Human small cell lung carcinoma and carcinoid tumor regulate dendritic cell maturation and function.

The induction of apoptosis in dendritic cells (DC) is a key mechanism by which tumors escape immune recognition and elimination. In fact, a number of studies have showed the correlation between the number of DC within the tumor and the clinical prognosis, suggesting that increased infiltration of tumor tissue by DC was associated with better patient survival and low incidence of metastatic disease. We compared the number of DC and their distribution pattern in human small-cell lung carcinoma and bronchial carcinoid tumor (CT) tissues. Immunohistochemical analysis revealed the presence of cells expressing DC markers CD1a and CD83 in small-cell lung carcinoma tissues and the complete absence of these cells in CT samples. Next, we examined whether human lung tumor cells produce soluble factors that inhibit differentiation of hematopoietic precursors into mature DC. The addition of small-cell lung carcinoma-conditioned medium to CD34+ precursor cell cultures significantly inhibited colony-forming units of DC formation when compared with nontreated control DC cultures. Furthermore, DC generation and differentiation was completely abrogated in CD34+ cell cultures treated with CT-conditioned medium, suggesting that CT-derived factors blocked CD34+ cell differentiation into DC or induced their apoptosis. Finally, flow cytometry analysis of cultured DC confirmed these results. Thus, analysis of our data suggests that human lung tumors produce factors that inhibit DC generation or maturation and may also induce apoptotic death of DC precursors in vitro.