Melanoma antigens

DFG-Project Pa 361/5-3

Melanoma is an aggressive skin tumour against which chemotherapy is essentially ineffective. However, most patients seem to have some immunity against their tumours, and clinical trials of immunotherapy have begun to exploit the possibility of using the patient´s own immune system to destroy the tumour. There are two types of immune cells required for rejection of the tumour. The first are designated CD8 killer T cells because they are able to lyse tumour cells. The second are designated CD4 helper T cells because they assist the CD8 cells to kill tumour as well as having anti-tumour affects themselves. In most melanoma patients, both types of T cells are present in very small numbers, which may be one of the reasons why the tumour eventually overcomes the patient´s defences. If it were possible to identify these anti-tumour T cells amongst all the other T cells in the patient´s blood, they could be separated from the latter and propagated in tissue culture. In this way, a large amount of tumour-specific T cells could be generated in the laboratory and re-infused into patients. This is called specific adoptive immunotherapy.

 

The present project addresses itself to the anti-tumour T cells, particularly the CD4 helper T cells, using new technology to identify, isolate and propagate such cells. First, the program is identifying antigens expressed by melanoma cells which can be recognized by the CD4 T cells on tumour cells but not on normal cells (tumour-specific antigens). Next, soluble complexes of these antigens (peptides) together with a molecule shared by tumour cell and T cell which enables the T cell to recognise the peptide (MHC molecule) are constructed ("tetramers"). Such complexes are bound by the antigen receptor on the tumour-specific T cells (but not by any other T cells) in the same way that it binds the antigen on the surface of the melanoma cells. Third, the soluble complexes of MHC molecules and tumour antigen will be labelled with a fluorescent dye. Then, T cells from patients´ peripheral blood will be mixed with these "tetramers" and examined under fluorescent light. Only the small numbers of tumour-specific T cells labelled with the tetramer will fluoresce, and these can be separated from the non-specific cells using a fluorescence-activated cell sorter. This yields a pure population of tumour antigen-specific T cells which can be propagated in the laboratory for extended periods. For those cells which can be established in culture (and not all of them can be) we estimate average longevity to be about 35 population doublings. This represents something like 3 x 1010 cells, which will be a useful number for immunotherapy.

 

This project is a pre-clinical study. However, in other projects, the feasibility of adoptive immunotherapy has been demonstrated, and we believe that the present project will bring effective immunotherapy of melanoma closer to clinical reality.