Leishmaniasis, a neglected tropical disease caused by protozoan parasites belonging to the genus Leishmania, results in a wide-ranging spectrum of clinical manifestations, from self-healing localized cutaneous lesions to fatal visceral forms of the disease. It has been estimated that up to one million new leishmaniasis cases arise annually, with a reported 12 million people currently infected worldwide1. Visceral leishmaniasis (VL), which can be fatal in over 95% of cases when left untreated, causes more than 50,000 deaths annually, affecting millions in South America, East Africa, South Asia, and the Mediterranean region2. The main etiological agent of VL in the new world, Leishmania infantum, is transmitted to humans by infected female sandflies during blood-feeding3. These parasites are recognized and internalized by phagocytes, e.g., macrophages and dendritic cells3,4,5. Inside these cells, parasites differentiate into their intracellular forms, known as amastigotes, which will then multiply and be transported via the lymphatic system and bloodstream to different host tissues6,7. However, the mechanisms by which Leishmania parasites are disseminated in the vertebrate host, as well as the role played by host cell migration in this process, remain unclear.
Cell migration is a complex process executed by all nucleated cells, including leukocytes8. According to the classic cycling model of cell migration, this process involves several integrated molecular events that can be divided into five steps: leading-edge protrusion; adhesion of the leading edge to matrix contacts; contraction of cellular cytoplasm; release of the rear edge of the cell from contact sites; and the recycling of membrane receptors from the rear to the front of the cell9.
For cell migration to occur, protrusions must be formed and then stabilized through attachment to the extracellular matrix. Among the different receptor types involved in the promotion of cell migration, integrins are notable. Integrin activation results in migration-related signaling; intracellular signaling then occurs via focal adhesion kinase (FAK) and Src family kinases, in addition to talin, vinculin, and paxillin molecues10,11,12. The phosphorylation of paxillin by activated kinases, including FAK, leads to the recruitment of effector molecules, which transduces external signals that prompt cell migration. It has been shown that paxillin is an intracellular molecule that is crucial to cell adhesion, actin polymerization, and cell migration processes13,14,15.
The actin cytoskeleton plays a central role in the polarization and migration of phagocytes16. During cell migration process, protrusions formed due to actin polymerization become stabilized through cell adhesion to the extracellular matrix. This process may be modulated by integrin receptors associated with the actin cytoskeleton17,18,19. Several actin-binding proteins regulate the rate and organization of actin polymerization in cellular protrusions20. Studies have shown that RhoA, Rac, and Cdc42 regulate actin reorganization after the stimulation of adherent cells by extracellular factors21,22. During migration, Rac1 and Cdc42 are located at the leading edge of the cell, controlling the extension of lamellipodia and filopodia, respectively, while RhoA, located at the rear of the cell, regulates the contraction of the actomyosin cytoskeleton15,23,24,25.
Studies have shown that Leishmania infection modulates host cell functions, such as adhesion to the cellular substrate and migration26,27,28,29,30,31. Immature DCs reside in peripheral tissues; upon interaction with PAMPS, these cells become activated and migrate to the draining lymph nodes, prompting antigen presentation to T cells. A previous study using a mouse model showed that L. amazonensis infection provokes a reduction in the migration of DCs to draining lymph nodes29. It was also demonstrated that the inhibition of the adhesion process reduced DC migration after infection with L. major30. Nonetheless, the impact of DC migration on parasite dissemination in the host, as well as the mechanisms involved in this process, remain poorly understood.
Here we present a compiled step-by-step protocol to perform an in vitro adhesion and migration assay involving human DCs infected by Leishmania. This method comprises not only the differentiation and infection of DCs, but also permits the analysis of host cell motility and migration, the formation of adhesion complexes, as well as actin dynamics. The presently described in vitro protocol allows researchers to further investigate the mechanisms involved in Leishmania dissemination within vertebrate host tissues and can also be manipulated and applied to other cell migration studies.