Immune checkpoint inhibitors unleash pathogenic immune responses against the microbiota.

Immune checkpoint inhibitors (ICIs) are essential components of the cancer therapeutic armamentarium. While ICIs have demonstrated remarkable clinical responses, they can be accompanied by immune-related adverse events (irAEs). These inflammatory side effects are of unclear etiology and impact virtually all organ systems, with the most common being sites colonized by the microbiota such as the skin and gastrointestinal tract. Here, we establish a mouse model of commensal bacteria-driven skin irAEs and demonstrate that immune checkpoint inhibition unleashes commensal-specific inflammatory T cell responses. These aberrant responses were dependent on production of IL-17 by commensal-specific T cells and induced pathology that recapitulated the cutaneous inflammation seen in patients treated with ICIs. Importantly, aberrant T cell responses unleashed by ICIs were sufficient to perpetuate inflammatory memory responses to the microbiota months following the cessation of treatment. Altogether, we have established a mouse model of skin irAEs and reveal that ICIs unleash aberrant immune responses against skin commensals, with long-lasting inflammatory consequences.

Prenatal maternal infection promotes tissue-specific immunity and inflammation in offspring.

The immune system has evolved in the face of microbial exposure. How maternal infection experienced at distinct developmental stages shapes the offspring immune system remains poorly understood. Here, we show that during pregnancy, maternally restricted infection can have permanent and tissue-specific impacts on offspring immunity. Mechanistically, maternal interleukin-6 produced in response to infection can directly impose epigenetic changes on fetal intestinal epithelial stem cells, leading to long-lasting impacts on intestinal immune homeostasis. As a result, offspring of previously infected dams develop enhanced protective immunity to gut infection and increased inflammation in the context of colitis. Thus, maternal infection can be coopted by the fetus to promote long-term, tissue-specific fitness, a phenomenon that may come at the cost of predisposition to inflammatory disorders.

Leishmania RNA virus exacerbates Leishmaniasis by subverting innate immunity via TLR3-mediated NLRP3 inflammasome inhibition.

Leishmania RNA virus (LRV) is an important virulence factor associated with the development of mucocutaneous Leishmaniasis, a severe form of the disease. LRV-mediated disease exacerbation relies on TLR3 activation, but downstream mechanisms remain largely unexplored. Here, we combine human and mouse data to demonstrate that LRV triggers TLR3 and TRIF to induce type I IFN production, which induces autophagy. This process results in ATG5-mediated degradation of NLRP3 and ASC, thereby limiting NLRP3 inflammasome activation in macrophages. Consistent with the known restricting role of NLRP3 for Leishmania replication, the signaling pathway triggered by LRV results in increased parasite survival and disease progression. In support of this data, we find that lesions in patients infected with LRV+ Leishmania are associated with reduced inflammasome activation and the development of mucocutaneous disease. Our findings reveal the mechanisms triggered by LRV that contribute to the development of the debilitating mucocutaneous form of Leishmaniasis.

CCR2 Plays a Protective Role in Rocio Virus-Induced Encephalitis by Promoting Macrophage Infiltration Into the Brain.

Rocio virus (ROCV) is a highly neuropathogenic mosquito-transmitted flavivirus responsible for an unprecedented outbreak of human encephalitis during 1975-1976 in Sao Paulo State, Brazil. Previous studies have shown an increased number of inflammatory macrophages in the central nervous system (CNS) of ROCV-infected mice, implying a role for macrophages in the pathogenesis of ROCV. Here, we show that ROCV infection results in increased expression of CCL2 in the blood and in infiltration of macrophages into the brain. Moreover, we show, using CCR2 knockout mice, that CCR2 expression is essential for macrophage infiltration in the brain during ROCV infection and that the lack of CCR2 results in increased disease severity and mortality. Thus, our findings show the protective role of CCR2-mediated infiltration of macrophages in the brain during ROCV infection.

Leishmania Lipophosphoglycan Triggers Caspase-11 and the Non-canonical Activation of the NLRP3 Inflammasome.

Activation of the NLRP3 inflammasome by Leishmania parasites is critical for the outcome of leishmaniasis, a disease that affects millions of people worldwide. We investigate the mechanisms involved in NLRP3 activation and demonstrate that caspase-11 (CASP11) is activated in response to infection by Leishmania species and triggers the non-canonical activation of NLRP3. This process accounts for host resistance to infection in macrophages and in vivo. We identify the parasite membrane glycoconjugate lipophosphoglycan (LPG) as the molecule involved in CASP11 activation. Cytosolic delivery of LPG in macrophages triggers CASP11 activation, and infections performed with Lpg1-/- parasites reduce CASP11/NLRP3 activation. Unlike bacterial LPS, purified LPG does not activate mouse CASP11 (or human Casp4) in vitro, suggesting the participation of additional molecules for LPG-mediated CASP11 activation. Our data identify a parasite molecule involved in CASP11 activation, thereby establishing the mechanisms underlying inflammasome activation in response to Leishmania species.

Dectin-1 Activation during Leishmania amazonensis Phagocytosis Prompts Syk-Dependent Reactive Oxygen Species Production To Trigger Inflammasome Assembly and Restriction of Parasite Replication.

Protozoan parasites of the genus Leishmania are the causative agents of Leishmaniasis, a disease that can be lethal and affects 12 million people worldwide. Leishmania replicates intracellularly in macrophages, a process that is essential for disease progression. Although the production of reactive oxygen species (ROS) accounts for restriction of parasite replication, Leishmania is known to induce ROS upon macrophage infection. We have recently demonstrated NLRP3 inflammasome activation in infected macrophages, a process that is important for the outcome of infection. However, the molecular mechanisms responsible for inflammasome activation are unknown. In this article, we demonstrate that ROS induced via NADPH oxidase during the early stages of L. amazonensis infection is critical for inflammasome activation in macrophages. We identified that ROS production during L. amazonensis infection occurs upon engagement of Dectin-1, a C-type lectin receptor that signals via spleen tyrosine kinase (Syk) to induce ROS. Accordingly, inflammasome activation in response to L. amazonensis is impaired by inhibitors of NADPH oxidase, Syk, focal adhesion kinase, and proline-rich tyrosine kinase 2, and in the absence of Dectin-1. Experiments performed with Clec7a-/- mice support the critical role of Dectin-1 for inflammasome activation, restriction of parasite replication in macrophages, and mouse resistance to L. amazonensis infection in vivo. Thus, we reported that activation of the Dectin-1/Syk/ROS/NLRP3 pathway during L. amazonensis phagocytosis is important for macrophage restriction of the parasite replication and effectively accounts for host resistance to Leishmania infection.

Nucleotide-binding oligomerization domain-containing protein 2 prompts potent inflammatory stimuli during Neospora caninum infection.

Neospora caninum is an apicomplexan parasite responsible for major economic losses due to abortions in cattle. Innate immune responses are crucial for host resistance against the infection, however the molecules involved in parasite recognition are still poorly understood. Nod2 is a cytosolic receptor that recognizes several pathogens and its role during N. caninum infection has not yet been described. In that sense, we evaluated the role of Nod2 in host response against this parasite. We found that infection of macrophages induced increased expression of Nod2, which colocalized with the parasites' vacuoles. Nod2-deficient macrophages showed an impaired induction of pro-inflammatory cytokines, increased production of modulatory molecules, and failure to restrict parasite replication. In vivo, Nod2-knockout mice showed a reduction of MAPK phosphorylation and proinflammatory cytokines, followed by decreased inflammation in target organs and increment in parasite burden. Surprisingly, these mice were partially resistant to lethal doses of tachyzoites. In addition, these phenomena were not observed in Rip2-/- mice. In conclusion, our study indicates that Nod2-dependent responses account for N. caninum elimination. On the other hand, the inflammatory milieu induced by this innate receptor provoked pathogenesis and death in severe experimental neosporosis.

CCR2 signaling contributes to the differentiation of protective inflammatory dendritic cells in Leishmania braziliensis infection.

In vertebrate hosts, Leishmania braziliensis parasites infect mainly mononuclear phagocytic system cells, which when activated by T helper cell type 1 cytokines produce nitric oxide and kill the pathogens. Chemokine (C-C motif) receptor 2 is a chemokine receptor that binds primarily chemokine (C-C motif) ligand 2 and has an important role in the recruitment of monocytic phagocytes. Although it has been reported that Leishmania braziliensis infection induces CCR2 expression in the lesions, the role of CCR2 during Leishmania braziliensis infection remains unknown. Here, we showed that CCR2 has a role in mediating protection against Leishmania braziliensis infection in mice. The absence of CCR2 resulted in increased susceptibility to infection and was associated with low amounts of Ly6C(+) inflammatory dendritic cells in the lesions, which we found to be the major sources of tumor necrosis factor production and induced nitric oxide synthase expression in C57BL/6 mice lesions. Consequently, CCR2(-/-) mice showed decreased tumor necrosis factor production and induced nitric oxide synthase expression, resulting in impaired parasite elimination. We also demonstrated that CCR2 has a role in directly mediating the differentiation of monocytes into inflammatory dendritic cells at the infection sites, contributing to the accumulation of inflammatory dendritic cells in Leishmania braziliensis lesions and subsequent control of parasite replication. Therefore, these data provide new information on the role of chemokines during the immune response to infections and identify a potential target for therapeutic interventions in cutaneous leishmaniasis.

Nucleosides from Phlebotomus papatasi salivary gland ameliorate murine collagen-induced arthritis by impairing dendritic cell functions.

Among several pharmacological compounds, Phlebotomine saliva contains substances with anti-inflammatory properties. In this article, we demonstrated the therapeutic activity of salivary gland extract (SGE) of Phlebotomus papatasi in an experimental model of arthritis (collagen-induced arthritis [CIA]) and identified the constituents responsible for such activity. Daily administration of SGE, initiated at disease onset, attenuated the severity of CIA, reducing the joint lesion and proinflammatory cytokine release. In vitro incubation of dendritic cells (DCs) with SGE limited specific CD4(+) Th17 cell response. We identified adenosine (ADO) and 5'AMP as the major salivary molecules responsible for anti-inflammatory activities. Pharmacologic inhibition of ADO A2(A) receptor or enzymatic catabolism of salivary nucleosides reversed the SGE-induced immunosuppressive effect. Importantly, CD73 (ecto-5'-nucleotidase enzyme) is expressed on DC surface during stage of activation, suggesting that ADO is also generated by 5'AMP metabolism. Moreover, both nucleosides mimicked SGE-induced anti-inflammatory activity upon DC function in vitro and attenuated establishment of CIA in vivo. We reveal that ADO and 5'AMP are present in pharmacological amounts in P. papatasi saliva and act preferentially on DC function, consequently reducing Th17 subset activation and suppressing the autoimmune response. Thus, it is plausible that these constituents might be promising therapeutic molecules to target immune inflammatory diseases.