so what are these tumor antigens we know that antigens are those foreign substances that are capable of stimulating an immune response against them particularly production of antibodies similarly tumor antigens are those molecules that are produced by tumor cells and they can potentially be recognized by the immune system many tumor cells produce antigens which may be released in bloodstream or remain on cell surface and they have been found in many carcinomas lymphomas and many solid cell cancers here a key role of the immune system is to detect these antigens so to permit subsequent targeting for eradication
there are a number of different types of tumor antigen including viral proteins the viral proteins that normally act as antigen for themselves can also act as tumor antigen if that invading virus is capable of tumor formation for example Epstein bar virus evv and human Pila virus produce specific proteins such as HPV protein E6 and E7 in cervical cancer cells these proteins then act as tumor antigens and help in tumor detection that are also called as oncoviral proteins next category is developmental proteins or enco fetal proteins these proteins are normally expressed during development process or during
fetal life only but are absent in normal developed adult cells however in case of tumor some enco fetal or developmental proteins reexpress themselves on tumor cells for example carcinoembryonic antigen C EA is normally expressed on many tissues during fetal life but absent in adult life but in gastro intestinal cancer CAA can be abnormally expressed on tumor cells that can generate immune response against them thus acting as tumor antigen next one is lineage specific proteins these proteins are not only expressed in cancer cells but also in the normal tissues from which those cancer cells arise for
example melanoma is a skin cancer of melanocytes normal ocytes and cancerous melanoma cells both Express the enzyme tyrosinase which is not expressed in any other cells normal or abnormal so these lineage specific antigens are present in either normal or abnormal cells of same lineage and thus they are detected as tumor antigen only based on their intensity now the last category of tumor antigen is proteins produced through translocations as we have have already discussed in lymphoid tumors that one of the reason of tumor formation is chromosomal translocation that alters Gene positions over chromosomes so proteins that
are produced through these translocations are those that are present only on tumor cells and thus can be detected and can generate immune responses for example as discussed under chromosomal translocation the BC AVL fusion protein that is formed as a result of BC A1 translocation is an oncoprotein that stimulates uncontrolled cell proliferation and also it can be detected as tumor antigen ideally a tumor antigen should be present or expressed only by tumor cells and not by normal cells in order to be recognized as foreign by the immune system by in reality most tumor antigens are overexpressed
in tumor cells but in small amounts they are also expressed in normal cells like Developmental and lineage specific proteins are poorly immunogenic because they are expressed on both in tumor cells and normal tissues as well but viral and fusion proteins tend to be more immunogenic because they are never present in the normal individual but only found in tumor cells so tumor antigens are expressed in two ways either they are tumor specific antigens or tumor Associated antigens tumor specific antigens tsas are found in cancer cells only and never in healthy cells for example point mutations in
tumor suppressor Gene p53 can lead to development of tumor specific peptides that do not occur in normal cells and tumor Associated antigens taas are those that are found in both healthy and cancerous cells but in cancerous cells elevated levels of these antigens are expressed and in normal cells they're expressed in small amounts for example Alpha feto protein are present in normal fetal cells but are overly expressed in adult patients with tumor of the liver so this was all about the introduction of tumor antigen in the last few years immunologists have put much effort in finding
different types of tumor antigens as they can be the main target for imun therapy against cancer if tumors do Express antigens that can be recognized by the immune system the big question is how do cancer cells avoid destruction and develop into full-blown tumors well we will answer this question in our next section [Music]
