Although previous attempts to utilize the immune system to treat cancer patients have had limited success, there has been considerable progress in understanding how cancer cells could escape from this system, and also how to amplify innate and adaptive immunity to treat cancer patients. The main focus of our research has been on the dissection of the molecular mechanisms regulating the transition from innate to adaptive immunity to provide a new strategy to enhance adaptive immunity against cancer. Neutrophils are the most abundant leukocyte population and comprise about two-thirds of peripheral blood leukocytes. Neutrophils are the first to arrive at sites of tissue injury in response to invasion by pathogens or noxious stimuli, and play an important role in innate immunity. In addition, studies using animal disease models suggest the involvement of neutrophils in the development of adaptive immunity. This led us to consider neutrophils as a reasonable source of secreted substances that attract monocytes and lymphocytes. We previously demonstrated that cytokine-activated human neutrophils express a high level of the CC-chemokine monocyte chemoattractant protein (MCP-1), a critical mediator capable of recruiting monocytes, T lymphocytes and immature dendritic cells (iDCs), during both innate and adaptive immune responses. Therefore, we hypothesized that neutrophil-derived MCP-1 could contribute to the transition from innate to adaptive immunity by attracting monocytes, T lymphocytes and iDCs to injury sites. To test our hypothesis, we are in the process of generating a genetically modified mouse model in which the gene for MCP-1 is specifically deleted in PMN by using the Cre/loxP system. Our model will provide a definitive answer as to whether PMN-derived MCP-1 contributes to the transition to adaptive immunity. During the immune responses, the fate and functions of tissue-infiltrating leukocytes are strongly influenced by up- or down-regulation of cell-surface receptor expression, and subsequent interaction of the receptors with their ligands. These ligands include a wide range of agents secreted at the sites and the components of the extracellular matrix (ECM). Discoidin domain receptor 1 (DDR1) is a receptor tyrosine kinase constitutively expressed in normal epithelial cells and also in tumor cells of epithelial origins and activated by binding to collagen of the ECM. Our previous cDNA array study indicated considerable up-regulation of DDR1 mRNA expression after in vitro activation of neutrophils. Further study of DDR1 led us to a discovery that the expression of DDR1 was inducible not only in neutrophils but also in monocytes and lymphocytes, important participants in adaptive immunity. Since DDR1 is activated by the binding to collagen, the most abundant protein in tissues, we hypothesized that the interaction of DDR1 with collagen could influence the migratory behavior and maturation/differentiation of leukocytes in a tissue microenvironment. In fact, we have previously demonstrated that overexpression of the DDR1a isoform in the human leukemic cell line, THP-1, promotes their migration through three-dimensional collagen lattices. We recently evaluated the role for DDR1 in the differentiation of THP-1 cells. Although activation of either the DDR1a or DDR1b isoform with collagen had no effect on the differentiation of THP-1 cells, activation of DDR1b, but not DDR1a, facilitated the differentiation of THP-1 cells when they were induced to differentiate in response to phorbol ester. Thus, we have identified a distinct role for DDR1a and DDR1b in cell migration and differentiation, respectively. Furthermore, activation of DDR1, most likely DDR1b, on human blood monocytes or iDCs with DDR1 agonists, such as collagen or agonistic anti-DDR1 antibody, resulted in the generation of highly differentiated macrophages or mature DCs, suggesting a potential use of DDR1 agonists to enhance adaptive immunity. To understand the underlying molecular mechanisms regulating the DDR1 effects, we studied signal transduction by DDR1 using human macrophages and DCs. DDR1b signaling targets p38 mitogen-activated protein kinase (MAPK) through TNF receptor associated factor 6 (TRAF6)/TGF-?-activated protein kinase 1-binding protein 1? (TAB1?)-mediated autophosphorylation of p38a MAPK. This indicates that the interaction of DDR1b with collagen of the ECM results in a requisite intracellular signaling cascade that enables leukocytes to mature/differentiate in a tissue microenvironment. We will attempt to further identify the molecules involved in the DDR1b signaling and also to characterize the signaling pathways downstream of DDR1a to better understand the role of DDR1 expressed on not only leukocytes but also epithelial cells, including cancer cells.
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