Immunology exam 1 notes

Uterus is an Immune privileged site

• fetus and mother are partial MHC mismatch, suggesting the gravid uterus is immune privileged site.

• Physical separation of maternal and fetal tissues

• Antigenic immaturity of the fetus

• Immune tolerance

• Maternal immunosuppression induced by pregnancy hormones and seminal fluid

Physical separation of fetal and maternal circulation

• (F) fetal endothelium

• (C) chorionic epithelium

• (E) Endometrial epithelium

• (M) Maternal endothelium

• Epitheliochorial

  • Pig, horse, cow

  • F, C, E, M

• Endotheliiochorial

  • Dog, cat

  • F, C, M

• Hemochorial

  • F, C

  • In hemochorial placentation, maternal immune cells are in direct contact with fetal tissue

Maternal immune tolerance

• CD4+ regulatory T cells (Treg)

  • Accumulate in decidua and elevated in maternal circulation during pregnancy

  • Treg insufficiency or dysfunction associated with infertility, miscarriage.

• Placental MHC class I expression

  • mainly primates

  • CD8+ T cells

• FasL expression

  • induces apoptosis of activated lymphocytes

Factors capable of inducing apoptosis in immune cells that reach conceptus/fetus

• Expression in conceptus/fetus

  • FasLG

  • TNFSF10

• Expression in endometrium

  • FasLG

  • TNFSF10

    

Fetal immune Development

• Thymus is the first lymphatic organ to develop

• B cells appear after development of spleen and lymph nodes

• Antibodies appear in late fetal, or early postnatal period.

• Gradual increase in the use of gene conversion or somatic mutation to increase antibody diversity

• Cytokine production low in the fetus and neonate.

Immune response to Intrauterine infection

• Infections that may be mild or inapparent in the mother, may be severe or lethal in the fetus

  • Blue tongue, BHV-1, BVD

• In general, the response to these viruses is determined by the state of immunological development of the fetus

Neonatal Immunity dependent on rate of immune development

• Longer gestation = more responsive at birth

• Domestic species

  • acquired immune system fully developed at birth, but naive. Thus, immune response is slow to respond and not fully functional until several weeks of age.

Neonatal Immunity

• All responses are primary; no memory responses when born

• Neonatal immune system development stimulated by intestinal microbiota

• Neonatal immune response skewed toward Th2 response

  • result of pregnancy hormones?

• Newborns vulnerable to infections for first few weeks of life

  • protection provided by passive immunity

Passive Immunity

• Maternal Antibody Transfer

  • colostrum

  • Maternal Ig crossover through placenta

• Short-term immunity which results from the introduction of relevant antibodies from another animal

Colostrum

• “first milk”

• mammary secretions accumulated over the last few weeks of pregnancy

• Rich in IgG, IgM, IgA

• Provides humoral immunity (passive immunity) to neonate

• contains immune cells

• contains cytokines

Colostrum - Mechanism of Entry

• Absorption

• newborn intestine has low protease activity

• colostrum has trypsin inhibitors

• Newborn enterocytes have receptors (FcRn) that bind colostral antibody

• Antibody endocytosed and absorbed into peripheral circulation

Time-dependent absorption of colostral antibodies

• Highest Ab uptake first 6 hours of life

• Negligible uptake after 24 hours = gut closure

• Exact time of gut closure varies, depending on feeding

• Feeding colostrum speeds up gut closure

• Delayed feeding colostrum slows down gut closure

Predominant immunoglobulin in colostrum and milk varies by host species

• Don’t forget that IgG, IgM and IgA are ALL present in colostrum

IgG transfer (and reliance on colostrum) varies according to placental type

• IgG transfer (not IgM, IgA, or IgE)

Situations that you might encounter

• Failure of Passive Transfer (FPT)

• Neonatal Isoerythrolysis (NI)

• Neonatal sepsis

Failure of Passive Transfer (FPT)

• Affected: calves and foals

• Cause: insufficient absorption of maternal antibodies

  • Quality, quantity, and timing all contribute

• Clinical presentation: failure to thrive, weakness, recurrent infection

• Prevention: ensure adequate ingestion of high-quality colostrum ASAP after birth

• 38% incidence of FPT (Negative test result)

• Increased disease incidence and poorer outcomes in cases of FPT

Neonatal Isoerythrolysis

• Affected: Foals

• Pathogenesis

  • Dam sensitized to foreign RBC antigens from sire (Aa, Qa) and alloantibodies produced

  • Antibodies concentrated in colostrum

  • If ingested by foal prior to gut closure, antibodies will target and lyse RBCs

  • More common in multiparous mares

• Diagnosis

  • circumstantial, indirect coombs test is definitive

• Treatment

  • Supportive care and alternative source of nutrition for foal, strip mare of colostrum; transfusion if PCV <12% using washed RBCs from mare

Neonatal Isoerythrolysis

• Affected: CAts

• Pathogenesis

  • Dam produces alloantibodies to RBC antigens from sire

    • Queen (blood type B) have naturally occurring ABs to type A antigens

    • Queen is multiparous or had unmatched transfusion

  • If ingested by kittens prior to gut closure, antibodies will target and lyse RBCs

• Diagnosis

  • Circumstantial, indirect Coombs test is definitive

• Treatment

  • Supportive care and alternative source of nutrition

Neonatal/early life infection - exemplars

• Feline Neonatal Sepsis

  • Wide variety of opportunities

    • E. coli, Klebsiella, Pseudomonas, Streptococcus, Enterobacter, Salmonella)

  • Secondary to other factors

    • poor nutrition, parasitism, heritable defects in immunity

• Equine Neonatal sepsis

  • Wide variety of opportunists

    • E.coli, Klebsiella, Pseudomonas, streptococcus, Enterobacter, Salmonella)

  • Secondary to other factors

    • inadequate or poor quality colostrum, prematurity, dirty environment

• Canine Neonatal Sepsis

  • 14.8% incidence with 25.6% mortality among affected

  • 75% occur during first week of life

  • Wide variety of opportunists

    • E. coli, Staphylococcus, Streptococus)

  • Often secondary to umbilical trauma, but other routes of infection possible.

• Bovine Neonatal Sepsis

  • Wide variety of opportunists

    • E. coli, Klebsiella, Campylobacter, Salmonella

  • Often secondary to FPT

  • Often occurs via gastrointestinal exposure

Critical Concepts

• Fetal immune system is immature and fetal environment is protective, but immunosuppressed

• Result of in utero viral infection depends on timing and viral pathogenicity

• Degree of passive transfer of IgG in utero (and dependence on colostrum) varies according to placentation type.

• Neonatal immune system is immature, resulting in “immunity gap” as passive immunity wanes.

• Bacterial infections during neonatal period are primarily opportunistic infections due to predisposing factors

Why do we have an immune system?

• The principal physiologic function of the immune system is defense against infectious microbes.

• The principal function of the veterinarian in defense against microbes is to take advantage of and aid the immune system.

Who are the enemies

• Parasites

  • protozoa

  • Helminths

  • Ectoparasites

• Intracellular bacteria and fungi

• Extracellular Bacteria

• Viruses

What do we have in our immune arsenal? ****

• Physical barriers

  • skin/epithelium

  • mucus

  • peristalsis

• “Immune” cells

  • Neutrophils, eosinophils

  • Mast cells

  • Monocytes/Macrophages

  • Dendritic cells (DCs)

  • NK cells/Innate lymphoid cells (ILCs)

  • T cells

  • B cells

  • Epithelial and other cells

• Cell products

  • Complement

  • Antibody

  • Cytokines

The pathogen’s perspective

• Make contact

  • skin, mucosal surfaces, tissue, blood

• Colonize and replicate

• Infect = disease

  • Breach physical and innate barriers

  • Spread or remain localized and cause damage

  • Some release products (toxins)

  • Establish extracellular or intracellular niche)

Immunity to infection

• Defense is mediated by both innate and adaptive immune mechanisms ***

• Somewhat artificial separation

  • not completely sequential

• Different types of microbes stimulate distinct responses and effector mechanisms ***

  • efficient use of complex system

Oversimplification of immune repertoire ***

• Innate

  • physical barrier front line

  • Innate response (macrophages, granulocytes, NK cells, complement, etc.) - killing

  • DCs (innate) present antigen to T cells (adaptive)

• Adaptive

  • T cells kill and/or help innate and B cells

  • Antibody has direct effects and supports innate

• Cytokines (innate and adaptive) used to communicate

• All are recruited to where they need to be

  • lymph node (production) or site of infection (action)

  • chemokines and adhesion molecule interactions.

• Goal is clearance or at least containment

Why do we have lymph nodes? (spatial response)

1. Adherence to epithelium

2. local infection, penetration of epithelium

3. local infection of tissues

4. lymphatic spread

5. adaptive immunity

• Protection against infection

  • Normal flora and local chemical factors inhibit microbial growth phagocytes activated (especially in lung)

  • Wound healing induced antimicrobial proteins and peptides, phagocytes, and complement destroy invading microorganisms.

  • Complement activation, dendritic cells migrate to lymph nodes, phagocytes action, NK cells activated, cytokines and chemokines produced.

  • Pathogens trapped and phagocytosed in lymphoid tissue. Adaptive immunity initiated by migrating dendritic cells

  • Infection cleared by specific antibody, T-cell dependent macrophage activation and cytotoxic T cells

    

Immunity to extracellular bacteria

• mucosal spaces, circulation, tissue spaces

• Physical defenses have been breached

• Disease

  • Tissue destruction

    • endotoxins (LPS)

    • Exotoxins

    • Adhesion/Effacement

  • Inflammation

Non-specific defense

• Physical ***

  • skin

  • mucociliary system

  • Intestinal motility

  • Temperature

• Chemical/enzymatic

  • lysozyme

  • pH

  • Iron competition

  • Defensins

• Histamine

• AA metabolites

• Clotting cascade

• Kinin System

• Complement

Innate immunity to extracellular bacteria

• Immediate

• components

  • neutrophils, monocytes/macrophages, DCs and NK cells/ILCs

• Cooperation with adaptive critical ***

  • SCID - control many infections with innate system, highlighting the latter’s importance, however, can’t always sterilize

• Neutrophils first line ***

• Macrophage phagocytosis and activation

  • Opsonization (via ab, complement fragments) increases activation over 1000-fold ***

  • cytokine production

  • stimulates inflammation

  

  **** repeats

  1. Mac binds microbe

  2. Secrete cytokines (IL-12, TNF-a)

  3. Stimulates NK cells

  4. Secrete cytokines (IFN-y)

  5. Activate macrophages

Complement

• Alternative and mannose pathways

• Stimulate inflammation (C3a, C5a) ****

• Opsonins (C3b, C4b) ****

Inflammation/vascular changes

• recruit more immune cells ***

• With inflammation, adhesion molecules (E and P selections and VCAM-1) upregulated on endothelium

• More neuts and monos attracted

  • monos —> macs — > activated macs

• chemokines also recruit

• Innate response proceeds

Leukocyte migration

Tissue Dendritic Cells (DCs)

• DC recognition of bacteria

  • Toll-like, mannose, scavenger receptors, etc

• Take up antigens

• Increased expression of MHCII and co-stimulatory molecules

• Then what?

• Antigen presentation to adaptive

  • Need to find specific T cells so travel to lymph node (adhesion molecule and chemokine mediated) ****

  • Immature dendritic cells reside in peripheral tissues

  • Dendritic cells migrate via afferent lymphatics to regional lymph nodes

  • Mature dendritic cell in the deep cortex

Hinge for dendritic cells

• DC with antigen and costimulators upregulated travels to the T cell zone of the lymph node

• Circulating naive T cells find lymph nodes by HEV adhesion molecules (e.g., L-selectin) and chemokines (e.g., CCR7)

  • migrate in and check out the antigens presented by DCs

  • No DC/T cell match - move one

  • Match - DC present antigen to T cell —> T cell activation

  • Activated T cell changes surface marker (decrease S1P1) to stay put for clonal Expansion and differentiation

• Pretty efficient dating system

  

Effector T cells - where to now?

• Back to the site of infection

  • decrease L-selection, decrease CCR7

  • Increase VLA-4 (and more) - binds to VCAM1 on inflamed endothelium

• Or stay behind to help B cells

  

T cell functions

• Macrophage activation ****

  • T cel diff to Th1 (IL-12 driven) —> IFN-y

• B cell help ****

  • antibody production

  • Th1 —> opsonizing antibody isotypes (mice

Th17 functions

• TH17 produce IL-17

  • IL-6+ TGF-b promote differentiation (mice)

  • IL-23 promote maintenance (mice)

• IL-17

  • Neutrophil proliferation, maturation and chemoattraction ***

  • induce proinflammatory cytokines, chemokines and MMPs

  • Anti-bacterial defense

• ILC3s - innate counterpart that produces IL-17

T Cell polarization

• Regulation

  • Treg

    • IL-10

    • TGF-b

• Antibody production & defense against helminths

  • Th2

    • IL-4

    • IL-5

    • IL-13

• Neutrophil activation, inflammation, defence against extracellular bacteria

  • Th17

    • IL-17

    • IL-6

• Macrophage activation, inflammation, defence against intracellular bacteria and protozoa

  • Th1

    • IFN-y

    • IL-2

    • TNF

    • GM-CSF

  

What about B cells?

• Some migrate to follicles, form germinal centers and undergo somatic hypermutation to optimize and diversify specificity of antibody (EXPAND)

  • ultimately produce antibody (as plasma cells***)

• Migration is chemokine-mediated

• B cells also directly bind antigen through membrane Ig —> cross-linked —> upregulates co-stimulatory molecules for ag presentation (B7-1, B7-2, and CD-40 )

Recirculating B cells enter lymphoid organs through high endothelial venules and migrate to the primary follicle

• Antigen-specific B cells are trapped at the border between the T zone and the follicle and are stimulated to proliferate

  • Proliferating B cells form a primary focus; some B cells migrate to medullary cords to medullary cords and secrete antibody

    • Several B cells migrate into a nearby follicle forming a germinal center where rapid proliferation and somatic mutation occur

      • Somatically mutated B cells that retain the capacity to survive, whereas other B cells die

Adaptive immunity to extracellular bacteria

• B cells produce antibody

  • T-independent antigens (polysaccharides; limited T cell activation)

  • T-dependent antigens (proteins)

  • Bacteria and toxin targeted

• Antibody function

  • Opsonization ***

  • Complement activation (classical) ***

  • Neutralization (toxins) ***

Adaptive immunity to extracellular bacteria

• IgM and IgG for most bacteria

  • Opsonization and complement activation; some neutralization

• IgA at mucosal surfaces (driven by IL-4, IL-5, TGF-b)

  • neutralization

Toxin neutralization

• Toxins bind to cellular receptor

  • Endocytosis of toxin:receptor complex

    • Dissociation of toxin to release active chain, which poisons cell

      • Antibody protects cell by blocking binding of toxin

Intracellular bacteria

• some bacteria can live in phagocytes

  • mycobacterium spp. or facultative intracellular bacteria

• Activated macrophages critical in defense ***

  • can form giant cells

• T cell component - primarily CD4+ Th1 cells provide help to macs (IFN-y, CD40L co-stim, isotype switching, etc)

• NK cells - IFN-y production ****

• ILC1s - IFN-y & TNF-a production

• Humoral role?

  • contributes, but can’t succeed alone

  • extracellular stages

Intracellular bacteria, part 2

• some intracellular bacteria end up in the cytoplasm (escape the phagosome) and thus can be presented by MHC1 to CD8+ T cells that help to kill the cell

• Examples: Listeria, Chlamydia, Rickettsia

• NK cells —> IFN-y —> activates macs —> cytokines —> CD4+ and CD8+ cells —> cycles —> clearance

• Both CD4 and CD8 involved, thus both MHC I and II presentation occurring (CD8 perhaps more important due to intracytoplasmic lifestyle)

• Nafe’s rule of 8: CD4 MHC2 = 8 and CD8 MHC1 = 8

A few more words about intracellular bacteria

• Immunopathology - immune response is the problem ***

• Granuloma - hallmark of mycobacterial disease

  • resist macrophage killing and decreases IFN-y mediated activation

• Are granulomas good or bad???

  • organism walled off but not cleared

  • Can stimulate fibrosis (TNF-a and TGF-b)

• In some cases, alternatives are more problematic.

Leprosy

• Two forms

  • Tuberculoid (Th1 predominates) - potent mac activation - controls bud does not eradicate - symptoms milder and associated with inflammatory response

  • Lepromatous form - (Th2 predominates and Th1 immunity is suppressed) - marked increase in bacterial numbers = more severe disease

• Sheep with Johne’s = similar phenomenon

  • Th1 may transition to Th2

Immunity to fungi

• often environmental “opportunists”

• Disease in immunocompromised hosts (esp. innate but both involved; e.g., AIDS patient susceptible)

• Granulomatous to ppyogranulomatous inflammation predominates ***

  • giant cells

• Innate (macs, neuts & ILCs) critical ***

  • cytokines and antimicrobial peptides

• Adaptive (CD4+) important too

  • CD8+ iff intracellular fungus such as Histoplasma sp.

• Antibody response occurs, but role is debatable

• Th1 & Th17 = good; Th2 = non-protective ***