B cells are one of the major forms of lymphocytes, the others being T cells and NK cells. Taken together, B and T cells make up make up 20 – 40% of circulating white blood cells (Leucocytes). Lymphocytes are immunologically competent cells that assist neutrophils and monocytes in the defence of the body against pathogenic infection and invasion by other foreign bodies. B cells play a major role in the humoral arm of the adaptive immune response.
B cells are small mononuclear cells with dense nuclei and very little cytoplasm. In adults, they mature mainly in the bone marrow and later migrate to the spleen. Maturation of B cells involves several stages in which gene rearrangement of V, D and J gene segments takes place to produce B cells with antigen specific ‘B Cell Receptors (Ig)’, which translates into a large antibody specificity repertoire, from a relative small number of genes. Immature B cells that produce Ig receptors which bind too strongly to self antigen undergo further gene rearrangement or are clonally deleted. Mature B cells are initially naive until they encounter antigen. Naive mature B cells are characterised by cell surface expression of immunoglobulin (IgM and IgD) and CD19.
Subsets of B cells include Mature B cells, antibody secreting Plasma cells, Memory B cells and Follicular B cells.
The diversity of the B cell receptor is such that virtually all foreign molecules can present determinants that can be recognised by the antigen binding sites of at least some B cells. B cells recognize their cognate antigen in its native form unlike T cells which only recognise antigen as peptides presented by MHC. In the normal immune response, upon B cell receptors binding antigen, the bound antigen is internalised, processed and presented on the cell surface as peptides bound to HLA class II molecules. These peptide bearing B cells continuously circulate through the peripheral lymphoid organs where the peptide-MHC class II complex can be recognised by antigen specific armed CD4+ helper T cells which provide the co-stimulatory signal needed alongside T Cell Receptor (TCR) binding, resulting in activation of the B cells, leading to proliferation and differentiation into antibody secreting plasma cells. The T cells are armed by being presented with peptide by professional antigen presenting cells recognising the same antigen initially recognised by the B cells.
In the alloimmune response such as results from pregnancy, transfusion or transplantation, B cell clones that are activated are primarily those with B cell receptors that recognise HLA antigens, leading to production of anti-HLA antibodies. Other antibodies, such as anti-endothelial antibodies, anti-HPA and anti-MICA antibodies are also produced depending on the sensitising antigen. The route of alloimmunisation influences the type of antibody produced. Transfusion tends to lead to transient production of IgM and IgG antibodies whilst pregnancy and transplantation leads to production of IgG antibodies and memory B cells.
T cells are one of the major forms of lymphocytes, the others being B cells and NK cells. Taken together, B and T cells make up make up 20 – 40% of circulating white blood cells (Leucocytes). Lymphocytes are immunologically competent cells that assist neutrophils and monocytes in the defence of the body against pathogenic infection and invasion by other foreign bodies. T cells play a major role in the humoral and cellular arms of the adaptive immune response.
T cells are small mononuclear cells with dense nuclei and very little cytoplasm. In adults, they mature mainly in the Thymus. Maturation of T cells involves several stages in which gene rearrangement of V, D and J gene segments takes place to produce T cells with antigen specific ‘T Cell Receptors (TCR)’.
As part of their maturation, T cells undergo both Positive and Negative selection to generate a self tolerant repertoire of T cells. Mature T cells are initially naive until they encounter antigen. Naive mature T cells are characterised by cell surface expression TCR, CD3 and in most cases either CD4 or CD8. The diversity of the TCR is such that virtually all foreign molecules can present determinants that can be recognised by the antigen binding sites of at least some TCRs. Unlike B cells which recognize their cognate antigen in its native form, T cells only recognise antigen as peptides presented by MHC. That is, the T cell response is MHC restricted.
Subsets of T cells include Helper T cells (TH cells), Cytotoxic T cells (CTLs), Memory T cells, Regulatory T cells (Treg cells) and Natural Killer T cells (NKT cells).
TH cells provide help to other cells in the adaptive immune response including B cells and cytotoxic T cells. TH cells are characterised by cell surface expression of CD4. They recognise peptide presented by class II MHC molecules. Depending on the cytokine environment at activation, TH cells can differentiate into different types of helper cells, including Th1, Th2, Th3 and Th17. Presence of IL-12 leads to a Th1 response with secretion of proinflammatory cytokines including IFNγ which drives the cellular immune response. In the normal immune reaction, the Th1 response targets obligate intracellular pathogens. Aberrant elevation of the Th1 response can lead to autoimmunity. Presence of IL-4 leads to a Th2 response with secretion of IL-4, 5, 9, 10 and 13 which mainly drive the humoral immune response. In the normal immune reaction, the Th2 response targets extracellular pathogen. Aberrant elevation of the Th2 response can lead to allergy. Presence of IL-10 and TGFβ in vitro leads to a Th3 response. Th3 type cells have a regulatory phenotype and are involved in mucosal immunity, protecting mucosal surfaces from infection. They secrete IL-10 and TGFβ which suppress Th1 and Th2 responses. Presence of IL-21 or TGFβ + IL-6 leads to a Th17 response with secretion of IL-17 and IL-21 overproduction of which leads to autoimmunity.
CTLs are killer cells characterised by cell surface expression of CD8. They recognise peptide presented by class I MHC molecules. Depending on the cytokine environment at activation, CTLs can differentiate into different types of cytotoxic cells, including TC1 which secrete TGFβ and TC2 which secrete IL-4, IL-5, and IL-10. In the normal immune response CTLs are involved in the killing of cells infected with obligate intracellular pathogens.
Treg cells play an important role in the induction and maintenance of immunological tolerance. They are characterised by cell surface expression of CD4, CD25 and FoxP3. In the normal immune reaction, Treg cells suppress IL-2 and IFNγ production to achieve immune homeostasis.
NKT cells bridge the innate and adaptive immune responses. They share properties of both normal T cells and of NK cell and are characterised by cell surface expression of an invariant TCR with limited TCR specificities and NK cell markers. They are CD1d-restricted, lipid antigen-reactive, regulatory T cell that can promote immunity to tumours and infection.
In the alloimmune response, Th1 cells are involved in acute and chronic cellular rejection in solid organ transplantation and in GvHD induction in stem cell transplantation and Th2 cell drive humoral antibody mediated rejection (AMR). Th17 cells are implicated in steroid resistant rejection. They contribute to glomerulonephritis in kidney and to rejection of cardiac allografts. They have also been detected in bronchoalveolar in lung transplant patients after acute rejection. Cytotoxic T cells are involved in acute and chronic cellular rejection following activation by helper T cells, which are themselves activated in the Direct and Indirect pathways of allorecognition.
Natural Killer (NK) cells are lymphoid cells involved primarily in the innate immune response but which also contribute to the adaptive immune response. They comprise approximately 15% of all circulating lymphocytes. NK cells are characterised by cell surface expression of a number of receptors including Killer cell Immunoglobulin like Receptor (KIR) and NKG2. They also express CD56. NK cells have a rapid effector function due to germline encoding of receptors. This contrasts with B and T cells which undergo gene rearrangement of V, D and J gene segments before producing antigen specific receptors. In contrast to resting T cells, NK cells express receptors for numerous cytokines constitutively and produce IFNγ and other NK-derived cytokines rapidly in response to stimulation.
NK cells interact with target cells through a complex competitive interaction of activatory and inhibitory signals of cell surface receptors, including KIR and NKG2, binding to their cogent ligands on the target cells. The ‘missing self’ hypothesis holds that NK cell reactivity occurs when the ligand for inhibitory receptors are down regulated or ‘missing’, leading to activation signals prevailing. The ligands for KIR receptors are HLA class I molecules. These include HLA-C locus antigens with either Asn (Group 1) or Lys (Group 2) at position 80, the HLA-Bw4 epitope and some HLA A antigens. The ligands for NKG2 include the MHC class I like antigens MICA and MICB. NK cells kill by releasing small cytoplasmic granules, perforins and granzymes, which cause target cell lysis or apoptosis. NK cells acquire their effector function by having receptors engaging their cogent ligands during maturation, a process known as ‘licensing’.
NK cells can be divided into two subsets based on their level of cell surface expression of CD56. These are CD56bright and CD56dim. There is some evidence to suggest distinct immunological roles for these subsets. CD56dim NK-cells are more naturally cytotoxic and expresses higher levels of Ig-like NK receptors and FCγ receptor III (CD16) than the CD56bright NK-cells. CD56bright cells produce abundant cytokines but have low natural cytotoxicity.
In the alloimmune response following stem cell transplantation, donor versus recipient alloreactivity can potentially be generated in HLA-C allele group and/or HLA-Bw4 group mismatched transplantation. Where the recipient does not possess a HLA class I allele group that the donor NK cell inhibitory KIR ligands recognise, the donor NK cell KIR receptors ‘sense’ the missing ligand in the recipient and can mediate an allo response given the presence of the right activating signals. This is the ‘missing ligand’ theory.
In solid organ transplantation, NK cells are known to infiltrate allograft, suggesting that activation of NK cells may be critical in the immediate post transplant period. However, one recent study found that there was no association between NK cell KIR ligand mismatch and the incidence of acute rejection. This reflects the outcome of a much larger CTS study involving over 2,700 deceased donor kidney transplants which found no effect of NK cell ligand matching on graft survival.
Dendritic cells are the most potent of the Antigen Presenting Cells (APC) responsible for priming the immune response. Distributed throughout the tissues of the body, they possess surface pattern recognition receptors that recognize pathogen associated molecular patterns making them capable of initiating an immune response to infection. Two main subsets of dendritic cells have been described, the myeloid dendritic cells (mDC) and plasmacytoid dendritic cells (pDC) with the mDC being the more abundant. These are traditionally believed to derive from myeloid and lymphoid precursors respectively, though more recently both cells lines have been proposed to derive from both cell types of precursors. Dendritic cells are capable of stimulating naive T cells and are therefore known as professional APC’s. More recently, an additional role for dendritic cells as important mediators of peripheral immune tolerance and maintenance of immune homeostasis has been described.
In the normal state, with absence of inflammation, dendritic cells reside in the peripheral tissue as immature interstitial cells. In this state, their main characteristics are an efficient capability of ingesting exogenous antigen and expressing them on the cell surface in association with MHC antigens as well as low levels of expression of costimulatory molecules. During an inflammatory response, maturation of dendritic cells is triggered leading to a reduced endocytotic capability as well as an increased level of expression of costimulatory molecules and distinct chemokine receptors and upregulation of intercellular adhesion molecules required for interaction with T cells. Mature dendritic cells migrate to T cell areas of the secondary lymphoid tissue where they stimulate naive T cells.
Dendritic cells play a central role in the alloimmune response. After kidney transplantation, donor derived dendritic cells, triggered into maturation by the transplant Ischemic and Reperfusion Injury (IRI) as well as locally released pro-inflammatory cytokines, migrate from the kidney to the recipient lymphoid organs where they stimulate the host immune response through the direct and indirect routes of allorecognition. In stem cell transplantation, the number of circulating plasmacytoid and myeloid dendritic cells and their origin, donor or recipient, have been shown to be associated with the initiation of acute Graft versus Host Disease (aGvHD), relapse and graft failure. The recognition of alloantigen presented by residual host dendritic cells to donor T cells in the direct pathway of allorecognition initiates GvHD. Ongoing antigen presentation involves donor derived dendritic cells presenting host antigen to donor T cells in the indirect pathway of allorecognition. Some studies have shown that the absolute numbers of circulating dendritic cells post stem cell transplantation is an independent predictor of aGvHD. Patients with aGvHD after stem cell transplantation have lower numbers of circulating mDC and pDC compared to healthy individuals but do have a higher number of dendritic cells in affected areas such as the skin.
Monocytes make up 2-6% of circulating leucocytes. They are the largest of the leucocytes. They are myeloid progenitor derived mononuclear phagocytic cells with bean or horseshoe shaped nuclei and abundant cytoplasm.
Monocytes circulate in the blood for 1 – 3 days before leaving to enter tissues where they mature into either macrophages or some dendritic cell subsets. Monocytes appear to be capable of taking up and processing antigen both from the bone marrow and also from the blood during their transit from the marrow through blood into tissues, for delayed presentation to T cells later in their cell cycle. Once they leave the circulation and enter tissues, the maturation process starts and they are no longer considered Monocytes but are instead considered Monocyte derived cells (macrophages or dendritic cells).
The effector functions of macrophages include phagocytoses and destruction of pathogens and cellular debris, production of cytokines which regulate and participate in haemopoiesis, inflammation and cellular responses and processing and presenting antigens to T cells, i.e. act as Antigen Presenting Cells (APC).
Monocytes can be divided into three main subsets based on their cell surface expression of CD14 (a component of lipopolysaccharide) and CD16 (FCγ receptor III). The Classical Monocyte subsets have high expression of CD14 and are CD16 neg., Intermediate Monocytes have high expression of CD14 and low expression of CD16 and Non Classical Monocytes have low expression of CD14 and high expression of CD16. In the normal immune response to inflammation, there is a gradual increase first of Intermediate Monocytes from Classical Monocytes, followed by an increase in the Non Classical Monocytes.
In the alloimmune response following kidney transplantation, activated monocytes in peripheral blood secrete proinflammatory cytokines which contribute to the development of transplant glomerulopathy. Following stem cell transplantation, the innate immune cells including granulocytes, NK cells and monocytes, reconstitute earlier than the adaptive immune cells including B and T cells. There is evidence to suggest that monocytes are the first cells to engraft. This reconstitution of the innate cell population, including monocytes, restores innate immunity, allowing bacterial prophylaxis to be lowered thus lowering the risk of post stem cell transplant infections.
Macrophages are leukocytes produced by differentiation and maturation of monocytes in tissues. Monocytes and macrophages are phagocytes. Macrophages function in both the innate and adaptive immune responses. Macrophages are resident in many tissues of the body including lymph nodes and the spleen as well as in the liver, kidneys, brain, bones, lungs and gastrointestinal tract. Macrophages are also recruited into tissue through positive chemotaxis during inflammation.
The effector functions of macrophages include 1) phagocytoses and destruction of pathogens and cellular debris, 2) production of cytokines which regulate and participate in haemopoiesis, inflammation and cellular responses and 3) processing and presenting antigens to T cells, i.e. act as Antigen Presenting Cells (APC).
The antigen presenting role of macrophages involves take up and presentation of processed peptide through MHC class II antigens mainly to armed effector helper T cells. Macrophages can stimulate naive T cells but not as effectively as dendritic cells can. Macrophages that phagocytose pathogens into intracellular vesicles become activated by Th1 T cells into secreting lysosomes into those vesicles thus destroying the pathogens. Cytokines produced by macrophages include TNFα, IL-1, IL-6, IL-8 and IL-12. These are proinflammatory cytokines which attract and activate neutrophils and promote fibrosis of endothelial cells.
Macrophages are characterised by cell surface expression of CD14 (a component of lipopolysaccharide), Epidermal growth factor module-containing Mucin-like Receptor 1 (EMR1) and CD68.
In the alloimmune response following renal transplantation, early post transplant macrophage activation has been associated with poorer long term graft and patient survival.
Granulocytes are myeloid progenitor derived polymorphonuclear cells with characteristic cytoplasmic granules as seen in blood films. They are highly motile phagocytic cells. They can be divided based on their cytoplasmic and nuclear appearances into Neutrophils, Basophils and Eosinophils. The most abundant of these are the Neutrophils. The name of Neutrophils derives from their characteristic staining when treated with haematoxylin and eosin. Whilst Basophils stain dark blue and Eosinophils stain bright red, Neutrophils remain a neutral pink.
Neutrophils play a central role in host defence to infection and tissue injury. They are recruited into tissue through positive chemotaxis during inflammation by chemokines including IL-8 and INFγ. They rapidly engulf and kill any antibody or complement opsonised pathogen, damaged cells or cellular debris. Once they have completed their function, their timely removal from sites of inflammation is essential for resolution of inflammation. Neutrophils do not re-circulate to the blood but instead die, perhaps through apoptosis and turn into Pus.
Basophils have been shown to play a role in the induction and maintenance of specific types of Th2 cytokine dependent immunity and inflammation, particularly those that cause hypersensitivity and allergic symptoms. The granules of Basophils contain vasoactive substances such as histamine as well as anticoagulants such as heparin. The granules also contain peroxidases and platelet activating factors. Basophils express IgE receptors on their cell surface. Presence of Th2 cytokines or binding of IgE antibodies triggers activation which caused Basophils to de-granulate and secrete their content, causing the hypersensitivity reactions and allergic disorders.
Eosinophils are proinflammatory cells which are recruited from the circulation by diverse stimuli. They modulate immune responses through an array of mechanisms including the secretion of an array of proinflammatory cytokines. They proliferate during an allergic reaction such as asthma, hayfever or eczema. Eosinophils are capable of either protecting or damaging the host. They can initiate antigen-specific immune responses by acting as Antigen Presenting Cells (APCs) or they can serve as major effector cells inducing tissue damage and dysfunction by releasing toxic granule proteins and lipid mediators. Eosinophil granules contain toxic basic protein and cationic proteins such as Cathepsin, peroxidases, as well as common lysosomal enzymes. They also contain substances that can neutralize mast cell and Basophil secretions
In the normal immune response, antibodies directed against antigens on the granulocyte membrane are known to cause a variety of disorders including neonatal alloimmune neutropenia, autoimmune neutropenia, and drug-induced immune neutropenia. In addition, alloantibodies to granulocyte antigens can contribute to transfusion related acute lung injury (TRALI), febrile transfusion reactions and refractoriness to granulocyte transfusions.
Granulocyte therapy contributes to stem cell transplantation and to transfusion. In stem cell transplantation, Granulocyte colony stimulating factor (G-CSF) is used in the mobilization of hematopoietic stem cells and progenitors prior to peripheral blood stem cell harvest. In transfusion, Granulocyte transfusion can be used as supportive therapy in patients with life-threatening neutropenia caused by bone marrow failure or in patients with neutrophil dysfunction.
Neutrophils represent the body’s primary line of defence against invading pathogens such as bacteria and against and tissue injury. Neutrophils are the most abundant (50-60%) of the leukocytes and the most abundant (60-70%) of the granulocytes. Neutrophils, like other granulocytes, are myeloid progenitor derived polymorphonuclear cells with characteristic cytoplasmic granules as seen in blood films. The other granulocytes are Basophils and Eosinophils. The name of Neutrophils derives from their characteristic staining when treated with haematoxylin and eosin. Whilst Basophils stain dark blue and Eosinophils stain bright red, Neutrophils remain a neutral pink colour.
Neutrophils are recruited into tissue through positive chemotaxis during inflammation by chemokines including IL-8 and INFγ. They are highly phagocytic, rapidly engulfing any cellular debris and engulfing and killing any damaged cells and antibody or complement opsonised pathogen. Once they have completed their function, their timely removal from sites of inflammation is essential for resolution of inflammation. Neutrophils do not re-circulate to the blood but instead die, perhaps through apoptosis and turn into Pus.
The granules of neutrophils contain the active killing agents. The granules of the neutrophil have traditionally been divided into two types, Primary and Secondary. More recent studies have however shown that there are in fact a continuum of several subtypes of granules with differences in protein content and propensity for mobilization. The granules contain cationic proteins, defensins, proteolytic enzymes, myeloperoxidases, lysozymes and other acid hydrolases which contribute to cell lysis and destruction.
Neutrophils are implicated in a number of diseases. Neutropeania can result from some forms of leukaemia or as a side effect of medication or some types of infection and can leave individuals highly susceptible to infections. In inflammatory conditions such as Rheumatoid Arthritis (RA) a potential role for Neutrophils in the persistence of inflammation and progression of joint damage has been identified. Neutrophils are found in high numbers within the synovial tissue and in joint fluid of rheumatoid joints.
Recovery of Neutrophil count following stem cell transplantation is one of the traditional indicators of immune reconstitution. Number of days post transplant to Neutrophil counts reaching 0.5 x 109/kg patient weight gives a measure of engraftment. Peripheral blood stem cell transplantation (PBSC) generally has the fastest recovery time of around 15 days, depending on a number of factors including induction therapy and cell dose transplanted. Bone marrow transplantation has a typical recovery time of around 21 days. Cord blood has the longest recovery time and can take up to 35 days. Patients are at increased risk of infection during this neutropaenic period.
In renal transplantation, Neutrophil activation appears to play a role in ischemia-reperfusion injury (IRI) and the consequent delayed graft function (DGF), contributing to a reduction in overall graft function.
Basophils contribute to the body’s response to allergens. Basophils are the least abundant cells in the circulation occurring at less than 1%. Basophils are myeloid progenitor derived polymorphonuclear (usually have a bi-lobal) granulocytes. They have characteristic cytoplasmic granules as seen in blood films. The other granulocytes are Neutrophils and Eosinophils. Basophils stain dark blue when treated with haematoxylin and eosin. Eosinophils stain bright red, while Neutrophils remain a neutral pink colour, hence the name.
Basophils have been shown to play a role in the induction and maintenance of specific types of Th2 cytokine dependent immunity and inflammation, particularly those that cause hypersensitivity and allergic symptoms. This initially caused Basophils to be considered a redundant mast cell like population. Studies later showed however that Basophils and mast cells had distinct differentiation pathways and cell surface markers.
The granules of Basophils contain vasoactive substances such as histamine as well as anticoagulants such as heparin. They also contain peroxidases and platelet activating factors. Basophils express IgE receptors on their cell surface. Presence of Th2 cytokines or binding of IgE antibodies triggers activation which caused Basophils to degranulate and secrete their content, causing the hypersensitivity reactions and allergic disorders.
One study into the potential role of Basophils in renal transplantation found evidence of sensitized Basophils in post transplant patients but there have been very few follow up studies.
Eosinophils have multiple biological functions and contribute to a variety of immune defence mechanisms, including defence against parasitic infections, particularly parasitic worms and protozoa. Eosinophils make up 1 – 4% of circulating leukocytes. Eosinophils are myeloid progenitor derived polymorphonuclear, usually 2 – 4 four lobed, granulocytes with characteristic cytoplasmic granules as seen in blood films. The other granulocytes are Neutrophils and Basophils. Eosinophils stain bright red when treated with haematoxylin and eosin. Basophils stain dark blue, while Neutrophils remain a neutral pink colour, hence the name.
Eosinophils are proinflammatory cells which are recruited from the circulation by diverse stimuli. They modulate immune responses through an array of mechanisms including the secretion of an array of proinflammatory cytokines. They proliferate during an allergic reaction such as asthma, hayfever or eczema.
Eosinophils are capable of either protecting or damaging the host. They can initiate antigen-specific immune responses by acting as Antigen Presenting Cells (APCs) or they can serve as major effector cells inducing tissue damage and dysfunction by releasing toxic granule proteins and lipid mediators. Eosinophil granules contain toxic basic protein and cationic proteins such as Cathepsin, peroxidases, as well as common lysosomal enzymes. They also contain substances that can neutralize mast cell and Basophil secretions, thereby down modulating the allergic response. Eosinophils predominantly secrete their granule protein by regulated exocytosis and degranulation, thereby avoiding tissue damage by releasing granular content while migrating to effector sites.
Eosinophils participate in the process of acute rejection in solid organ allografts and their levels in kidney allografts do have a diagnostic value for acute rejection. Graft eosinophilia (high numbers of Eosinophils) is a sensitive and specific marker of acute rejection in liver allografts.
Mast cells are derived from the same common myeloid progenitor as the myeloblasts that differentiate into granulocytes and monocytes. Mast cells are resident in many tissues, particularly near surfaces exposed to the environment. They are involved in allergy reactions and in anaphylaxis. They also play an important role on wound healing, in clearance of enteric pathogens, visceral hypersensitivity and in intestinal cancer.
The granules of Mast cells contain vasoactive substances such as histamine as well as anticoagulants such as heparin. This characteristic is shared with Basophils, which initially caused Mast cells to be considered Basophils with specialised functions. Studies later showed however that Basophils and Mast cells had distinct differentiation pathways and cell surface markers. Mast cells express IgE receptors on their cell surface. Binding of IgE antibodies and many other stimuli, including products derived from either pathogens or the host during innate immune responses, triggers activation which causes Mast cells to degranulate and secrete their content, causing allergy reactions and anaphylaxis.
Mast cells are at the interface between the innate and acquired immune systems. They are capable of producing a vast array of both pro- and anti-inflammatory molecules, of acting as antigen presenting cells (APC) and of expressing a spectrum of costimulatory molecules. Thus depending on circumstances, mast cells are capable of either protecting or damaging the host or both.
In the alloimmune response in renal transplantation, Mast cells are believed to have a protective effect with regards to acute alloreactive responses through activation of Treg cells. However Mast cells are thought to have an adverse role in the progression of chronic organ rejection through the release of different profibrotic mediators and through increased collagen deposition.
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