Data

Copy and paste the text below. It is read-only. Select All

Host Defense Against Tumors--Tumor Immunity Immune surveillance: the concept that a normal function of the immune system is to survey the body for emerging malignant cells and destroy them. This is supported by many observations: Lymphocytes around tumors and in lymph node drainage, experimental results, increased cancer in immunodeficient, tumor-specific T cells and antibodies in cancer Cancer Immunoediting: Immune system plays not only a protective role in tumor development but also can select for tumor variants. These variants have reduced immunogenicity and can more easily escape immunological detection and rejection. Immune system can prevent tumor formation and also "sculpt" the immunogenic properties of tumors to select tumor cells that escape immune elimination. Tumor antigens: Products of mutated genes, overexpressed or aberrantly expressed cellular genes, tumor antigens produced by oncogenic viruses, oncofetal antigens, altered cell surface glycolipids & glycoproteins, cell type-specific differentiation antigens. Tumor Antigens Tumor-Specific antigens: are present only on tumor cells and not on any normal cells Tumor-associated antigens: are present on tumor cells and also on some normal cells. Important advance in field was development of techniques for identifying tumor antigens that were recognized by cytotoxic T cells because CTLs are the major immune defense mechanism against tumors. (CTLs recognize peptides from cytoplasmic proteins on MHC I). Products of mutated genes: Genetic alterations in oncogenes and tumor suppressor genes produce mutated proteins that are presented as antigens that have never been seen by the immune system and thus can be recognized as non-self. (MHCI and CD8+ or MHCII/CD4 thru macrophages as they digest dead tumor cells). Some cancer patients have circulating T cells responsive to products of mutated Ras, p53, and Bcr-abl proteins. (in animals--immunization with mutated Ras of p53 induces CTLs and rejection against tumors with that mutant). Tumor Antigens (tyrosinase) Overexpressed or aberrantly expressed cellular genes: In a subset of melanomas, some tumor antigens are structurally normal protein that are produced at low levels in normal cells and overexpressed in tumor cell. Tyrosinase, an enzyme involved in melanin biosynthesis, is expressed only in normal melanocytes and melanomas. T cells from melanoma patients recognize peptides derived from tyrosinase, raising the possibility that tyrosinase vaccines may stimulate such responses to melanomas. A probable explanation is that tyrosinase is normally produced in such small amounts and in so few cells that it is not recognized by the immune system and fails to induce tolerance. (Others are "cancer-testis" antigens that are in sperm but not normally exposed and Melanome antigen gene (MAGE) family for many tumor types) Tyrosinase expression by melanoma cells can cause them to become immunogenic and the target of killing by CTLs. MAGE-1: normally only seen in placenta. Expressed in many tumors: immunogenic in melanoma The melanoma patient shown here, who was dark-skinned priot to the onset of melanoma, has lost almost all of his skin pgiment except for several isolated areas (face, armpit) due to the autoimmune attack incited by the melanoma cells. This condition of pigment loss, known as vitiligo, is often correlated with a longer survival of melanoma patients Tumor Antigens, cont'd Tumor antigens produced by oncogenic viruses: Made by HPV, EPV. CTLs recognize and will survey (concept of immune surveillance best established for DNA virus-induced tumors). Vaccines against HPV antigens are effective in preventing cervical cancers in young females. Oncofetal antigens: Oncofetal antigens are proteins that are expressed at high levels on cancer cells and in normal developing (fetal) but not (but with a few exceptions) adult tissues. (Genes silenced in development. ID'd with antibodies raised in other species and they're importance to to provide markers for tumor diagnosis. Can be increased in some inflammatory conditions). The two most thoroughly characterized oncofetal antigens are carcinoembryonic antigen (CEA) and a-fetoprotein (AFP). Altered Cell surface glycolipids & glycoproteins: Most tumors have higher numbers and abnormal surface markets (diagnostic). These altered molcules includ gangliosides, blood group antigens, and mucins. Antibodies have been raised against these molecules, which are not specifically expressed on tumors, but they are present at high levels on cancer cells than on normal cells (tumor-associated). This class of antigens is a target for cancer therapy with specific antibodies. (Melanomas have increased GM2, GD2, GD3). Mucins (high MW glycoproteins are altered in tumors giving tumor specific epitopes on the carbohydrate side chains or polypeptide core). Several mucins have been the focus: CA-125 and CA-19-9 in ovarian carcinomas and MUC-2 on breast carcinomas. Tumor Antigens, cont'd Cell Type-specific differentiation antigens: tumors express molecules normally on cells of origin. They are specific for a particular lineage or differentiation stages of various cell types. Tumor differentiation antigens are typically normal self-antigens and therefore they do not induce immune response in tumor-bearing hosts. They are potential targets for immunotherapy and ID tissue of origin. Lymphomas may be diagnosed as B-cell derived tumors by the detection of surface markers characteristic of this lineage such as CD20. Antibodies against CD20 are used for tumor immunotherapy. Anti-CD20 antibodies kill normal B cells, however, but because hemopoeitic stem cells are spared, new B cells emerge eventually. (can use idiotypes) Melanoma: MAGE, tyrosinase; Colon cancer: CEA; Breast & Ovarian: HER2/Neu; CML: Bcr-abl; prostate: PSA alpha-fetoprotein: Liver cell cancer, noneminomatous germ cell tumors of testis Carcinoembryonci antigen: colon, pancreas, lung, stomach, heart PSA: prostate CA-125: ovarian; CA-19-9: colon, pancreatic, CA-15-3: breast p53, APC, RAS Mutants (stool): colon p53 and RAS (stool): pancreatic p53 and RAS (sputum): lung p53 (urine): bladder Clinical Relevance of Tumor Markers Prostate-Specific Antigen or Prostate-Specific Membrane Antigen (PSA): One of the most applied and successful tumor markets in clinical practice Used to screen for prostatic adenocarcinoma PSA is a product of prostatic epithelium and is normally secreted in the semen. It is a serine protease whose function is to cleave and liquefy the seminal coagulum formed after ejaculation In normal men, only minute amounts of PSA circulate in the serum Prostatic carcinoma can be suspected when elevated levels of PSA are found inthe blood (cutoff point is 4ng/ml between normal and abnormal) However, PSA screening also highlights problems encountered with virtually every tumor marker: Although PSA levels are often elevated in cancer, PSA levels may also be elevated in benign prostatic hyperplasia (BPH). PSA is organ-specific, not cancer-specific Other factors such as prostatitis can increase PSA 20-40% of patients with organ-confined prostate cancer have a PSA value of 4ng/ml or less Men with enlarged hyperplastic prostate glands have higher PSA levels than men with small glands. As men age, their prostates tend to enlarge with BPH. (no PSA level ensures no cancer, low sensitivity and specificity, density measurements factor out contribution of benign tissue to serum levels, upper age PSA reference ranges are 2.5 to 6.5 ng/ml. PSA velocity (rate of change): 0.75 ng/ml for men with cancer, must be 3 measurement over a period of 1.5-2 yrs, significant rise in PSA even if below 4 ng/ml needs work up. PSA detected by antibody test: 2 forms: major fraction bound to alpha1=chymotrypsin and minor free fraction. Free percent lower in men with cancer. Free higher than 25% lowers risk of cancer (as compared to less than 10%). Tumor Markers: CEA & AFP CEA: colon, pancreas, stomach, breast cancer. AFP: hepatocellular carcinomas, teratocarcinomas, embryonal cell carcinomas. Can be made by non neoplastic conditions. Lack specificy and sensitivity in test for early detection. Useful in detection of recurrences after excision. With successful resection the markers disappear from serum and reappearnce means terminal disease. Devetests in blood/body fluid: Detection of mutated APC, p53, Ras in stool for colorectal carcinomas. mutated p53 and hypermethylated genes in sputum for lung cancer and salive for head/neck. mutated p53 in urine for bladder cancers Anti-Tumor Effector Mechanisms Cytotoxic T cells (CTLs): react to umor antigens. CD8+ CTLs play a protective role against virus associated neoplams (EBV, HPV). CD8+ CTLs have been demonstrated in the blood and tumor infiltrates of cancer patients. Natural Killer cells (NKs): NK cells are lymphocytes that are capable of destroying tumor cells without prior sensitization and thus may provide the first line of defense against tumor cells. (after activation w/ IL2, IL15 can lyse a wide range of tumors). Tumors that fail to express MHC class I antigens cannot be recognized by T cells, but these tumors may trigger NK cells. Recognize antigens on stressed cells. Macrophages:T cells, NK cells, and macrophages may collaborate in antitumor reactivity, because IFN-G, a cytokine secreted by T cells and NK cells is a potent activator of macrophages. Activated macrophages may kill tumors by mechanisms similar to those used to kill microbes (ROS) or by secretion of TNF. Antibodies: Not protective, but monoclonal antibodies can be therapeutically effective. Monoclonal antibody against EGF receptor (Herceptin) is effective against breast cancer and a monoclonal antibody against CD20, a B-cell antigen is widely used for treatment of lymphomas. Immune Surveillance & Escape The strongest argument for the existence of immune surveillance is the increased frequency of cancers in immunodeficient hosts. (5% with deficiency develop cancer--200x normal. transplant recipients and those AIDS have increased chances. Most of these are lymphomas (diffuse large B-cell lymphoma). Rare X-linked immunodeficiency disorder XLP (xlinked lymphoproliferative syndrome) has increase chances of B cell lymphoma) Most cancers occur in persons who do not suffer from any over immunodeficiency. It is evident that tumor cells must develop mechanisms to escape or evade the immune system in immunocompetent hosts. Experiment: Mice inject with irradiated killed cancer cells from chemically-induced tumor. Injected with live cells from same tumor--failed to grow tumor. If injected with live cells of different tumor--formed a mass. Tumor-infiltrating lymphocytes: reveals tumor-infiltrating lymphocytes are abundant in certain areas of tumor. Cells are there tyring to infect the tumor (fight off). They are also found in invasive non-small-cell lung carcinoma: the expression here of the CD8 antigen indicates that these cells are largely cytotoxic lymphocytes Thos patients whose ovarian tumors had high levels of tumor-infiltrating lymphocytes fared significantly better than did those whose tumors lacked significant TILs. Immune Surveillance and Escape Mechanisms Selective growth of antigen-negative variants, loss or reduced expression of MHC molecules, lack of costimulation, immunosuppression, antigen masking, apoptosis of cytotoxic T cells Selective growth of antigen negatives (immunoediting): immunogenic tumor cells are eliminated and those with weak immunogenic antigens survive and multiply. The missing, strongly immunogenic tumors are eliminated early in tumor progression by host immune system and therefore never grow into tumor masses. Immunoediting can be thought of as a type of Darwinian selection, in which the selective force is created by the directed attacks of the immune system on incipient tumors. Suppression of MHC Class I: Cancer cells will often down-regulate expression of the MHC class I molecules, in order to avoid recognition and attack by the adaptive immune response. Immune Surveillance & Escape Mechanism, cont'd Loss of costimulation (B2 microglobulin and TAP): Defects in either B2M or a TAP protein preclude presentation of oligopeptide antigens on the cell surface. In 5 of 17 microsatellite unstable colorectal tumors, B2M gene was mutated & inactive. TAP in cancer may be absent, seen in patches, or normally expressed. Immunosuppression: Regulatory T cells. Tumor cells release CCL22 to attrack CCR4 on Tregs bringing them to the tumor. Once they are recrutied the Tregs which expressed the CD4 and CD25 cell-surface antigens are able to inhibit two types of "effector" cells--the CD4+CD25- helper T cells and the CD8+CD25- cytotoxic T cells. This can cripple major components of the immune response. In normal individuals, the Tregs represent only 5-10% of the population of CD4 cells. In cancer patients, this number increases to 25-30%. Tregs have been found in large numbers among the tumor infiltrating lymphocytes present in lung, ovarian, breast, and pancreatic carcinomas. Advanced ovarian cancer patients (stage IV), the concentration of tumor infiltrating Tregs in tumor sections is a strong predictor of long-term survival. Other mechanisms: Many oncogenic agents (e.g. chemcials and ionizing radiation) suppress host immune responses. TGF-B secreted in large quantities by many tumors, is a potent immunosuppresant. Immune Surveillance & Escape Mechanism, cont'd Antigen Masking: The cell surface antigens of tumors may be hidden, or masked, from the immune system by glycocalyx molecules, such as sialic acid-containing mucopolysaccharides. This may be a consequence of the fact that tumor cells often express more of these glycocalyx molcules than normal cell do. Apoptosis of CTLs: Make fas ligand to interact with Fas receptors on CTLs. Some cancer cells are able to protect themselves from killing by a two-step process: First they acquire resistance to killing by FasL, the ligand of the Fas death receptor. Second, they acquire the ability to produce and release FasL, which allows them to kill lymphcotyes and other cells that may stray too close to them. Approaches to Cancer Immunotherapy (3 of 6) Bacterial Immunostimulants: Bacillus Calmette Guerin and Corynebacterium parvum (heat killed) cause inflammatory response at tumor site (aid rejection). Non specific stimulation of macrophages and NK cells. BCG is frequently injected into the bladders of patients suffering from early-stage bladder carcinomas. It is usually effective in halting or delaying the progression of these tumors. The BCG treatment functions through its ability to attract CD4 and CD8, macrophages, and NK cells to the bladder, where these cells create localized inflammatory responses. Cytokine manipulations: IL2 and interferons (IFN), high toxicity, IL2 to stimulate their CTls and NK response. Both IFNa and IFNB can increase MHC class I expression, enabling improved tumor antigen presentation. IFNs also have direct anti-proliferative effects on tumor cells. Tumor vaccines: killed tumor cells and antigens. Dendritic cell vaccines: dendritic cells transfected with genes encoding tumor antigens. Injection of tumor cells transfected with cytokine genes (GM-CSF) into patients (autologous tumor cells irradiated are injected; Dendritic cells are known to be actived by GM-CSF). DNA vaccine: immunize with plasmids encoding tumor antigens. Cancer vaccines do not directly kill cancer cells, but generate specific, targeted immune response, activate immune system to overcome suppression, improve disease-free intervals in patients and increase survival without major side effects. Approaches to Cancer Immunotherapy (1 of 6) serotherapy: Monoclonal antibodies to tumor antigens: Monoclonal antibodies coupled with toxin--Abs bind the tumor antigen and activate host mechanisms (phagocytosis, etc.) or toxin is delivered to tumor cells. 3 types of immunoconjugates: radioimmunoconjugate (effector is isotope), immunotoxin conjugate (effector is toxin), antibody-drug conjugate (effect is small drug) Herceptin treatment: collaborative effects of herceptin and radiation act synergistically The utility of the Rituxan/Rituximab monoclonal antibody derives in part from the fact that the CD20 antigen that it recognizes is displayed as a cell surface transmembrane protein by B cells at different stages of differentiation, beginning with early B cells, the immediate precursors of the antibody-secreting plasma cells. A variety of B-cell derived tumors express CD20 making rituxan a highly useful agent for treating these tumors. (there are other CD with potential) Concept summary: Bone Marrow Transplant (1 of 6) The original rationale for this treatment came from the discovery that the immune system of a mouse or human cell can be ablated (eliminated) through drug treatments or X-irradiation. The subsequently introduced donor marrow graft, because it contains hematopoietic stem cells, can repopulate the recipients bone marrow and regenrate all of the cell lineages required for normal hematopoiesis and immune function. In the case of hematopoietic malignancies, notably lymphomas and leukemias, the intent of BMT was to rid the body of neoplastic stem cells that were present throughout the body, in particular the marrow itself. Accordingly, BMT would prevent the tumor from ever regenerating itself because the tumor stem cells would be eliminated from the patients' marrow However, much of the therapeutic effects of BMTs in treating hematopoietic malignancies derives from the graft versus tumor response, in which donor immunocytes identify and attack residual tumor cells--those that have survived the radiation and chemotherapy used to ablate the patient's bone marrow. The BMT response does not involve a graft-versus-host attack, which would, if it occurred to widespread inflammation and destruction of the recipient's normal tissues. BMT has not proven to be an effective strategy for treating patients with solid tumors. Concept Summary: Adoptive Cell Transfer Immunotherapy (1 of 6) Procedure: isolation of lymphocytes from blood or tumor infiltrate of the patient and expand NK cells by culture in IL2 Lyphocyte activated killer (LAK) cells are derived from expanded NK cells LAK cells have a greatly enhanced non-specific cytotoxic effects on tumors Infusion of expanded LAK cells back into the patient Adoptive Cell Transfer Immunotherapy, cont'd One version: take TILs and identify subclone making IFN-G and expand in vitro. Treat patient's bone marrow with cytotoxic agents that reduce resident lymphocytes, creating a biological niche for lymphocytes expanded in vitro. Infusion (introduced cells, establish in marrow launch attack on tumor cells) May have dramatic responses. This is not typical--suggest that immune systems of cancer patients may one day be manipulated to generate outcomes like these reproducibly. Anticancer immunotherapy: focus on 3 major steps: enhancing antigen presentation, suppressing immunological checkpoints, increasign the migration of activated T cells to tumor masses.