Production of both interleukin-1 alpha and beta by newborn mouse calvarial cultures

The conditioned medium (CM) from 4-6 day newborn mouse calvarial cultures was found to contain thymocyte comitogen proliferation activity. This activity was blocked by an antiserum to murine interleukin-1 alpha (IL-1 alpha) but not by an antiserum to murine interleukin-1 beta. The release of thymocyte comitogen proliferation activity from the cultures did not appear dependent on endotoxin and was not associated with detectable interleukin-2 activity in the CM. Activity in the CM eluted from a gel filtration column with a peak Mr of 16-18 kD (the Mr of mature murine IL-1 alpha and beta is 17 kD). Western immunoblots of 100-fold concentrated CM demonstrated only a single 33 kD band with an antiserum to murine IL-1 beta and no bands with an antiserum to murine IL-1 alpha. However, this assay was relatively insensitive (limit of detection 1-10 ng compared with 1-10 pg for the thymocyte comitogen proliferation assay). Immunoprecipitation of [35S]methionine-labeled CM with three different anti-IL-1 alpha antisera, a more sensitive assay, demonstrated 15-17 kD bands in all cases. These results demonstrate that 4-6 day newborn mouse calvarial cultures spontaneously release 17 kD IL-1 alpha and 33 kD IL-1 beta into their conditioned medium. It appears that although 17 kD IL-1 alpha is the major bioactive form in the CM, 33 kD IL-1 beta is present in greater amounts. These results also suggest that local production of IL-1 can regulate bone cell function and may play a role in bone growth and remodeling.