Lecture Summaries

"Humanization" can occur by genetic means, whereby a human gene is inserted into the mouse locus, or by the engraftment of human cells in immunocompromised mice. For this class, we will use the term humanized mice to refer to the latter: immunocompromised mice engrafted with human cells and/or tissues. As we discovered in Session 4, the earliest attempts to create mouse models of human cancer relied on the engraftment of human cancer cells in immunocompromised mice. Several different genetic backgrounds of immunocompromised mice have been used for xenograft assays. However, there are several limitations to these models, e.g. i) the presence of residual mouse innate and adaptive immune cells, and ii) the lack of functional human T cells. The absence of functional human T cells that are selected on human major histocompatibility complex-I (MHC-I) molecules is a major disadvantage for the development of models that rely on T cells as a mode of therapeutic intervention. During T cell development in the thymus, naïve thymocytes undergo several rounds of selection by thymic epithelial cells, which express MHC-I molecules in complex with peptide antigens. This interaction, also referred to as the education of the T cells, is important to select for T cells that are able to differentiate between self and non-self peptide antigens.

In humanized mouse strains that do not express human MHC-I molecules, the engrafted human T cells are selected on mouse MHC-I molecules. This is not ideal, as these T cells are a suboptimal model to study the role of human T cells, via interaction with human MHC-I: Peptide complexes, on disease progression and therapy efficacy. To address this problem, several research groups have attempted to generate improved humanized mouse strains that allow the development and education of human T cells by human MHC-I molecules. Lan et al. reasoned that the engraftment of human thymic tissue along with human hematopoietic stem cells in immunocompromised mice should allow for the development of T cells that are selected on human thymic epithelial cells. The model they developed is commonly referred to as the BLT (blood-liver-thymus) model and marks a momentous step forward in the field of humanized mice. Shultz and colleagues took a different approach by introducing the human MHC-I locus into an immunocompromised mouse strain that is devoid of mouse adaptive immunity and natural killer cells (NSG strain). This new strain of mice, known as HLA-A2-NSG, is a powerful tool to study the development and function of human T cells in an MHC-matched environment. We will compare and contrast the different approaches taken by the two groups to develop a better humanized mouse model.

Students will have the opportunity of visiting the Chen research lab to learn about some of the techniques used to generate humanized mice with de novo human cancer and a matching immune system.

Reminder: Written assignment due at the start of class

In the early 1900s, Paul Ehrlich was one of the first people to hypothesize that organisms with a long lifespan would have a high cancer burden if not for the protective mechanisms imposed by the body's natural defense, the immune system. This led to the development of the cancer immunosurveillance concept, which was later refined into the cancer immunoediting hypothesis by Robert Schreiber and colleagues in the early 2000s. In its most basic form, cancer immunoediting can be defined as an extrinsic tumor suppressive role of the immune system. There are three stages to immunoediting: Elimination, equilibrium and escape. In the first stage, a tumor is recognized and eliminated by the immune system before disease onset. In the event that there are malignant cells that survive the elimination stage, the adaptive immune system steps in and keeps these cells in a dormant state constituting the second stage–equilibrium. A person can remain cancer free as long as this equilibrium phase is unperturbed and the tumor is not allowed to grow. However, upon external cues, environmental stressors and changes in the tumor, the immune system becomes suppressed and the tumor escapes dormancy and grows uncontrollably, also known as escape, which manifests as apparent disease.

We will start today's class with a discussion of a seminal paper by Shankaran et al. that helped shaped the cancer immunoediting hypothesis by proving, using mouse models, that the long-held concept that the immune system can control cancer was indeed true. We will then discuss a second, older paper that shows evidence for cancer immunoediting in patients, providing evidence for the presence of an immune response against melanoma in patients. The Muul et al. paper describes the discovery of tumor-infiltrating lymphocytes capable of specifically lysing melanoma cells, thus proving that the immune system elicits a response against tumor cells.

Most of the chemotherapeutic agents used in the clinic today are decades old DNA-damaging agents that lack specificity for the tumor and work by attacking rapidly dividing cells, including tumor cells. However, due to this lack of targeting, these agents also cause a large amount of collateral damage in the form of unwanted side effects. Recently, several targeted immune-based therapies have been developed that aim to preferentially eliminate tumor cells. As we discussed in Session 9, of these therapies, antibody-based therapies have been the most promising. In this session, we will be discussing two papers that use antibody based therapies in conjunction with conventional chemotherapy to eliminate minimal residual disease in the bone marrow. Recall from Session 2 the concept of minimal residual disease: in most patients, disease relapse is due to a reservoir of cells that are highly resistant to conventional chemotherapy.

The first paper from Rambaldi et al. introduces us to a combination chemotherapy regimen, R-CHOP, which is widely used in the treatment of a type of B cell lymphoma. R-CHOP comprises of an antibody therapy (Rituximab) and four conventional chemotherapeutic agents (cyclophosphamide, doxorubicin hydrochloride, oncovin and prednisone). The second paper attempts to improve on this highly toxic chemotherapy regimen by testing new combination strategies in a humanized mouse model of B cell lymphoma. Pallasch and colleagues demonstrate that a lower dose of the highly toxic cyclophosphamide is effective at eliminating MRD in this humanized mouse model. This work was instrumental in altering the dose and administration strategy of R-CHOP in the clinic, and clinical trials are currently underway to implement this new strategy in the treatment of B cell lymphomas.

Traditionally, clinical trials are initiated based on the efficacy of drugs on tumors generated either in genetically engineered mouse models (GEMM) or xenograft models with patient tumor or cell lines. While both models have their downsides, several advances in developing sophisticated GEMMs with complex mutational backgrounds have suggested the feasibility of this approach in accurately predicting the efficacy of drug or drug combinations for human cancers. About 15 years ago, Pandolfi and colleagues found a combination of drugs that could eradicate acute promyelocytic leukemia in a GEMM that recapitulates the human disease and tested this combination in the clinic in 2004. Remarkably, the drug combination was successful in eradication of a leukemia that until then was deemed uncurable. This success prompted the conception of the co-clinical trial to speed up the discovery process and uncover new combinations of therapies to eradicate various cancers. A co-clinical trial is a GEMM drug trial run in parallel with a human clinical trial with similar drug regimens. Co-clinical trials can be held in parallel with Phase I, II or III clinical trials and aim at fine-tuning the clinical trials based on data obtained from GEMMs treated with similar chemotherapy regimens.

The co-clinical trial concept is relatively new, and these papers are by pioneers in the field setting the stage for future studies. It is important to note that as we have previously discussed, engineered mouse models have their limitations, especially the lack of human cells and a human immune system. Therefore, while these models will be important for the idenfitication of drug combinations for non-immune based therapies, their utility in developing and testing immunotherapies will remain limited. Today we will review two papers describing co-clinical trials, both of non-small cell lung cancer (NSCLC) but with different genetic lesions and different drug combinations.