Adam Snook, PhD

My research has focused on the discovery and clinical development of novel cancer immunotherapeutics targeting the self/tumor antigen guanylyl cyclase C (GUCY2C; GCC). Current research projects focus on:

Cancer Mucosa Antigens as Immunotherapeutic Targets for Metastatic Tumors

Immunotherapy for human cancers is hindered, in part, by a lack of suitable target antigens. This is particularly relevant in tumors derived from mucosal tissues such as colorectal cancer, in which antigens that are sufficiently immunogenic, tumor-restricted and shared among patients are lacking, and for which conventional therapeutics are poorly efficacious. We have explored a novel class of tumor-associated antigens fulfilling these criteria by exploiting immune compartmentalization, which restricts cross-talk between systemic and mucosal immune compartments. This compartmentalization limits systemic tolerance to mucosa-restricted self-antigens and shields mucosa from systemic autoimmune responses. Thus, a novel paradigm suggests that targeting self-antigens expressed by normal mucosal tissues and by derivative neoplasms should permit effective immunotherapy against systemic metastases, without inducing autoimmunity in normal mucosa. We have explored targeting the first of these antigens, termed cancer mucosa antigens (CMAs), in animal models of metastatic cancer. Adenoviral vector vaccines were generated containing guanylyl cyclase C (Ad5-GUCY2C), expressed in normal intestinal epithelium and all primary and metastatic human colorectal cancer (CRC) specimens. Immunization elicited CD8+ T, but not CD4+ T or B, cell responses in multiple strains of mice. Moreover, responses effectively prevented the development of lung and liver CRC metastases and treated established lung CRC metastases. This occurred in the absence of autoimmunity against normal GUCY2C-expressing tissues. These results suggest the utility of GUCY2C-specific immunotherapy for gastrointestinal malignancies, as well as the potential for CMA-targeted immunotherapy for malignancies of other mucosae such as oral, respiratory, mammary, and urogenital tissues, to treat head and neck, lung, breast, and bladder cancers, respectively. Currently, I am collaborating with a large multi-disciplinary team of investigators, including laboratory-based scientists, medical and surgical oncologists, pathologists, statisticians, and clinical trial professionals on testing the GUYC2C vaccine in gastric, esophageal, pancreatic, and colon cancer patients, with the hope that this vaccine can improve the survival of patients with these cancers.

Tolerance Mechanisms Restricting GUCY2C-Specific Immunotherapy

Immune tolerance limits immune responses to self-proteins, preventing autoimmunity and creating a significant barrier to developing cancer immunotherapeutics. Defining tolerance mechanisms for self/tumor antigens will allow for the development of new therapeutics by augmenting tolerance to treat autoimmune disorders or by inhibiting it to enhance tumor immunotherapeutic efficacy. In that context, recent murine studies have revealed a novel tolerance mechanism limiting GUCY2C-specific immune responses and antitumor efficacy. Rather than eliminating all three adaptive immune lineages (CD4+ T, CD8+ T, and B lymphocytes), GUCY2C tolerance was characterized by selective CD4+ T cell tolerance, while CD8+ T and B cells were unaffected. Importantly, CD4+ T cells serve a critical "helper" role in immune responses, and in the absence of GUCY2C-specific CD4+ T cell responses, CD8+ T and B cell response were also inhibited. However, this creates a unique opportunity to engage latent CD8+ T and B cells, restoring antitumor immunity, through the addition of CD4+ T cell epitopes from foreign antigens (such as influenza virus). Those epitopes activate foreign antigen-specific CD4+ T cells, which “help” GUCY2C-specific CD8+ T and B cells. That vaccination approach is being employed in the ongoing clinical trials in which patients receive a GUCY2C vaccine containing the helper epitope PADRE (Ad5-hGCC-PADRE). Further defining mechanism(s) underlying select CD4+ T cell tolerance to GUCY2C will create strategies for next-generation GUCY2C vaccines.

GUCY2C-Targeted Adoptive T Cell Therapy

Development of innovative paradigms beyond vaccines is needed to safely and effectively treat patients with advanced, bulky tumor metastases. While a vaccine relies on a patient's immune system to create an antigen-specific immune response, adoptive cell therapy (ACT) using engineered T cells offers unique advantages. Specifically, tolerance removes antigen-specific T cells during their maturation, limiting the number and the potency of T cells that escape tolerance and can be harnessed by vaccination. In contrast, T cells can be engineered and expanded ex vivo to produce large numbers of cells expressing receptors of high affinity, maximizing their potency. Importantly, we have shown that while GUCY2C vaccines possess antitumor activity, they are limited by tolerance, reducing their efficacy against large, established tumors. In mouse models, GUCY2C-targeted ACT is superior to GUCY2C-targeted vaccines in the context of established tumors. Similar techniques also have produced remarkable clinical successes in melanoma, neuroblastoma, and leukemia, but ACT approaches for adenocarcinomas remain undefined. Currently, we are interested in identifying antigen-targeted ACT approaches which can be immediately translated to patients with metastatic disease originating in mucosal tumors. Ideally, physicians would have in their therapeutic armamentarium both antigen-targeted vaccines and ACT, providing a universal strategy for patients. In this paradigm, early-stage patients can be safely and effectively treated with antigen-targeted vaccines to eradicate micrometastases underlying disease recurrence, while late-stage patients can be treated more aggressively with antigen-targeted ACT to eradicate metastatic disease. 

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• Dr. Snook's Research Lab Site