Lymphatic distribution considerations for subunit vaccine design and development.

Subunit vaccines are an important platform for controlling current and emerging infectious diseases. The lymph nodes are the primary site generating the humoral response and delivery of antigens to these sites is critical to effective immunization. Indeed, the duration of antigen exposure within the lymph node is correlated with the antibody response. While current licensed vaccines are typically given through the intramuscular route, injecting vaccines subcutaneously allows for direct access to lymphatic vessels and therefore can enhance the transfer of antigen to the lymph nodes. However, protein subunit antigen uptake into the lymph nodes is inefficient, and subunit vaccines require adjuvants to stimulate the initial immune response. Therefore, formulation strategies have been developed to enhance the exposure of subunit proteins and adjuvants to the lymph nodes by increasing lymphatic uptake or prolonging the retention at the injection site. Given that lymph node exposure is a crucial consideration in vaccine design, in depth analyses of the pharmacokinetics of antigens and adjuvants should be the focus of future preclinical and clinical studies. This review will provide an overview of formulation strategies for targeting the lymphatics and prolonging antigen exposure and will discuss pharmacokinetic evaluations which can be applied toward vaccine development.

A connection between the ribosome and two S. pombe tRNA modification mutants subject to rapid tRNA decay.

tRNA modifications are crucial in all organisms to ensure tRNA folding and stability, and accurate translation. In both the yeast Saccharomyces cerevisiae and the evolutionarily distant yeast Schizosaccharomyces pombe, mutants lacking certain tRNA body modifications (outside the anticodon loop) are temperature sensitive due to rapid tRNA decay (RTD) of a subset of hypomodified tRNAs. Here we show that for each of two S. pombe mutants subject to RTD, mutations in ribosomal protein genes suppress the temperature sensitivity without altering tRNA levels. Prior work showed that S. pombe trm8Δ mutants, lacking 7-methylguanosine, were temperature sensitive due to RTD, and that one class of suppressors had mutations in the general amino acid control (GAAC) pathway, which was activated concomitant with RTD, resulting in further tRNA loss. We now find that another class of S. pombe trm8Δ suppressors have mutations in rpl genes, encoding 60S subunit proteins, and that suppression occurs with minimal restoration of tRNA levels and reduced GAAC activation. Furthermore, trm8Δ suppression extends to other mutations in the large or small ribosomal subunit. We also find that S. pombe tan1Δ mutants, lacking 4-acetylcytidine, are temperature sensitive due to RTD, that one class of suppressors have rpl mutations, associated with minimal restoration of tRNA levels, and that suppression extends to other rpl and rps mutations. However, although S. pombe tan1Δ temperature sensitivity is associated with some GAAC activation, suppression by an rpl mutation only modestly inhibits GAAC activation. We propose a model in which ribosomal protein mutations result in reduced ribosome concentrations, leading to both reduced ribosome collisions and a reduced requirement for tRNA, with these effects having different relative importance in trm8Δ and tan1Δ mutants. This model is consistent with our results in S. cerevisiae trm8Δ trm4Δ mutants, known to undergo RTD, fueling speculation that this model applies across eukaryotes.

A connection between the ribosome and two S. pombe tRNA modification mutants subject to rapid tRNA decay.

tRNA modifications are crucial in all organisms to ensure tRNA folding and stability, and accurate translation in the ribosome. In both the yeast Saccharomyces cerevisiae and the evolutionarily distant yeast Schizosaccharomyces pombe, mutants lacking certain tRNA body modifications (outside the anticodon loop) are temperature sensitive due to rapid tRNA decay (RTD) of a subset of hypomodified tRNAs. Here we show that for each of two S. pombe mutants subject to RTD, mutations in ribosomal protein genes suppress the temperature sensitivity without altering tRNA levels. Prior work showed that S. pombe trm8Δ mutants, lacking 7-methylguanosine, were temperature sensitive due to RTD and that one class of suppressors had mutations in the general amino acid control (GAAC) pathway, which was activated concomitant with RTD, resulting in further tRNA loss. We now find that another class of S. pombe trm8Δ suppressors have mutations in rpl genes, encoding 60S subunit proteins, and that suppression occurs with minimal restoration of tRNA levels and reduced GAAC activation. Furthermore, trm8Δ suppression extends to other mutations in the large or small ribosomal subunit. We also find that S. pombe tan1Δ mutants, lacking 4-acetylcytidine, are temperature sensitive due to RTD, that one class of suppressors have rpl mutations, associated with minimal restoration of tRNA levels, and that suppression extends to other rpl and rps mutations. However, although S. pombe tan1Δ temperature sensitivity is associated with some GAAC activation, suppression by an rpl mutation does not significantly inhibit GAAC activation. These results suggest that ribosomal protein mutations suppress the temperature sensitivity of S. pombe trm8Δ and tan1Δ mutants due to reduced ribosome concentrations, leading to both a reduced requirement for tRNA, and reduced ribosome collisions and GAAC activation. Results with S. cerevisiae trm8Δ trm4Δ mutants are consistent with this model, and fuel speculation that similar results will apply across eukaryotes.

Modeling the kinetics of lymph node retention and exposure of a cargo protein delivered by biotin-functionalized nanoparticles.

Generation of protective immunity through vaccination arises from the adaptive immune response developed primarily in the lymph nodes drained from the immunization site. Relative to the intramuscular route, subcutaneous administration allows for direct and rapid access to the lymphatics, but accumulation of soluble protein antigens within the lymph nodes is limited. Subunit vaccines also require immune stimulating adjuvants which may not accumulate in the same lymph nodes simultaneously with antigen. Herein we report the use of biotinylated poly (lactic-co-glycolic acid) nanoparticles (bNPs) to enhance delivery of a model protein antigen to the lymphatics. bNPs provide dual functionality as adjuvant and vehicle to localize antigens with stimulated immune cells in the same draining lymph node. Using streptavidin as a model antigen, which can be loaded directly onto the bNP surface, we evaluated the kinetics of lymph node occupancy and adaptive immune responses in wildtype C57BL/6 mice. Antigen exposure in vivo was significantly improved through surface loading onto bNPs, and we developed a working kinetic model to account for the retention of both particles and antigen in draining lymph nodes. We observed enhanced T cell responses and antigen-specific B cell response in vivo when antigen was delivered on the particle surface. This work highlights the advantage of combining intrinsic adjuvant and antigen loading in a single entity, and the utility of kinetic modeling in the understanding of particle-based vaccines. STATEMENT OF SIGNIFICANCE: Development of safe and effective subunit vaccines depends on effective formulations that render optimized exposure and colocalization of antigens and adjuvants. In this work, we utilize a nanoparticle system which features self-adjuvanting properties and allows for surface loading of recombinant protein antigens. Using in vivo imaging, we demonstrated prolonged co-localization of the antigen and adjuvant particles in draining lymph nodes and provided evidence of B cell activation for up to 21 days following subcutaneous injection. A pharmacokinetic model was developed as a step towards bridging the translational gap between particulate-based vaccines and observed outcomes. The results have implications for the rational design of particle-based vaccines.