Generation of an interleukin-I-like lymphocyte-stimulating factor at inflammatory sites: correlation with the infiltration of polymorphonuclear leucocytes.

Using a mouse thymocyte co-stimulation assay, we demonstrated thymocyte-stimulating activity in murine peritoneal fluid obtained from the early stage (3 to 9 h) of casein-induced inflammation. This early inflammatory stage coincided with the time at which an influx of polymorphonuclear leucocytes (PMN) into the inflamed site was observed. Similar thymocyte-stimulating activity was produced in vitro by PMN purified from 4-h peritoneal exudate but not by purified PMN obtained at a later stage (24 h) of the inflammation. The inflammatory factor was interleukin (IL)-I-like; it was devoid of IL-2 activity when tested with IL-2-dependent cells. It could stimulate murine thymocytes to produce IL-2. On a Sephadex G-75 column, the factor was eluted between the molecular sizes of 10 000 and 30 000; its peak activity was at 21 000. The factor mainly consisted of two (pI 6.5 and pI 5.0) iso-electrophoretically different factors.