Small is beautiful: N-trimethyl chitosan-ovalbumin conjugates for microneedle-based transcutaneous immunisation

To provoke an immune response, a transcutaneously administered vaccine has to diffuse into the skin, reach the lymph nodes and be taken up by dendritic cells (DCs). To study these three steps we immunised mice transcutaneously (with microneedles), intradermally and intranodally. The effect of the formulation was investigated by formulating ovalbumin (OVA) in three ways with N-trimethyl chitosan (TMC): TMC+OVA mixtures, TMC-OVA conjugates and TMC/OVA nanoparticles. Both the percentage OVA(+) DCs in the lymph node and the resultant immunogenicity (serum IgG titres) were studied. Transcutaneously, the TMC-OVA conjugates induced the highest IgG levels and resulted in more OVA(+) DCs in the lymph nodes after 24h than the other TMC formulations. Intradermally, all TMC-adjuvanted OVA formulations increased IgG titres compared to plain OVA. These formulations formed a depot in the skin, prolonging OVA delivery to the lymph nodes. The prolonged delivery of TMC-adjuvanted OVA to lymph node resident DCs was also observed after intranodal immunisation, but in this case the higher uptake did not correspond with elevated antibody titres compared to plain OVA. In conclusion, after transcutaneous administration, TMC-OVA conjugates are most immunogenic among the tested formulations, likely because they penetrate the skin more easily than nanoparticles and consequently are better delivered to DCs, while they show higher uptake by DCs than TMC+OVA mixtures.