Role Of Neurophils In The Fight Against Bacteria Infection

Project and Seminar Material for Microbiology

Abstract

This study was carried out on the role of neurophils in the fight against bacteria infection. Neutrophils are the first immune cell population recruited to sites of infection, including viral infections, and exhibit both protective and pathologic functions. In antibacterial and antifungal immunity, the role of neutrophils is well defined.

Neutrophils are endowed with a plethora of toxic molecules that are mobilized in immune responses. These cells evolved to fight infections, but when deployed at the wrong time and in the wrong place, they cause damage to the host. Here, the study reviewed the generalities of these cells as well as the difficulties encountered when trying to unravel them mechanistically.

However, recent studies have shown that neutrophils participate in both protective and detrimental responses to a diverse array of inflammatory and infectious diseases. Although the contribution of neutrophils to extracellular infections has been investigated for decades, their specific role during intracellular bacterial infections has only recently been appreciated. Most bacteria are killed readily by neutrophils, some bacterial pathogens have the capacity to circumvent destruction by these host leukocytes.

The ability of bacterial pathogens to avoid killing by neutrophils often involves multiple attributes or characteristics, including the production of virulence molecules. These molecules are diverse in composition and function, and collectively have the potential to alter or inhibit neutrophil recruitment, phagocytosis, bactericidal activity, and/or apoptosis.

Chapter One

Introduction

1.1 Background of the study

The human innate immune system is comprised of many components that function in concert to defend against microorganisms. These components include physical barriers, antimicrobial peptides, freely secreted antimicrobial proteins present in the blood, mucous secretions, and interstitial fluid, and leukocytes. Together, these innate immune system elements protect against infection from many types of microorganisms, including bacteria.
Neutrophils are the most abundant white blood cells, and deficiencies in these cells, inherited or acquired, often result in severe infections (Klein, 2011).

Curiously, in spite of their obvious relevance in immunity, in comparison to other immune cells, we know relatively little about how they function. Neutrophils are easy to recognize because of their uniquely lobulated nucleus, which has earned these cells the alternative name of polymorphonuclear cells (PMNs), and we use these two names indistinctively. They contain different types of granules packed with molecules that allow them to fulfil their antimicrobial function.

Neutrophils develop in the bone marrow and emerge as terminally differentiated cells in circulation, where they live a short life (whether it is hours or days is currently debated) (Pillay et al., 2010; Tak et al., 2013), unless called into action at an inflammatory site (Nathan, 2006; Amulic et al., 2012).

Neutrophils are hematopoietic-derived immune cells that are generated, and continue to develop, in the bone marrow until recruited into circulation and then to sites of infection or inflammation.

Neutrophils (also known as neutrocytes) are the most abundant type of granulocytes and the most abundant (60% to 70%) type of white blood cells in most mammals. They form an essential part of the innate immune system. Their functions vary in different animals.

They are formed from stem cells in the bone marrow and differentiated into subpopulations of neutrophil-killers and neutrophil-cagers. They are short-lived and highly motile, or mobile, as they can enter parts of tissue where other cells/molecules cannot. Neutrophils may be subdivided into segmented neutrophils and banded neutrophils (or bands). They form part of the polymorphonuclear cell family (PMNs) together with basophils and eosinophils.
The name neutrophil derives from staining characteristics on hematoxylin and eosin (H&E) histological or cytological preparations. Whereas basophilic white blood cells stain dark blue and eosinophilic white blood cells stain bright red, neutrophils stain a neutral pink. Normally, neutrophils contain a nucleus divided into 2–5 lobes.

Neutrophils are a type of phagocyte and are normally found in the bloodstream. During the beginning (acute) phase of inflammation, particularly as a result of bacterial infection, environmental exposure, and some cancers, neutrophils are one of the first-responders of inflammatory cells to migrate towards the site of inflammation. They migrate through the blood vessels, then through interstitial tissue, following chemical signals such as Interleukin-8 (IL-8), C5a, fMLP, Leukotriene B4 and H2O2 in a process called chemotaxis. They are the predominant cells in pus, accounting for its whitish/yellowish appearance.

Neutrophils are recruited to the site of injury within minutes following trauma and are the hallmark of acute inflammation; however, due to some pathogens being indigestible, they can be unable to resolve certain infections without the assistance of other types of immune cells.

Neutrophils are recruited from the circulation to an infection site in response to the call of microbial molecules and cytokines produced by tissue-resident cells, like interleukin-1b (IL-1b), IL-6, and tumor necrosis factor a (TNF-a), or chemokines, like IL-8. Neutrophils are the first cells to arrive at an inflammatory site, and they do that in massive numbers. T cells and other immune cells also recruit neutrophils during more chronic inflammation, for example by secreting IL-17. Regardless of the cue, neutrophils in circulation first recognize signals in the endothelium close to an inflammatory site and, after rolling on the endothelium, extravasate into the tissue in a process that has been well described and reviewed (Borregaard, 2010; Kolaczkowska and Kubes, 2013). Once entered into tissues, neutrophils are fully equipped to fight infections and to interact with other cells of the immune system.

Neutrophils have a well-established role during fungal and extracellular bacterial infections where they promote bacterial clearance through phagocytosis, production of ROS and reactive nitrogen species (RNS), neutrophil extracellular trap formation, and production of proinflammatory cytokines. Recently, studies have focused on more nontraditional roles for neutrophils in disease including detrimental effects during inflammatory conditions, ranging from seasonal allergies to diabetes, protection against viral infections, and both protective and damaging effects during cancer.

Neutrophils are hematopoietic-derived immune cells that are generated, and continue to develop, in the bone marrow until recruited into circulation and then to sites of infection or inflammation. Steady-state neutrophil granulopoiesis is modulated by common stem cell cytokines, such as IL-3 and IL-6, as well as G-CSF and GM-CSF.

Under infectious or inflammatory conditions, neutrophil granulopoiesis can be increased, typically termed “emergency granulopoiesis,” to restore homeostasis in the bone marrow after recruitment of neutrophils to peripheral sites. Although IL-3, IL-6, G-CSF, and GM-CSF have all been shown to contribute to emergency granulopoiesis, it has also been demonstrated that the production of reactive oxygen species (ROS) by bone marrow myeloid cells is critical for this process during infection.

Neutrophils that traffic into tissues in the absence of infection or inflammation commonly become apoptotic rather than returning to circulation. It has been shown that these dendritic cells in the liver, which could potentially induce a feedback loop that decreases further granulopoiesis. Alternately, the chemokine receptor CXCR4 is upregulated as circulating neutrophils age, leading to trafficking back to the bone marrow where they are ingested by macrophages. Although neutrophil production is constitutive during homeostasis, an enhanced neutrophil response is often essential for host survival.

1.2 Statement of the problem

A bacterial infection is a proliferation of a harmful strain of bacteria on or inside the body. Bacteria can infect any area of the body. Pneumonia, meningitis, and food poisoning are just a few illnesses that may be caused by harmful bacteria.The complement system, antibody, collectins, ficolins, and pentraxins are important noncellular antimicrobial factors that contribute significantly to the host defense against invading microbes. For example, individuals with complement deficiencies are more susceptible to recurrent bacterial infections than people with a fully functional complement system.When a neutrophil meets a microbe, it can respond through various mechanisms, and here we will concentrate on degranulation, phagocytosis, or the generation of neutrophil extracellular traps (NETs).

Microbes might also trigger other mechanisms, like autophagy, apoptosis, or pyroptosis, which we will not review here because of space limitations. Degranulation is an exocytosis process, whereby neutrophil granules fuse with the cytoplasmic membrane, releasing an arsenal of enzymes, antimicrobial peptides, and other molecules into the surrounding tissue. These include proteases that degrade virulence factors and toxins, lysozyme that degrades the bacterial cell wall, and antimicrobials like bactericidal/permeability-increasing protein (BPI), cathelicidins, and defensins that can kill bacteria directly, at least at high concentrations and in buffer solutions in vitro. Once released, these molecules have powerful antimicrobial capacities, but they also harm the host tissue by collateral damage. During phagocytosis, microbes or other particles are recognized by pattern recognition receptors (PRRs) or, even more efficiently, by antibody or complement receptors if the particles are opsonised. When recognized, particles are first engulfed in a phagosome, which later fuses with granules to make a phagolysosome. In this process, the NADPH oxidase is assembled to convert oxygen into oxidizing molecules like superoxide, hydrogen peroxide, and halic acids, collectively called reactive oxygen species (ROS).

The combination of granule contents and ROS leads to an efficient killing of microbes, and it is likely that these processes act in concert and potentiate each other. Besides phagocytosis and degranulation, neutrophils stimulated by microbes or by specific antibodies can also undergo an unusual form of cell death where chromatin gets processed, studded with antimicrobial proteins, and released in the form of NETs. Indeed, the extrusion of chromatin likely occurs through different mechanisms, including the fascinating nuclear exclusion recently described (Yipp et al., 2012). NETs expose a concentrated form of antimicrobials, including histones, which can trap and kill microbes as well as activate other immune cells. Neutrophils might be triggered to respond differently to distinct microbes, and the relevance of these antimicrobial processes in specific diseases is not entirely clear.

The occurrence of life-threatening infections in neutropenic patients illustrates the importance of neutrophils in antimicrobial defense. In addition, several rare immune deficiencies have been described as affecting particular antimicrobial functions of neutrophils (Bouma et al., 2010). Patients with such deficiencies often suffer from infections caused by opportunistic pathogens that rarely cause severe infections in healthy individuals. These ‘‘experiments of nature’’ show that neutrophils are crucial cells in host defense against microbes. The specific phenotypes of these patients can help us understand neutrophil function.This study therefore examines the role of neurophils in the fight against bacteria infection.

1.3 Objectives of the study

The main objective of the study is to examine the role of neurophils in the fight against bacteria infection.
Specifically, the study sought to;

Examine the function of neutrophils.
Examine the mechanism of neutrophil development.
Establish the role of neutrophils in infections (with a focus on immunodeficiency)
Identify how neutrophil activation can also be detrimental to the host, as demonstrated by their involvement in the development of autoimmunity.

1.4 Significance and justification of the study

The immune system protects the body from microbes that invade and harm the host. In humans roughly 100 billion neutrophils enter and leave circulating blood every day and constitute the dominant leukocyte population in the circulation, mediate the earliest innate immune responses to infection, and play a pivotal role in the resolution of microbial infections. Neutropenia, an acquired or inherited neutropenia, and neutrophil malfunction result in recurrent, life-threatening infections with bacteria. Neutrophils originate and mature in the bone marrow and are subsequently released into the peripheral vasculature. After a pathogen has breached the epithelial barriers, neutrophils are the first innate immune cells that are rapidly recruited from the bloodstream to sites of infection. Pathogens entry and replication in host tissues lead to the release of exogenous products, such as formyl peptides, lipoproteins, or peptidoglycan.

Traditionally, based on their impressive antimicrobial capacity in vitro and the susceptibility of patients with few or defective neutrophils, we think of PMNs primarily as microbe hunters. This view might well be correct, but in the last decade neutrophils are emerging, not surprisingly, as instructors of other immune cells like dendritic cells (DCs), macrophages, natural killer (NK) cells, B cells, and T cells. These advances were recently reviewed (Mo´ csai, 2013), and we will only touch upon them in the context of neutrophils as the culprit of jumpstarting autoimmunity after infections. Due to these diverse functions, on top of the collateral damage occurring during their antimicrobial action, neutrophils are now implicated in many diseases, including cancer, metabolic diseases, and circulatory disturbances.

Chapter Three

Conclusion

Neutrophils are considered the front line of host defense against pathogen attack, as they are the first immune cells to arrive at the site of infection and are equipped with an arsenal of weapons to ensure successful clearance of pathogens. The importance of neutrophils in antimicrobial defense has been outlined here by the sheer numbers of new neutrophils generated on a daily basis to ensure the numbers required to clear infection are always available. This has been further confirmed by the fact that individuals with mutations in proteins important for neutrophil function or neutropenic patients succumb to certain infections more readily than healthy people.

However, one must be aware that even though neutrophils aim to be helpful in the immune response, as they will do anything to ensure death of pathogens, this can also result in the release of potentially harmful self molecules that are known to be antigens in the progression of an autoimmune disease (Figure 3). Therefore, while it is clear that neutrophils are important in host defense, they must also be thought of as potentially dangerous to the host and as a potential therapeutic target in autoimmunity.