Links between Immunologic Memory and Metabolic Cycling [BRIEF REVIEWS]

Treatments for metabolic diseases, such as diet and therapeutics, often provide short-term therapy for metabolic stressors, but relapse is common. Repeated bouts of exposure to, and relief from, metabolic stimuli results in a phenomenon we call “metabolic cycling.” Recent human and rodent data suggest metabolic cycling promotes an exaggerated response and ultimately worsened metabolic health. This is particularly evident with cycling of body weight and hypertension. The innate and adaptive immune systems have a profound impact on development of metabolic disease, and current data suggest that immunologic memory may partially explain this association, especially in the context of metabolic cycling. In this Brief Review, we highlight recent work in this field and discuss potential immunologic mechanisms for worsened disease prognosis in individuals who experience metabolic cycling.

Cutting Edge: Identification of Marginal Reticular Cells as Phagocytes of Apoptotic B Cells in Germinal Centers [CUTTING EDGE]

Germinal centers (GCs) in secondary lymphoid organs generate large numbers of apoptotic B cells that must be eliminated by phagocytes to prevent the development of autoimmune diseases. Although tingible body macrophages engulf apoptotic GC B cells, whether stromal cells are also involved in this process is unclear. In this study, we identified marginal reticular cells (MRCs) as novel nonprofessional phagocytes for the clearance of apoptotic GC B cells in the spleen. We used CD19eGFP (CD19creZ/EG) mice, which express enhanced GFP (eGFP) under the control of CD19cre expression, to track B cells in the GCs after immunization with NP-chicken globulin plus aluminum salt. We demonstrated that the MRC population, as determined by expression of podoplanin or Rankl, specifically showed an eGFP signal in the cytoplasm after immunization. These results suggest that MRCs contribute to the clearance of apoptotic B cells in GCs.

Vasoactive Intestinal Peptide Ameliorates Acute Myocarditis and Atherosclerosis by Regulating Inflammatory and Autoimmune Responses [AUTOIMMUNITY]

Vasoactive intestinal peptide (VIP) is a neuropeptide that exerts various vascular and cardioprotective functions and regulates immune function and inflammatory response at multiple levels. However, its role in inflammatory cardiovascular disorders is largely unknown. Myocarditis and atherosclerosis are two inflammatory and autoimmune cardiovascular diseases that cause important adverse circulatory events. In this study, we investigate the therapeutic effects of VIP in various well-established preclinical models of experimental autoimmune myocarditis and atherosclerosis. Intraperitoneal injection of VIP during the effector phase of experimental autoimmune myocarditis in susceptible BALB/c mice significantly reduced its prevalence, ameliorated signs of heart hypertrophy and injury, attenuated myocardial inflammatory infiltration, and avoided subsequent profibrotic cardiac remodeling. This effect was accompanied by a reduction of Th17-driven cardiomyogenic responses in peripheral lymphoid organs and in the levels of myocardial autoantibodies. In contrast, acute and chronic atherosclerosis was induced in apolipoprotein E–deficient mice fed a hyperlipidemic diet and subjected to partial carotid ligation. Systemic VIP treatment reduced the number and size of atherosclerotic plaques in carotid, aorta, and sinus in hypercholesterolemic mice. VIP reduced Th1-driven inflammatory responses and increased regulatory T cells in atherosclerotic arteries and their draining lymph nodes. VIP also regulated cholesterol efflux in macrophages and reduced the formation of foam cells and their presence in atherosclerotic plaques. Finally, VIP inhibited proliferation and migration of smooth muscle cells and neointima formation in a mouse model of complete carotid ligation. These findings encourage further studies aimed to assess whether VIP can be used as a pharmaceutical agent to treat heart inflammation and atherosclerosis.

PD-L1/B7-H1 Inhibits Viral Clearance by Macrophages in HSV-1-Infected Corneas [IMMUNE REGULATION]

Immune privilege helps protect the cornea from damaging inflammation but can also impair pathogen clearance from this mucosal surface. Programmed death-ligand 1 (PD-L1 or B7-H1) contributes to corneal immune privilege by inhibiting the function of a variety of immune cells. We asked whether programmed death-1 (PD-1)/PD-L1 interaction regulates HSV-1 clearance from infected corneas. We show that PD-L1 is constitutively expressed in the corneal epithelium and is upregulated upon HSV-1 corneal infection, with peak expression on CD45+ cells NK cells, dendritic cells, neutrophils, and macrophages and CD45 corneal epithelial cells at 4 d postinfection (dpi). As early as 1 dpi, HSV-1–infected corneas of B7-H1–/– mice as compared with wild-type mice showed increased chemokine expression and this correlated with increased migration of inflammatory cells into the viral lesions and decreased HSV-1 corneal titers. Local PD-L1 blockade caused a similar increase in viral clearance, suggesting a local effect of PD-1/PD-L1 in the cornea. The enhanced HSV-1 clearance at 2 dpi resulting from PD-1/PD-L1 blockade is mediated primarily by a monocyte/macrophage population. Studies in bone marrow chimeras demonstrated enhanced viral clearance when PD-L1 was absent only from nonhematopoietic cells. We conclude that PD-L1 expression on corneal cells negatively impacts the ability of the innate immune system to clear HSV-1 from infected corneas.

Identification of a Transcriptionally Forward {alpha} Gene and Two {upsilon} Genes within the Pigeon (Columba livia) IgH Gene Locus [IMMUNOGENETICS]

Compared with mammals, the bird Ig genetic system relies on gene conversion to create an Ab repertoire, with inversion of the IgA-encoding gene and very few cases of Ig subclass diversification. Although gene conversion has been studied intensively, class-switch recombination, a mechanism by which the IgH C region is exchanged, has rarely been investigated in birds. In this study, based on the published genome of pigeon (Columba livia) and high-throughput transcriptome sequencing of immune-related tissues, we identified a transcriptionally forward α gene and found that the pigeon IgH gene locus is arranged as μ-α-1-2. In this article, we show that both DNA deletion and inversion may result from IgA and IgY class switching, and similar junction patterns were observed for both types of class-switch recombination. We also identified two subclasses of genes in pigeon, which share low sequence identity. Phylogenetic analysis suggests that divergence of the two pigeon genes occurred during the early stage of bird evolution. The data obtained in this study provide new insight into class-switch recombination and Ig gene evolution in birds.

Cyclooxygenase-1 and -2 Play Contrasting Roles in Listeria-Stimulated Immunity [IMMUNOTHERAPY AND VACCINES]

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit cyclooxygenase (COX) activity and are commonly used for pain relief and fever reduction. NSAIDs are used following childhood vaccinations and cancer immunotherapies; however, how NSAIDs influence the development of immunity following these therapies is unknown. We hypothesized that NSAIDs would modulate the development of an immune response to Listeria monocytogenes–based immunotherapy. Treatment of mice with the nonspecific COX inhibitor indomethacin impaired the generation of cell-mediated immunity. This phenotype was due to inhibition of the inducible COX-2 enzyme, as treatment with the COX-2–selective inhibitor celecoxib similarly inhibited the development of immunity. In contrast, loss of COX-1 activity improved immunity to L. monocytogenes. Impairments in immunity were independent of bacterial burden, dendritic cell costimulation, or innate immune cell infiltrate. Instead, we observed that PGE2 production following L. monocytogenes is critical for the formation of an Ag-specific CD8+ T cell response. Use of the alternative analgesic acetaminophen did not impair immunity. Taken together, our results suggest that COX-2 is necessary for optimal CD8+ T cell responses to L. monocytogenes, whereas COX-1 is detrimental. Use of pharmacotherapies that spare COX-2 activity and the production of PGE2 like acetaminophen will be critical for the generation of optimal antitumor responses using L. monocytogenes.

Nasopharyngeal Exposure to Streptococcus pneumoniae Induces Extended Age-Dependent Protection against Pulmonary Infection Mediated by Antibodies and CD138+ Cells [INFECTIOUS DISEASE AND HOST RESPONSE]

Streptococcus pneumoniae commonly resides asymptomatically in the nasopharyngeal (NP) cavity of healthy individuals but can cause life-threatening pulmonary and systemic infections, particularly in the elderly. NP colonization results in a robust immune response that protects against invasive infections. However, the duration, mechanism, and cellular component of such responses are poorly understood. In this study, we found that repeated NP exposure of mice to S. pneumoniae TIGR4 strain results in pneumococcal-specific Ab responses that protect against lethal lung challenge. Abs were necessary and sufficient for protection because Ab-deficient μMT mice did not develop postexposure protection, only becoming resistant to lung infection after transfer of immune sera from NP-exposed mice. T cells contributed to immunity at the time of NP exposure, but neither CD4+ nor CD8+ T cells were required. The protective activity was detectable 20 wk after exposure and was maintained in irradiated mice, suggesting involvement of long-lived Ab-secreting cells (ASC), which are radioresistant and secrete Abs for extended periods of time in the absence of T cells or persistent Ag. CD138+ bone marrow cells, likely corresponding to long-lived ASC, were sufficient to confer protection. NP exposure of aged mice failed to protect against subsequent lung infection despite eliciting a robust Ab response. Furthermore, transfer of CD138+ bone marrow cells or sera from NP-exposed old mice failed to protect naive young mice. These findings suggest that NP exposure elicits extended protection against pneumococcal lung infection by generating long-lived CD138+ ASC and that the protective efficacy of these responses declines with age.

The Respiratory Environment Diverts the Development of Antiviral Memory CD8 T Cells [INFECTIOUS DISEASE AND HOST RESPONSE]

Our understanding of memory CD8+ T cells has been largely derived from acute, systemic infection models. However, memory CD8+ T cells generated from mucosal infection exhibit unique properties and, following respiratory infection, are not maintained in the lung long term. To better understand how infection route modifies memory differentiation, we compared murine CD8+ T cell responses to a vesicular stomatitis virus (VSV) challenge generated intranasally (i.n.) or i.v. The i.n. infection resulted in greater peak expansion of VSV-specific CD8+ T cells. However, this numerical advantage was rapidly lost during the contraction phase of the immune response, resulting in memory CD8+ T cell numerical deficiencies when compared with i.v. infection. Interestingly, the antiviral CD8+ T cells generated in response to i.n. VSV exhibited a biased and sustained proportion of early effector cells (CD127loKLRG1lo) akin to the developmental program favored after i.n. influenza infection, suggesting that respiratory infection broadly favors an incomplete memory differentiation program. Correspondingly, i.n. VSV infection resulted in lower CD122 expression and eomesodermin levels by VSV-specific CD8+ T cells, further indicative of an inferior transition to bona fide memory. These results may be due to distinct (CD103+CD11b+) dendritic cell subsets in the i.n. versus i.v. T cell priming environments, which express molecules that regulate T cell signaling and the balance between tolerance and immunity. Therefore, we propose that distinct immunization routes modulate both the quality and quantity of antiviral effector and memory CD8+ T cells in response to an identical pathogen and should be considered in CD8+ T cell–based vaccine design.

NKp46 Calibrates Tumoricidal Potential of Type 1 Innate Lymphocytes by Regulating TRAIL Expression [INNATE IMMUNITY AND INFLAMMATION]

NK cells are a subset of group 1 innate lymphocytes that recognize and eliminate virus-infected and transformed cells. During the course of their development, NK cells acquire a repertoire of activating and inhibitory receptors, which ultimately define their reactivity against target cells. The array of receptors and their specificity during early developmental stages will control and imprint functional properties of NK cells, a process known as “NK cell education.” Innate lymphoid cells (ILCs) are a diverse group of lymphocytes, which, like NK cells, do not rely on somatically rearranged Ag receptors for recognition. Among ILC subsets, ILC1s are most like NK cells functionally. Prototypic ILC1s reside in the liver, and a large part of their function is attributed to the expression of TRAIL, a TNF superfamily member with a well-documented antitumor activity. In this article, we show that TRAIL expression on mouse ILC1s is controlled by an activating receptor NKp46, which has been previously shown to control NK cell education. In the absence of NKp46, ILC1s fail to express normal levels of TRAIL on the surface, which results in diminished cytotoxicity toward TRAIL receptor-positive targets. To our knowledge, these findings provide the first evidence of a role of NKp46 in ILC1s that calibrates their antitumor response.

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