Cassian Yee

Appointments and Affiliations

Fred Hutchinson Cancer Research Center
Clinical Research Division
Program in Immunology
Member, Appointed: 2009
University of Washington
School of Medicine
Professor, Appointed: 2010
Fred Hutchinson Cancer Research Center
Immune Monitoring Laboratory
Director, Appointed: 2003
Professional Headshot of Cassian  Yee

Mailing Address

Fred Hutchinson Cancer Research Center
1100 Fairview Avenue N.
P.O. Box 19024
Seattle, Washington 98109-1024
United States


Phone: (206) 667-6287
Fax: (206) 667-7983


M.D., University of Manitoba, 1986.
B.Sc., University of Manitoba, Medicine, 1986.

Research Interests

Adoptive T Cell Therapy

Introduction and Synopsis

Adoptive cellular therapy involves the ex vivo isolation, enrichment and expansion of tumor-reactive T cells for infusion into patients with cancer. Unlike most conventional therapies, adoptive cellular therapy represents a non-cross-resistant modality that offers minimal toxicity and the potential for long-term immunoprotection from disease recurrence.

The T cells can come from the peripheral blood, tumor infiltrating lymphocyte population or may be engineered to express the antigen receptor by genetic modification.

Our work over the last 15 years has been focused on the generation of autologous antigen-specific T cells from the peripheral blood of patients; we have performed several first-in-man studies using ex vivo expanded antigen-specific T cells. 

We developed strategies to isolate rare high-affinity tumor-reactive T cells from the peripheral blood of patients, expand these to several tens of billions and infuse them for the treatment of refractory, progressive metastatic melanoma. Our studies demonstrate long-term persistence of infused T cells with encouraging clinical results, sometimes complete long-lasting responses, and more often a significant delay in time to progression.

We have formulated a path forward to translate our work into a treatment modality that can become more widely applicable to other malignancies (breast cancer, ovarian cancer, sarcoma and others), to a larger pool of patients, and with relatively modest infrastructure requirements.  Our research is focused on developing adoptive T cell therapy in combination with other immunomodulatory reagents including checkpoint inhibitors, vaccines and biologicals, and defining the intrinsic and extrinsic immune parameters for an effective, durable response. Below, we summarize our work to date and include a few selected footnote references. 

Tumor-reactive T cells circulate in peripheral blood of patients

These cells are present in rare numbers in the circulation but can be identified using peptide-MHC tetramers, which are several cross-linked monomers of the peptide-MHC complex. The peptide-MHC complex is the natural ligand for a specific T cell receptor (TCR). As a monomer, its dissociation rate is too high for binding; as a multimer conjugated to a fluorescent molecule, it can be used to tag antigen-specific T cells for flow cytometry and cell sorting.

We demonstrated in 1996 that melanoma-specific T cells could be isolated from patients with melanoma.1

We demonstrated in 1999 that there are circulating populations of melanoma-specific T cells in patients that can be detected using these tetramers2 and that it was possible to sort them, retain their viability and expand them to large numbers, sufficient for adoptive therapy. 3

Adoptive therapy using CD8 T cell clones

At that time (1980s)  the NCI was pursuing TIL therapy for patients, using lymphocytes collected from excisional tumor biopsies from patients with melanoma, growing these in vitro with high dose IL-2 (6000 U/ml) and treating patients with expanded TIL and high-dose IL-2.

We pursued a different approach by starting with patient peripheral blood as a source of T cells and trying to grow T cell clones so that we could examine more rigorously the fate of the infused T cell clones we had expanded in the lab.  We used autologous dendritic cells pulsed with a peptide representing the target epitope of melanoma associated antigens, MART-1, tyrosinase or gp100, to stimulate the  T cells in vitro. Because the antigen-specific T cells were present at frequencies of 1:10,000 or less, this required iterative cycles of in vitro stimulation, followed by limiting dilution cloning and then two cycles of expansion. We were able to generate 10-20 billion T cell clones from a single clone (4-5000 fold expansion per cycle) and then infuse these into patients with metastatic melanoma 4. Although it took 2-3 months to get a T cell product for therapy, we were able to treat several patients and demonstrate

  1. T cells, even after more than 20 doublings can still persist in vivo and respond to exogenous low-dose IL-2
  2. Transferred T cells recognizing a tumor-associated melanocyte antigen traffic to skin, destroy melanocytes causing vitiligo
  3. Transferred T cells traffic to tumor sites
  4. A modest clinical response. In 5 of 10 patients with refractory progressive metastatic melanoma, disease stabilization was observed for 3-33 months.

The CD8+ T cell clones survived in vivo and did respond to low-dose IL-2, but they only persisted for 2-3 weeks at most.

Adoptive therapy with CD4 T cells

We then pursued a study using CD4 + T cell clones reasoning that CD4 T cells produce their own IL-2 and may persist longer by  an antigen-driven autocrine mechanism.5  We targeted tyrosinase, a melanocyte-associated tumor antigen and NY-ESO-1 a cancer-testis antigen expressed in melanoma but also many other solid tumors, such as gastric cancer, breast cancer and head and neck cancer.

We found that these antigen-specific tumor-reactive CD4 T cell clones lasted for more than 3 weeks (up to 8 months)  in the patient’s peripheral circulation and that no IL-2 was required.

We found clinical responses in 5 of 9 patients including one patient with a complete response within 4 weeks of therapy. In that patient, we discovered that although we were targeting only NY-ESO-1, endogenous responses to MART-1 and MAGE-A3 were also elicited, likely through a process of antigen-spreading whereby, the initial tumor damage led to the release of non-targeted tumor antigens that were processed and presented by local antigen presenting cells and could elicit a broad endogenous T cell response to non-targeted antigens like MART-1, and MAGE-A3.

This was a first-in-man study using adoptively transferred CD4 T cells for the treatment of cancer.

More recently, we implemented three clinical trials to address three specific questions about adoptive T cell therapy:

  1. is lymphodepletion required ?
  2. will an immunomodulatory reagent such as anti-CTLA4 augment efficacy?
  3. will a post-infusion vaccine augment the transferred T cell response ? 

Lymphodepletion and Adoptive therapy with CD8 T cells leads to clinical response

The first study has been completed and will be published in the Proceedings of the National Academy of Science.6 We demonstrate that a short relatively low-toxicity course of cyclophosphamide conditioning was sufficient and necessary to mediate significant clinical responses (see  CT/PET scans on right). We observed one durable complete response with elimination of transverse colon metastasis within 4 weeks (now disease-free with no other therapy 3 years later) and  7 other patients presenting with progressive refractory cancer whose disease has  stabilized) .

More importantly, this study demonstrated that T cells could revert to a more de-differentiated state and impart central memory-like properties that render these tumor-reactive CTL, helper-independent.

The second study using anti-CTLA4 has already produced provocative data demonstrating the ability to boost anti-tumor T cell responses, both transferred and endogenous, by anti-CTLA4 administration. In this example the frequency of transferred NY-ESO-1-specific CTL which had plateaued at < 1 %, within a week, demonstrated a rapid 3-fold increase to more than 4% with anti-CTLA4 alone.

Path Forward

T cell therapy has been highly effective in metastatic melanoma. It has induced clinical responses, sometimes durable complete responses where conventional therapy  failed.

Treatment of other cancers with T cell therapy is feasible. We have treated patients with ovarian cancer and breast cancer and are beginning studies in patients with sarcoma.  Antigens, such as NY-ESO-1 are expressed in many solid tumor malignances and can be targeted with T cell therapy.

Future studies will address two major challenges:

  1. Increase the availability of adoptive T cell therapy as a treatment modality by investigating strategies to broaden the eligible patient pool, and improve the efficiency of generating antigen-specific T cells
  2. Develop combinational strategies to enhance the persistence and anti-tumor efficacy of adoptively transferred T cells

1. Increase the availability
One major limitation to broader application of adoptive T cell therapy for the treatment of patients with cancer has been the length of time required to isolate rare tumor-reactive T cells.
We have developed a novel strategy to accelerate the time from leukapheresis to T cell infusion from 3 months to 24 days.
This strategy is comprised of three parts:

  1. Depletion of CD25+ regulatory T cells  from responder T cell population
  2. Cytokine modulation with IL-21 during in vitro priming
  3. Tetramer-based selection with clinical grade cell sorter.

CD8 T cells that can be generated.7,8

To further enrich the population of NY –ESO-1 specific CTL, we recently acquired, modified and received regulatory approval to use a BD Influx cell sorter under clinical grade conditions.  With a cell sorter, we have been able to enrich, from a single round of in vitro stimulation,  a 0.2% population of tetramer-staining antigen-specific T cells to > 80% after sorting and in vitro expansion. We have succeeded in generating NY-ESO-1-specific CTL for adoptive therapy from  5 patients so far using this 3-pronged strategy; an autologous T cell infusion product can now be attained in 24-28 days.

2. Develop combinational strategies
The development of a coherent strategy to enhance the efficacy of adoptive T cell therapy will involve judicious manipulation of intrinsic and extrinsic factors. Intrinsic factors describe cell-associated features such as subset selection, for CD4/CD8 phenotypes, memory properties, cytokine or biological modulation or genetic modification to enhance safety, efficacy or function. One example of intrinsic manipulation is exposure to IL-21 (described above).

Extrinsic features include pre- or post-infusion immunomodulation and represent a combinational approach that encompasses the use of reagents that influence the immune response in order to:
  1. remove regulatory constraints
  2. provide homeostatic upregulation of growth factors
  3. facilitate eradication of stromal barriers
  4. augment the transferred and endogenous immune response

These extrinsic components are represented by previously approved chemotherapy or radiation therapy (e.g. for lymphodepletion conditioning) or  the newly emerging class of biologic and chemical reagents, that can augment the survival, function and anti-tumor efficacy of the transferred T cells.

With the advent of these novel immunomodulators,  a means to evaluate their in vivo activity can best be achieved using a well-defined population of effector T cells. Using methods described above to efficiently and routinely generate an expanded population of antigen-specific tumor-reactive T cells of defined specificity, phenotype, function and magnitude, we can investigate the influence of a given immunomodulator on an adoptively transferred population of T cells, track the immune response and correlate with clinical outcome. Within this framework we can build, design,  and re-invent how adoptive cellular therapy can be used for specific malignancies, disease stage and clinical condition and develop highly informative and effective treatment protocols for patients with cancer.

Future Research

1. Adoptive T cell therapy of melanoma
2. Adoptive T cell therapy of ovarian cancer
3. Murine model of tumor immunity (or lack thereof)
4. Novel methods in immune monitoring
5. Novel methods in isolation and expansion of tumor-specific T cells


American Association for Cancer Research
American Association of Immunologists
American Society for Clinical Investigation
Society for Biological Therapy

Previous Positions

1995-1998, Associate, Fred Hutchinson Cancer Research Center, Clinical Research
1989-1991, Resident, Stanford University
1987-1989, Postdoctoral Fellow, Ontario Cancer Institute




  • Burroughs Wellcome Fund Career Award
  • Cancer Research Institute Investigator Award
  • Cancer Research Institute - Melanoma Initiative - Clinical Trials Grant
  • NIH, NCI R01
  • Howard Hughes Medical Institute Pilot Research Grant
  • Damon Runyon Walter Winchell (Eli Lilly) Clinical Investigator Award


Recent Publications

Iyer JG, Afanasiev OK, McClurkan C, Paulson K, Nagase K, Jing L, Marshak JO, Dong L, Carter J, Lai I et al..  2011.  Merkel Cell Polyomavirus-Specific CD8+ and CD4+ T-cell Responses Identified in Merkel Cell Carcinomas and Blood.. Clinical cancer research : an official journal of the American Association for Cancer Research. 317(21):6671-6680. Abstract
Yee C.  2011.  Tumor Immunotherapy. The Molecular Biology of Cancer.

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