Revitalizing systemic immune responses in advanced NSCLC using FLT3L and SBRT

Non-small cell lung cancer (NSCLC) is the leading cause of cancer mortality worldwide. For patients who develop progressive disease after treatment with chemotherapy and immunotherapy, treatment options are limited. Here, we report the results of a Phase II clinical trial where twenty-nine patients with advanced, previously treated NSCLC were treated with stereotactic body radiotherapy (SBRT) targeting a single site of disease along with CDX-301, a recombinant human Fms-like tyrosine kinase 3 ligand (FLT3L). The primary study endpoint was progression-free survival four months after study entry (PFS4), which was achieved for 14 patients (48%). Abscopal responses were observed on fludeoxyglucose-18 positron emission tomography (FDG-PET) in nine patients (31%). The median overall survival for all participants was 18 months, with a median overall survival for participants with abscopal response of 31 months. To identify underlying immune signatures of response, we developed a high dimensional flow cytometric approach that quantified 31 distinct cell subsets from peripheral blood, and performed multiplex proteomic analysis of 92 proteins from plasma. SBRT combined with CDX-301 significantly increased circulating myeloid cells, including monocytes, myeloid derived suppressor cells (MDSCs) and dendritic cells (DCs). Among DCs, FceR1-expressing DC2 and DC3 subsets changed most robustly upon treatment. These dynamic changes in DCs and MDSCs following initiation of SBRT and CDX-301 returned to baseline by 8 weeks. However, these changes were also associated with treatment-induced T cell activation with quantitatively robust activation of CD4 T cells. Abscopal responses were associated with a prolonged increase in DC1 cells, Th1-like CD4 T cells, circulating IL-12 and FLT3L. Integrated analysis of multiple types of immune parameters revealed a strong association between coordinated DC induction, CD4 T cell and CD8 T cell activation, and inflammatory cytokine activity in patients with abscopal responses, compared to a stark lack of immune coordination in patients without abscopal responses. These coordinated immune responses were sustained for over 4 weeks in responders, highlighting a potential therapeutic axis engaged in a systemic immune response against multiple lesions in NSCLC. Overall, these findings underscore the potential of combining in situ vaccination, using SBRT, with strategies to enhance the activation of innate immune cells, such as DCs through FLT3L, to potentiate robust anti-tumor T cell responses.