BL 03-06-14 11am-Noon Tumor Immunology - Cohen Flashcards
Cancer - epidemiology
- 2nd leading cause of death in US (after heart disease)
- 2nd leading cause of years of life lost (after accidents)
- BUT, US cancer death rates decreased between 2002 & 2008 (in the rest of the world, rising)
Mutations in humans
- human will undergo up to 10^16 mitoses in a lifetime
- mutations happen at a rate of 10^-8 / bp / mitosis
- –> 3000 mutations/hr
- about 1 per hour is potentially oncogeneic
Immune Surveillance
The view of Lewis Thomas in 1959, that the adaptive immune response evolved not so much for dealing with foreign substances, but as a way of detecting changes in the body’s own cell surfaces.
- These changes would probably be due to damage or mutation.
- The true role of the immune system, esp. of T cells, would be to constantly monitor cell surfaces in the body
—> if one was detected as abnormal, that cell would be destroyed before a mutant (possibly malignant) clone developed.
= incredibly ahead of its time, since T cell’s preoccupation with “self” was not discovered for ~20 yrs
- Looked at in this light, the development of cancer could be seen as a failure of immunity.
Evidence for cancer immune surveillance
- Ppl w/ immunodeficiencies, esp. of T cells, have a higher incidence of tumors
- AIDS patients
- Organ transplant recipients
- People treated w/ chemotherapy - Activated T cells that recognize tumor-associated antigens can easily be IDed
- Small % of tumors (mainly melanomas & some lymphomas) spontaneously regress, presumably due to an immunologic response.
Tumor-infiltrating lymphocytes (TIL)
= activated T cells that recognize tumor-associated antigens
- presence of lymphocytes in a tumor (TIL), many of which are tumor-specific, is a good prognostic sign.
Limitations to immune surveillance hypothesis
- Tumors that immunodeficient/suppressed ppl get:
- NOT a random sample of all tumors that can occur
- rather, tend to be tumors of lymphoid system & of the skin, but rarely lung or breast - Nude mice (mice w/ no thymus)
- should get tumors very readily
- BUT in fact spontaneous tumors are rare
- Why? Probably b/c these mice have very high levels of NK cells, which are not part of traditional (T & B cell) immune system but can be quite tumoricidal (directly or via ADCC)
Immunotherapy in Cancer
- B/c of shortcomings, immune surveillance hypothesis fell somewhat into disrepute.
- But newer ideas have revived the study & application of immunological principles to cancer
- Compared to traditional cancer treatment modalities (radiation, chemo, surgery) immunotherapy promises the new concept of specificity.
Immunoediting
One can think about the role of the immune system in neoplastic development as a series of stages in a process that has been called “immunoediting.”
Stages:
- Elimination
- Equilibrium
- Escape: the tumor cells fight back
Why Immun Surveillence must be involved in the Elimination step of Immunoediting
Going from 1 cell to 1 gram of tumor (109 cells) is 31 generations.
- would take 30 days if tumor cell cycle were 24 hrs
- BUT epidemiology suggests it takes ~20 years!
- –> implies cycle time of 240 days = unreasonable
Something must be eliminating most cells that get initiated by a mutagenic event.
= immune surveillance?
Immunoediting - Step 1 - Elimination
When clone becomes malignant, most likely will be recognized as abnormal by both innate & adaptive immune systems
—> eliminated
Tumor cells exhibit a variety of metabolic abnormalities compared to normal cells
- –> expression of DAMPs
- –> activate innate immunity
Cytokine secretion & Ag presentation on DCs activate T cells
—> macrophages & CTLs infiltrate tumor
If the abnormal clone is successfully eradicated, process ends.
Immunoediting - Step 2 - Equilibrium
In most clinically relevant tumors, lymphocytes infiltrate tumor, but do not fully destroy it.
- Instead tumor & lymphocytes exist in equilibrium.
May be analogous to situation w/ Epstein-Barr virus in bone marrow, or Varicella in dorsal root ganglia:
- As long as immune response is strong, latent
But biologic equilibria are dynamic
- Changing conditions (drop in host immunity or further mutations accumulate in residual tumor cells)
- –> reactivation
Immunoediting - Step 3 - Escape
= the tumor cells fight back via mutations and immunosupressive mechanisms
3 Experiments to answer the question of whether the immune system has something to do with tumor regression:
Traditional cancer treatments can’t remove every last cell– so is immune system involved ?
Experiments studied pts w/ malignant melanoma:
- Some had been cured by therapy = regressors
- Most had growing tumors = progressors
EXPERIMENT 1:
- T cells from regressors added to melanoma cells
- Result: T cells killed tumor cells.
- Conclusion: As expected, if immune system really helped these people get rid of their tumors.
EXPERIMENT 2:
- T cells from pts w/ progressively growing tumors added to melanoma cells, as a CONTROL
- Result: T cells killed tumor cells.
- Conclusion: Unexpected in light of Experiment 1!
Other controls quickly showed this killing was tumor-specific in both experiments…
*Both groups really had killer T cells against melanoma; normals don’t.
So why did tumors keep on growing in progressor pts who had developed CTL against them?
- Tumor cells found means to escape surveillance.
EXPERIMENT 3:
- T cells from either group added to melanoma cells, now in presence of serum from progressor pts
- Result: NO killing.
- Conclusion: Serum contains blocking factors, which specifically block the killing of melanoma cells by melanoma-specific killer T cells
Blocking factors - types
Blocking factors include:
- shed tumor antigen (or antigen-MHC complexes)
- sometimes antibody against tumor antigens (don’t harm the tumor while shielding it from T cells)
Blocking factors - what they remind us
Existence of blocking factors (though not seen in every tumor type) is instructive:
- Reminds us that tumor & host are co-evolving systems
- –> When we see clinically-relevant tumor in pt, we know it has already come up w/variety of tricks to avoid immunity.
Escape Mechanism of Tumor cells
►Tumors evolve many escape mechanisms.
- Some modify their tumor-associated antigens (see
below) until the host does not have T cells against them with highly avid receptors. - Others make immunosuppressive factors like TGFbeta
- Almost all, as they progress, reduce the expression of MHC Class I so there is less and less for CTL to recognize.
Tumor antigens
All tumor cells can be shown to have antigens not readily found on the corresponding normal cell.
Often the tumor Ags are found on normal cells, but in much lower quantities
—> over- or abnormally expressed by tumor
►Such antigens are called tumor-associated antigens (TAA)
Subclass of TAA are those that can be recognized by immune system, in a way that could lead to the destruction of the tumor. = called tumor rejection antigens
Tumor-associated antigens (TAA)
- antigens on tumor cells that are not readily found on corresponding normal cells
- often these Ags ARE found on normal cells, but in much lower quantities
- –> over- or abnormally expressed by tumor
Tumor rejection antigens
= subclass of TAA = those TAA that can be recognized by immune system, in a way that could lead to tumor destruction
Viral gene products
Many tumors are known to be caused by tumor viruses
= ~20% of tumors in humans (directly or indirectly caused by viruses)
Tumor viruses - HTLV-1 & -2
Esp. noteworthy tumor viruses
Strongly implicated in:
- Sézary syndrome/ mycosis fungoides
- Similar epidemic lymphoma in Japan & the Caribbean
Tumor viruses - HPV
Cervical cancer (human papilloma virus) = currently best-known virally-induced human tumor
Tumor viruses - Hepatitis virus
Much liver cancer in developing world follows hepatitis virus infection.
Tumor viruses - Epstein Barr
can induce Burkitt lymphoma & nasopharyngeal carcinoma
Tumor bacteria - Helicobacter pylori
associated with gastric carcinomas
Mutant gene products
Chemical & physical carcinogens lead to cellular transformation.
Mutated proteins will be processed & presented to immune system.
Mutations contributing to development of tumors:
- not always identical from pt to pt.
- –> immunotherapy designed against these Ags may not be as generalizable as might be w/ viral or normal gene products
►These antigens are called tumor-specific antigens, TSA.
Tumor specific antigens (TSA)
= Mutated proteins that contribute to development of tumors
- not always identical from pt to pt.
- difficult to create generalizable immunotherapy against them
Normal gene products
Oncofetal antigens (carcinoembryonic antigen (CEA))
Differentiation antigens
Clonal antigens
Oncofetal antigens
- made in normal fetal tissues
- NOT found in normal adult tissues
- can be re-expressed in tumor
Most familiar = carcinoembryonic antigen (CEA)
Carcinoembryonic antigen (CEA)
- an oncofetal antigen (normal gene product only in fetal tissues, re-expressed in tumore cells)
- found in blood of pts w/ colon carcinoma & other cancers
Commercially available kits detect CEA in blood
- Should NOT be used as routine screening test
- –> Too many false positives
Proper use of measurement of CEA in blood:
- when have high index of suspicion of colon cancer
- when such a cancer has been removed, to confirm complete excision (levels fall to 0 and remain there)
- to warn of recurrence
Differentiation antigens
= lineage-specific Ags
= can be greatly overexpressed in tumors
= represent most frequently IDed TAA
Best studied
= those from malignant melanoma (tyrosinase, gp100, MelanA/MART-1)
Also:
= overexpressed human ECFR-1 gene product (HER-2/neu) in breat/ovarian cancers
= prostate specific antigen (PSA) in prostate cancer
HER-2/neu in breast/ovarian cancer
Overexpresssion of human EGFR-2 gene product (HER-2/neu) In 30% of breast & ovarian cancers
Therapeutic Ab & T cell responses to HER-2/neu can be induced
Clonal antigens
- Expressed uniquely on malignant clone
Most familiar examples:
- idiotype of surface Ig in B cell malignancies
- idiotype of TCR in T cell malignancies
How the Immune System Kills Tumor Cells
Cytotoxic T cells Th1 cells Natural Killer (NK) cells Macrophages and neutrophils Antibody and complement
Cytotoxic T cells
- CD8+ T cells (CTL) = most important cells in tumor resistance - can recognize TAA presented by MHC class I (from APCs such as DCs)
Activation of T cells against tumors
Naive T cells are activated in lymph nodes, NOT at tumor site, via interactions w/ APCS such as DCs
Following initial activating event, CD8+ T cells undergo clonal expansion & acquire lytic function.
Activated TAA-specific T cells leave lymph node & migrate to tumor
Action of T cells against tumors after activation/migration to tumor site
CTL can kill tumor cells by inducing apoptosis, via either:
- perforin
OR
- Fas-mediated pathways
CTL also secrete IFNgamma upon engagement of their TCR
—> attracts & stimulates macrophages
However, pt-derived T cells that recognize TAAs are often ineffective in controlling tumor growth.
Study examining T cells specific for melanoma antigens
Study examined biological properties of T cells specific for melanoma antigens from patients.
T cells divided in presence of tumor cells BUT did not produce cytokines nor were they cytotoxic.
Abnormal behavior NOT INHERENT in T cell, but is INDUCED by the tumor.
CTL have surface molecule, PD-1
—> when engaged, it inactivates the CTL
= part of normal regulatory interactions
- BUT many tumors upregulate PD-1 ligands which suppress CTL, so they escape lysis
Th1 cells
- CD4+ T cells
- recognize tumor Ags, make lymphokines, & attract angry M1 macrophages
- also help get CTL activated
How can we make people with tumors get this system going better than it is? - Th1 stimulation may be part of the goal for the development of therapeutic cancer vaccines
Tumor cell protection against Th1
Tumors frequently protect themselves by creating an environment in which M2, not M1,
macrophages are favored.
—> M2 seem to protect tumors, even encourage their growth.
Natural Killer (NK) cells - appearance
Look like large lymphocytes
BUT have peculiar granules in their cytoplasm
—> usually called LGLs (large granular lymphocytes)
Natural Killer (NK) cells - which part of immune system
Do NOT need to come from immunized host to recognize & destroy wide range of tumors (mostly of hematopoietic origin)
= part of innate immunity
Natural Killer (NK) cells - action
Recognize small number of “stress-related” markers on tumor cells
<— Dr Cohen Likes This Idea!!!
Macrophages and neutrophils
Activated in vitro w/ foreign products (e.g., bacteria) to kill tumor cells
- much antitumor activity attributable to TNF
BUT, tumors frequently learn to subvert macs & even recruit them to support tumor growth (turning M1 into M2, for example)!
Antibody and complement
Antibody response is commonly made in tumor-bearing hosts, but it is not commonly effective.
Opsonization of tumor cells by Ab & complement can kill some leukemias in vitro
- but a strong B cell response to tumor Ags does not seem to correlate w/ resistance to tumor
Tumor resistance to Ab & complement
Tumors that have survived immunoediting:
- Cells of surviving tumor likely to have down-regulated Ag expression as much as possible
- Others have become resistant to complement, or can inactivate it.
Immunotherapy for cancer
There’s nothing as specific, & nearly nothing as powerful at cell-killing, as the immune system.
Types of Immunotherapy against Cancer:
- Specific immunization, a.k.a., a tumor vaccine
- Innocent bystander killing via BCG
- Antibody therapy (immunotoxins)
- Adoptive cell transfer therapy
Specific immunization (tumor vaccine)
- so far, not much success in this area
- BUT as more defined TAAs are being IDed, keep an eye out for breakthroughs
- Initially shots will be therapeutic, not preventative
Most interesting vaccine experiments:
- use pt’s own DCs mixed w/ tumor extracts or purified Ags as highly potent immunogens.
- One such treatment is the 1st to be approved (2010) by FDA = Provenge® (sipuleucel-T)
Some people designing improved epitopes w/ higher affinity to MHC or to TCR, or both, than the ones that the tumor itself chooses to use
Provenge® (sipuleucel-T)
1st such treatment to be approved (2010) by FDA
- Combines pt’s own DCs w/ proprietary fusion protein containing prostate cancer TAA prostatic acid phosphatase (highly potent immugens)
- –> extended survival in phase III tests, but a series of immunizations costs $93,000
Innocent bystander killing as tumor therapy
BCG is injected directly into tumor
- –> ferocious delayed-type hypersensitivity rxn to BCG
- –> tumor cells killed the way lung is killed in TB
Used to some degree in cutaneous tumors like melanoma,
►BCG instilled directly into bladder is treatment of choice for superficial bladder carcinoma
Antibody therapy - Abs to TAAs
Many mAbs to TAAs are already available
Can be used as is:
- –> possibly activate complement, causing tumor to be lysed / phagocytosed
- –> more likely invoke ADCC
Can be tagged w/ poison such as ricin, diphtheria toxin, or a radioisotope
= such modified Abs are called “immunotoxins”
Antibody therapy - Abs to growth factors & their receptors
ABs to growth factors or their receptors
—> to try to inhibit autocrine (self-stimulating) tumors
EX: anti-IL-2 receptor in T lymphomas
EX: Herceptin, a mAb to HER2 surface growth stimulatory molecule on some breast cancers
EX: mAb to VEGF (vascular endothelial growth
factor)
Antibody therapy: Coupled antibody constructs
Coupled together 2 single-chain engineered Abs:
- one against CD19
- one against CD3
—> can bind T cells via their CD3 to CD19+ B cell lymphoma cells
Small doses given to lymphoma patients resulted in some cases in complete clearance of tumor cells!
Antibody therapy - Nivolumab
= human mAb against PD-1
—> binds/blocks CTL inactivation by tumors that express PD-1 ligands
In Phase III trials in 2013.
Antibody therapy - Ipilimumab
- approved 2011
- mAb blocker of CTLA-4, another downregulator of CTL activity
Adoptive cell transfer therapy
= utilizes cells from pt’s immune system to destroy cancerous cells that cannot be surgically removed
Immune cells w/ potential to fight tumor are isolated from pt’s blood, tumor, or lymph nodes.
T cells are expanded greatly in culture using cytokines like IL-2.
Pt’s immune system may then be partially destroyed by irradiation to make “room” for expanded anti-tumor clones.
They are reintroduced into immune-depleted pt to kill remaining tumor cells
Tumor-infiltrating lymphocytes (TIL)
T Cells that have potential to fight tumor that are isolated directly the pt’s tumor, in order to be used in adoptive transfer therapy
