Researchers at the University of Notre Dame discovered that amino acid nitration can inhibit the activation of T cells employed in immunotherapy against cancer and that suppression of reactive nitrogen species (RNS) responsible for nitration can significantly boost the effectiveness of immunotherapy .
Potent immunosuppressive mechanisms within the tumor microenvironment contribute to the resistance of aggressive human cancers to immune checkpoint blockade (ICB) therapy. One of the main mechanisms for myeloid-derived suppressor cells (MDSCs) to induce T cell tolerance is through secretion of reactive nitrogen species (RNS), which nitrates tyrosine residues in proteins involved in T cell function. However, so far very few nitrated proteins have been identified. Here, using a transgenic mouse model of prostate cancer and a syngeneic cell line model of lung cancer, we applied a nitroproteomic approach based on chemical derivation of 3-nitrotyrosine and identified that lymphocyte-specific protein tyrosine kinase (LCK), an initiating tyrosine kinase in the T cell receptor signaling cascade, is nitrated at Tyr394 by MDSCs. LCK nitration inhibits T cell activation, leading to reduced interleukin 2 (IL2) production and proliferation. In human T cells with defective endogenous LCK, wild type, but not nitrated LCK, rescues IL2 production. In the mouse model of castration-resistant prostate cancer (CRPC) by prostate-specific deletion of Pten, p53, and Smad4, CRPC is resistant to an ICB therapy composed of antiprogrammed cell death 1 (PD1) and anticytotoxic–T lymphocyte-associated protein 4 (CTLA4) antibodies. However, we showed that ICB elicits strong anti-CRPC efficacy when combined with an RNS neutralizing agent. Together, these data identify a previously unknown mechanism of T cell inactivation by MDSC-induced protein nitration and illuminate a clinical path hypothesis for combining ICB with RNS-reducing agents in the treatment of CRPC.
Immunotherapy, RNS, and MDSCs
Immunotherapy is the current cutting-edge approach to curing cancer; it relies on instigating the immune system to attack cancer cells, and while it shows great promise, it isn’t yet equally effective in all patients—indeed, some patients don’t seem to respond to it at all. Scientists are constantly at work to improve immunotherapy so that it will benefit the largest possible number of patients, and this includes figuring out which patients will or won’t respond and, most importantly, what causes a lack of response.
In their recent study, Professor Xin Lu and his team discovered that the activation of T cells employed in immunotherapy can be inhibited by the action of reactive nitrogen species, or RNS. Together with reactive oxygen species, these are free radicals, highly reactive molecules that each contain an unpaired electron; this leads to a tendency to steal electrons from nearby molecules, which may ultimately result in tissue damage. Both ROS and RNS can be produced by both internal and external processes; RNS, in particular, are produced by myeloid-derived suppressor cells, or MDSCs—immune cells that originate from bone marrow stem cells that interact with other immune cells, including T cells.
The results of the study show that RNS may cause the nitration—the addition of a nitro functional group (-NO2)—of an amino acid in tyrosine kinase, a protein specific to T cells. This protein is essential for successful T-cell activation, and nitration may alter its structure, thereby changing its function and inhibiting T-cell activation. The team further looked for ways to disable this inhibition mechanism so that immunotherapy may work.
In a mice model of prostate cancer, the scientists tried three different ways to block nitration; taken singularly, none of them was very effective, but the combination of immune checkpoint blockade and uric acid, both of which can neutralize RNS to a certain extent, successfully allowed T cell activation and yielded far better immunotherapy outcomes.
The abundant MDSCs in the solid tumors of prostate cancer also exist in other cancers, so Prof. Lu thinks that the results of this study might apply to them as well. If so, detection of the nitrated protein biomarker employed in this study might reveal whether administration of a MDSC inhibitor to prevent RNS production will be needed for immunotherapy to work. However, MDSCs in different tissues differ from each other, so the approach will need to be fine-tuned for each specific tissue.
 Feng, S., Cheng, X., Zhang, L., Lu, X., Chaudhary, S., Teng, R., … Lu, X. (2018). Myeloid-derived suppressor cells inhibit T cell activation through nitrating LCK in mouse cancers. Proceedings of the National Academy of Sciences, 201800695.