Alterations in marginal zone macrophages and marginal zone B cells in old mice

p8 Deficiency causes siderosis in spleens and lymphocyte apoptosis in acute pancreatitis

OBJECTIVES

The gene p8 was initially described in pancreatic tissue during acute experimental pancreatitis, a disease that is characterized by a systemic immune response. Although early reports suggested that p8 affects leukocyte migration during acute pancreatitis (AP), no studies revealing its immune-modulatory effects have been performed.

METHODS

We investigated the composition of the cellular immune system in naive p8 knockout (p8(−/−)) mice and compared with matched wild-type mice during pancreatitis.

RESULTS

In young mice, there were no relevant differences in the composition of peripheral and splenic CD3(+), CD3(+)CD4(+), CD3(+)CD8(+), CD11b(+)Gr-1(-), and Gr-1 cells. In mature p8(−/−) mice, increased splenic CD4CD25FoxP3 cells, spleen siderosis, and increased marginal zones in the splenic white pulp were found. During AP, peripheral and splenic CD3(+) and CD3CD4 declined stronger in the p8(−/−) mice. The spleen of the p8(−/−) mice showed severe hypoplasia of the white pulp and mild hyperplasia of the red pulp. This was associated with a significantly increased rate of apoptosis.

CONCLUSIONS

We conclude that p8 has no impact on the cellular composition of the adaptive and innate immune systems in noninflammatory conditions. However, it may limit apoptosis and maintain homeostasis of the immune reaction during AP.

Mucosal addressin cell adhesion molecule-1 of rhesus macaques: molecular cloning, expression, and alteration after viral infection

BACKGROUND

Mucosal addressin cell adhesion molecule-1 (MAdCAM-1), a member of the immunoglobulin superfamily, is essential for gut-specific homing of leukocytes; however, it has not been well characterized in rhesus macaques.

AIMS

To obtain the complete nucleotide sequence of rhesus macaque MAdCAM-1 cDNA and determine its distribution in gut-associated lymphoid tissues (GALT) and its alteration in duodenal mucosa after simian/human immunodeficiency virus (SHIV) infection.

METHODS

MAdCAM-1 cDNA was cloned from the colon mucosa of a rhesus macaque by 3'- and 5'-RACE. The distribution and abundance of MAdCAM-1 mRNA in the GALT were examined by nested and real-time RT-PCR. The alterations of MAdCAM-1 mRNA levels in SHIV-infected duodenal mucosa were determined by real-time RT-PCR.

RESULTS

The nucleotide sequence of rhesus macaque MAdCAM-1 cDNA (1,503 bp nucleotides) including the 5'- and 3'-untranslated regions was obtained. The coding region (1,086 bp) showed 87.56% and the Ig-like domain 1, 2 and TM + cytoplasmic domains showed >93% nucleotide sequence identity to that of humans. Like humans, rhesus macaques lacked MAdCAM-1 IgA1-like domain, which could be a common feature for all primates appeared later during vertebrate evolution. Two species of MAdCAM-1 mRNA were detected and high-level transcripts were observed primarily in the GALT. The full-length MAdCAM-1 expressed in vitro could bind to human α4β7. MAdCAM-1 mRNA levels were statistically significantly reduced in SHIV-infected duodenal mucosa.

CONCLUSIONS

These data provided a basis for using rhesus macaques in pathological and therapeutic studies on leukocyte homing related diseases such as inflammatory bowel disease and HIV/AIDS.

Marginal Zone Macrophage Receptor MARCO Is Trapped in Conduits Formed by Follicular Dendritic Cells in the Spleen

The marginal zone (MZ) region of the spleen plays an important role in leukocyte traffic and the removal of blood-borne pathogens by resident macrophages. Macrophage receptor with a collagenous structure (MARCO), expressed by MZ macrophages, recognizes several microbial ligands and is also involved in the retention of MZ B cells. Here, we report that MARCO is also associated with follicular dendritic cells (FDCs) in the spleen. In its FDC-associated form MARCO is arranged in 0.3-0.5-μm diameter granular-fibrillar structures with an appearance similar to the white pulp conduit system formed by fibroblastic reticular cells (FRCs), but with different compartment preference. The follicular display of MARCO resists irradiation and requires the presence of both MZ macrophages and differentiated FDCs. The follicular delivery of MARCO is independent from the shuffling of marginal zone B cells, and it persists after clodronate liposome-mediated depletion of MZ macrophages. Our findings thus indicate that MARCO is distributed to both MZ and follicles within the spleen into conduit-like structures, where FDC-bound MARCO may mediate communication between the stromal microenvironments of MZ and follicles.

Evidence of subclinical prion disease in aged mice following exposure to bovine spongiform encephalopathy

The occurrence of variant Creutzfeldt-Jakob (vCJD) disease in humans was almost certainly the result of consumption of food contaminated with bovine spongiform encephalopathy (BSE) prions. Despite probable widespread exposure of the UK population to BSE-contaminated food in the 1980s, vCJD has been identified predominantly in young individuals, and there have been fewer cases of clinical disease than anticipated. The reasons for this are uncertain. Following peripheral exposure, many prions replicate within the lymphoid tissues before infecting the central nervous system. We have shown that the effects of host age on the microarchitecture of the spleen significantly impair susceptibility to mouse-adapted prions after peripheral exposure. The transmission of prions between different mammalian species is considered to be limited by the 'species barrier', which is dependent on several factors, including an intact immune system. Thus, cross-species prion transmission may be much less efficient in aged individuals. To test this hypothesis, we compared prion pathogenesis in groups of young (6-8 weeks old) and aged (600 days old) mice injected with primary BSE brain homogenate. We showed that prion pathogenesis was impaired dramatically in aged mice when compared with young animals. Whereas most young mice succumbed to clinical prion disease, all aged mice failed to develop clinical disease during their lifespans. However, the demonstration that prion accumulation was detected in the lymphoid tissues of some aged mice after injection with primary BSE brain homogenate, in the absence of clinical signs of prion disease, has important implications for human health.

Age-dependent dysregulation of innate immunity

As we age, the innate immune system becomes dysregulated and is characterized by persistent inflammatory responses that involve multiple immune and non-immune cell types and that vary depending on the cell activation state and tissue context. This ageing-associated basal inflammation, particularly in humans, is thought to be induced by several factors, including the reactivation of latent viral infections and the release of endogenous damage-associated ligands of pattern recognition receptors (PRRs). Innate immune cell functions that are required to respond to pathogens or vaccines, such as cell migration and PRR signalling, are also impaired in aged individuals. This immune dysregulation may affect conditions associated with chronic inflammation, such as atherosclerosis and Alzheimer's disease.

Immunosenescence and pneumococcal disease: an imbalance in host-pathogen interactions

Respiratory infections are among the most important causes of morbidity and mortality from infectious diseases worldwide. The most common causative bacterium, Streptococcus pneumoniae, frequently colonises the upper respiratory tract, where it resides mostly asymptomatically. Occasionally, however, S pneumoniae can cause severe disease such as pneumonia. Local host immunity is essential to control colonising pathogens by preventing overgrowth, spread, and invasion. However, age-related immune deficits in elderly people, known as immunosenescence, might contribute to increased disease burden. We review present knowledge about immunosenescence in the respiratory tract against Gram-positive bacteria, particularly S pneumoniae. We discuss the possible underdetection of pneumococcal colonisation in elderly people, and suggest changes to present surveillance methods to improve understanding of the relation between colonisation and disease. We conclude that present knowledge about alteration of host-pathogen interactions by immunosenescence in the respiratory tract is insufficient, and that research is needed to enable improved measures for prevention.

A comparative review of aging and B cell function in mice and humans

Immune system function declines with age. Here we review and compare age-associated changes in murine and human B cell pools and humoral immune responses. We summarize changes in B cell generation and homeostasis, as well as notable changes at the subcellular level; then discuss how these changes help to explain alterations in immune responses across the adult lifespan of the animal. In each section we compare and contrast findings in the mouse, arguably the best animal model of the aging immune system, with current understanding of B cell immunity in humans.

Activated αvβ3 integrin regulates αvβ5 integrin-mediated phagocytosis in trabecular meshwork cells

PURPOSE

To investigate the roles of αvβ3 and αvβ5 integrins in phagocytosis in human trabecular meshwork (HTM) cells.

METHODS

Immunofluorescence microscopy and FACS analysis were used to determine levels of αvβ3 and αvβ5 integrins in TM tissue and cultures of normal and immortalized TM cells. Phagocytosis was measured using pHrodo-labeled S. aureus bioparticles followed by FACS analysis. The role of αvβ5 integrin in phagocytosis was evaluated by knocking down αvβ5 integrin expression with siRNA against the human β5 gene. Signaling from focal adhesion kinase (FAK) was blocked using FAK inhibitor 14. The role of αvβ3 integrins in phagocytosis was determined by treating HTM cells with dexamethasone (DEX) or ethanol (EtOH) and by generating stable cell lines that overexpressed either wild type (WT) or constitutively active (CA) β3 integrin subunit.

RESULTS

Both TM tissue and cell lines expressed αvβ3 and αvβ5 integrins. Knockdown of αvβ5 integrin reduced phagocytosis by ∼60% and FAK inhibition significantly reduced phagocytosis up to 84%, in a dose-dependent manner. DEX treatment increased αvβ3 integrin expression in HTM cells but reduced phagocytosis by ∼50% compared with untreated and EtOH-treated cells. The CA β3 integrin-expressing cell line showed increased αvβ3 integrin levels and decreased phagocytosis by ∼50% compared with the control.

CONCLUSIONS

The αvβ5 integrin-FAK-mediated pathway regulates phagocytosis in TM cells and this pathway is inhibited by activation of αvβ3 integrins. This suggests that changes in integrin expression and activity may be responsible for alterations in phagocytosis observed in steroid induced glaucoma.

Immunosenescence: a product of the environment?

The major function of the immune system is to provide protection against pathogens, in order to prevent infections and potential death. However, with increasing age the immune system undergoes alterations culminating in a progressive deterioration in the ability to respond to infection and vaccination. The precise mechanisms associated with immunosenescence have not been fully elucidated although extensive analyses have suggested that intrinsic defects within immune cells are potentially involved. Despite the stromal niche playing a critical role in the development and activation of immune cells, the role of extrinsic factors within the microenvironment in immunosenescence is less well understood. Moreover, emerging evidence suggests that the aged microenvironment contributes significantly to the age-associated decline of immune function and additionally may offer a potential target for rejuvenating the immune system. Indeed, rejuvenation strategies which have targeted the thymic stromal microenvironment have proved to be successful in recovering thymic function in the aged.

Causes, consequences, and reversal of immune system aging

The effects of aging on the immune system are manifest at multiple levels that include reduced production of B and T cells in bone marrow and thymus and diminished function of mature lymphocytes in secondary lymphoid tissues. As a result, elderly individuals do not respond to immune challenge as robustly as the young. An important goal of aging research is to define the cellular changes that occur in the immune system and the molecular events that underlie them. Considerable progress has been made in this regard, and this information has provided the rationale for clinical trials to rejuvenate the aging immune system.

Reduced neutrophil chemotaxis and infiltration contributes to delayed resolution of cutaneous wound infection with advanced age

Advanced age is associated with alterations in innate and adaptive immune responses, which contribute to an increased risk of infection in elderly patients. Coupled with this immune dysfunction, elderly patients demonstrate impaired wound healing with elevated rates of wound dehiscence and chronic wounds. To evaluate how advanced age alters the host immune response to cutaneous wound infection, we developed a murine model of cutaneous Staphylococcus aureus wound infection in young (3-4 mo) and aged (18-20 mo) BALB/c mice. Aged mice exhibit increased bacterial colonization and delayed wound closure over time compared with young mice. These differences were not attributed to alterations in wound neutrophil or macrophage TLR2 or FcγRIII expression, or age-related changes in phagocytic potential and bactericidal activity. To evaluate the role of chemotaxis in our model, we first examined in vivo chemotaxis in the absence of wound injury to KC, a neutrophil chemokine. In response to a s.c. injection of KC, aged mice recruited fewer neutrophils at increasing doses of KC compared with young mice. This paralleled our model of wound infection, where diminished neutrophil and macrophage recruitment was observed in aged mice relative to young mice despite equivalent levels of KC, MIP-2, and MCP-1 chemokine levels at the wound site. This reduced leukocyte accumulation was also associated with lower levels of ICAM-1 in wounds from aged mice at early time points. These age-mediated defects in early neutrophil recruitment may alter the dynamics of the inflammatory phase of wound healing, impacting macrophage recruitment, bacterial clearance, and wound closure.

Revisiting the B-cell compartment in mouse and humans: more than one B-cell subset exists in the marginal zone and beyond

The immunological roles of B-cells are being revealed as increasingly complex by functions that are largely beyond their commitment to differentiate into plasma cells and produce antibodies, the key molecular protagonists of innate immunity, and also by their compartmentalisation, a more recently acknowledged property of this immune cell category. For decades, B-cells have been recognised by their expression of an immunoglobulin that serves the function of an antigen receptor, which mediates intracellular signalling assisted by companion molecules. As such, B-cells were considered simple in their functioning compared to the other major type of immune cell, the T-lymphocytes, which comprise conventional T-lymphocyte subsets with seminal roles in homeostasis and pathology, and non-conventional T-lymphocyte subsets for which increasing knowledge is accumulating. Since the discovery that the B-cell family included two distinct categories — the non-conventional, or extrafollicular, B1 cells, that have mainly been characterised in the mouse; and the conventional, or lymph node type, B2 cells — plus the detailed description of the main B-cell regulator, FcγRIIb, and the function of CD40⁺ antigen presenting cells as committed/memory B-cells, progress in B-cell physiology has been slower than in other areas of immunology. Cellular and molecular tools have enabled the revival of innate immunity by allowing almost all aspects of cellular immunology to be re-visited. As such, B-cells were found to express “Pathogen Recognition Receptors” such as TLRs, and use them in concert with B-cell signalling during innate and adaptive immunity. An era of B-cell phenotypic and functional analysis thus began that encompassed the study of B-cell microanatomy principally in the lymph nodes, spleen and mucosae. The novel discovery of the differential localisation of B-cells with distinct phenotypes and functions revealed the compartmentalisation of B-cells. This review thus aims to describe novel findings regarding the B-cell compartments found in the mouse as a model organism, and in human physiology and pathology. It must be emphasised that some differences are noticeable between the mouse and human systems, thus increasing the complexity of B-cell compartmentalisation. Special attention will be given to the (lymph node and spleen) marginal zones, which represent major crossroads for B-cell types and functions and a challenge for understanding better the role of B-cell specificities in innate and adaptive immunology.

CCN1 expression in hepatocytes contributes to macrophage infiltration in nonalcoholic fatty liver disease in mice

Our objective was to investigate the potential roles of CCN1 in the inflammation and macrophage infiltration of nonalcoholic fatty liver disease (NAFLD). The regulation of hepatic CCN1 expression was investigated in vitro with murine primary hepatocytes treated with free fatty acids or lipopolysaccharide (LPS) and in vivo with high-fat (HF) diet-fed mice or ob/ob mice. CCN1 protein and a liver-specific CCN1 expression plasmid were administered to mice fed a normal diet (ND) or HF diet. Myeloid-derived macrophages and RAW264.7 cells were also treated with CCN1 in vitro to determine the chemotactic effects of CCN1 on macrophages. LPS treatment significantly increased hepatic CCN1 expression in HF diet-fed mice and ob/ob mice. LPS and FFAs induced CCN1 expression in primary murine hepatocytes in vitro through the TLR4/MyD88/AP-1 pathway. CCN1 protein and overexpression of CCN1 in the liver induced more severe hepatic inflammation and macrophage infiltrates in HF mice than in ND mice. CCN1 recruited macrophages through activation of the Mek/Erk signaling pathway in myeloid-derived macrophages and RAW264.7 cells in vitro. Endotoxin and FFA-induced CCN1 expression in hepatocytes is involved in the hepatic proinflammatory response and macrophage infiltration in murine NAFLD.

Marginal zone B-cells, a gatekeeper of innate immunity

To maintain the integrity of an organism constantly challenged by pathogens, the immune system is endowed with a variety of cell types. B lymphocytes were initially thought to only play a role in the adaptive branch of immunity. However, a number of converging observations revealed that two B-cell subsets, marginal zone (MZ) and B1 cells, exhibit unique developmental and functional characteristics, and can contribute to innate immune responses. In addition to their capacity to mount a local antibody response against type-2 T-cell-independent (TI-2) antigens, MZ B-cells can participate to T-cell-dependent (TD) immune responses through the capture and import of blood-borne antigens to follicular areas of the spleen. Here, we discuss the multiple roles of MZ B-cells in humans, non-human primates, and rodents. We also summarize studies – performed in transgenic mice expressing fully human antibodies on their B-cells and in macaques whose infection with Simian immunodeficiency virus (SIV) represents a suitable model for HIV-1 infection in humans – showing that infectious agents have developed strategies to subvert MZ B-cell functions. In these two experimental models, we observed that two microbial superantigens for B-cells (protein A from Staphylococcus aureus and protein L from Peptostreptococcus magnus) as well as inactivated AT-2 virions of HIV-1 and infectious SIV preferentially deplete innate-like B-cells – MZ B-cells and/or B1 B-cells – with different consequences on TI and TD antibody responses. These data revealed that viruses and bacteria have developed strategies to deplete innate-like B-cells during the acute phase of infection and to impair the antibody response. Unraveling the intimate mechanisms responsible for targeting MZ B-cells in humans will be important for understanding disease pathogenesis and for designing novel vaccine strategies.

A molecular mechanism for TNF-α-mediated downregulation of B cell responses

B cell function with age is decreased in class switch recombination (CSR), activation-induced cytidine deaminase (AID), and stability of E47 mRNA. The latter is regulated, at least in part, by tristetraprolin (TTP), which is increased in aged B cells and also negatively regulates TNF-α. In this study, we investigated whether B cells produce TNF-α, whether this changes with age, and how this affects their function upon stimulation. Our hypothesis is that in aging there is a feedback mechanism of autocrine inflammatory cytokines (TNF-α) that lowers the expression of AID and CSR. Our results showed that unstimulated B cells from old BALB/c mice make significantly more TNF-α mRNA and protein than do B cells from young mice, but after stimulation the old make less than the young; thus, they are refractory to stimulation. The increase in TNF-α made by old B cells is primarily due to follicular, but not minor, subsets of B cells. Incubation of B cells with TNF-α before LPS stimulation decreased both young and old B cell responses. Importantly, B cell function was restored by adding anti-TNF-α Ab to cultured B cells. To address a molecular mechanism, we found that incubation of B cells with TNF-α before LPS stimulation induced TTP, a physiological regulator of mRNA stability of the transcription factor E47, which is crucial for CSR. Finally, anti-TNF-α given in vivo increased B cell function in old, but not in young, follicular B cells. These results suggest new molecular mechanisms that contribute to reduced Ab responses in aging.

Age-related Dysregulation of Inflammation and Innate Immunity: Lessons Learned from Rodent Models

In the elderly patient population, it has become increasingly evident that immune dysregulation is a contributing factor to age-related pathologies and their associated morbidity and mortality. In particular, elderly subjects are plagued by poor responses to infectious challenge and immunization and are at heightened risk for the development of autoimmune, neuroinflammatory and tumor-associated pathologies. Rodent models of aging and age-related disorders have been utilized to better describe how innate immune cell dysfunction contributes to these clinical scenarios. As the elderly population continues to increase in size, use of these aging rodent models to study immune dysregulation may translate into increased healthy living years for these individuals.

Aging impairs murine B cell differentiation and function in primary and secondary lymphoid tissues

Age-related changes in humoral immunity are responsible for the reduced vaccine responses observed in elderly individuals. Although aging has been shown to affect T cells, dendritic cells and macrophages and these effects significantly impact humoral responses, intrinsic alterations in B cells also occur. We here provide an overview of age-related changes in mouse B cells. In particular, we summarize data from the literature showing age-related changes in B cell differentiation in the bone marrow, in B cell marker expression and cell survival in the periphery and in the ability to make specific antibodies in both splenic and mucosal tissues. Moreover, we summarize the results from our studies showing that the ability to undergo class switch recombination, the enzyme activation-induced cytidine deaminase and the transcription factor E47 are all decreased in stimulated B cells from old mice. The defects presented in this review for aged B cells should allow the discovery of strategies for improvement of humoral immune responses in both humans and mice in the near future.