Any good scientist knows that correlation does not necessarily equal causation. So, when two patients experienced complete remission of HPV-associated cervical cancer following adoptive therapy with tumor-infiltrating lymphocytes (TILs) that were selected for HPV reactivity, Stevanović et al. dug deeper to challenge the notion that a T cell response against the viral antigens was solely responsible for the therapeutic success. Supporting their hypothesis is recent, mounting evidence highlighting the importance of TILs that target neoantigens arising from somatic mutations, and germline antigens arising from genetic dysregulation.
The research team first confirmed that the infused TILs displayed on-target reactivity against the HPV-encoded proteins, then moved on to exploring other targets. In a hunt for neoantigen responses, the TIL samples from the first patient were screened for 222 predicted neoantigens and T cell activity was observed in response to three; no response was observed to seven germline antigens. In the second patient TILs displayed no immune response to 72 predicted neoepitopes but did display reactivity against the germline antigen KK-LC-1.
Single cell TCR sequencing revealed that in the first patient, mutated neoantigen-specific TCR clonotypes accounted for 35% of the TIL sample, while HPV-specific clonotypes represented only 14%. Similarly, for the second patient the germline antigen KK-LC-1-specific TCR clonotype represented 67% of the sample, while the HPV protein-specific TCR clonotype represented 14%.
Evaluation of peripheral blood samples showed that infused viral and nonviral tumor antigen-specific T cells expanded dramatically during tumor regression, with mutated neoantigen-specific TCRs expanding at a comparable rate to HPV-reactive T cells, while KK-LC-1-specific T cells expanded at a notably higher rate. A year after therapy, when both patients were in remission, tumor-specific T cells had reduced, but remained at elevated levels compared to pre-infusion peripheral blood samples.
Researchers were not able to characterize 100% of the TILs from these two samples, so while the current data does not prove which T cells were directly responsible for the eradication of tumor cells in these patients, the predominance of the non-viral tumor antigen-specific T cells in the TILs, their expansion during regression, and persistence during remission provides evidence that they may have contributed to cancer regression in these two patients. These frequencies and kinetics also provide evidence that viral antigens are not necessarily immunodominant over mutation-derived neoantigens or germline antigens.
Although HPV-associated cervical cancer is induced by the oncoproteins of HPV, accumulated mutations produce unique neoantigens that could serve as viable targets for personalized treatments. Additionally, the shared KK-LC-1 germline antigen is expressed in about 40% of metastatic cervical cancers, suggesting it could become a broader therapeutic target.
This study only explored the T cell landscapes in patients whose adoptive T cell therapy was successful; analysis of patients who did not respond successfully to immunotherapy may provide additional insight. In a nod to future research, Stevanović et al. analyzed pretreatment blood samples and found that most of the tumor-specific T cell clones expressed PD-1, suggesting that PD-1 checkpoint blockade could enhance a diverse T cell response in HPV-associated cancers.
by Lauren Hitchings
