Immune Tolerance
The other major direction my laboratory has taken includes investigating mechanisms underlying tumor-mediated immunosuppression/tolerance as seen in animals with high tumor burdens. This represents a major obstacle in immune modulated cancer therapy. We have found a significant increase in the number of myeloid suppressor cells (MSC) and T regulatory cells in mice with large tumors. MSC can inhibit T-cell proliferation mediated by anti-CD3 and anti-CD28 (Kusmartsev et al., J. Immunol. 2000), HA peptide- (antigen-specific) mediated proliferation of TCR transgenic T cells, and tumor-specific CTL responses. Interestingly, MSC can be induced to differentiate into mature DCs (Li et al., 2003). Recently, we have established a conditionally-immortalized hematopoietic stem cell system to investigate the development, accumulation, and migration of MSC in tumor-bearing animals. A tumor model with an artificial tumor antigen (HA) has been established and will be utilized to study the underlying mechanisms of immune suppression and tolerance mediated by MSC and T regulatory cells in vivo.
My laboratory is also developing regimens capable of re-activating anergic T cells and identifying immunosuppressive factors secreted by tumor cells or MSC, which may regulate DC development and induce T regulatory cell development in mice and patients with large tumor burdens.
By identifying the molecules involved in immune suppression and tolerance, and studying their underlying mechanisms in hosts with large tumor burdens, we are reassessing a critical avenue to effective and persistent anti-tumor immunity for cancer immune therapies. My ultimate goal is translate the knowledge gained from animal models to comparable studies in clinical trial patients and thus work toward treatments for metastatic diseases. Already, we are expanding collaborations to translate the results of our preclinical research into clinical application for metastatic melanoma, colon, and breast cancers.
Malignant metastases present one of the most challenging roadblocks to cancer treatment. In order to develop a treatment modality for metastatic tumors, my lab is focusing on strategies for immune modulated cancer gene therapies. This promising approach has shown up to 100-1000 times higher concentrations of cytokines in situ, attracted dendritic cells (DCs) and inflammatory cells to the tumor site, yet reduced the systemic toxicity mediated by the cytokines. Specifically, my laboratory is pursuing two major directions, immune enhancement and immune tolerance.
Immune Enhancement
Our goal is to combine activation of innate and adaptive immunity. We are investigating the relevant immune activation mechanisms, e.g. IL-12 alone (Manuel et al., PNAS 1996, Khiem et al., Int. J. Cancer 1999), or in combination with agonism of co-stimulatory molecules 4-1BB (Martinet et al., J. of National Cancer Ins. 2000, Chen et al., Mol. Therapy 2000, Xu et al., 2003 Int. J. of Cancer), OX40 (Pan et al., Mol. Therapy 2002), and/or CD40 (manuscript in preparation). We have observed a dramatic synergistic therapeutic effect with the combination of IL-12 + 4-1BB activation, which translated to greater therapeutic efficacy than with either reagent alone. This allowed a greater than 18-fold reduction of the IL-12 viral dose while still achieving treatment goals.
These immune enhancing strategies have been actively studied in metastatic models of colorectal carcinoma, breast cancer, and melanoma with similar results. We hypothesize that IL-12 activated NK cells initiate the innate immune response by regulating the activation of and proliferation of DCs. More importantly, these activated DCs can be further activated and maturation induced by ligation of the co-stimulatory molecules with agonistic 4-1BB or CD40 antibodies or the corresponding natural ligands. Subsequently, these activated DCs can migrate into lymphoid organs where they activate nave T cells. These primed CD8+ and CD4+ T cell can be further activated and induced to undergo clonal expansion by 4-1BB and OX40 co-stimulation.
Results from studies detailing the in vivo immune mechanisms underlying these treatment modalities have been accepted or are under the revision for publication (Li et al., 2003, Pan et al., 2003, Xu et al., 2003). Currently, Phase I clinical trials are investigating adenoviral human IL-12 gene delivery for breast and colon cancer metastases to the liver. Also, a combination therapy trial for breast cancer is waiting final development of GMP-grade reagents. Clinical response and relevant information from treated patients will help improve our treatment paradigms and thinking.
Future directions in my laboratory include continued focus upon the interaction between NK and DCs; the effect of 4-1BB, OX40, and CD40 ligation on DC activation; and identification of receptors involved in NK/DC activation and linkage of innate and adaptive immune responses.
