Immuno-oncology drug development challenges researchers to model the interaction between novel immunotherapy agents, functional T cells, and in vivo models of cancer. In this introduction to T cells and immunotherapy studies, we’ll explore the function and analysis of T cells in preclinical drug development.
What Are T Cells?
T cells belong to the subset of leukocytes responsible for cell mediated immunity. Their main function is to fight viruses by killing virally infected cells, thereby stopping virus replication. This is achieved through a complex network of proteins expressed on both the T cell and the target cell.
These proteins coordinate both the recognition of the infected target cell by the T cell, and the subsequent activation of the T cell:
- Recognition is achieved through the target cell presenting viral peptides, which T cells distinguish as foreign.
- Activation is achieved through stimulation of a complex network of cell surface and soluble ligands, and cell surface receptor signaling molecules.
T Cell Subsets and Functions
There are subtypes of T cells, too, defined by cellular function and cell surface markers.
- The cell surface marker CD4 highlights T helper cells, whose role is to coordinate an immune response by the secretion of a broad range of cytokines.
- CD8 positive cells are the T killer cells, while the role of the T regulatory cell, characterized by the expression of the FOXP3 transcription factor, is to suppress an immune response.
- Finally, T memory cells (as the name implies) are responsible for establishing long term memory, thereby forming the basis of vaccination.
There are other T cell subtypes, as well, and each contributes to the coordinating and carrying out an effective, cell-mediated immune response.
Cancers Hijack Regulatory Systems to Suppress T Cells
From the above description, T cells seem to be the perfect host defense against cancer. T cells generate a coordinated effective cytotoxic response, and they remember. Yet, cancer still happens. We now know that cancers leverage a number of regulatory systems designed to diminish an immune response – and this has given rise to the currently approved immuno-oncology agents targeting the so-called checkpoint pathways (e.g. antibodies against PD-1, PD-L1 and CTLA-4).
Understanding how novel experimental therapies augment T cell function is critical to progress in cancer immunotherapy. This is where many preclinical challenges still reside. Syngeneic animal models of cancer, models in which there is a complete immune system, have been used widely to illuminate different mechanisms of cancer immunotherapy.
Preclinical Immunotherapy Models
How do we best use these models? Our goal is to kill cancer cells, and this typically translates to %TGI endpoints in animal studies. Making tumors smaller is always a good goal, and eliminating them is ideal, but understanding how this is achieved is another matter. Making immunologically “cold” tumors “hot” is also a critical goal.
Analyzing whether enhanced cancer cell killing is related to T cell activation, proliferation, or migration, and what T cell subset is being affected by an experimental therapy, will help us better evaluate candidates for combination therapy. For instance, combining two agents, one that enhances T cell migration and one that enhances T cell killing would be desirable.
Monitoring Immunotherapy PD Endpoints and T Cell Spectrum with Flow Cytometry
For immunotherapy studies, these pharmacodynamic endpoints are typically measured through flow cytometry. Multicolor staining and gating approaches allows researchers to profile many different cell types in a tumor sample, including a complete spectrum of T cells.
Cellular activation can be measured through the assessment of cytokine production, and cell proliferation can be measured through markers expressed in dividing cells. For a comprehensive assessment of cell populations within a tumor mass, flow cytometry cannot be beaten.
Maintaining Spatial Information with Immunohistochemistry
There is one drawback to flow cytometry, however. To facilitate the analysis, the tissue sample must be processed into a single cell suspension. This processing destroys any spatial information about the cells within the sample. Therefore, one might not be able to distinguish between “hot” tumors and “cold” tumors, where the immune cells are present but have been excluded.
In this case, it is always a good idea to have access to histological samples to make any assessments of the spatial distribution of cells within a tumor. While not as high throughput, immunohistochemical approaches can highlight all of the same markers used in flow cytometry, with the advantage of maintaining spatial orientation of the various cell populations in the tumor.
Understand the Immune System to Enhance Immunotherapy
Immunotherapies are indirect approaches to cancer treatment; the drugs augment the function of immune cells, which then affect the cancer. With this in mind, it becomes critical to go beyond %TGI to better understand how to harness the immune system in cancer immunotherapy.