Aspects of the invention provides methods for preparing and using adipose-tissue-derived stem and progenitor cells, adipose-tissue-derived lymphatic endothelial cells, and cells capable of differentiating into lymphatic endothelial cells to treat disorders of the lymphatic system and to modulate expansion, repair, and/or regeneration of the lymphatic system. The invention further provides using adipose-tissue-derived lymphatic endothelial cells and cells capable of differentiating into lymphatic endothelial cells for delivery of therapeutic agents to tumor cells as a means for treating malignant disease, and assays to screen for drugs that modulate lymphatic system expansion, repair or regeneration.