Antitumor immunity and T-cell avidity

In this issue of Blood, Roex et al analyzed the frequency and functional capabilities of self-restricted tumor-associated antigen (TAA) specific T-cell clones isolated from peripheral blood of healthy donors.¹  They found that high-avidity T cells were not detectable, whereas a very limited number of intermediate avidity T cells, with the ability to recognize malignant cells, could be identified. In addition, most of the analyzed clones were nonfunctional and of low avidity, suggesting that a strong and meaningful immune response has to be elicited in cancer patients using a combination of complementary treatment options designed to increase T-cell responses and to enhance anti-tumor activity of vaccinations containing such TAAs.² 

Most TAAs that were identified and used in immunotherapeutic approaches represent nonmutated self-antigens that are preferentially expressed or overexpressed in malignant cells.³  Accordingly, epitopes from these TAAs are recognized by T lymphocytes with low or intermediate affinity T-cell receptors (TCRs) that require high levels of antigen to become activated and often fail to eliminate cells endogenously expressing the cognate antigen.^3,4  However, based on preclinical studies and results from vaccination trials, it was observed that T cells generated against these TAAs are able to recognize malignant cells and contribute to tumor regression. In addition, infusion of isolated tumor infiltrating lymphocytes as well as the adoptive transfer of ex vivo-generated TAA-specific T lymphocytes were shown to induce clinical responses in some patients with relapsed/refractory diseases, demonstrating that, under certain conditions, self-reacting T cells can mediate tumor cell elimination while sparing their nonmalignant counterparts.^5,6 

T lymphocytes recognize via the TCRs the cognate antigens that are presented as short antigenic peptides in the groove of the major histocompatibility complex on the surface of infected cells, malignant cells or antigen-presenting cells (APCs). T cells bearing receptors that strongly react with self-peptides are removed in the thymus (negative selection), referred to as central tolerance.⁷  A key mechanism that mediates the elimination of autoreactive cells from the developing polyclonal T-cell repertoire is the ectopic expression of tissue-restricted antigens by medullary thymic epithelial cells. Deletion of most aggressive T lymphocytes is also accomplished by immature dendritic cells or stromal cells in the lymph nodes that present the self-antigens. In addition, there is convincing evidence that regulatory T cells play a crucial role in maintaining tolerance.

Thus, thymic and peripheral tolerance ensure that high-avidity autoreactive T cells are negatively selected and deleted, but both spare cells that weekly recognize antigenic peptides. Low-avidity T lymphocytes can enter the periphery and persist without losing their effector functions, indicating that the threshold for their activation in the periphery is below of that required for negative selection. A weak peptide antigen that failed to delete self-reacting T lymphocytes can induce T-cell activation in the periphery because of higher ligand density, inflammation, or presentation by activated professional APCs (see figure).⁸ 

Roex et al analyzed the escape of TAA-specific T cells from negative selection and the avidity of these T cells by functional characterization of T-cell clones isolated from peripheral blood of healthy individuals. To accomplish this, they first used as a model minor antigen (MiHA) specific T cells that recognize the HLA-A2 binding HA-1H peptide. MiHA are peptides derived from polymorphic genes and can be recognized as foreign in the setting of allogeneic transplantation when there is a disparity between the donor and recipient. MiHA-reactive T cells were shown to mediate the elimination of leukemic cells and to induce the graft-versus-host disease. High-avidity T-cell clones could be isolated wonly from HA-1H⁻ patients where the used peptide antigen represented a foreign antigen. In contrast, in HA-1H⁺ individuals (self-antigen situation), only 1 clone showed limited cytotoxic reactivity, demonstrating that a very low number of autoreactive functional T-cell clones with intermediate avidity can escape from the thymic and peripheral tolerance.

In the next set of experiments, T-cell clones specific for several HLA-A2 binding epitopes derived from WT1, PRAME, RHAMM, proteinase, and NY-ESO-1 were isolated from the peripheral blood of healthy individuals. In line with the results using the HA-1H peptide, no high-avidity TAA-specific T cells were identified, and only a small number of intermediate avidity clones was isolated. Of 663 T-cell clones directed against the used TAAs, only 3 PRAME- and 1 NY-ESO-specific T-cell clone secreted interferon-γ or were able to lyse HLA-A2⁺ malignant cells in response to endogenous levels of antigen.

Importantly, similar to previous reports, the authors found that tetramer-positive T cells comprise many cells that are unable to recognize tumor cells and a large number of nonfunctional low-avidity T-cell clones specifically stained with the corresponding tetramer. Detection of vaccine-induced T cells as a surrogate parameter for efficacy using tetramer staining is broadly used for immunological monitoring and may overestimate the generation of efficient anti-tumor-specific responses, supporting the additional application of functional assays such as ELISpot assays, intracellular interferon-γ staining, or cytotoxic assays.

This study shows that a small proportion of self-reacting T cells can escape from negative selection and only a few TAA-specific T cells can be isolated from healthy donors with the ability to recognize malignant cells in an antigen-specific and HLA-restricted manner. This is probably different in cancer patients where the frequency of tumor-specific T cells might be higher (especially in the tumor microenvironment) and T cells might have different phenotypes and effector functions because of chronic exposure to tumor-derived antigens and the immunological composition of the tumor microenvironment. Most of these T cells have an exhausted phenotype characterized by the expression of check point molecules PD-1, LAG-3, and TIM-3. Some of these T lymphocytes can be invigorated by the application of check point inhibitors and mediate tumor regression.⁹  Combination of vaccination therapies with strong adjuvants and compounds that stimulate T-cell priming and activation such as check point inhibitors could enhance the induction of anti-tumor-mediated immunity and therapeutic efficacy.²