Skin delivery of a hybrid liposome/ISCOM vaccine implicates a role for adjuvants in rapid modulation of inflammatory cells involved in innate immunity before the enhancement of adaptive immune responses

There is now compelling evidence that intradermal vaccination with an efficacious adjuvanted antigen triggers a series of coordinated responses characterized initially by the rapid mobilization and recruitment of granulocytes to the lung. Activation of effector cells of the innate immune system is intended to provide surveillance and temporary protective cover at vulnerable mucosal sites while both T and B cell precursors, as well as haematopoietic progenitor cells, are undergoing dramatic reductions in numbers during the first 2–4 days post‐vaccination. Some of these events recapitulate those seen after infection with a pathogen. Initial decreases in cell numbers in the thymus and bone marrow (BM) are followed by rapid increases in cellular proliferation in these organs, probably in response to peripheral signals. Vaccine‐induced cell death (by apoptosis) in the thymus may provide one of many stimuli needed to up‐regulate BM production of progenitor cells, and cells of the B, myeloid and monocytic lineages so that depleted peripheral compartments are replenished. Reconstitution of the latter cell population is critical in ensuring sufficient numbers of APC are generated to deal with extraneous antigen resulting from either vaccination or proliferation of a pathogen. Ultimately, these APC, as effector cells of the innate immune system, must provide pattern recognition of dangerous pathogens and serve to activate appropriate T cell responses. Vaccination not only educates both the innate and adaptive arms of the immune response but also more interestingly, appears to regulate subsequent innate immune responses following exposure to a lethal challenge dose of bacteria. Under these conditions, the rate of loss of BM precursors is greatly attenuated in mice previously vaccinated with adjuvanted antigen compared to unvaccinated controls or mice that had received only antigen. Mice intradermally vaccinated with adjuvanted antigen also displayed increased rates of granulocyte and monocyte recruitment in the lung and spleen. These events occurred very rapidly within 12–36 h of challenge and may be crucial in providing complete protection in vaccinated mice against a challenge dose that was otherwise lethal for unvaccinated controls. Therefore, an important characteristic of an efficacious intradermal vaccine may be the ability to deplete T and B precursors in the thymus and BM lymphoid compartments followed by increased rates of haematopoiesis to re‐supply peripheral requirements for granulocytes/monocytes, and T and B cells. Adaptive immunity elicited by intradermal vaccination is, therefore, dependent upon prior activation of the innate immune system.