As a critical component of the innate immune system, natural killer (NK) cells are widely known for destroying virally infected cells while also detecting and controlling early signs of cancer.1 However, NK cells are not always reliable anti-cancer actors, as they rely on signals from activating and inhibitory receptors on the NK cell surface to dictate whether or not to kill certain cells. Stimulating receptors recognize molecules that are expressed on the surface of cancer and infected cells and can then “switch on” the NK cell, while inhibitory receptors act as a check on NK cell killing.1 Thus, NK cells may receive inhibitory signals which instruct them not to kill cancerous cells, leading to continued cancer progression. More research is needed into the mechanisms of these signaling processes, and what treatments may promote NK cell activating receptors for enhanced function against cancerous cells.
A recent study published in Communications Biology used IsoPlexis’ functional proteomics to provide insight into how the tumor microenvironment undergoes immune evasion through secretion of sMIC, which elicits a strong pro-inflammatory response in NK cells, leading to continued cancer progression. This work fills a critical gap in researchers’ understanding of the mechanisms of inhibitory NK cell signaling and how these signals can be prevented with novel treatments.
sMIC Deters NK Cells from Eliciting an Anti-Tumorigenic Response
Activation of NK cells occurs through the engagement of both activating and inhibitory surface receptors. One type of activating receptor is Natural killer group 2, member D (NKG2D), which is expressed on nearly all NK cells and is engaged through interaction with its specific ligand NKG2D-L. This receptor-ligand interaction activates NK cell cytotoxic function in mice.2 The expression of NKG2D ligands are typically triggered by cells experiencing an inflammatory reaction such as oncogene development or viral infection. One type are the MIC ligands that are expressed during early-stage cancer development.3 However, the relationship between MIC ligand-NKG2D receptor interaction and NK cell activity has remained unclear.
In the study published in Communications Biology, researchers Dhar, et al. sought to explain the molecular mechanisms of soluble and cell surface-bound MIC (sMIC and mMIC, respectively) on NK cell function, as both have been discovered in human tumor environments.4 Previous work has demonstrated that sMIC downregulates NKG2D expression and NK cell cytotoxic function.5 Therefore, the team performed multi-omic analysis, revealing that in the presence of sMIC, NK cells demonstrated an enrichment in gene transcripts associated with inflammation and pro-tumorigenic cytokines and chemokines such as IL-10, CCL1, and CCL3. In contrast, genes associated with cytotoxic function were downregulated.
Using IsoPlexis’ single-cell functional proteomics platform, researchers found that NK cells treated with sMIC showed a reduction in their polyfunctional strength compared to untreated NK cells, as well as a depletion in the secretion of cytotoxic effector cytokines. The depletion in polyfunctional strength index (PSI) and effector cytokines by sMIC interaction could be recovered through the addition of an antibody that specifically clears sMIC. Consequently, the researchers concluded that sMIC regulates NK cell cytotoxicity through the CBM signalosome pathway and that interaction with sMIC reduces NK cell polyfunctionality.
In contrast, co-cultures of NK cells with mMIC-expressing cancer cells revealed enhanced cytotoxic activity through both tumor-killing and immunoblot assays. Therefore, mMIC does not appear to reduce NK cell function, unlike sMIC.
To confirm that the in vitro findings were replicated in vivo, the team developed mouse xenografts with human pancreatic cancer cells expressing both sMIC and mMIC and subsequently performed adoptive NK cell therapy. As expected, these mice exhibited increased tumor volume and reduced survival. However, mice treated with NK cell therapy and an anti-sMIC antibody showed significant reductions in tumor volume and overall greater survival.
Single-Cell Proteomics Reveals a Potential Target for Increasing NK Cell Therapy Efficacy
The work presented in this study4 provides insight into a mechanism of pro-inflammatory response in NK cells which leads to increased tumor burden and cancer progression. IsoPlexis’ Single-Cell Secretome solution, a fully automated solution for single-cell functional phenotyping of immune cells, revealed that these NK cells showed a significant reduction in cytotoxic effector protein secretions. In addition, this solution helped demonstrate that such a state is remedied through antibodies targeting sMIC, which disrupts the sMIC-NK cell interaction.
The findings of this study improve our understanding of how the anti-cancer function of NK cells is mediated by inhibitory and activating signals, and how the inhibitory signals which enable cancer progression can be counteracted. The preclinical value of IsoPlexis’ single-cell solution is shown in the in vivo work that demonstrated that treating mouse xenografts with an anti-sMIC antibody combined with NK cell therapy leads to a significant reduction in tumor volume and promotes long-term survival.
With IsoPlexis’ single-cell functional proteomics platform, researchers are able to uncover the mechanisms of cancer progression and regression to optimize and identify novel treatments.
- Eissmann, P. (n.d.). Natural killer cells. British Society for Immunology. https://www.immunology.org/public-information/bitesized-immunology/cells/natural-killer-cells
- Cerwenka, A et al. “Ectopic expression of retinoic acid early inducible-1 gene (RAE-1) permits natural killer cell-mediated rejection of a MHC class I-bearing tumor in vivo.” Proceedings of the National Academy of Sciences of the United States of America 98,20 (2001): 11521-6. doi:10.1073/pnas.201238598
- Dhar, Payal, and Jennifer D Wu. “NKG2D and its ligands in cancer.” Current opinion in immunology 51 (2018): 55-61. doi:10.1016/j.coi.2018.02.004
- Dhar P, et al. Tumor-derived NKG2D ligand sMIC reprograms NK cells to an inflammatory phenotype through CBM signalosome activation. Communications Biology 2021; 4(905).
- Wu, Jennifer D et al. “Prevalent expression of the immunostimulatory MHC class I chain-related molecule is counteracted by shedding in prostate cancer.” The Journal of clinical investigation 114,4 (2004): 560-8. doi:10.1172/JCI22206