Abstract 72: Radiation induces LAP, latency-associated peptide of TGF-beta, on the surface of lymphoid cells in the tumor microenvironment

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