University of Colorado, School of Pharmacy,
Keywords: gene delivery, immune cells
Summary:It is well established that intravenously-injected nanoparticles are cleared by resident macrophages in the lung, liver, and spleen. However, prior to deposition in these tissues, nanoparticles are exposed to cells circulating in the blood, i.e., erythrocytes and leukocytes. In contrast to the well-known effects of blood proteins on the stability of nanoparticles, it is generally not appreciated that interactions with circulating cells have the potential to limit distribution to target tissues. Our experiments with lipid/DNA complexes (lipoplexes) indicate that these particles are rapidly associated with the cell fraction of blood after intravenous administration. Surprisingly, the particle charge has little impact on uptake. Additional experiments using flow cytometry demonstrate that lipoplexes are specifically taken up by blood leukocytes upon intravenous injection, and that this uptake limits distribution to other tissues. Uptake by leukocytes can be significantly reduced by PEGylation, and this corresponds with increased deposition in all tissues, including the tumor. However, previous studies have shown that the immunogenicity of PEG prevents subsequent injections from being used to increase expression in target tissues. It follows that delivery systems should be designed to elicit a minimal immune response in order to progressively increase gene expression via repetitive administration. Our data indicate that this is possible, and that improved strategies for reducing both immune cell uptake and immunogenicity should be pursued.