During chronic infection, memory T cells acquire a unique phenotype and become dependent on different survival signals than those needed for memory T cells generated during an acute infection. The distinction between the role of effector and memory T cells in an environment of persistent antigen remains unclear. Here, in the context of chronic Toxoplasma gondii infection, we demonstrate that a population of CD8 T cells exhibiting a tissue-resident memory (TRM) phenotype accumulates within the brain. We show that this population is distributed throughout the brain in both parenchymal and extraparenchymal spaces. Furthermore, this population is transcriptionally distinct and exhibits a transcriptional signature consistent with the TRM observed in acute viral infections. Finally, we establish that the CD103+ TRM population has an intrinsic capacity to produce both IFN-γ and TNF-α, cytokines critical for parasite control within the central nervous system (CNS). The contribution of this population to pro-inflammatory cytokine production suggests an important role for TRM in protective and ongoing immune responses in the infected CNS.Accession number: GSE95105

bResistin-like molecules (RELMs) are highly expressed following helminth infection, where they impact both the host and helminth. While RELM␣ (Retnla) impairs helminth expulsion by inhibiting protective Th2 immunity, RELM␤ (Retnlb) can promote its expulsion. We employed Retnla ؊/؊ and Retnlb ؊/؊ mice to delineate the function of both proteins following infection with Nippostrongylus brasiliensis, a hookworm that infects the lung and intestine. Whereas wild-type (WT) and Retnlb ؊/؊ mice exhibited equivalent infection-induced inflammation, Retnla ؊/؊ mice suffered a heightened inflammatory response, including increased mortality, weight loss, and lung inflammation. In the intestine, Retnla ؊/؊ mice had low parasite egg burdens compared to those of WT mice, while Retnlb ؊/؊ mice exhibited high egg burdens, suggesting that RELM␣ and RELM␤ have functionally distinct effects on immunity and inflammation to N. brasiliensis. To test the importance of both proteins, we generated Retnla ؊/؊ Retnlb ؊/؊ mice. Infected Retnla ؊/؊ Retnlb ؊/؊ mice exhibited similar responses to Retnla ؊/؊ mice, including increased mortality and lung inflammation. This inflammatory response in Retnla ؊/؊ Retnlb ؊/؊ mice negatively impacted N. brasiliensis fitness, as demonstrated by significantly lower worm ATP levels and decreased intestinal worm burden and fecundity. Lung cytokine analysis revealed that Retnla ؊/؊ and Retnla ؊/؊ Retnlb ؊/؊ mice expressed significantly increased levels of interleukin-4 (IL-4). Finally, we generated Retnla ؊/؊ mice on the Rag ؊/؊ background and observed that the effects of RELM␣ were abrogated in the absence of adaptive immunity. Together, these data demonstrate that RELM␣ but not RELM␤ significantly impacts the immune response to N. brasiliensis infection by downregulating the Th2 adaptive immune response in the lung, which protects the host but allows improved parasite fitness. Soil-transmitted helminths (STHs) afflict over 2 billion individuals worldwide and can cause anemia and debilitating symptoms in the lung and intestine (1). Host protection against STHs is dependent on the balance between effector immune responses that promote worm expulsion and immunoregulatory pathways that control excessive infection-induced inflammation (2). Resistin-like molecules (RELMs) are secreted proteins with putative effector and immunoregulatory functions against helminth infections (3, 4). In humans, significant increases in resistin expression occur in both filarial nematode and gastrointestinal nematode infections (5, 6). In mice, RELM␣ and RELM␤, which share sequence homology with human RELM␤, are induced in response to Schistosoma mansoni and the gastrointestinal helminths Trichuris muris, Heligmosomoides polygyrus, and Nippostrongylus brasiliensis (7-11).Despite sharing sequence identity and expression patterns, the putative functions of RELM␣ and RELM␤ in helminth infection are different and involve both host-specific and parasite-specific effects. S. mansoni and N. brasiliensis infection led to increased RELM␣ expr...

Parasitic helminth infections can be associated with lifelong morbidity such as immune-mediated organ failure. A better understanding of the host immune response to helminths could provide new avenues to promote parasite clearance and/or alleviate infection-associated morbidity. Murine resistin-like molecules (RELM) exhibit pleiotropic functions following helminth infection including modulating the host immune response; however, the relevance of human RELM proteins in helminth infection is unknown. To examine the function of human resistin (hResistin), we utilized transgenic mice expressing the human resistin gene (hRetnTg+). Following infection with the helminth Nippostrongylus brasiliensis (Nb), hResistin expression was significantly upregulated in infected tissue. Compared to control hRetnTg− mice, hRetnTg+ mice suffered from exacerbated Nb-induced inflammation characterized by weight loss and increased infiltration of inflammatory monocytes in the lung, along with elevated Nb egg burdens and delayed parasite expulsion. Genome-wide transcriptional profiling of the infected tissue revealed that hResistin promoted expression of proinflammatory cytokines and genes downstream of toll-like receptor signaling. Moreover, hResistin preferentially bound lung monocytes, and exogenous treatment of mice with recombinant hResistin promoted monocyte recruitment and proinflammatory cytokine expression. In human studies, increased serum resistin was associated with higher parasite load in individuals infected with soil-transmitted helminths or filarial nematode Wuchereria bancrofti, and was positively correlated with proinflammatory cytokines. Together, these studies identify human resistin as a detrimental factor induced by multiple helminth infections, where it promotes proinflammatory cytokines and impedes parasite clearance. Targeting the resistin/proinflammatory cytokine immune axis may provide new diagnostic or treatment strategies for helminth infection and associated immune-mediated pathology.

Helminths trigger multiple immunomodulatory pathways that can protect from sepsis. Human resistin (hRetn) is an immune cell-derived protein that is highly elevated in helminth infection and sepsis. However, the function of hRetn in sepsis, or whether hRetn influences helminth protection against sepsis, is unknown. Employing hRetn-expressing transgenic mice (hTg) and recombinant hRetn, we identify a therapeutic function for hRetn in lipopolysaccharide (LPS)-induced septic shock. hRetn promoted helminth-induced immunomodulation, with increased survival of ()-infected hTg mice after a fatal LPS dose compared with naive mice or -infected hTg mice. Employing immunoprecipitation assays, hTg mice, and human immune cell culture, we demonstrate that hRetn binds the LPS receptor Toll-like receptor 4 (TLR4) through its N terminal and modulates STAT3 and TBK1 signaling, triggering a switch from proinflammatory to anti-inflammatory responses. Further, we generate hRetn N-terminal peptides that are able to block LPS proinflammatory function. Together, our studies identify a critical role for hRetn in blocking LPS function with important clinical significance in helminth-induced immunomodulation and sepsis.

Helminths have coevolved with their hosts, resulting in the development of specialized host immune mechanisms and parasite-specific regulatory products. Identification of new pathways that regulate helminth infection could provide a better understanding of host-helminth interaction and may identify new therapeutic targets for helminth infection. Here we identify the endocannabinoid system as a new mechanism that influences host immunity to helminths. Endocannabinoids are lipid-derived signaling molecules that control important physiologic processes, such as feeding behavior and metabolism. Following murine infection with , an intestinal nematode with a life cycle similar to that of hookworms, we observed increased levels of endocannabinoids (2-arachidonoylglycerol [2-AG] or anandamide [AEA]) and the endocannabinoid-like molecule oleoylethanolamine (OEA) in infected lung and intestine. To investigate endocannabinoid function in helminth infection, we employed pharmacological inhibitors of cannabinoid subtype receptors 1 and 2 (CBR and CBR). Compared to findings for vehicle-treated mice, inhibition of CBR but not CBR resulted in increased worm burden and egg output, associated with significantly decreased expression of the T helper type 2 cytokine interleukin 5 (IL-5) in intestinal tissue and splenocyte cultures. Strikingly, bioinformatic analysis of genomic and transcriptome sequencing (RNA-seq) data sets identified putative genes encoding endocannabinoid biosynthetic and degradative enzymes in many parasitic nematodes. To test the novel hypothesis that helminth parasites produce their own endocannabinoids, we measured endocannabinoid levels in by mass spectrometry and quantitative PCR and found that parasites produced endocannabinoids, especially at the infectious larval stage. To our knowledge, this is the first report of helminth- and host-derived endocannabinoids that promote host immune responses and reduce parasite burden.

Resistin-like molecule α (RELMα) is a highly secreted protein in type 2 (Th2) cytokine-induced inflammation including helminth infection and allergy. In infection with Nippostrongylus brasiliensis (Nb), RELMα dampens Th2 inflammatory responses. RELMα is expressed by immune cells, and by epithelial cells (EC); however, the functional impact of immune versus EC-derived RELMα is unknown. We generated bone marrow (BM) chimeras that were RELMα deficient (RELMα ) in BM or non BM cells and infected them with Nb. Non BM RELMα chimeras had comparable inflammatory responses and parasite burdens to RELMα mice. In contrast, both RELMα and BM RELMα mice exhibited increased Nb-induced lung and intestinal inflammation, correlated with elevated Th2 cytokines and Nb killing. CD11c lung macrophages were the dominant BM-derived source of RELMα and can mediate Nb killing. Therefore, we employed a macrophage-worm co-culture system to investigate whether RELMα regulates macrophage-mediated Nb killing. Compared to RELMα macrophages, RELMα macrophages exhibited increased binding to Nb and functionally impaired Nb development. Supplementation with recombinant RELMα partially reversed this phenotype. Gene expression analysis revealed that RELMα decreased cell adhesion and Fc receptor signaling pathways, which are associated with macrophage-mediated helminth killing. Collectively, these studies demonstrate that BM-derived RELMα is necessary and sufficient to dampen Nb immune responses, and identify that one mechanism of action of RELMα is through inhibiting macrophage recruitment and interaction with Nb. Our findings suggest that RELMα acts as an immune brake that provides mutually beneficial effects for the host and parasite by limiting tissue damage and delaying parasite expulsion.

The role of macrophages in homeostatic conditions and the immune system range from clearing debris to recognizing and killing pathogens. While classically activated macrophages (CAMacs) are induced by T helper type 1 (Th1) cytokines and exhibit microbicidal properties, Th2 cytokines promote alternative activation of macrophages (AAMacs). AAMacs contribute to the killing of helminth parasites and mediate additional host-protective processes such as regulating inflammation and wound healing. Yet, other parasites susceptible to Th1 type responses can exploit alternative activation of macrophages to diminish Th1 immune responses and prolong infection. In this review, we will delineate the factors that mediate alternative activation (e.g. Th2 cytokines and chitin) and the resulting downstream signaling events (e.g. STAT6 signaling). Next, the specific AAMac-derived factors (e.g. Arginase1) that contribute to resistance or susceptibility to parasitic infections will be summarized. Finally, we will conclude with the discussion of additional AAMac functions beyond immunity to parasites, including the regulation of inflammation, wound healing and the regulation of metabolic disorders.

IntroductionMucosal surfaces, such as the intestine, are constantly exposed to the external environment, and development of a balanced immune response is essential to prevent pathogen invasion while controlling excessive or unnecessary inflammation. Notably, macrophages, which constitute a significant proportion of the leukocytes within the gut, serve as initiators to polarize immune effector or regulatory responses following a variety of infectious or inflammatory stimuli. RELMα is a secreted protein that is most commonly associated with alternatively activated macrophages (AAMac), which are recruited in T helper type (Th) 2 cytokine-dominated environments, such as helminth infection and allergy. [4][5][6] In Th2 cytokine-biased immune responses, RELMα exhibited critical immunomodulatory functions. [7][8][9] In contrast, studies by Rothenberg and colleagues uncovered a pro-inflammatory function for RELMα in mouse models of inflammatory bowel disease. 10,11 Our recent study focused on examining the role of RELMα in bacterial infection-induced inflammation.