Levels of CSP-antibodies in mice were determined by ELISA. T cells and CD8+ dendritic cells compared to IV-I mice. Our results indicate that the lower protection efficacy obtained by intradermal sporozoite administration is not linked to low hepatic parasite numbers as presumed before, but correlates with a shift towards regulatory immune responses. Overcoming these immune suppressive responses is important not only for live-attenuated malaria NB001 vaccines but also for other live vaccines administered in the skin. Introduction Malaria remains a major threat to the lives of more than 3 billion people world-wide. There is a pressing and yet unmet need for an effective vaccine that provides a high degree of sustained protection. Despite decades of clinical testing of (recombinant) sub-unit vaccines, only modest protection has been achieved so far. As a consequence, the interest in whole organism malaria vaccine approaches has been renewed1C4. Induction of complete protective immunity in humans has only been achieved by immunization with live attenuated sporozoites1, 5, 6 or by (non-attenuated) sporozoites that are administered under chemoprophylaxis7, 8. Attenuated sporozoites induce strong protective immune responses both in rodents9, 10 and in humans5, 6, 11. Injected sporozoites need to be alive and to retain capacity to invade hepatocytes to induce protective immunity. Most immunization studies in rodent models have been conducted using the intravenous (IV) route of administration of sporozoites and only a few studies have analyzed alternative techniques such as intradermal (ID), intramuscular (IM) or subcutaneous (SC) injection of sporozoites12C18. However, the latter techniques will be more amenable for large-scale administration to infants in endemic countries. For vaccines in general there is renewed interest in the intradermal route of administration driven by the fact that the dermis and epidermis of human skin are rich in antigen-presenting cells, suggesting that delivery of vaccines to these layers should be more efficient and induce protective immune responses with smaller amounts of vaccine antigen19. Unfortunately, immunization by ID, IM or SC injections of attenuated sporozoites of both rodent (and human (malaria parasites induced lower levels of protective immunity compared to IV administration16, 20C23. In rodent malaria models, reduced potency was linked to a lower number of parasites in the liver (30C50 fold) after ID immunization (ID-I) compared to IV immunization (IV-I)12, 13, NB001 17, 24. The importance of the number of sporozoites in the liver, i.e. the parasite liver load, for protective immunity is emphasized by the observations that high level protection can be achieved after ID-I provided that sufficiently high numbers of sporozoites are injected17, 24. This suggests that induction of protection mainly associates with the number of attenuated sporozoites reaching the liver and infecting hepatocytes25C31. Protective immunity induced by immunization with sporozoites is associated with expansion of IFN- producing CD8 memory T cells in the liver13, 32C35. Lower CD8 T cell responses were found after ID-I compared to IV-I which was explained by the lower parasite loads in the liver after ID-I13. Therefore, it has been speculated that the differences between ID-I and Rabbit Polyclonal to RFWD2 IV-I are the result of fewer parasites entering the liver after ID-I14. However, it is unknown whether the differences in protective immunity between ID-I and IV-I can be exclusively explained by differences in parasite liver loads or whether other immunological factors associated with the route of administration of sporozoites can also influence the induction of protective immune responses. Some authors favor the view that sporozoites deposited in the skin use the lymphatic system and thereby pass through lymph nodes to NB001 reach the liver36, 37. In order to study the effect of the route of sporozoite administration on development of protective immune responses we developed a mouse model to compare.