on M-MDSC peaked at day 7.
These data are the first demonstration that radiation increases cell-associated latent TGFβ in the tumor microenvironment. Radiation significantly altered both the distribution of immune cells in the tumor microenvironment and the expression of LAP on the surface of those cells. The increased LAP/TGFβ expression is anticipated to offset the effects of increased CD8+ effector cells. These observations suggest testing the use of an anti-LAP antibody in combination with radiation therapy to enhance effective immune responses against the tumor.
Citation Format: Stavros Kopsiaftis, Xiufeng Song, Patricia E. Rao, Barbara S. Fox. Radiation induces LAP, latency-associated peptide of TGF-beta, on the surface of lymphoid cells in the tumor microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 72.