Leishmaniasis is a parasitic, widespread, and neglected disease that affects more than 90 countries in the world. More than 20 Leishmania species cause different forms of leishmaniasis that range in severity from cutaneous lesions to systemic infection. The diversity of leishmaniasis forms is due to the species of parasite, vector, environmental and social factors, genetic background, nutritional status, as well as immunocompetence of the host. Here, we discuss the role of the immune system, its molecules, and responses in the establishment, development, and outcome of Leishmaniasis, focusing on innate immune cells and Leishmania major interactions.

Chagas disease (CD) is a neglected tropical disease caused by Trypanosoma cruzi infection that, despite being discovered over a century ago, remains a public health problem, mainly in developing countries. Since T. cruzi can infect a wide range of mammalian host cells, parasite–host interactions may be critical to infection outcome. The intense immune stimulation that helps the control of the parasite’s replication and dissemination may also be linked with the pathogenesis and symptomatology worsening. Here, we discuss the findings that support the notion that excessive immune system stimulation driven by parasite persistence might elicit a progressive loss and collapse of immune functions. In this context, cellular stress and inflammatory responses elicited by T. cruzi induce fibroblast and other immune cell senescence phenotypes that may compromise the host’s capacity to control the magnitude of T. cruzi-induced inflammation, contributing to parasite persistence and CD progression. A better understanding of the steps involved in the induction of this chronic inflammatory status, which disables host defense capacity, providing an extra advantage to the parasite and predisposing infected hosts prematurely to immunosenescence, may provide insights to designing and developing novel therapeutic approaches to prevent and treat Chagas disease.

The SARS-CoV-2 pandemic has had a social and economic impact worldwide, and vaccination is an efficient strategy for diminishing those damages. New adjuvant formulations are required for the high vaccine demands, especially adjuvant formulations that induce a Th1 phenotype. Herein we assess a vaccination strategy using a combination of Alum and polyinosinic:polycytidylic acid (Poly(I:C)) adjuvants plus the SARS-CoV-2 spike protein in a prefusion trimeric conformation by an intradermal (ID) route. We found high levels of IgG anti-spike antibodies in the serum by enzyme linked immunosorbent assay (ELISA) and high neutralizing titers against SARS-CoV-2 in vitro by neutralization assay, after one or two boosts. By evaluating the production of IgG subtypes, as expected, we found that formulations containing Poly(I:C) induced IgG2a whereas Alum did not. The combination of these two adjuvants induced high levels of both IgG1 and IgG2a. In addition, cellular immune responses of CD4 ⁺ and CD8 ⁺ T cells producing interferon-gamma were equivalent, demonstrating that the Alum + Poly(I:C) combination supported a Th1 profile. Based on the high neutralizing titers, we evaluated B cells in the germinal centers, which are specific for receptor-binding domain (RBD) and spike, and observed that more positive B cells were induced upon the Alum + Poly(I:C) combination. Moreover, these B cells produced antibodies against both RBD and non-RBD sites. We also studied the impact of this vaccination preparation (spike protein with Alum + Poly(I:C)) in the lungs of mice challenged with inactivated SARS-CoV-2 virus. We found a production of IgG, but not IgA, and a reduction in neutrophil recruitment in the bronchoalveolar lavage fluid (BALF) of mice, suggesting that our immunization scheme reduced lung inflammation. Altogether, our data suggest that Alum and Poly(I:C) together is a possible adjuvant combination for vaccines against SARS-CoV-2 by the intradermal route.