Characterization of an inhibitory factor derived from epithelial cells of the small intestine.

Rat small intestine epithelial cell (EC) culture conditioned media (ECCM) contains a factor that has inhibitory activity against: a) the response of freshly isolated lymphocytes to Concanavalin A (Con A) or IL-2, b) the proliferative response to antigen of primed lymphocytes, and c) the normal growth of transformed cell lines derived from several species. Inhibition is reversible, and not the result of a cytotoxic effect. The inhibitory activity is enterocyte derived, effective at low concentration (1-2% in growth media), and can be derived from freshly isolated EC. Biochemical analysis indicates the inhibitory activity is associated with a protein with an approximate molecular weight of 32 kd, and an isoelectric point (PI) in the range of 3-5. The protein is trypsin sensitive, and labile with prolonged heating at 56 degrees C. The described activity differs from previously reported mucosally-derived inhibitory activities on the basis of molecular weight, and its ability to inhibit the growth of several cell lines. We suggest that this factor can provide the immunomodulatory activity necessary to produce the low level of intestinal T cell reactivity that is observed in vivo.