Basic Pathological Mechanisms April 6-10 Flashcards
What are the causes of organ failure?
genetic predisposition, inflammation, infection, toxins, and trauma
What organs are commonly transplanted?
kidney, liver, heart, pancreas, and lung
What are potential barriers to transplantation?
genetic, immunological/inflammatory, physiological, psychological, surgical, social, financial, and ethical
Describe the ABO blood group system.
Group A population has A antigens on red blood cells (and endothelial cells) and produces antibodies to B
Group O population expresses no antigens on red blood cells and produces both A and B antibodies
What is the role of ABO antigens in transfusion medicine?
For a blood donor and recipient to be ABO-compatible for a transfusion, the recipient must not be able to produce Anti-A or Anti-B antibodies that correspond to the A or B antigens on the surface of the donor’s red blood cells
What are major histocompatibility antigens?
class 1 and class 2 MHC
typically more potent inducers of rejection than are minor H antigens (disparities require more immunosuppression than minor H antigen disparities)
In the HLA map, Class II MHC locus is closer to centromere or telomere? MHC class I?
centromere
telomere
What is the clinical significance of HLA complexes?
solid organ & hematopoetic cell transplation; disease association; pharmocogenomics; paternity testing
What is an autograft? How is this different from a xenograft?
a graft transplanted from one site to another site in the same individual; a xenograft is between individuals of different species
How is a syngraft different from a allograft?
syngraft is between genetically identical individuals (monozygotic twins) and allograft is between genetically non-identical individuals with a species
What is the fundamental immunogenetic principle?
Grafts that express histocompatibility antigens on donor cell surfaces that are not expressed by recipient cells
will be rejected.
What is GVHD?
graft versus host disease following blood and marrow transplantation
With GVHD, the newly transplanted donor cells attack the transplant recipient’s body.
How do recipient alloreactive T cells recognize donor HLA antigens?
direct recognition: donor HLA antigen (on donor cell) and donor peptide; occurs initially following allogeneic transplantation
indirect: recipient HLA antigen (on recipient cell) and donor HLA peptide; occurs later in transplantation
What are minor H antigens?
Minor histocompatibility antigens are due to normal proteins that are in themselves polymorphic in a given population. Even when a transplant donor and recipient are identical with respect to their major histocompatibility complex genes, the amino acid differences in minor proteins can cause the grafted tissue to be slowly rejected.
Describe hyperacute rejection.
(minutes to hours) preformed anti-donor antibodies and complement (type II hypersensitivity: neutrophils and lytic enzymes; C4d)
What are pathological features of antibody-mediated rejection?
vasculitis, glomerulitis, and fibrinoid necrosis (chronic rejection)
In kidney transplants what is the main target of cellular rejection?
renal tubule; neutrophils in peritubular capillaries
glomerulitis is not specific for rejection
What are the forms of solid-organ graft rejection?
hyperacute: preformed antibodies (caused by previous blood transfusions, previous transplants, and pregnancy)
acute: T cell respone
acute vascular: de novo antibodies (immunoglobulins)
chronic: T and B cell response (fibroproliferative changes)
How is histocompatibility tested?
HLA typing: microlymphoctotoxicy assay, flow cytometry, PCR with sequence-specific primers (SSP/SSO)
crossmatching
antibody screening
What are approaches to eliminating graft rejection?
Minimization of genetic disparity between donor and recipient (HLA matching)
Pharmacologic immunosuppression: drugs and antibodies
Clonally-specific immunosuppression: blockade of co-stimulation
(to limit the effect of pathogens and prevention of neoplastic transformation)
Describe microlymphocytotoxicity assay.
A blood specimen from the patient is processed
to separate the lymphocytes from other cellular
blood components (separation of T and B lymphocytes)
antibody directed against a known MHC antigen is added to a well (then lymphocytes are added): during this incubation, if the lymphocyte possesses
the antigen that is the target of the antibody, the antibody will coat the cell.
complement proteins are added: During this incubation,
complement will attach to any cell that is coated with
antibody
In regards to MHC complexes, which allele has the highest variability? least?
DRB1; DRA
What are passenger leukocytes?
professional APCs: simulate immune response against the allograft (dendritic cells, B cells, macrophages, and monocytes)
What is the result of a second transplantation following an initial rejection?
second rejection is more rapid and more potent (result of immunized T cells to the MHC complexes exhibited on the allograft)
Which is better: treatment of antibody-mediated rejection or T-cell mediated rejection?
T cell
How long can a donor organ be out of the body before incurring damage? Why?
at most two days; ROS from ischemia-reperfusion injury
What is the source of antigens that cause hypersensitivity reactions?
xenoimmune; alloimmune (transplant rejection and GVHD); autoimmune
What are effectors of hypersensitivity reactions?
humoral: antibody and/or complement
cell mediated: T cells and perhaps other leukocytes
mixed
Describe type 1 hypersensitivity
anaphylaxis
recognized by IgE bound to the surface of leukocytes (Two or more IgE must be able to bind to the antigen and bring the two IgE immunoglobulins together (requirement for cross-linking of two Fc regions of IgE; ITAM signaling))
effector cells are basophils and mast cells (histamine, heparin, leukotrienes (LTD4 and LTE4), and PGD2); have receptor for constant region of IgE or constant heavy 2 (Fc)); PLC signaling (calcium mobilization and DAG)
effects: degranulation, histamine, leukotrienes and PGD2 stimulate tetanic muscle contraction, vasoconstriction, and postcapillary venule relaxation (inflammation)
symptoms: wheezing, diarrhea, runny nose, and teary eyes
duration: minutes to hours
Describe type 2 hypersensitivity
recognized by circulating IgG or IgA (cross-linking required); localized immune response resulting in cell necrosis
effectors cells are monocytes, NK cells, neutrophils, and platelets
Neutrophils and platelets release superoxide, hydrogen peroxide, hydroxyls, nitric oxide, and myeloperoxidase (cathepsins and granzymes)
Monocytes and natural killer cells release perforin and death domain receptor agonists
leukocytes release cytokines and chemokines
duration: many hours to 1-2 days
examples: autoimmune thyroiditis, rheumatic mitral valve myocarditis and pericarditis
How is complement-mediated cytotoxicity different from other type 2 hypersensitivity? examples?
recognized by IgG or IgM
Membrane attack complex, sub-lethal metabolic change; chemotaxis, leukocyte activation, and phagocytosis
attack of squamal epidermis and blistering
Describe type 3 hypersensitivity.
recognized by circulating IgG, IgA, or IgM; cell death is not necessary and the complexes are diffusely expressed in the organism
effector cells are granulocytes (including platelets) and target cells via complement
glomerular nephritis, vasculits
Describe type 4 hypersensitivity.
recognized by T cell antigen receptor (TCR)
effector cells are factor-producing T cells
de novo synthesis of cytokines and chemokines
Methicillin can bind class II MHC molecules and cause T helper cells to stimulate cytokine production
duration: 2-4 days
How is cell-mediated cytotoxicity type 4 hypersensitivity different from factor-producing?
effector cells are cytotoxic T cells (lytic enzymes including granzymes, cathepsins,and perforin; TNF and lymphotoxin; FasL; chemokines and cytokines
Transitional metals can bind class I MHC molecules and cause a cytotoxic T cell response (also occurs with elastics)
multiple sclerosis
Describe type 5 hypersensitivity
Chaperone antibody: IgG, IgA, IgM; blocks the physiological interaction of natural ligand; myasthenia gravis)
imposter antibody: mimics the physiologic agonist; Grave’s disease
handcuff antibody: antibody prevents a ligand-receptor complex from dissociating; C3 nephritic factor (membranoproliferative glomerulonephritis type II from constitutive activation of C3 convertase)
What are immunologically privileged sites?
brain, eye, testis, uterus (fetus)
Tissue grafts placed on these sites often last indefinitely, and antigen placed in these sites do not elicit destructive immune responses.
However damage to an immunologically privileged site can induce an autoimmune response.
How autoimmunity induced?
cross-reaction between self/non-self (the GAD 65/Coxsackie virus P2-C paradigm)
drug-conjugated to self protein; foreign protein with sequence similarity to self protein; and cross-reaction
Describe X-linked (Bruton’s) Agammaglobulinemia.
characterized by the failure of B-cell precursors (pro-B cells and pre-B cells) to develop into mature B cells
defect in a src family tyrosine kinase (btk) required for signal transduction in the B cell lineage (associated with the Ig receptor; light chains are not produced) pre-B cells cannot mature to express IgM on the surface
X-linked: more common in males
low immunoglobulin of all classes (A, G, E, M), no circulating B cells, preB cells in bone marrow in normal number; absence of plasma cells
normal cell-mediated immunity
Germinal centers of lymph nodes, Peyer’s pathches, the appendix, and tonsil are underdeveloped
Describe hyper-IgM syndrome
defect in CD154, also known as gp39 or CD40L, a T cell surface molecule that stimulates Ig class switching/affinity maturation in B cells (X-linked); OR AID mutation
high serum titers of IgM without IgG, IgA, and IgE
normal B- and T-cell numbers
susceptible to extracellular bacteria infections
Describe Common Variable Immunodeficiency.
B-cell maturation defect and hypogammaglobulinemia (low levels of immunoglobulins; sometimes only IgG)
in contrast to X-linked agammaglobulinemia, most individuals with common variable immunodeficiency have normal or near-normal numbers of B cells in blood and lymphoid tissues (not able to differentiate into plasma cells)
the clinical manifestations are caused by antibody deficiency (resemble X-linked agammaglobulinemia)
affects both sexes equally
hyperplastic lymphoid follicles in nodes, spleen, and gut (B cells can proliferate in response to antigen but do not produce antibodies)
Describe Severe Combined Immunodeficiency (SCID).
complete functional B- and T-cell deficiency (defects in both humoral and cell-mediated immune responses)
X-linked: defects in common gamma chain of IL-2 (cytokine) receptor; T cell numbers are greatly reduced, and antibody synthesis is impaired (also have NK deficiency)
autosomal recessive:
adenosine deaminase deficiency (toxic to rapidly dividing immature lymphocytes, especially those of the T-cell lineage);
other causes: RAG somatic gene rearrangement machinery; purine
nucleoside phosphorylase deficiency; JAK3 mutation
histology: small thymus devoid of lymphoid cells
opportunistic infections with fungus, viruses, and parasites