A major parameter limiting immune responses to vaccination is the number of activated antigen presenting cells (APCs) that capture antigen from the vaccine site and migrate to draining lymph nodes (LNs), the site where T and B cell priming occurs. Currently, a quantitative non-invasive technique for monitoring in vivo antigen capture and dehvery is laclting. The use of cellular magnetic resonance imaging (MRI) is a promising approach for this purpose;however, cellular imaging currently requires ex vivo pre-labeling of cells with contrast agents followed by reintroduction of cells into the subject being monitored. Using mouse models, we have developed an in vivo labehng method which relies upon the capture of vaccine antigen-associated superparamagnetic iron oxide (SPIO) by endogenous antigen presenting cells, in situ, in order to quantify AFC delivery to LNs. In this system, MRI is capable of monitoring the trafficking of magnetically labeled APCs in vivo that are responsible for inducing tumor-specific immune responses. Analysis of lymph node MR images using dedicated software enables signal quantification through the generation of pixel intensity histograms. Excellent correlation is observed between in vivo and ex vivo quantification of vaccine antigen-loaded APCs, with resolution sufficient to detect increased APC trafficking elicited by an adjuvant. Furthermore, APCs that capture SPIO (and antigen) and traffic to LN can subsequently be magnetically recovered ex vivo, allowing for detailed cellular and molecular studies of the upstream parameters that influence the afferent arm of vaccine-induced immunity. Using murine vaccine models targeting lung cancer, we now propose to examine the correlation between the extent of APC trafficking as measured by MRI and the quantity and quality of the downstream immune response. This information will be used to evaluate candidate vaccine adjuvants for their ability to augment this critical step In generating systemic immunity. We will also extend this technology to additional vaccine platforms setting the stage for clinical application.
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