Kelsey Kubelick , Krishnendu Roy, Ph.D
Development of particulate vaccines, termed pathogen-like particles (PLPs), is of interest to stimulate a more robust immune response compared to traditional, soluble formulations. Physical and other properties of PLPs can be tailored to impact trafficking to the lymph node, cell-particle interactions, and ultimately mechanism of action. To expedite development of PLPs, a minimally invasive longitudinal monitoring tool is needed to understand the impact of vaccine particle design on the immune response in vivo. To address this need, we have developed an ultrasound/photoacoustic (US/PA) imaging platform to track different types of PLPs, as they are trafficked and localized to the lymph node. The platform provides critical information on how to more effectively design PLPs to stimulate a robust immune response.
Two types of PLPs, composed of poly(lactide-co-glycolic acid) (PLGA), were synthesized and loaded with different dyes to allow PA imaging. In unloaded or dye-loaded PLPs, diameter was approximately 1 µm or 300 nm for micro- and nano-PA-tagged-PLPs, respectively (Fig. 1A), and UV-vis confirmed encapsulation of dyes (Fig. 1B). For in vivo studies, PLPs were injected subcutaneously in BALB/c mice. Spectroscopic US/PA imaging and analysis was conducted at the inguinal lymph node at t=2-72 hours to assess PLP arrival and particle localization, indicating cellular trafficking or passive drainage.
Spectroscopic US/PA imaging and analysis showed nano-PA-tagged-PLPs were detected at the periphery of the lymph node by 2 hours post-injection and continued to accumulate for at least 24 hours in vivo (Fig. 1C). PLP transport was indicative of passive drainage in this case. Prior to injection, no PLPs were detected, proving accuracy of the spectroscopic PA imaging. Changes in blood flow were also observed (Fig. 1C).
Results show proof-of-concept that the developed US/PA imaging platform can be used to track kinetics and localization of PA-tagged-PLPs. Currently, we are coating PLPs with different combinations of antigen and adjuvant to better mimic vaccine particles.
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