Typical targets of checkpoint inhibitors (usually humanized monoclonal antibodies) are molecules such as(PD-1), , . Applying checkpoint inhibitors ( ) when such molecules are expressed by cells within a given tumor is akin to releasing the brakes of tumor antigen-specific (TILs) present within them, empowering them to eliminate such tumors. This is the purported mechanism of action (MOA) of checkpoint inhibitors. However, their major problem is non-specificity, as in potential for off-target responses since they could release the brakes off of all T cells that express them, not just those that are tumor antigen-specific. To be able to effectively harness TILs to destroy any given tumor,
- Tumor should have TILs within it in the first place.
- Tumor should express ‘neoantigens’, i.e., tumor-specific antigens.
- TILs being used in immunotherapy (1) should be specific for peptides derived from antigens specifically expressed by tumor (2).
2 and 3 present formidable challenges because unless both conditions are met, cancer immunotherapy could end up attacking the body itself, with tragic results, since even death is possible. For example, while details haven’t been divulged, higher than anticipated clinical trial deaths have led leading cancer immunotherapy company,, to recently scrap its lead CAR-T therapy ( ).
Thus, checkpoint inhibitors notwithstanding, to assess whether TILs might be induced to specifically target a given tumor, the experimental process entails
- Assessing whether the tumor has TILs and if yes, then isolating them.
- Assessing whether tumor expresses tumor-specific antigens by comparing tumor and matched healthy tissue.
- Confirming tumor cells themselves can indeed process and present these tumor-specific antigens within MHC class I (to tumor-specific cytotoxic CD8+ T cells) or MHC class II (to tumor-specific helper CD4+ T cells) molecules, an extremely technically challenging task made even more arduous by the following caveats,
- Even if a given tumor expresses several tumor-specific antigens, it might not present peptides derived from them since tumors are known to have aberrant antigen processing and presentation pathways ( ).
- Differentiating immunologically relevant from irrelevant peptides is turning out to be more complex than envisaged ( ).
- MHC class II-binding peptides are longer and bind more promiscuously compared to those that bind class I, making binding predictions much more technically challenging.
- Tumor cells rarely express MHC class II.
- Assessing whether isolated TILs are tumor-specific, i.e., specifically bind tumor-derived pMHC (peptides bound to (MHC)).
- Assessing effect of checkpoint inhibitors on such tumor-specific TILs. If tumor-specific TILs don’t express the targets of such checkpoint inhibitors and yet also don’t attack and eliminate the tumor,
- They could be T regs ( ) in which case it may be difficult to impossible to engineer effective anti-tumor immunity using them.
- If not Tregs, may need to assess whether such TILs express novel ‘brakes’, targets for novel checkpoint inhibitors.
Plenty of obstacles stand in the way of these experimental imperatives. Steps 3 and 4 the most fiendishly complicated, no one method exists to reliably assess both in a high-throughput fashion nor are existingapproaches fool-proof ( ). There are other hurdles as well.
- Not all tumors express abundant tumor-specific antigens ( , , , ).
- Some tumors can mutate ferociously, i.e., cancer ( , ), which reduces the likelihood of finding tumor-specific TILs.
- Rather than being tumor antigen-specific, some TILs may be specific for antigens that both tumor and normal tissue cells express (cross-reactivity).
- T cells function by recognizing and binding pMHC through their (TCR). , the developmental process of eliminating T cells reactive to cells of the body in which they develop, is incomplete.
- The TCR is inherently cross-reactive, capable of binding >1 pMHC, i.e., can bind peptides derived from different antigens presented by the same MHC molecule (cross-reactivity) ( ).
- Thus, possible for checkpoint inhibitors to trigger autoimmunity.
However, promise of this approach is sustained by examples of tumor-specific TILs () having been found in
- Human non-small cell lung cancer ( ).
- Human melanoma ( , , , , , 18, ).
- Human AML (Acute Myeloid Leukemia) (20).
- Human CLL (Chronic Lymphocytic Leukemia) ( , ).
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