In summary, basal IP-10 secretion is induced by monocyte-epithelial cell interactions, with a presence of lymphocytes, most likely to provide a source of IFN-γ. The interaction of monocytes and lung epithelial cells are made by direct cell-cell contact. (A). Addition of recombinant IFN-γ induces strong IP-10 secretion in co-cultures even in absence of lymphocytes. Moreover, a secreted factor from lung epithelial cells augments the IFN-γ mediated secretion of IP-10 from monocytes (B). Addition of recombinant IL-12 induces IFN-γ independent IP-10 secretion in Calu-3/PBMC co-cultures which cannot be blocked by IFN-γ antibodies. Moreover, no detectable IFN-γ is present and Calu-3 cells secrete a factor which augments IP-10 secretion from monocytes in response to IL-12 (C). Addition of recombinant IL-12 induces IP-10 secretion both by inducing IFN-γ secretion from lymphocytes and by an IFN-γ independent pathway, which cannot be blocked by IFN-γ antibodies (D).