Understanding Experimental LCMV Infection of Mice: The Role of Mathematical Models

Kidney double positive T cells have distinct characteristics in normal and diseased kidneys

Multiple types of T cells have been described and assigned pathophysiologic functions in the kidneys. However, the existence and functions of TCR+CD4+CD8+ (double positive; DP) T cells are understudied in normal and diseased murine and human kidneys. We studied kidney DPT cells in mice at baseline and after ischemia reperfusion (IR) and cisplatin injury. Additionally, effects of viral infection and gut microbiota were studied. Human kidneys from patients with renal cell carcinoma were evaluated. Our results demonstrate that DPT cells expressing CD4 and CD8 co-receptors constitute a minor T cell population in mouse kidneys. DPT cells had significant Ki67 and PD1 expression, effector/central memory phenotype, proinflammatory cytokine (IFNγ, TNFα and IL-17) and metabolic marker (GLUT1, HKII, CPT1a and pS6) expression at baseline. IR, cisplatin and viral infection elevated DPT cell proportions, and induced distinct functional and metabolic changes. scRNA-seq analysis showed increased expression of Klf2 and Ccr7 and enrichment of TNFα and oxidative phosphorylation related genes in DPT cells. DPT cells constituted a minor population in both normal and cancer portion of human kidneys. In conclusion, DPT cells constitute a small population of mouse and human kidney T cells with distinct inflammatory and metabolic profile at baseline and following kidney injury.

Mitochondrial metabolic flexibility is critical for CD8+ T cell antitumor immunity

Mitochondria use different substrates for energy production and intermediatory metabolism according to the availability of nutrients and oxygen levels. The role of mitochondrial metabolic flexibility for CD8+ T cell immune response is poorly understood. Here, we report that the deletion or pharmacological inhibition of protein tyrosine phosphatase, mitochondrial 1 (PTPMT1) significantly decreased CD8+ effector T cell development and clonal expansion. In addition, PTPMT1 deletion impaired stem-like CD8+ T cell maintenance and accelerated CD8+ T cell exhaustion/dysfunction, leading to aggravated tumor growth. Mechanistically, the loss of PTPMT1 critically altered mitochondrial fuel selection-the utilization of pyruvate, a major mitochondrial substrate derived from glucose-was inhibited, whereas fatty acid utilization was enhanced. Persistent mitochondrial substrate shift and metabolic inflexibility induced oxidative stress, DNA damage, and apoptosis in PTPMT1 knockout cells. Collectively, this study reveals an important role of PTPMT1 in facilitating mitochondrial utilization of carbohydrates and that mitochondrial flexibility in energy source selection is critical for CD8+ T cell antitumor immunity.

Neuropilin-1 identifies a subset of highly activated CD8+ T cells during parasitic and viral infections

Neuropilin-1 (Nrp-1) expression on CD8+ T cells has been identified in tumor-infiltrating lymphocytes and in persistent murine gamma-herpes virus infections, where it interferes with the development of long-lived memory T cell responses. In parasitic and acute viral infections, the role of Nrp-1 expression on CD8+ T cells remains unclear. Here, we demonstrate a strong induction of Nrp-1 expression on CD8+ T cells in Plasmodium berghei ANKA (PbA)-infected mice that correlated with neurological deficits of experimental cerebral malaria (ECM). Likewise, the frequency of Nrp-1+CD8+ T cells was significantly elevated and correlated with liver damage in the acute phase of lymphocytic choriomeningitis virus (LCMV) infection. Transcriptomic and flow cytometric analyses revealed a highly activated phenotype of Nrp-1+CD8+ T cells from infected mice. Correspondingly, in vitro experiments showed rapid induction of Nrp-1 expression on CD8+ T cells after stimulation in conjunction with increased expression of activation-associated molecules. Strikingly, T cell-specific Nrp-1 ablation resulted in reduced numbers of activated T cells in the brain of PbA-infected mice as well as in spleen and liver of LCMV-infected mice and alleviated the severity of ECM and LCMV-induced liver pathology. Mechanistically, we identified reduced blood-brain barrier leakage associated with reduced parasite sequestration in the brain of PbA-infected mice with T cell-specific Nrp-1 deficiency. In conclusion, Nrp-1 expression on CD8+ T cells represents a very early activation marker that exacerbates deleterious CD8+ T cell responses during both, parasitic PbA and acute LCMV infections.

Intestinal colonization regulates systemic anti-commensal immune sensitivity and hyperreactivity

Healthy host-microbial mutualism with our intestinal microbiota relies to a large degree on compartmentalization and careful regulation of adaptive mucosal and systemic anti-microbial immune responses. However, commensal intestinal bacteria are never exclusively or permanently restricted to the intestinal lumen and regularly reach the systemic circulation. This results in various degrees of commensal bacteremia that needs to be appropriately dealt with by the systemic immune system. While most intestinal commensal bacteria, except for pathobionts or opportunistic pathogen, have evolved to be non-pathogenic, this does not mean that they are non-immunogenic. Mucosal immune adaptation is carefully controlled and regulated to avoid an inflammatory response, but the systemic immune system usually responds differently and more vigorously to systemic bacteremia. Here we show that germ-free mice have increased systemic immune sensitivity and display anti-commensal hyperreactivity in response to the addition of a single defined T helper cell epitope to the outer membrane porin C (OmpC) of a commensal Escherichia coli strain demonstrated by increased E. coli-specific T cell-dependent IgG responses following systemic priming. This increased systemic immune sensitivity was not observed in mice colonized with a defined microbiota at birth indicating that intestinal commensal colonization also regulates systemic, and not only mucosal, anti-commensal responses. The observed increased immunogenicity of the E. coli strain with the modified OmpC protein was not due to a loss of function and associated metabolic changes as a control E. coli strain without OmpC did not display increased immunogenicity.

Immunosuppressive glucocorticoids at epithelial barriers in the regulation of anti-viral immune response

The anti-inflammatory action of adrenal-derived glucocorticoids has been recognized since several decades. This knowledge has found broad application in the clinics and today synthetic glucocorticoids are widely used in the treatment of various inflammatory diseases. However, the use of synthetic glucocorticoids in the treatment of diseases associated with viral infections of epithelial surfaces, like the lung or the intestine, is still under debate and seems not as efficient as desired. Basic research on the anti-viral immune responses and on regulatory mechanisms in the prevention of immunopathological disorders, however, has led us back again to focus on endogenous glucocorticoid synthesis. It has become established that this synthesis is not restricted to the adrenal glands alone, but that numerous tissues also produce glucocorticoids in situ. Extra-adrenal derived glucocorticoids have the capacity to locally control and maintain immune homeostasis under steady-state and inflammatory conditions. Here, we discuss the current knowledge of extra-adrenal glucocorticoid synthesis in the lung and the intestine, and its role in the regulation of anti-viral immune responses.

TCR Transgenic Mice: A Valuable Tool for Studying Viral Immunopathogenesis Mechanisms

Viral infectious diseases are a significant burden on public health and the global economy, and new viral threats emerge continuously. Since CD4⁺ and CD8⁺ T cell responses are essential to eliminating viruses, it is important to understand the underlying mechanisms of anti-viral T cell-mediated immunopathogenesis during viral infections. Remarkable progress in transgenic (Tg) techniques has enabled scientists to more readily understand the mechanisms of viral pathogenesis. T cell receptor (TCR) Tg mice are extremely useful in studying T cell-mediated immune responses because the majority of T cells in these mice express specific TCRs for partner antigens. In this review, we discuss the important studies utilizing TCR Tg mice to unveil underlying mechanisms of T cell-mediated immunopathogenesis during viral infections.

MATHEMATICAL MODELLING OF IMMUNE PROCESSES AND ITS APPLICATION

The aim of the study was to develop a mathematical model to research hypoxic states in case of simulation of an organism infectious lesions. The model is based on the methods of mathematical modeling and the theory of optimal control of moving objects. The processes of organism damage are simulated with the mathematical model of immune response developed by G.I. Marchuk and the members of his scientific school, adapted to current conditions. This model is based on Burnet’s clone selection theory of the determining role of antigen. Simulation results using the model are presented. The dependencies of infectious courses on the volumetric velocity of systemic blood flow is analyzed on the complex mathematical model of immune response, respiratory and blood circulation systems. The immune system is shown to be rather sensitive to the changes in blood flow via capillaries. Thus, the organ blood flows can be used as parameters for the model by which the respiratory, immune response, and blood circulation systems interact and interplay.

Strength of tonic T cell receptor signaling instructs T follicular helper cell-fate decisions

T follicular helper (T_FH) cells are critical in adaptive immune responses to pathogens and vaccines; however, what drives the initiation of their developmental program remains unclear. Studies suggest that a T cell antigen receptor (TCR)-dependent mechanism may be responsible for the earliest T_FH cell-fate decision, but a critical aspect of the TCR has been overlooked: tonic TCR signaling. We hypothesized that tonic signaling influences early T_FH cell development. Here, two murine TCR-transgenic CD4⁺ T cells, LLO56 and LLO118, which recognize the same antigenic peptide presented on major histocompatibility complex molecules but experience disparate strengths of tonic signaling, revealed low tonic signaling promotes T_FH cell differentiation. Polyclonal T cells paralleled these findings, with naive Nur77 expression distinguishing T_FH cell potential. Two mouse lines were also generated to both increase and decrease tonic signaling strength, directly establishing an inverse relationship between tonic signaling strength and T_FH cell development. Our findings elucidate a central role for tonic TCR signaling in early T_FH cell-lineage decisions.

Retinoic Acid Modulates Hyperactive T Cell Responses and Protects Vitamin A-Deficient Mice against Persistent Lymphocytic Choriomeningitis Virus Infection

Vitamin A deficiency (VAD) is a major public health problem and is associated with increased host susceptibility to infection; however, how VAD influences viral infection remains unclear. Using a persistent lymphocytic choriomeningitis virus infection model, we showed in this study that although VAD did not alter innate type I IFN production, infected VAD mice had hyperactive, virus-specific T cell responses at both the acute and contraction stages, showing significantly decreased PD-1 but increased cytokine (IFN-γ, TNF-α, and IL-2) expression by T cells. Compared with control mice, VAD mice displayed excessive inflammation and more severe liver pathology, with increased death during persistent infection. Of note, supplements of all-trans retinoic acid (RA), one of the important metabolites of vitamin A, downregulated hyperactive T cell responses and rescued the persistently infected VAD mice. By using adoptive transfer of splenocytes, we found that the environmental vitamin A or its metabolites acted as rheostats modulating antiviral T cells. The analyses of T cell transcriptional factors and signaling pathways revealed the possible mechanisms of RA, as its supplements inhibited the abundance of NFATc1 (NFAT 1), a key regulator for T cell activation. Also, following CD3/CD28 cross-linking stimulation, RA negatively regulated the TCR-proximal signaling in T cells, via decreased phosphorylation of Zap70 and its downstream signals, including phosphorylated AKT, p38, ERK, and S6, respectively. Together, our data reveal VAD-mediated alterations in antiviral T cell responses and highlight the potential utility of RA for modulating excessive immune responses and tissue injury in infectious diseases.

Chronic Lymphocytic Choriomeningitis Infection Causes Susceptibility to Mousepox and Impairs Natural Killer Cell Maturation and Function

Chronic viral infections. like those of humans with cytomegalovirus, human immunodeficiency virus (even when under antiretroviral therapy), and hepatitis C virus or those of mice with lymphocytic choriomeningitis virus (LCMV) clone 13 (CL13), result in immune dysfunction that predisposes the host to severe infections with unrelated pathogens. It is known that C57BL/6 (B6) mice are resistant to mousepox, a lethal disease caused by the orthopoxvirus ectromelia virus (ECTV), and that this resistance requires natural killer (NK) cells and other immune cells. We show that most B6 mice chronically infected with CL13 succumb to mousepox but that most of those that recovered from acute infection with the LCMV Armstrong (Arm) strain survive. We also show that B6 mice chronically infected with CL13 and those that recovered from Arm infection have a reduced frequency and a reduced number of NK cells. However, at steady state, NK cells in mice that have recovered from Arm infection mature normally and, in response to ECTV, get activated, become more mature, proliferate, and increase their cytotoxicity in vivo Conversely, in mice chronically infected with CL13, NK cells are immature and residually activated, and following ECTV infection, they do not mature, proliferate, or increase their cytotoxicity. Given the well-established importance of NK cells in resistance to mousepox, these data suggest that the NK cell dysfunction caused by CL13 persistence may contribute to the susceptibility of CL13-infected mice to mousepox. Whether chronic infections similarly affect NK cells in humans should be explored.IMPORTANCE Infection of adult mice with the clone 13 (CL13) strain of lymphocytic choriomeningitis virus (LCMV) is extensively used as a model of chronic infection. In this paper, we show that mice chronically infected with CL13 succumb to challenge with ectromelia virus (ECTV; the agent of mousepox) and that natural killer (NK) cells in CL13-infected mice are reduced in numbers and have an immature and partially activated phenotype but do respond to ECTV. These data may provide additional clues why humans chronically infected with certain pathogens are less resistant to viral diseases.

Lipid order and charge protect killer T cells from accidental death

Killer T cells (cytotoxic T lymphocytes, CTLs) maintain immune homoeostasis by eliminating virus-infected and cancerous cells. CTLs achieve this by forming an immunological synapse with their targets and secreting a pore-forming protein (perforin) and pro-apoptotic serine proteases (granzymes) into the synaptic cleft. Although the CTL and the target cell are both exposed to perforin within the synapse, only the target cell membrane is disrupted, while the CTL is invariably spared. How CTLs escape unscathed remains a mystery. Here, we report that CTLs achieve this via two protective properties of their plasma membrane within the synapse: high lipid order repels perforin and, in addition, exposed phosphatidylserine sequesters and inactivates perforin. The resulting resistance of CTLs to perforin explains their ability to kill target cells in rapid succession and to survive these encounters. Furthermore, these mechanisms imply an unsuspected role for plasma membrane organization in protecting cells from immune attack.

Analyzing Mouse B Cell Responses Specific to LCMV Infection

B cell responses play a central role in humoral immunity, which protects an individual from invading pathogens by antigen-specific antibodies. Understanding the basic principles of the B cell responses during viral infection is of substantial importance for anti-viral vaccine development. In inbred mice, lymphocytic choriomeningitis virus (LCMV) infection elicits robust and typical T cell-dependent B cell responses, including germinal center reaction, memory B cell formation, and a long-lived plasma cell pool in bone marrow. Therefore, this system represents an ideal model to investigate anti-viral B cell responses. In this protocol, we describe how to propagate and quantify LCMV and successfully establish an acute LCMV infection in mice. This protocol also provides three different techniques to analyze B cell responses specific to an acute LCMV infection: the identification of germinal center (GC) B cells and follicular helper CD4 T (T_FH) cells from the spleens and lymph nodes via flow cytometry, titration of LCMV-specific IgG in the serum after LCMV infection using an enzyme-linked immunosorbent assay (ELISA) analysis, and detection of LCMV-IgG secreted plasma cells from bone marrow with an enzyme-linked immunospot (ELISPOT) assay.