Chemically Or Physically Induced Tumor Antigens

Methylcholanthrene and ultraviolet light are two carcinogens that have been used extensively to generate lines of tumor cells. When syngeneic animals are injected with killed cells from a carcinogen-induced tumor-cell line, the animals develop a specific immunologic response that can protect against later challenge by live cells of the same line but not other tumor-cell lines (Table 22-2). Even when the same chemical carcinogen induces two separate tumors at different sites in the same animal, the tumor antigens are distinct and the immune response to one tumor does not protect against the other tumor.

The tumor-specific transplantation antigens of chemically induced tumors have been difficult to characterize because they cannot be identified by induced antibodies but only by their T-cell-mediated rejection. One experimental approach that has allowed identification of genes encoding some TSTAs is outlined in Figure 22-7. When a mouse tumorigenic cell line

(tum+), which gives rise to progressively growing tumors, is treated in vitro with a chemical mutagen, some cells are mutated so that they no longer are capable of growing into a

Immune response to methyl-cholanthrene (MCA) or polyoma virus (PV)*

Immune response to methyl-cholanthrene (MCA) or polyoma virus (PV)*

Transplanted killed tumor cells

Transplanted killed tumor cells

Live tumor cells for challenge

Tumor growth

CHEMICALLY INDUCED

MCA-induced sarcoma A MCA-induced sarcoma A -MCA-induced sarcoma A MCA-induced sarcoma B +

VIRALLY INDUCED

PV-induced sarcoma A PV-induced sarcoma A PV-induced sarcoma A

PV-induced sarcoma A -PV-induced sarcoma B -SV40-induced sarcoma C +

*Tumors were induced either with MCA or PV, and killed cells from the induced tumors were injected into syngeneic animals, which were then challenged with live cells from the indicated tumor-cell lines. The absence of tumor growth after live challenge indicates that the immune response induced by tumor antigens on the killed cells provided protection against the live cells.

Prepare cDNA library from tum- cells

Transplant

Tum+ cells i

Mutagenize and clone

Tum+ cells do not express TSTA

Tumor growth

Mutagenize and clone

Tsta Antigen

Transfect tum- gene into tum+ cells

Observe for lysis

Transfect tum- gene into tum+ cells

TSTA-specific CTLs 1

Incubate transfected tum+ cells with TSTA specific CTL clone

Observe for lysis

No lysis (cells do not express TSTA)

No lysis (cells do not express TSTA)

Lysis (cells express TSTA)

Tum+ cells do not express TSTA

Tumor growth

FIGURE 22-7

One procedure for identifying genes encoding tumor-specific transplantation antigens (TSTAs). Most TSTAs can be detected only by the cell-mediated rejection they elicit. In the first part of this procedure, a nontumorigenic (tum-) cell line is generated; this cell line expresses a TSTA that is recognized by syngeneic mice, which mount a cell-mediated response against it. To isolate the gene encoding the TSTA, a cosmid gene library is prepared from the tum- cell line, the genes are transfected into tumorigenic tum+ cells, and the transfected cells are incubated with TSTA-specific CTLs.

tumor in syngeneic mice. These mutant tumor cells are designated as tum- variants. Most tum- variants have been shown to express TSTAs that are not expressed by the original tum+ tumor-cell line. When tum- cells are injected into syngeneic mice, the unique TSTAs that the tum- cells express are recognized by specific CTLs. The TSTA-specific CTLs destroy the tum- tumor cells, thus preventing tumor growth. To identify the genes encoding the TSTAs that are expressed on a tum- cell line, a cosmid DNA library is prepared from the tum- cells. Genes from the tum- cells are transfected into the original tum+ cells. The transfected tum+ cells are tested for the expression of the tum- TSTAs by their ability to activate cloned CTLs specific for the tum- TSTA. A number of diverse TSTAs have been identified by this method.

In the past few years, two methods have facilitated the characterization of TSTAs (Figure 22-8). In one method, peptides bound to class I MHC molecules on the membranes of the tumor cells are eluted with acid and purified by high-pressure liquid chromatography (HPLC). In some cases, sufficient peptide is eluted to allow its sequence to be deduced by Edman degradation. In a second approach, cDNA libraries are prepared from tumor cells. These cDNA libraries are transfected transiently into COS cells, which are monkey kidney cells transfected with the gene that codes for the SV40 large-T antigen. When these cells are later transfected with plasmids containing both the tumor-cell cDNA and an SV40 origin of replication, the large-T antigen stimulates plasmid replication, so that up to 104-105 plasmid copies are produced per cell. This results in high-level expression of the tumor-cell DNA.

The genes that encode some TSTAs have been shown to differ from normal cellular genes by a single point mutation. Further characterization of TSTAs has demonstrated that many of them are not cell-membrane proteins; rather, as indicated already, they are short peptides derived from cytosolic proteins that have been processed and presented together with class I MHC molecules.

Was this article helpful?

0 0
How To Bolster Your Immune System

How To Bolster Your Immune System

All Natural Immune Boosters Proven To Fight Infection, Disease And More. Discover A Natural, Safe Effective Way To Boost Your Immune System Using Ingredients From Your Kitchen Cupboard. The only common sense, no holds barred guide to hit the market today no gimmicks, no pills, just old fashioned common sense remedies to cure colds, influenza, viral infections and more.

Get My Free Audio Book


Responses

  • polo
    Why tumor specific transplantation antigen is difficult to characterize?
    3 years ago
  • david
    Why tumor specific transplantation aantigen is difficult to characterize?
    3 years ago
  • steven
    How are tumor antigens induced?
    2 years ago

Post a comment