Counteracting resistance to cancer immunotherapy with neoadjuvant immune checkpoint blockade and targeting of novel regulators of T cell exhaustion

The interactions between cancer and the immune system reveal a dual role of the latter, best described with the concept of immunoediting: initially, the immune system readily eliminates nascent tumor cells, some which gain the ability to circumvent immune recognition, resulting in equilibrium, and eventually, immune escape. Recognition of cancerous cells rests on both genetic and epigenetic mechanisms: mutations give rise to novel protein sequences that are recognized as neoantigens, while epigenetic dysregulation enables expression of proteins that are normally restricted to distinct developmental phases or to immunoprivileged sites. Dendritic cells process such antigen from dying cancer cells, migrate to secondary lymphoid organs and prime naïve T cells. T cells seed back to the tumor bed and exert direct and indirect cytotoxicity onto cancer cells, which in turn promotes release of new antigen to dendritic cells. However, the anti-tumor T cell response is blunted by several mechanisms, including T cell exhaustion, an adaptive response to chronically elevated antigen levels which preserves some level of immune protection without causing immunopathology that is also observed in virus-specific T cells. Exhausted T cells express inhibitory receptors such as PD-1 and CTLA4 which dampen signaling through their T cell receptor. Antibodies against these receptors, so called immune checkpoint inhibitors, have shown impressive therapeutic potential in some cancers. However, only a minority of patients profit from this treatment so far, encouraging further studies to optimize treatment strategies and to acquire a deeper molecular understanding of the anti-tumor immune response in general and of T cell exhaustion in particular.
Neoadjuvant administration, that is, systemic therapy prior to surgical tumor resection, has enhanced the efficacy of checkpoint inhibitors, presumably by stimulating the immune response in presence of abundant antigen material. The SAKK 16/14 clinical trial tested a regimen of neoadjuvant chemoimmunotherapy in patients with locally advanced lung cancer and compares well to historical data. A translational analysis of this trial is reported as a first project here. Digital pathology analysis of tissue from pre-treatment biopsies and post-treatment surgical resections shows enhanced infiltration and intratumoral positioning of CD8+ T cells in patients responding well to treatment. These patients also had a more diverse array of T cell clones and systemic proliferation of CD8+ T cells. Furthermore, selected patients had elevated levels of serum CCL15, pointing towards a yet unrecognized signaling axis with the potential to attract tumor-specific T cells to the tumor site. In sum, the results presented here support the view that checkpoint inhibition is more efficacious when administered in a neoadjuvant manner by inducing a systemic immune response.
To further elucidate the molecular drivers of T cell exhaustion, existing studies have overwhelmingly relied on mouse models of chronic infection and transplantable tumors. As chronic antigen exposure is both necessary and sufficient to induce exhaustion, repetitive in vitro stimulation of primary human T cell is increasingly used as a model system. Here, the proteome of in vitro generated exhausted T cells was profiled and found to resemble the proteome of in vivo peers. Genes encoding for overexpressed proteins were used to build a CRISPR/Cas9 knock out library. Enrichment of gene knock outs in cells that had retained the ability to produce effector cytokines after repetitive stimulation included some well characterized exhaustion-related genes. Additionally, ablation of previously uncharacterized exhausted regulators ataxin-1 and leucine carboxyl methyltransferase 1 partially rescued T cells from exhaustion as evidenced by improved cytokine secretion, tumor cell killing capacity and signaling flux. First adoptive T cell transfer experiments support the relevancy of both genes in vivo. Both projects offer numerous observations that will benefit future studies into the systemic interplay of anti-cancer immunity as well as the molecular pathways limiting the efficacy of T cells in the tumor context.