6.7.2 Types of Antigens Used in TCR-T-Cell Therapy
In practice, there are different ways of categorizing tumor antigens targeted in TCR-T-cell therapy.Generally, human tumor antigens are classified into 2 main types: shared tumor-associated antigens(shared TAAs) and unique tumor-associated antigens (unique TAAs), or tumor-specific antigens.The former includes cancer-testis (CT) antigens, overexpressed antigens, and differentiation antigens, while the latter includes neoantigens and oncoviral antigens.Human papillomavirus(HPV) E6 protein and HPV E7 protein belong to the category of oncoviral antigens.The diversity and complexity of tumor antigens often give rise to “on-target, off-tumor” toxicity.Every type of antigen has not only its own characteristic advantages for TCR targeting but also its particular disadvantages [187].
Cancer-testis antigens are expressed in various tumor types as well as in testis, during times of spermatogenesis, and placenta.Normally they are silent in adult tissues, but they are active for transcription in different tumor types.In theory, because many tumor types express CT antigens at a high level and normal tissues rarely express them, they are deemed attractive and safe immunotherapy targets, but in practice, this is not always the case.Melanoma-associated antigen(MAGE)-A gene family is a group of CT antigens: MAGE-A1, MAGE-A2, MAGE-A3, MAGE-A4,MAGE-A6, MAGE-A10, and MAGE-A12 are expressed at a frequency of more than 1/10 000.In an NCI MAGE-A3 trial, TCR targeted at MAGE-A3 unexpectedly cross-reacted with the related peptide, MAGE-A12, which is expressed in the brain, resulting in the death of 2 of the first 9 patients and severe mental damage of the third.It was found that the human brain might also express MAGE-A1, MAGE-A8, and MAGE-A9.In another trial conducted by adaptive immune,the TCR targeting MAGE-A3 bound to an unrelated protein, titin, in the heart.These interactions between TCRs and normal tissues show the potential danger of targeting CT antigens [187].(https://www.daowen.com)
Differentiation antigens are encoded by genes that express in a tissue-specific manner.As a result,they are shared between tumor cells and healthy cells of corresponding tissues of origin, unlike overexpressed antigens, which are expressed in certain, if not all, healthy tissue types.Most such antigens, including glycoprotein (gp100), melanoma antigen recognized by T cells (MART-1), and tyrosinase, are mainly found in melanomas and normal melanocytes.However, differentiation antigens do include carcinoembryonic antigen, which is often highly expressed in colon cancer, and others are found in some epithelial tissues and tumors, including prostate cancers, in which case prostate-specific antigen is identified as one type of differentiation antigen.Targeting such differentiation antigens will be likely to induce “on-target, off-tumor toxicity” on normal cells in critical organs.The first 2 trials of TCR-T-cell therapy against MART-1 protein used 2 different proteins that were class I MHC-restricted but that were targeting similar epitopes, DMF4 and DMF5.Although DMF5-engineered T cells proved to be more avid than DMF4-engineered ones in functional assays, expanded trials with DMF5 T cells reported autoimmune toxicity on the eye, ear,and skin not reported with DMF4 T cells.In one trial, such toxicity consisted of erythematous skin rash observed in 14 of 20 patients, anterior uveitis observed in 11 of 20 patients, and hearing loss observed in 10 of 20 patients.Lethal cardiac toxicity was also observed in 3 patients undergoing trials against MART-1.On the one hand, to effectively eliminate solid tumors, TCR should be extremely potent in order to make any progress.On the other hand, when a therapeutic is highly effective, it would have the same effect upon tissues with low expression level as well as upon those with high expression level.Thus, the potency of TCR should be controlled in a precise range to not induce immune response up to a certain level but be potent enough to kill tumor cells after the threshold has been reached to reduce “on-target, off-tumor” toxicity [179,188,189].
Overexpressed antigens are antigens that are expressed in many normal tissues as well as different types of tumor.They are expressed at a higher level in tumor cells to reach the threshold of T-cell recognition.Because such antigens are also expressed, although, at a lower level, there are risks of“on-target, off-tumor” toxicity in normal tissues.For example, Wilms tumor 1 (WT1) is one kind of overexpressed antigen that is highly expressed in most acute myeloid leukemia (AML), acute lymphoid leukemia, and almost every type of solid tumor.It is also present in several critical tissues,such as heart tissues.Mesothelin, another kind of overexpressed antigen, is highly expressed in mesothelioma but is also present on mesothelial cells of several tissues, including trachea.Despite common apprehension of the “on-target, off-tumor” toxicity by targeting such antigens, reports have suggested that WT1-targeting T cells are able to differentiate between tumor cells and normal tissues.Using murine models, it was also suggested that enhanced-affinity TCRs targeting WT1 antigens, though surpassing the threshold for thymic selection, showed no autoimmune toxicity when transferred into wild-type mice.It thus could be deducted that WT1, and other overexpressed antigens, might have the ability to attack tumor cells but largely spare normal cells, making them ideal targets for TCR-T therapy [190,191].