Donna A. Chow

Natural Immunity and Neuroimmune Host Defense

Abstract: Innate resistance is mediated by non‐immune defense and by natural immunity. Non‐immune defense includes diverse mechanisms (e.g., physico‐chemical defense by bile acids). Natural killer (NK) cells, γδ T lymphocytes and CD5+ B lymphocytes are key mediators of natural immunity. These cells utilize germ‐line coded receptors that recognize highly conserved, homologous epitopes (homotopes). Typically, it is not the antigen, but cytokines and hormones that regulate the level of NK‐mediated cytotoxicity. These include interleukin‐2, interferons, prolactin and growth hormone. Less is known about γδ T lymphocytes. CD5+ B lymphocytes produce germ‐line coded antibodies (predominantly IgM) that are polyspecific, and able to recognize a great variety of microorganisms, cancer cells and self‐components. Antigen is not an effective stimulus for natural antibody (NAb), but bacterial lipopolysaccharide (LPS) is. During the acute phase response (febrile illness) the T‐cell‐regulated adaptive immune response is switched off and natural immune mechanisms are amplified several hundred to a thousand times within 24‐48 hours (immunoconversion). This immunoconversion is initiated by immune‐derived cytokines, and involves profound neuroendocrine and metabolic changes, all in the interest of host defense. Immune recognition is assured by natural antibodies and by some liver‐derived acute phase proteins, such as C‐reactive protein or endotoxin‐binding protein, the level of which is elevated in the serum. Thus, natural immunity is essential for a first and last line of defense and the neuroendocrine system is an important promoter of this activity.

Neuroimmunoregulation and natural immunity

The development and function of the immune system is regulated by neuroendocrine factors. Immune function may be divided into adaptive and natural immunity. Adaptive immune responses are driven by specific determinants of the antigen (epitopes), require 5-10 d to fully develop, and show an accelerated or memory response after repeated exposure to the same antigen. Natural immunity may be divided into host defense mediated by non-immune factors (e.g., antimicrobial proteins, enzymes, mucus etc.) and polyspecific responses of the immune system. This polyspecific response relies on natural antibodies and on some other serum proteins (e.g., lipopolysaccharide-binding protein-LBP, C-reactive protein-CRP), and on surface receptors of macrophages, natural killer cells and B and T lymphocytes for activation. Highly conserved homologous (crossreactive) epitopes, or homotopes for short, are recognized by the natural immune system. Natural antibodies, LBP, and CRP are capable of activating the entire immune system after combination with the appropriate homotope. During febrile illness natural immune host defense is promptly elevated because of the rapid rise of natural antibodies, LBP, and CRP in the serum. This is known as the acute phase response (APR), which is initiated by a sudden rise of cytokines in the circulation, such as IL-1, IL-6, and TNF-alpha. The cytokines act on the brain, the neuroendocrine system, and on other tissues and organs, which leads to fever and profound hormonal and metabolic changes. The hypothalamus-pituitary adrenal axis is activated and serves as the primary regulator of immune and inflammatory reactions. Insulin, glucagon, and catecholeamine levels are also raised. Bone marrow activity and leukocyte function are high and the liver is converted to the rapid production of acute-phase proteins (APP). APP include LBP, CRP, fibrinogen, some complement components, enzyme inhibitors, and anti-inflammatory proteins, which may rise in the serum from several hundred to a thousand times within 24-48 hr. Therefore, natural immunity is a polyspecific response to homotopes, which functions as an instantaneous defense mechanism in health and which is rapidly boosted by cytokines and hormones during febrile illness. This is a highly successful defense reaction, as in the overwhelming majority of cases, febrile illness leads to recovery and the development of adaptive immunity in man and higher animals.

Host Defence: An Interaction of Neuroendocrine-, Metabolic- and Immune Mechanisms in the Interest of Survival

ABSTRACT The term natural resistance refers to the capacity of living organisms to withstand injury caused by physical, chemical and biological agents that may be present in the external or internal environment. This protection is mediated by the natural, or innate, immune system, a multi-factorial and polyspecific defence system. Evolutionarily preserved germ-line receptors mediate the activation of natural immune cells that recognize genetically preserved, cross-reactive homologous epitopes (homotopes) in micro-organisms, cancer cells, virus-infected cells and distressed cells. In general, protection is based on balancing the defence mechanisms of the organism with the damaging effects of harmful agents. This defence comprises epithelial, secretory and endogenous mechanisms in addition to the cellular and humoural components of the natural immune system. In recent years, a continuing surge of exploration and discussion has helped to crystallize our appreciation of the molecular mechanisms of this innate system, their basis in evolution, physiological, pathological and behavioural significance and their regulation, in particular their intimate connection with the neuroendocrine system. In higher animals natural immune mechanisms are boosted profoundly during acute febrile illness leading to the release of pro-inflammatory cytokines, IL-1, TNF-alpha and IL-6, which in turn activate the neuroimmune regulatory network. The HPA axis and the sympathetic nervous system is activated and catabolism prevails. IL-6, glucocorticoids and cathecolamines induce the production of acute phase proteins permitting a rapid activation of phagocytic and cytotoxic mechanisms under the command of natural antibodies and other recognition molecules (e.g. C-reactive protein, endotoxin binding and mannose binding proteins). The acute phase response is a highly co-ordinated emergency defence reaction, which relies on the interaction of neuroendocrine, immune and metabolic mechanisms in the interest of maximum host defence during emergency situations, such as sepsis. In most cases febrile illness leads to healing and recovery, which attests to the effectiveness of the natural immune system and excites the desire for the benefits which should accrue from mastering the manipulation of this system.

Natural Immune Activation: Stimulators/Receptors

ABSTRACT The activation of the natural immune response is initiated through multiple recognition molecules representing numerous structural families including, several lectin families, pentraxins, leucine-rich repeats, many members of the IgSF, integrins, scavenger receptors and the seven transmembrane receptor family, which have been maintained through evolution. Invading pathogens exhibit a range of different repeating epitopes. Host molecules, soluble or on cell surfaces, express a variety of receptors capable of recognising these epitopes and act in a combinatorial manner which confers specificity to the host response. The number, diversity and ancient evolutionary origin of these receptors argues for the essential nature of their functions. While providing a first line of defence against invading pathogens is clearly crucial for organism survival, evidence is accumulating that these same receptors participate in essential functions unrelated to defence against invaders.

Physiological Activities of the Natural Immune System

ABSTRACT Our understanding of the extent of the role of the innate immune response against pathogens has expanded enormously with the knowledge of an increasing number of mediators, an increasing variety and complexity of function, and central roles in the initiation and mediation/resolution of the adaptive immune response. However, this search for understanding has also revealed the extensive underlying physiological role of the innate immune system in the development and homeostasis of the organism. The impact on development is evident during embryogenesis and also during normal cyclical changes in reproductive tissues in the adult. Mediators of the innate immune system are essential for normal tissue renewal and healing. This chapter explores, as examples of its physiological role, the critical contributions of the innate immune system in wound healing, reproduction and normal tissue homeostasis including, regeneration, air breathing, cell signalling and cancer control. Natural immunity contributes to the normal physiology of the organism in many and diverse ways arguing for an evolutionary selection centred on self-organization for survival.

RNK granule extract cytolysis : differential inhibitor production by an NK-resistant vs an NK-sensitive murine lymphoma

Natural killer (NK) cell-resistant tumors exist despite their ability to bind cells from the effector population. Tumor sensitivity to NK activity was therefore examined at the level of susceptibility to cytolysin-containing NK cell cytotoxic granule extracts. The NK-sensitive SL2-5 murine lymphoma was markedly more susceptible than the NK-resistant L5178Y-F9 to solubilized granule preparations from the rat NK tumor cell line RNK-16, and this corresponded also with tumor sensitivity to hypotonic lysis. However, the resistant L5178Y-F9 was better able to inhibit the extract activity than the SL2-5. Dissociation of the binding and lysis phases of the cytolysin reaction based on their differential temperature requirements, 4 degrees C for binding and 37 degrees C for lysis, permitted an examination of the cytolysin/tumor interaction prior to lysis. The residual cytotoxic activity was lower after extract exposure to the L5178Y-F9 compared with the SL2-5 consistent with possible inhibitor production. Finally, supernatant material collected from the L5178Y-F9 was a better inhibitor of granule extract lysis and acted preferentially in the extract-binding phase. The inhibitor appears to be protein in nature, relatively stable, and exhibits molecular weight heterogeneity ranging from 2000 to greater than 300,000.

Natural immune regulation of activated cells

Abstract The natural immune system provides a prompt first-line of defense against invading pathogens. It is comprised of both cellular and humoral mediators including macrophages, NK and T cells, natural antibodies, complement and interferon. Unlike the exquisitely specific, adaptive immune system, natural immunity is polyspecific, recognizing highly conserved homologous or crossreactive epitopes (homotopes), and does not require previous exposure to antigen to generate a response. This evolutionarily conserved, innate system is also considered important in homeostatic regulation of the organism through a highly connected receptor network and through the removal of aberrant cells, including nascent tumor cells. It provides the immunological environment for the development of the adaptive T cell response. Furthermore, activation of the acute phase response (APR) through immune derived cytokines when adaptive immunity has failed to eradicate an invader, provides a massive stimulus to the natural immune system including the polyspecific acute phase mediators in an all-out final natural defense. Several in vivo and in vitro syngeneic murine tumor models of natural resistance haveprovided evidence to support polyclonal natural antibody (NAb), NK cell and activated macrophage surveillance of developing tumors. A study of NAb, those antibodies in the circulation of normal individuals that are not intentionally immunized, revealed a consistent inverse correlation between NAb binding by tumors and tumor incidence in a threshold s.c. tumor inoculum assay of natural resistance. Considering that LPS induces APR, a role for neuroendocrine regulation of NAb surveillance was suggested by correlating LPS-induced increases in NAb levels and reductions in tumorigenicity of threshold s.c. inocula of an NK-resistant tumor. Furthermore, a strong influence of sex hormones on the natural resistance was evident in the reduced tumor incidence of threshold s.c. tumor inocula in female versus male mice. T lymphomas treated with the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) to generate variants and repeatedly selected for high serum NAb IgG- plus IgM-binding, exhibited a reduced s.c. tumorigenicity. Surface expression analysis also revealed that the selected cells bound more monoclonal antibodies against activation-associated molecules including CD25, the IL2-Rα chain, and asialo forms of CD45RA, a transient marker of T cell activation. This argues that NAb also regulates T cell activation. The observations in the murine models raised the possibility that direct down-regulation ofT cell activation may contribute to the beneficial effects of intravenous immunoglobulin (IVIg) against inflammatory and autoimmune diseases. In order to identify the T cell surface targets of IVIg in humans, we produced a model human T cell through TPA treatment of Jurkat T leukemia cells with repeated selection for high serum IgG- plus IgM-binding which mimics the conditions for IVIg action in patients. Cells produced from the 3rd and 4th sequential selections exhibited stable increases in serum IgG- and IgM-binding of 28% for J3.1 and 45% for J4.1. The J4.1 bound at least 45% more IVIg than the parental cells consistent with their increased binding of pooled human IgG-plus IgM. The J4.1 exhibited a 120% increase in expression of CD45RA, a marker which correlates with cytokine and hormone expression in women. The results provide the first evidence that human serum natural IgG-plus IgM antibodies and IVIg react with a CD45RA epitope. The conserved nature of many NAb and the contribution of endogenous NAb and passively administered IVIg to the control of immune activation within the context of neuroendocrine regulation, raise the possibility that the epitope on CD45RA may be a highly conserved homologous epitope or homotope of the neuroimmune system involved in health and disease.

RNK granule extract cytolysis: Increased tumor susceptibility and release of proteochondroitin sulphate inhibitor in high NaCl

The insensitivity of the natural killer (NK)-resistant L5178Y-F9 murine T-cell lymphoma to granule extracts from a rat NK leukemia could be preferentially reversed in increased NaCl (0.25 M) compared with the NK- and granule extract-sensitive SL2-5. The high salt effect predominated in the binding rather than the lytic phase of the extract reaction similar to the activity of extract inhibitory supernates preferentially produced from L5178Y-F9 cells. Exposure of the L5178Y-F9 to 0.25 M NaCl was associated with an increased production of inhibitory supernate and an increased sensitivity of the cell as an extract target. Pretreatment of inhibitor-containing supernatant or inhibitor-producing L5178Y-F9 cells with pronase or chondroitinase AC reduced the inhibitory activity of the resultant supernates, and L5178Y-F9 supernates treated with anti-chondroitin sulphate AC antibodies exhibited reduced inhibitory activity. These observations and the previously reported molecular weight heterogeneity and protease sensitivity of the inhibitor argue that chondroitin sulphate AC-containing proteoglycans released from the tumor cell surface may inhibit cytolysin activity, contributing to the preferential resistance of the L5178Y-F9 to rat NK granule extract cytolysis.

Natural Antibodies in Tumor Development

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