Complement

1
Complement

• Complement
• complex group of plasma proteins that are
  pre-formed (not made in response to infection)
• found in serum and body fluids
• produced mainly by liver cells
• can be thought of as a form of innate humoral
  immunity
•  
• Activation of complement results in a cascade of
  molecular events, which results in
• enhanced phagocytosis of microbes
• recruitment of inflammatory cells
• direct lysis of bacteria
•  

2

• There are three different activation pathways for
  complement
• alternative pathway (induced by direct
  interactions with molecules in the surface of
  pathogens)
• classical pathway (antibody-mediated)
• lectin pathway (induced by the mannose-binding
  protein, an acute-phase reactant)
• The poorly-named alternative pathway is perhaps
  the most active and important means of activating
  complement, in that it can act early in
  infection, before the stimulation of other
  responses.
•  

3
(No Transcript)
4
(No Transcript)
5

• Some complement components are activation/proteoly
  tic enzymes.
• Other complement components are membrane-binding
  proteins that act as opsonins.
• Other complement components are mediators of
  inflammation.
• Still others form part of what is called the
  membrane-attack complex, which can lyse microbes.

6

• All three complement pathways intersect at the
  point of having some enzyme (a C3-convertase)
  that can cleave a molecule called C3 into C3b and
  C3a, as well as an enzyme that can cleave C5 (C5
  convertase).
•  
• C3b is a potent opsonin.
• C3a and C5a are peptide mediators of
  inflammation.
• C5b forms part of the membrane attack complex

7
Macrophage-driven inflammatory responses, Acute
Phase Response
8
(No Transcript)
9
Antibody Effector Functions

• Antibodies can exert a wide variety of effector
  functions, including helping induce complement
  activation via the classical pathway, enhancing
  phagocytosis, as well as arming NK cells in ADCC
•  
• In addition to this, antibody molecules can carry
  out a wide range of biological functions.
  Antibodies can directly neutralize viruses,
  bacteria, or bacterial toxins

10
(No Transcript)
11
(No Transcript)
12

• The different immunoglobulin isotypes have
  different biological properties, and are
  selectively distributed throughout the body

13

• IgM is found primarily in blood, and is very
  efficient at activating complement.
• IgE can activate mast cell degranulation,
  inducing allergic responses.
• IgA is readily transported across epithelial
  barriers and serves as the primary immunoglobulin
  type in gut and mucosal surfaces. IgA also is
  found in breast milk.
• IgG is the only isotype that is transported
  across the placenta.

14

• Together, these passively-transferred antibodies
  provide a significant level of immune protection
  in the newborn, as newborn humans cannot produce
  their own antibodies for some time after birth

15
(No Transcript)
16
Cancer - interactions with the immune system

• Cancer refers to a collection of diseases
  characterized by
• abnormal cellular proliferation
• invasion of normal tissues
• Cancers are primarily diseases of older people.
• In developed countries where life expectancy has
  increased significantly over the last century,
  due to control of infectious disease, cancer is a
  significant clinical problem.
• Nearly a quarter of deaths in developed
  countries, such as the United States, are due to
  cancer.
•  

17
5 2 10 4 1 3 8
7 1 9 3 5 8
18
All Cancers, White Males, 1950-1994
from NIH-NCI Atlas of Cancer Mortality
19
Melanoma, White Males, 1950-1994
20
Lung, White Males, 1950-1994
21

• Cancer cells are similar enough to normal cells
  to make eliminating cancer quite difficult.
• Commonly used approaches, such as surgery,
  radiation therapy and cytotoxic drugs either
  affect normal, non-cancerous cells, or cannot
  eliminate all tumor cells, making treatment
  difficult.
• Tumors arise from mutations that result in
  uncontrolled growth.
• In particular, mutations that subvert mechanisms
  normally involved in normal controls on cell
  division and cellular survival can result in
  cancer.
• Examples of this are mutations in which
  proto-oncogenes are activated, or tumor
  suppressor gene function is lost.
•  

22

• These mutations provide a selective growth
  advantage to nascent tumor cells, and these cells
  then accumulate further mutations, which give
  them a great growth and viability advantage.
• The cumulative effect of these multiple mutations
  is malignant transformation .
• Therefore, cancer arises from a single cell that
  has accumulated the most optimal set of
  mutations.
•  

mutations acquired during the development of
colorectal cancer
23

• Cancer can arise from exposure to carcinogens
• mutagenic chemicals
• radiation (ultraviolet light)
• Certain viruses have the potential to transform
  cells
• oncogenic viruses
• Such viruses account for a significant minority
  of cancers.  
• Oncogenic viruses can promote cancer by a variety
  of means, including virus-encoded homologues that
  mimic or subvert human genes involved in
• cellular activation (EBV LMP1 CD40 on B
  cells)
• cellular growth control (HPV E6 E7 other
  viruses - cytokine homologues)
• induction of resistance to apoptosis (bcl2
  homologues)

24

• Oncogenic DNA viruses usually induce long-term
  infection in their host cells, and override
  normal growth/viability control in these cells.
• Simpler viruses, such as retroviruses, can induce
  cancer by inserting into a host cell chromosome
  in the wrong place, resulting in uncontrolled
  host gene expression, since these viruses are
  inserted into the host chromosome as proviral
  DNA.
• Finally, some viruses may cause cancer by
  inducing elevated levels of cellular
  proliferation and replacement, enhancing the
  chances of a genetic error that could result in
  cancer.

25
(No Transcript)
26

• Immune surveillance theory the theory that the
  immune system plays a significant role in tumor
  surveillance.
• Based on increasing knowledge of immune function,
  and the understanding that immune responses could
  distinguish between self and non-self, and could
  exert cytotoxic responses that could kill tumor
  cells.
• A correlate of this theory is that cancer results
  from some failure in immune surveillance.
•  
• Does immune surveillance play a central role in
  cancer control?

27

• People who have immune deficiency syndromes do
  not show a significant increase in the most
  common cancers.
• While there is a marked increase in cancer in
  such immune-deficient people, they tend to
  develop certain rare cancers (Kaposis sarcoma,
  lymphoid malignancies), most of which are
  virus-associated cancers.
• Therefore, immune responsiveness almost certainly
  plays a key role in controlling tumor cell growth
  and development in the case of virus-associated
  cancers, by eliminating virus-infected cells
  (CTL, NK cells, etc).
• However, loss of immune surveillance may not be
  as important in the genesis of more common forms
  of cancer.

28

• Curiously, tumor cells are easily killed by
  cytotoxic T cells that are responding to
  allogeneic differences
• tumor cells that are not of the same MHC type as
  the host are readily killed by normal cellular
  immune responses

29

• Therefore, tumor cells are not inherently
  resistant to cytotoxic effector cells.
• Tumor cells are not killed in an animal that
  shares MHC type with the tumor cell because the
  tumor cell is not antigenic.
•  
• Since the genetic changes that occur in tumor
  cells are slight (in tumors not associated with
  virus infection), the host immune system may not
  be able to distinguish these cancer cells,
  antigenically, from other cells in the body.
• In spite of this, there has been much research
  examining the potential role of immune responses
  in fighting cancer.

30

• Tumor antigens are molecules expressed on tumors
  that can elicit an immune response
• Tumor antigens
• tumor-specific antigens - antigens that are
  unique to a given tumor
• tumor-associated antigens - antigens that are
  widely expressed on tumor cells, but which are
  not unique (these same antigens may be expressed
  on normal cells of other tissue types, or at
  other stages of development)
• Tumor-specific antigens are often seen in
  chemically-induced tumors, in which a mutagen has
  induced a change that is sufficiently large to
  result in a clear antigenic change.
•  

31

• Some tumor-associated antigens are products of
  re-activated embryonic genes, which are not
  normally expressed on mature cells.
• In other cases, tumor-associated antigens
  represent over-expression of normal molecules
  that are expressed at much lower levels in
  non-malignant cells
•  

32
(No Transcript)
33

• It is possible that there is some level of immune
  response initiated against a nascent tumor clone
  - as the progeny of the original tumor cells
  accumulate further mutations, some rare cells
  evolve the ability to evade host immune
  responses.
• Some tumor cells have been seen to have lost
  expression of MHC class I genes, which would
  allow them to evade CTL killing.
• However, such cells would be more susceptible to
  NK cell killing.
• In any case, it is possible that enhancing
  anti-tumor immune responses, either humoral
  (antibody) or cellular (CTL, NK), might allow
  control of tumor cell growth.

34

• Many experimental immune-based therapies are
  being examined for their ability to result in
  anti-cancer effects.
• Recently, monoclonal antibodies directed to tumor
  antigens (i.e., Rituximab anti-CD20,
  Herceptin anti-HER2), sometimes conjugated to
  toxins or radioactive tags, have been utilized in
  treatment of cancer, with significant positive
  results

35

• Other experimental approaches include attempts to
  enhance anti-tumor T cell responses by enhancing
  the effectiveness of tumor antigen presentation
  to potentially cytotoxic cells

36
There are tumors of immune system cells, and
these mirror the various stages of these cells
development
37
Table B1. Common NHL subtypes characteristics
and molecular lesions NHL subtype
NHL hypermutated characteristic
associated
in US Ig V regions molecular
lesions immune dysfunction ______________________
__________________________________________________
___ Burkitts lymphoma (BL) 1-3
yes MYCIgH h Ig isotype
switching ? follicular lymphoma 35
yes BCL2IgH h somatic recombination
? diffuse large B cell 30-40 yes
h BCL6 h somatic hypermutation ?
(DLBCL) BCL2IgH h somatic
recombination ? mantle cell lymphoma 3-10
no CYCLIN D1IgH h somatic recombination
? ________________________________________________
___________________________
38
Table B1. Common NHL subtypes characteristics
and molecular lesions NHL subtype
NHL hypermutated characteristic
associated
in US Ig V regions molecular
lesions immune dysfunction ______________________
__________________________________________________
___ Burkitts lymphoma (BL) 1-3
yes MYCIgH h Ig isotype
switching ? follicular lymphoma 35
yes BCL2IgH h somatic recombination
? diffuse large B cell 30-40 yes
h BCL6 h somatic hypermutation ?
(DLBCL) BCL2IgH h somatic
recombination ? mantle cell lymphoma 3-10
no CYCLIN D1IgH h somatic recombination
? ________________________________________________
___________________________
39

• Isotype switching - change in the type of H-chain
  that is used by a given B cell

40

• Chromosomal translocations can result in the
  uncontrolled expression of a normal gene that is
  involved in inducing cellular proliferation or
  viability (myc proto-oncogene), inducing the
  over-expression of that gene, resulting in
  uncontrolled cellular growth
•  

41
Table B1. Common NHL subtypes characteristics
and molecular lesions NHL subtype
NHL hypermutated characteristic
associated
in US Ig V regions molecular
lesions immune dysfunction ______________________
__________________________________________________
___ Burkitts lymphoma (BL) 1-3
yes MYCIgH h Ig isotype
switching ? follicular lymphoma 35
yes BCL2IgH h somatic recombination
? diffuse large B cell 30-40 yes
h BCL6 h somatic hypermutation ?
(DLBCL) BCL2IgH h somatic
recombination ? mantle cell lymphoma 3-10
no CYCLIN D1IgH h somatic recombination
? ________________________________________________
___________________________
42
(No Transcript)
43
Table B1. Common NHL subtypes characteristics
and molecular lesions NHL subtype
NHL hypermutated characteristic
associated
in US Ig V regions molecular
lesions immune dysfunction ______________________
__________________________________________________
___ Burkitts lymphoma (BL) 1-3
yes MYCIgH h Ig isotype
switching ? follicular lymphoma 35
yes BCL2IgH h somatic recombination
? diffuse large B cell 30-40 yes
h BCL6 h somatic hypermutation ?
(DLBCL) BCL2IgH h somatic
recombination ? mantle cell lymphoma 3-10
no CYCLIN D1IgH h somatic recombination
? ________________________________________________
___________________________
44

• Somatic hypermutation
• rapid mutation (hypermutation) of immunoglobulin
  genes
• results in antigen-binding affinity that is
  higher, or lower, than its original binding
  affinity
• selection by antigen results in the generation
  of BCR with increased affinity for antigen
•  
• Only those B cells that have enhanced their
  antigen receptors binding affinity survive.

45
Mutation of proto-oncogenes occurs in B cells
undergoing somatic hypermutation This
contributes to the development of some
non-Hodgkins lymphomas.
46
(No Transcript)
47
Transplantation and the immune system

• An increasing number of diseases are being
  treated by transplantation

in the US
48

• A significant clinical problem in tissue
  transplantation has been the development of
  immune responses to the grafted tissue
  transplant rejection

49

• Another potential transplant-associated problem
  are anti-recipient responses by immune cells in
  the grafted tissue to the host (graft vs host
  disease GVH).
• A lot of research in immunology was driven by the
  need to understand transplantation rejection.
•  

50

• In fact, MHC, or major histocompatibility complex
  molecules, were first studied because they were
  associated with graft rejection, and only later
  were seen to play key roles in the generation of
  immune responses.
• It was seen that matching MHC type between donor
  tissues and the host resulted in decreased
  rejection, and in significantly increased graft
  survival

51

• Autograft - tissue transplanted from one site
  to another on the same person not rejected.
• Syngeneic transplants -transplants between
  genetically-identical animals (twin humans or
  inbred mouse strains) not rejected
• Allograft - transplants between genetically
  different individualsrejected
•  

52

• The exception to this was pregnancy, in which the
  fetus, which is genetically non-identical to the
  mother (fetal allograft) is not rejected (why
  this is so is still not completely clear).
•  

53

• Graft rejection was seen to involve several
  immune mechanisms.
• Subjects who had pre-formed antibodies that
  reacted with antigens on the graft often showed a
  very rapid antibody-mediated form of rejection -
  hyperacute rejection

54

• Other forms of graft rejection were mediated by
  host T cells that were stimulated by foreign
  antigens on the graft to become activated and
  develop into effector cells.
•   
• Some graft-reactive T cells are cells that
  respond to nonself MHC 1-10 of all T cells in
  an individual will respond to stimulation by
  cells from another, unrelated, member of the same
  species.

55

• In addition to this, host T cells respond, in a
  more conventional manner, to foreign graft
  antigens presented by APC.
•   
• This form of T cell-mediated rejection, acute
  rejection, is due to alloreactions to graft MHC
  class I and class II molecules that are different
  from those of the host.

56

• The development of effective immunosuppressive
  drugs has been of great clinical importance in
  prolonging graft survival.
• One of the most important immunosuppressive drugs
  is cyclosporin, which is a potent T
  cell-inhibitory drug.
• Cyclosporin inhibits signaling induced following
  ligation of the TCR complex, preventing induction
  of IL-2 and IL-2 receptor gene expression
•  

57
(No Transcript)
58
(No Transcript)
59

• This cyclosporin-mediated inhibition of IL-2 and
  IL-2 receptor gene expression effectively
  inhibits T cell activation and proliferation in
  response to antigen