Intradermal immunization with Plasmodium berghei late-arresting genetically attenuated sporozoites induces PD-L1 expression on regulatory macrophages and dendritic cells

The development of effective and long-lasting malaria vaccines remains a key goal. Late-arresting genetically attenuated sporozoite (LA-GAP SPZ) vaccines, such as Pf∆mei2 (GA2), have shown strong protective potential. While GA2 delivered via mosquito bites has demonstrated up to 90% protection in humans, practical vaccine deployment will require alternative administration routes. Intravenous (IV) delivery of whole SPZ vaccines has been highly effective, but intradermal (ID) administration though easier, offers significantly reduced protection, with unclear underlying mechanisms. In this study, we used a Plasmodium berghei GA2 SPZ rodent model to compare immune responses following ID and IV immunization across multiple organs. ID immunization resulted in lower frequencies of CD8⁺ tissue-resident memory T cells (Trm) in the spleen (1.3%, p = 0.3), lungs (8.1%, p = 0.005) and liver (2.5%, p = 0.07), along with reduced activation markers (Granzyme A, Ki67, and KLRG1). In the liver, Granzyme B (p = < 0.0001) and perforin (p = < 0.0001) were significantly decreased after ID immunization, indicating diminished cytotoxic potential. Importantly, ID immunization induced a regulatory myeloid phenotype in the skin and skin-draining lymph nodes, marked by low CD86 and high PD-L1 expression, potentially impairing T cell priming. A similar regulatory profile in the liver suggests systemic immunosuppression. These findings highlight key immunological differences between ID and IV GA2 SPZ delivery and suggest that optimizing ID administration, such as adjusting injection volume to enhance sporozoite migration, may improve efficacy. Understanding how myeloid regulation and T-cell activation vary across organs is essential for enhancing malaria vaccine strategies.