Abstract SY24-02: Imaging immune responses in cancer patients

Abstract

Abstract Antigen-specific immunotherapy has recently progressed through the development of effective therapeutic vaccinations in advanced melanoma (1,2) and prostate cancer (3). Antigen-specific immune responses in cancer patients can also be induced by exploiting autologous dendritic cells (DCs) that are “educated” ex vivo; i.e., DCs that are appropriately activated and loaded with tumor antigens. DCs are the most potent antigen-presenting cells of the immune system and play a central role in the induction and maintenance of antigen-specific immunity (4). These cells capture and process antigen and migrate to the lymph nodes (LNs), where they present the antigen to the adaptive arm of the immune system, inducing antigen-specific T- and B-cell responses. The detection of vaccine-induced immune responses in vivo using a clinically applicable means is critical for the optimization of novel immunotherapies. Positron emission tomography (PET) is a widely available, highly sensitive imaging modality for the in vivo visualization and quantification of molecular processes at a cellular level. Furthermore, whole body imaging allows localization in a longitudinal fashion. These features are necessary to measure immune responses by quantification of the low numbers of proliferating T and B cells early after vaccination in relevant LNs. Thus far, investigators have mainly exploited [18F]-labeled fluoro-2-deoxy-2-d-glucose ([18F]FDG) for PET imaging of proliferating cells, based on the increased glucose metabolism of these cells. Recently, novel tracers have been developed that facilitate imaging of other cellular processes: [18F]-labeled 3α-fluoro-3α-deoxy-thymidine ([18F]FLT) was designed as a tracer for cell proliferation (5) and is increasingly being applied in oncology. However, it has been recognized that enhanced nucleoside demand is not restricted to tumor cells (6). A successful vaccination results in the proliferation of activated lymphocytes in a highly controlled manner within LNs. This proliferation is accompanied by a large metabolic switch in lymphocytes and could serve as a marker of immune responsiveness. We demonstrate that [18F]FLT PET can be used to directly monitor antigen-specific immune responses in vivo shortly after therapeutic vaccination, because it offers a sensitive tool to study the kinetics and localization of induction of antigen-specific lymphocyte activation upon vaccination with antigen-loaded autologous DCs (7). Melanoma patients with lymph node (LN) metastases received DC vaccine therapy, injected intranodally, followed by [18F]FLT PET at varying time points after vaccination. Control LNs received saline or DCs without antigen. De novo immune responses were readily visualized in treated LNs early after the prime vaccination, and these signals persisted for up to 3 weeks. This selective [18F]FLT uptake was markedly absent in control LNs, although tracer uptake in treated LNs increased profoundly with as little as 4.5 × 105 DCs. Immunohistochemical staining confirmed injected DC dispersion to T-cell areas and resultant activation of CD4+ and CD8+ T cells. The level of LN tracer uptake significantly correlates to the level of circulating antigen-specific IgG antibodies and antigen-specific proliferation of T cells in peripheral blood. Furthermore, this correlation was not observed with [18F]FDG. Therefore, [18F]FLT PET offers a sensitive tool to study the kinetics, localization, and involvement of lymphocyte subsets in response to vaccination. Recently published large phase III trials, demonstrate that immunotherapies can be effective even in advanced cancer patients. Both antigen-specific (2,3) and nonantigen specific agents (2) have been approved by the Food and Drug Administration. Considering the amount of effort poured into the development of these novel agents, PET-based monitoring will advance our knowledge of the immunological processes that precede the failure or success of novel immunotherapies. Furthermore, it should vastly improve efficient application by aiding in individualized decision making.