Tissue regulation of dendritic cells : with focus on chemokines, function and migration

Abstract: Tissue-specific cells, such as fibroblasts and epithelial cells in local microenvironments have been recognized to influence the function and phenotype of hematopoietic cells, such as dendritic cells (DC). The interaction and cooperation between DC and the cells of the tissue is important for the maintenance of immune homeostasis as well as orchestrating immune responses against pathogens. However, a majority of studies on human DC are performed under conditions absent of a relevant physiological milieu allowing interactions between different cell types. Thus, there is a need to develop in vitro human tissue models with immune cells that can capture cellular responses under conditions similar to those found in real tissue. In my thesis work, I have developed a human three-dimensional (3D) lung tissue model that has morphological and functional features mimicking those of human lung epithelial tissue. The model has a stratified epithelial layer with human DC that are situated closely to the epithelium and an underlying collagen matrix rich in fibroblasts. We have found that the lung tissue model supports DC survival for at least eleven days in the absence of exogenous growth factors. The tissue model also regulates chemokine production by DC leading to enhanced production of CCL18 and downregulation of CCL17 and CCL22, which resemble chemokine production under physiological conditions in lung tissue. In addition, using live cell imaging, we could observe that stimulation with toll-like receptor-ligands and CCL2 attracted DC to the epithelial layer as well as increased their speed and their ability to survey a larger area in the tissue model. We also found, using our newly established 3D tumour spheroid tissue model of non-small cell lung cancer, that DC are recruited to the tumour area and engulf tumour cells more readily than normal epithelial cells. Another major focus of this thesis work is the study of stromal cell-derived chemokines supporting regulatory DC development during L. donovani infection. Stromal cells are known to regulate hematopoiesis in the bone marrow and spleen by secretion of chemokines, cytokines and growth factors. Studies have shown that murine splenic stromal cells have the ability to support differentiation of hematopoietic stem and progenitor cells (HSPC) into regulatory DC and this ability is enhanced during L. donovani infection. We further showed that stromal cell-derived chemokines CXCL12 and CCL8 cooperate to recruit HSPC with the ability to differentiate into regulatory DC. In addition, we observed that direct infection of MBA-1 cells by L. donovani enhanced their ability to support regulatory DC as well as their ability to produce CCL8. Interestingly, CCL8 expression was strongly induced in splenic stromal cells of mice infected with L. donovani, which enhanced their ability to attract HSPC. Our findings suggest that L. donovani infection modulates the ability of stromal cells to recruit and support HSPC differentiation into regulatory DC, and this may be a mechanism used by the parasite to establish persistent infection. Together, the studies in this thesis show the impact of tissue specific cells on DC differentiation and function, and highlights the importance of taking into account tissue-specific components when studying DC biology.